Installing a Subpanel

Contents

  1. 1. Preface
    1. Disclaimer - I'm not an Electrician
  2. 2. Planning
    1. 2.1 Do you need a Subpanel
    2. 2.2 Get a Permit
    3. 2.3 The NEC
  3. 3. Panel Location
  4. 4. Choose your Hardware
    1. 4.1 The Breaker
    2. 4.2 The Subpanel
      1. 4.2.1 An Outbuilding Subpanel
        1. 4.2.1.1 A Main Breaker is Needed
        2. 4.2.1.2 An Earth Ground is Needed
      2. 4.2.2 A Subpanel in the House
      3. 4.2.3 Converting a Main Panel to a Subpanel
    3. 4.3 The Feeder
      1. 4.3.1 Material - Copper or Aluminum
      2. 4.3.2 Insulator type
        1. 4.3.2.1 Feeder Insulator Type for an Outbuilding Installation
          1. 4.3.2.1.1 Overhead Feeder Run
          2. 4.3.2.1.2 Underground Feeder Run
        2. 4.3.2.2 Feeder Insulator Type for an In House Installation
      3. 4.3.3 Feeder Size
        1. 4.3.3.1 Initial Sizing
          1. 4.3.3.1.1 Factoring in the panel and breaker temperature ratings
        2. 4.3.3.2 Adjust for Length
        3. 4.3.3.3 Adjust for Ambient Temperature
        4. 4.3.3.4 Sizing the grounding Conductor
  5. 5. Installation Notes
    1. 5.1 My Particular Installation.
      1. 5.1.1 Preparing the main panel
        1. 5.1.1.1 Some careful inside demolition and a big hole to drill
        2. 5.1.1.2 Knocking a hole in the main panel and into the house
      2. 5.1.2 Preparing the subpanel
      3. 5.1.3 Running the Feeder
  6. 6. Appendix

    Preface

    This guide assumes the reader knows how to plan and install a circuit, which you can learn from the many home wiring books in the library or store. Few of these books, however, include any detail about installing a subpanel. So after a lot of research, I wrote this guide to clarify my own knowledge and record it all in one place for future reference. It almost reads like a blog. Now that you’ve run across it somehow, I hope it helps you with your subpanel project. As I read over this guide myself, it seems to double as a how-to-read-the-NEC guide, though that aspect just came to the surface as I was writing. (In case you don’t know, the NEC, National Electrical Code, is a book full of rules governing electrical work.) If you don’t share my NEC obsession, you’ll still get your panel installed without a copy.

    Disclaimer - I’m not an Electrician

    I'm just a DIY'er so this guide lacks the authority a real electrician could lend. An electrical engineer friend passed it for accuracy but an electrician on an electrical forum found a couple of errors, which I've fixed. Another electrician from the same forum stridently suggested that anything authored by a DIY'er was worthless, if not a menace. The guy was a little scary; enough to make me add this disclaimer.

    I've read a lot of material written by professionals, which, although being 100% accurate, is unfathomable to the layman. I hope this guide has value beyond guaranteed accuracy in that I can speak to a DIY'er, being one myself. So read on but check all the information you glean with your local inspector. This guide will just be one of several resources you'll use to get your project done.

    Planning

    Do you need a Subpanel?

    There are two reasons you might need a subpanel.

    One is that you’ve run out of breaker slots in your main panel AND you’ve already traded out some common 1-inch wide breakers for dual breakers, which take up half the space. You could almost double the number of breakers that can fit in your main panel by using the duals, but it’s not a good idea as your panel starts to get too crowded with wires, a possible code violation. Your main panel must be designed to accept the duals, so make sure yours is if you decide they are the way to go.

    The second is that you have a sufficiently large number of new loads to service that are located some distance from the main panel.Instead of running individual circuit wires, it’s often easier, and slightly more energy efficient, to run one thick feeder cable from the main panel to a subpanel located near the loads.If you need power in a detached garage or a house addition or even an attached garage, a subpanel may be the way to go.

    Or maybe there’s a third reason, which applied to me as much as the other two. You’re a hopeless DIY’er and this is just another project to take on.

    Get a Permit

    In most communities a permit for a subpanel installation job is required, and you can be fined if you don’t comply with city hall. If you want to skip out on the permit fee, mine was $40, consider this: Where I live, when a homeowner sells a house, a list of all un-permitted work must be compiled by the seller. The item, “subpanel installation”, might worry a prospective buyer, especially in a buyer's market.

    I think of the permit fee as the cost of professional advice at a deep discount. If your inspector is as friendly as mine was, he or she will be happy to help make your project go smoothly. Books help to a point, but inevitably, your unique installation will raise a question that can only be answered by a professional. I must have been at the building department a half dozen times to ask questions before I finalized my plan. Do your homework though; I wouldn’t head off to city hall knowing nothing about electrical work. Visiting an inspector is like traveling in France: If you show respect and at least try to speak the language, you can expect friendly service in return. In some parts of the country an inspector is referred to as the AHJ, for Authority Having Jurisdiction or Authority Housing Jurisdiction. I had to Google AHJ when I ran across it in forums.

    To get a permit you will need to submit to your city’s building department a floor plan of your house, showing all the planned electrical work you want to do. (Most electrical books show the symbols to use on an electrical drawing.) Your inspector will review it and if all is well, you pay your fee and begin your work.

    The NEC

    The National Electrical Code, NEC, is a professional’s reference and a difficult read for the layman. It’s about 700 pages long; it has an index and is broken into numbered articles, each addressing another topic. I’ve found that to use a technical reference effectively, you need to be familiar with the whole work to look up just one topic and the NEC is no different. This is mainly because the information you need may be spread around so you need to be familiar with the reference’s organization. For the layman particularly, and even the professional, the NEC can leave room for interpretation for several reasons. It sometimes uses words like, prolonged, which is difficult to translate into, say, some number of hours. One paragraph may seem to contradict another, (for me an example of this is NEC 2005, 110.26(E) Exception, and 110.26(F)). General language is often used, in what I imagine is an attempt to address an almost infinite number of situations in the real world. It’s your local inspector that will be the final authority.

    So, if you just want to get your subpanel installed, it may not be practical to spend hours digging around in the NEC, as you can probably get all the information you need from other sources, including this guide, the Internet and your inspector, but I’d recommend it if you want a deeper understanding of your project or a glimpse of the professional electrician’s world. After trying unsuccessfully to pry clarity from the NEC, you can resort to several NEC companion books written to help the reader interpret the code, but clarity can remain elusive.

    Here and there I’ve referenced NEC articles in case you’re interested in looking them up. My frustration with the NEC will surface from time to time, though surprisingly, over time, I’ve developed affection for it. It can grow on you.

    Panel Location

    The first thing you should do is figure out exactly where to locate the subpanel, as that will effect decisions down the line. My experience shows how the electrical codes can make this decision more complicated than you might think.

    I wanted to add lighting in my attached garage, install landscaping lights and re-wire my living room as part of a remodel. The living room is next to the garage so it was looking like the garage would be the obvious place to locate the subpanel.

    After eliminating a couple of locations in the garage due to NEC clearance restrictions, which I list just below, I decided the wall shared by the garage and living room would be a good spot. But during a visit to city hall I was told that walls shared with a garage are considered firewalls and you can't install subpanels in them. Then I remembered and explained that my garage happens to extend three feet beyond the living room, leaving three feet of the wall unshared. How about there? The inspector said if I put the panel in that unshared section of the wall, I'd be OK.

    For fun, let’s say I didn’t have this 3-foot stretch of unshared wall. In that case, I could have mounted the subpanel on the surface of the firewall. I would have had to build a chute or channel to accommodate the panel and the wires running to it, by nailing edge-on, 2, 2 x 4’s against the firewall, for the height of the wall, 16 inches apart. I could then mount the panel between the two 2x4's and drywall over and under the panel.

    Important note: Recently, a friend had a subpanel installed in a fire wall. His AHJ allowed it if they surrounded the panel box with 5/8" drywall and finished the joints with drywall tape and mud, forming a box into which the panel would be set. Their studs were 2x6's though, which allowed the space for the extra layer of drywall inside the wall cavity. My house has 2x4 studs so this wouldn't have worked for me. Though on second thought, I might have asked my inspector if I could build the drywall box inside a 2x4 stud cavity and mount the panel so that it was 1" or so proud of the wall surface. (The 5/8" drywall layer in the back of the cavity would prevent the panel from sliding all the way in.) I don't see why that would be a problem, but it's a question for your local AHJ.

    Here are few rules concerning panel location and where I learned about them:

    • A working space clearance 3 feet deep and 30 inches wide must exist in front of the panel. (NEC Article 110.26(A)(1 and 2))

    • The working space must be kept clear. (NEC Article 110.26(B))

    • The uppermost breaker in the panel must be not located higher than 6’ 7”. NEC Article 240.24(A) and maybe a little of (NEC Article 404.8), as far as this diy'er can figure.

    • Do not install a panel in a cabinet, or shelving. (Ray Mullin, Residential Wiring, 14th Edition, p. 503)

    • Do not install a panel in a damp location. (bathroom). (NEC Article 408.37, 312.2(A); Ray Mullin, 114th Ed.)

    • Do not install a panel in a firewall.  (My inspector)

    • There’s a headroom requirement of 6 ½’ which doesn’t apply for a panel rated at less than 200 amps. See the exception in NEC Article 110.26(E).

    As you can see, these rules are found all over the NEC and a couple I found in other books I never did find in the NEC . It’s almost impossible to know that you’ve found all the rules that apply to what you’re doing. Here’s how my reading of the NEC to learn about panelboard location went.

    "Circuit breaker box" wasn't in the index , but I did find "panelboard", which sounded promising, so I followed that trail. It pointed me to NEC article 408, "Panelboards and Switchboards", which are covered separately, panelboards beginning with Article 408.30. When I didn't find anything relating to panel location I started skimming through the "switchboard" section and found the heading "Clearances", Article 408.18, which sounded relevant. So why did the panelboards section lack a Clearances subarticle? What is a switchboard anyway? I then turned to the definitions section of the NEC to look up switchboards and panelboards and found, at least from this layman’s reading, that they are different, but very much alike too. Hummm. In classic NEC style, 408.18 referred me to another part of the code, Article 110.26, "Spaces About Electrical Equipment", a subarticle of Article 110, "Requirements for Electrical Insallations", where I found that some clearance rules mention “switchboards and panelboards” explicitly, though other rules use the more general phrase, “equipment operating at 600 volts or less”. So, finally I decided that 100.26, for the most part, does apply to panelboards. But the code search went on. Panelboards contain switches and breakers, so I went to Article 404, concerning "Switches", where I found another rule, the one on the maximum height of 6’7”. And so on.

    Like I say, the NEC is a challenging read though maybe you’ll catch on more quickly than I did. There are so many rules, (I don’t mean to imply that there are too many), I began to develop a sort of paranoia, where I’d stop to question whether every little aspect of the job complied with the NEC. When I was boring holes in framing members to run wires through, something which is also regulated by code, I wondered if I could drill a hole at an angle, an issue I hadn’t seen mentioned during my research. It seemed to me that an angled hole would remove more wood and might weaken the structure too much. For all I knew, borehole angles were dictated by the codes but I just couldn’t find the reference. Off to city hall again. By the way, an angled borehole is OK.

    One last note on location: You'll want to minimize the length of the feeder cable between the panels and avoid a route that makes feeder installation difficult. As I emphasize in the next section, the planning of your installation may ebb and flow and you may find yourself back here to change your location choice.

    Choose your Hardware

    1. Circuit breaker
    2. Subpanel
    3. Feeder conductors

    Then there are the odds and ends like staples, nail plates and various connectors and fittings, which are as important as the big stuff, and the codes dictate their use.

    You will need to coordinate the parts with one another, a process that hopefully will become clear as you read on, so don’t run out and buy anything until your plan is complete. You will however, want to go see what’s available in the local stores and you might even find knowledgeable help where you shop.

    The Breaker

    You’ll need a 220-240 volt breaker, the same type that is used on any circuit that serves a 220-240 volt appliance, like a range. It plugs into your main panel and you attach the conductors that feed the subpanel to it.

    To determine the amperage rating for your breaker, you need to estimate how much power you need. Most home wiring books show you how to plan circuits in the home so I won't cover that here. For my project, the planned loads included a dryer, (30 amps at 240 volts or 7200 watts), lights, (maybe 700 watts), and hand held power tools, (let's say 30 amps at 120 volts or 3600 watts). I'd need 7200 + 700 + 3600 = 11,500 watts. I’ve read that for safety’s sake, you should keep the power on a circuit 20% below the breaker rating. A 60 amp breaker for the feeder to the subpanel could supply 60 amps * 240 volts * .8 (safety factor) or 11,520 watts, which would be enough for me, but I went with a 100 amp breaker for expandability. Maybe I’ll get interested in pottery someday and I’ll need a kiln.

    So basically I just said that you should rate your breaker so it can handle ALL your planned loads, plus 20%. That would rub some people the wrong way in that you're probably not going to be running all your loads at once, which makes a lot of sense to me. If you do design a system where your breaker is rated less than all your loads and you do run so many loads at once that the breaker trips, then no real harm done I suppose. I don't know of any NEC requirements for sizing a breaker in a sub panel, but this is an issue I'd ask my inspector about. I went with safety. A breaker could fail and besides, to me, tripping breakers is a sign of a poorly designed system. As I'll emphasize later, no matter what size breaker you use, it's the feeder that the breaker protects and it's very important your breaker is sized to protect your feeder. I know, copper wire is getting very expensive and if cost is a major constriant, then you may have to go with a thinner feeder and so, a breaker rated at a correspondingly lower amperage. More on feeder sizing and how the feeder and breaker are related later.

    Somewhere on the breaker in fine print is the breaker’s temperature rating. Breakers are commonly rated at 75 C, though you may find 60 C or 90 C ratings. For now, make a note of what’s available and save it for when you size your feeder.  It’s possible the temperature ratings of the breaker, and your panel, will have an impact on your feeder size, but more about that in the feeder section.

    Also in fine print is the range of allowable wire sizes you can connect to the breaker. This is normally not an issue if you size your feeder correctly as higher amperage breakers are made to accept thicker wires. There may be one circumstance, almost not worth mentioning, where this could be an issue and that will be covered in the subpanel section.

    The Subpanel

    I won't cover the anatomy of a panel here, so if you don't know what a bus bar is, you'll need to read pick up a home wiring book. Panels have amperage ratings, just as breakers do, and they contain varying numbers of breaker slots. My local hardware store had 70-amp and 125-amp subpanels with 4 and 10 breaker slots respectively. The 70 amp panel was too low for my 100 amp breaker so I chose the 125-amp rated subpanel. The 125-amp rating is a maximum; you can use a 125-amp panel with the intention of only using, say, 100 amps, which was my plan.

    Panels, like breakers, also come with temperature ratings, usually 75 degrees C. Again, as with your breaker, the temperature rating may come into play during feeder sizing. The panel can have a different temperature rating from the breaker though most equipment you’ll find will be rated at 75 C.

    Your subpanel components and installation will differ depending on whether it will be located in the same structure as the main panel or in an out building, like a detached garage or shed. However, common to subpanels in either location is the requirement that the neutral bus bar be isolated from both the equipment ground bus bar and the metal box of the panel. (This assumes you're using a 4-wire feeder, which is the most common. If your feeder is 3-wires, the neutral bar should be bonded. See section 4.3 on feeders. Important Note: The 2008 NEC no longer allows 3-wire feeders - you'll need to use 2 hots and a neutral from now on, unless your local AHJ says otherwise.) The ground bus bar, on the other hand, needs to be bonded to the box. Only in a main panel are the ground and neutral bus bars connected to one another. It is basically this difference that distinguishes a main panel from a sub and in a later section you can see how to convert a main to a sub. You may need to buy an equipment ground bus bar as a separate item as many panels are sold without them. If the ground bus bar kit comes with a strap to connect the ground and neutral bus bars, don’t use it for that purpose. You could however, use it to connect the ground bus bar to the box, if that bond doesn’t already exist.

    The following sections cover the two possible subpanel locations separately, but some issues are shared so read both sections.

    An Outbuilding Subpanel

    A Main Breaker is required

    A subpanel in an out building must have a main breaker if it will contain more than six branch-circuits and you should probably have one even if you’re planning fewer circuits. Emergencies may occur where quick access to a panel shutoff would be handy. It’s easiest to get what’s called a panel with main breaker, which means the main breaker is integrated into the panel.

    Another option is to use a main lug only panel, commonly known by the acronym, MLO, which lacks a main breaker. For an MLO panel you’ll need what’s called a “back-fed” main breaker, which you install much like a regular breaker accept it has a screw or clip attachment so it can't be pulled out while it's energized from the main panel, at least not without some effort. Also, back fed breakers don't have "line" and "load" markings on them, (you connect the two feeder hots to the terminals). A back-feed breaker will take up two slots, so you may want to stick with a slightly more expense panel with a built-in main breaker. The amperage rating of the back-fed main breaker in the subpanel can be larger than the one servicing the feeder in the main panel, (though I’m not sure why you’d do this, unless you have one lying around and wanted to use it). Just remember that breakers have specifications on what size wires you can attach to them so you need to make sure the size of the feeder conductors are within the specified range for the breakers. This is the one situation that I mentioned earlier, where you’ll need to be particularly careful about the breaker’s wire size rating.

    An Earth Ground is required

    The earth ground requirement doesn’t affect any panel characteristics, but because the earth ground system is connected to the ground bus bar in the panel, this is a good time to cover it.

    Earth grounding is covered in NEC Article 250 but I was never able to understand it to the point where I could translate it into layman's terms. What follows is the best I could do with Article 250 and will serve as a starting point for your discussion with your inspector.

    There are several ways to achieve an earth ground, but most commonly, two separate 8 foot long grounding rods are used. Your main panel probably incorporates such a grounding system. Drive the rods flush into the ground a minimum of six feet from each other and anywhere you like in relation to the building. Avoid any gravel fill near the foundation, as dirt, especially wet dirt, is a better conductor. You might even choose a low spot where the dirt stays wet longer after a rain. Run #6 AWG bare copper wire from the ground bus bar to the rods, using clamps rated for direct burial, “DB”, to connect the wire to the rods.Make sure you understand how to use the clamps properly; the wire usually goes opposite the screw. It’s best to use one continuous length of wire as splices introduce opportunity for resistance. If you have any metal water pipes in the building, again, run #6 AWG bare copper wire from the ground bus bar to the pipes and make the connection to the pipes using clamps within the first 5 feet of where the water line enters the building. All connections must be accessible, even a ground clamp so if you cut into a wall, add an access plate.

    A Subpanel in the House

    For a subpanel in the same structure as the main panel, no main circuit breaker is required, so you can use a main lugs only, MLO, panel. (MLO panels were introduced in the outbuilding section.) The other good news is that you don’t need to ground the subpanel to anything other than the main panel. No additional ground rods or water pipe connections are required. All you need to do is connect the feeder's ground conductor to the ground bus bars in the main service panel and the subpanel.

    Converting a Main Panel to a Subpanel

    To clarify the differences between a main panel and a subpanel, here are three steps to follow to convert a main panel to a subpanel.

    1. Remove the tie bar connecting the neutral and ground bus bars.
    2. If present, remove the bond between the neutral bus and the metal box.
    3. If not present, add a bond between the ground bus and the metal box.

    So if you’ve got a main panel lying around, you can, with modifications, use it for your subpanel.

    The Feeder

    A 240v subpanel feeder consists of four separate conductors; two hots, one neutral and a ground. (It used to be that a 3-wire feeder could be used if you were installing your sub panel in a detached building with no other conducting pathways, pipes, phone, etc., connecting the outbuilding and the main house. As of the 2008 NEC, this is no longer allowed; you'll need 2 hots and a neutral for everything.) You can buy these separately or all in one bundle, in which case you’ve bought what’s called a cable, usually the easiest way to go unless you’re running conduit underground to an out-building. To determine what feeder to use, you need to know about conductors and factor in your installation particulars.

    Conductor characteristics include:

    • Material
    • Insulator type
    • Size
    Installation particulars include:
    • Distance between the main-panel and subpanel
    • Temperature of the air around the feeder
    • Subpanel location

    Material - Copper or Aluminum

    Aluminum is brittle, so small wires are almost always made of copper to reduce the chance of breakage. However, for larger conductors, like those in a feeder, aluminum can be used, even though copper has several advantages and is sometimes recommended over aluminum. Copper is more conductive than aluminum, so for the same amount of current capacity, a copper feeder can be thinner. The radius of a bend in a copper conductor will be a little smaller, so making a tighter turn will be easier. Aluminum will oxidize, so you need to use anti-corrosion grease where it’s been stripped at the connections. Aluminum will expand and contract under varying loads, possibly creating a tiny but dangerous gap at the connections, which need to be checked yearly or so, and the screws snugged down, however, I’ve read that using the newer style lug connections can eliminate this possible loose connection issue. What aluminum has going for it is that it’s lighter in weight and cheaper. Many professionals will tell you either will work just fine. My home center had only copper in stock so copper it was for me. Also, an electrical engineer I know told me DIY’ers should always use copper.

    Insulation type

    Conductors come with designations referring to the type of insulating sheath around the wire. Many insulation types exist, designed for various conditions such as moisture, sunlight and heat, so the location of your subpanel will be very important to your feeder choice. One type, THHN is sheathed with heat-resistant thermoplastic, which is rated to withstand prolonged temperatures of 90 C. THHW is moisture and heat resistant thermoplastic. There are about 25 types but many have very specialized uses and you won’t find them at your hardware store. For a subpanel project you might learn about 3 or 4 at the most.

    Feeder Insulator Type for an Outbuilding Installation

    There are two obvious choices here; laying a feeder on the ground isn’t going to pass inspection so it's got to go underground or overhead.

    Overhead Feeder Run

    For an overhead run, sunlight and moisture will dictate the use of a cable labeled SE, for Service Entrance. There are cables called triplex or quadplex, that include what's called a messenger wire, which doesn't conduct current but acts as support for the conductors. Unless you're spanning a short distance, you'll to need to look for this. NEC article 225 covers Outside branch circuits and feeders; good luck interpreting it. You’ll need an entrance head and conduit and the cable needs to be supported at least 12 feet off the ground and 18 feet over a driveway. See your inspector for the local codes concerning an overhead feeder run.

    Underground Feeder Run

    For an underground run, moisture dictates the use of a cable labeled UF, Underground Feeder or USE, Underground Service Entrance. Or you could bury conduit, though you’ll still be dealing what’s still considered a wet location, so you’ll need to pull individual THW, THWN or XHHW conductors through it. Notice they all have a “W”, for wet, in their type designation.

    Let’s say you choose to bury a UF or USE cable. Your trench needs to be 24” deep and the cable needs to be protected by conduit at either end where the trench meets a building. Begin this conduit run at the bottom of the trench, maybe a foot or two from the wall, and use a 90-degree elbow section to continue the conduit vertically till it enters the building or the panel.

    If you choose individual conductors inside conduit, your trench needs to be 18” deep. It’s easiest to use PVC schedule 80 and the conduit diameter needs to be large enough to provide sufficient air space for cooling. 1 1/4” is large enough to accommodate conductors carrying 100 amps, which is usually enough for most subpanel requirements. Your trench only needs to be 6” deep if you use rigid metallic conduit but it should be coated to resist corrosion. For any long conduit, the friction you encounter while pulling the wires may require a pull box or junction box near the center of the run.

    Feeder Insulator Type for an In House Installation

    If the feeder will be inside the same structure as your main panel, such as in an attached garage or addition, it’s probably easiest to use type NM, non-metallic cable. Romex, a trademarked term of the General Cable Company, is often used to refer to NM cable, like you’d use Kleenex for facial tissue. NEC Article 334, "Nonmetallic-Sheathed Cable: Types NM,NMC,and NMS", is the motherload of installation rules for Romex. To run the feeder, you’ll most likely need to bore holes in framing members. The NEC say’s you only need a nail plate if the edge of the borehole is less than 1 ½” to the edge of the wood, but that seems too relaxed if you ask me. I wouldn’t assume a future homeowner isn’t going to hammer a 16D nail into the stud for some reason, so I’d always use plates. In an accessible attic, guard strips at least as high as the cable is thick are required if you’re laying the cable perpendicular across any joists. You can probably omit the strips if you’re close to the perimeter where the roof rafters meet the ceiling joists, where people won’t be stumbling around. If you decide to staple the feeder to the rafters or other attic framing, there are more guard strip rules for you in NEC article 320.23. Staple the cable down every four feet when running along side a framing member and keep it 1 ½” from the board’s edge. Don’t bend the feeder in a radius tighter than 5 times the feeder’s diameter. Use lots of pulling lube; I use it not only for pulling wires through conduit but though boreholes too. In an unfinished basement, per NEC 334.15(C), "Where cable is run at angles with joists in unfinished basements, it shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists. Smaller cables shall be run either through bored holes in joists or on running boards." Your feeder will probably be large enough so you can simply staple it to the bottoms of the joists. I assume a finished basement rule works for a crawlspace too.

    You might need to run conduit for part of an inside run to protect the cable. According to NEC rule 334.15(B), protection is required when NM cable passes through a floor. It’s OK to run NM through conduit, but usually people do it only for short distances where protection is required. Add appropriate fittings to the ends of the conduit to cover any sharp edges that can damage the sheath.

    If you have a reason to use individual conductors instead of NM cable, they need to be inside conduit runs from box to box, even if the run is entirely inside a structure. Big reason to use NM cable.

    Feeder Size

    Sizing your feeder requires four steps:

    1. Initial sizing
    2. Adjust for length
    3. Adjust for Ambient temperature
    4. Sizing the grounding conductor

    Many experts will tell you that you can ignore steps 2 and 3 for a basic residential subpanel installation that will service lights, a power tool or two and maybe an electric dryer. I’d urge you to read through and apply all the steps listed above and if the outcome of the initial feeder sizing doesn’t need adjusting, then at least you learned a little more about electricity in you home. I find it all interesting.

    Initial Sizing

    When current flows, heat is generated due to resistance and if a wire gets too hot it’s insulation could melt or degrade over time. A fatter wire runs cooler because it offers less resistance to the flow. In the section on insulation type, it was hinted that each type of insulating sheath has a temperature rating, below which the conductor inside the sheath should be kept. Three insulation temperature ratings exist; 60 C, 75 C and 90 C. THHN is rated at 90 C, for example, THHW at 75 C and THHW-2 at 90 C. A “-2” suffix means a 90 C rating. Arranged across the top of a table in the NEC, Table 310.16, are the three temperature categories with the insulator types grouped appropriately. (This table is reproduced all over the place and you’ll find it on the web. Google it.) Along the left side of the table are sizes, in AWG values. (If you don’t know what AWG is, go back to your basic wiring books.) Inside the table, at the intersection of temperature rating and size, are allowable amperages. To use the table you first find the temperate rating of your feeder’s insulation. Let’s say you’re using THHW, it’s grouped with some others in the 75 C column. You go down the column till you hit the amperage rating you want, say you want 100 amps going to your subpanel. Then head across to the left side to get the size, which in this example is 3 AWG.

    Here’s some of the information from table 310.16, for a copper conductor.

    SIZE

    60 C

    UF, TW

    75 C

    THHW, XHHW

    90 C

    THHN, USE-2

    8 AWG

    40 AMPS

    50 AMPS

    55 AMPS

    6 AWG

    55 AMPS

    65 AMPS

    75 AMPS

    4 AWG

    70 AMPS

    85 AMPS

    95 AMPS

    3 AWG

    85 AMPS

    100 AMPS

    110 AMPS

    2 AWG

    95 AMPS

    115 AMPS

    130 AMPS

    If you have the full NEC table, notice that “NM” doesn’t appear in any of the temperature groupings. The NEC states in article 334.80, that “NM” must be considered to have a 60 C temperature rating, even though the individual conductors inside the outermost sheath may be rated at 90 C. So for NM you have to use the 60 C column. I’m not sure why this is but one guess is that since NM is so widely used, the NEC authors wanted to build a large margin of safety around it.

    My local big box store had no #3 AWG but they sold a lot of 2 AWG NM-B copper cable. The clerk told me the #2 was what I wanted to feed the 100 amps, despite the fact that the table says the limit is 95amps. Turns out the guy was right, at least as far as my local inspector was concerned. I guess residential inspectors are comfortable applying some wiggle room to Table 310.16. (Using 100 amps worth of equipment, lights, dryer, table saw, etc. all at once, would be unlikely afterall.) But there's more to consider so read on.

    Factoring in the panel and breaker temperature ratings

    You also need take into account what the feeder attaches to: the breaker in the main panel and connectors in the subpanel. As mentioned in the Breaker and Panel sections, they also have temperature ratings, which are usually 75 C. If your feeder conductors are rated at 90 C, say you’re using THHN, and your panel and breaker are rated at 75 C, you need to use the 75 C column of Table 310.16 when sizing your THHN feeder conductors. So here’s the rule:

    Of all your system component temperature ratings, use the lowest when using NEC Table 310.16 to size your feeder.

    Adjust for Length

    A certain voltage must be maintained for loads to operate properly. Under low voltage a light bulb will dim or a motor will run hot. The longer a conductor and the more current you’re pushing through it, the more the voltage decreases at the end of the run, a phenomenon called voltage drop. The NEC suggests that at the end of a feeder, the voltage should drop no more than 3% from the 120-volt source. This suggestion is for performance and efficiency, not safety, but I’d still take it into account, especially if off the subpanel, you’ll be running bigger power tools or motors or loads that run continuously, say for 3 or more hours at a time.

    Table 310.16 doesn’t take length into account so you should apply what’s called a derating formula to the size you got from the initial sizing. If the formula says the voltage drop is greater than 3%, you should increase the size of the feeder.

    Here is the simplest formula to calculate voltage drop for use in a residential setting:

    VD = I * R

    VD - voltage drop

    I - maximum current on the feeder, which for my project is 100 amps

    R - resistance of the feeder, which you need to calculate with the help of NEC Table 9, Chapter 9 which for my #2 AWG copper feeder, shows a resistance of .19 Ohms/1000 feet.

    To calculate R:

    My run was 80 feet, which I needed to double, as current flows to and back from the subpanel. So, R, for me was:

    (.19 Ohms/ 1000 feet) * 160 feet = .0304 Ohms

    VD = 100 amps * .0304 Ohms = 3.4 Volts

    3.4 volts / 240 volts = .014 or 1.4 %, which is well within the NEC suggestion of 3%.

    Say my subpanel was going to be 200 feet away from the main panel:

    VD = 100 amps * (.19 Ohms / 1000 feet) * 400 feet = 7.6 volts

    7.6 volts / 240 volts = 3.1%, which just exceeds the NEC suggestion. I’d need to go up in size.

    The appendix has an alternative method for calculating voltage drop, which is a bit more complicated, but you might be interested.

    Adjust for Ambient Temperature

    Not only are conductors subjected to heat generated from the resistance inside, but also from the environment around them. The bottom of NEC Table 310.16 shows “correction factors” for ambient temperature you need to apply to the allowed amperages found in the upper half of the table. According to the NEC, in the case of an NM cable, with individual conductor sheaths rated at 90 C, which is usually the case, even though we needed to use the 60 C column to size it, we can use the 90 C column to correct for ambient temperature. This is good news because if we had to use the 60 C column to correct, we could never use an NM cable in an attic that gets hotter than 56 C or 132 F. The correction factor would bring the allowable ampacity at 132 F to zero. Ambient temperature corrections can be significant, so depending on your choice of feeder and whether your run includes a hot attic and if you’ll be regularly nearing the maximum amperage on you subpanel, this can be an important step.

    I guessed my attic wouldn’t get hotter than 55 degrees C or 131 F, so I’d need to multiply the allowable ampacity of my #2 AWG NM-B cable, (the “B” indicates it has 90 C conductors inside), 130 amps, by .76, giving 98.8 amps. This was under my 100-amp target but was close enough. (I've since measured the temperature on a hot day and it was only about 103, not much hotter than the front yard.) NEC article 310.10, states that temperature ratings reflect the “maximum temperature at any point along a conductor’s length that it can stand for a prolonged period of time without serious degradation”. “Prolonged” is not defined and since my attic is only hot for a few hours during the heat of the day, and the likelihood of me running 100 amps through my subpanel during that time is slim, this 1.2 amp shortfall seemed safe to this layman.

    In the end, the feeder cable I bought was labeled like this:

    ROMEX® AWG 2 CU 3 CDR WITH AWG 8 GROUND

    TYPE NM-B 600 VOLTS

    Romex and the type, NM, are listed, and the “-B” after the “NM” means the insulation is rated to 90 degrees C, the maximum temperature the insulation can endure without damage. The “CU” indicates copper. AWG 2 indicates the size, 3 CDR means 3 conductors, (2 hots and a neutral), and the AWG 8 Ground indicates the size of the ground wire. Cable size requirements apply only to the hots and the neutral; the ground can be thinner. Based on the NEC, the ground for a #2 AWG conductor only needs to be a #8 AWG, so that’s what’s provided in the cable by the manufacturer. (See the next section.) This type of cable is often just referred to as 2/3 Romex. The 2 refers to the gauge of the conductors inside and the 3 is the number of conductors. The fact that there is a #8 ground also present is assumed.

    Sizing the grounding Conductor

    NEC table 250.122 dictates grounding conductor size based on the amperage rating of the breaker protecting the circuit. Here’s part of it, for copper

    Breaker rating

    Ground size

    30 amps

    10 AWG

    40 amps

    10 AWG

    60 amps

    10 AWG

    100 amps

    8 AWG

    My NM feeder came with a #8 ground, my needs anticipated by the manufacturer, and I was all set.

    Installation Notes

    This section describes some the work I did to install the system with some tips on how to deal with situations you may run into even if your installation differs from mine.

    My Particular Installation

    My subpanel went into an attached garage of a single story house sided with wood. The main panel is mounted on the outside of the house. My cable route starts through the back of the main panel, into the wall behind it, up through the wall’s top plate, into the attic, across the ceiling joists along the attic’s perimeter, through the top plate of the garage wall, into a stud cavity and finally into a cutout in the top of the subpanel. I could have avoided drywall work inside, behind the main panel, by routing the feeder out the top of the main panel, through a conduit running up the outside the house and then through the blocking between the roof rafters and into the attic.  If I’d have chosen to run the cable through the crawlspace, the conduit would run down the outside wall, through the rim joist and in under the house. I was planning to tear the drywall out of that room anyway so I chose the route running inside the wall.

    Preparing the main panel

    Some careful inside demolition and a big hole to drill

    Note where the main panel is attached to the outside of the house and transfer the approximate location to the inside of the wall. Working inside the house, locate the 2 studs on either side of the panel. Being careful not to damage any wires that may be inside the wall, remove the drywall between the studs, starting from a bit below the panel up to the ceiling. Extend the opening onto the ceiling for a foot or two, between the ceiling joists to allow you to work with the feeder cable while you're in the room and not in the attic, where claustrophobia rules. Drill a hole in the wall’s top plate between the appropriate studs to accommodate the cable. The hole I drilled was 1½ inch in diameter. Code requires that a nail plate needs to be attached to a framing member in front of any hole that leaves less than 1 1/4 inches of wood along the board’s edge. This metal plate protects the cable from any nails driven into the wall.

    Knocking a hole in the main panel and into the house

    Turn off the power to the main panel using the main breaker. While outside the house, locate a large circular cutout inside the main panel that you want to route the cable through. Create a small indent at the center point of the knockout with a punch or a nail to prevent a drill bit from drifting. Drill a small hole through the center point and through the house siding behind it. Now when you’re inside you can see daylight and know exactly where the knockout disk is.Working inside of the house, use a hole-cutting bit ½” larger than the diameter of the knockout and drill a hole in the house siding, using the small hole you made from the outside as a center point. Remove the wood from the hole, exposing the knockout. Back on the outside of the house; punch out the knockout from the back of the panel.

    Preparing the subpanel

    I mounted my subpanel between two studs in the garage using screws. I wanted the option to be able to use a couple of the side cutouts so I bored a couple of holes in the mounting studs to line up with the cutouts, but left the cutouts in place. In the future I might want to run wire in the adjacent stud cavity and this would be where that wire would enter the panel. For now though, I would run the feeder and branch circuits through the top plate of the stud cavity where the panel was mounted. Instead of boring separate holes for the feeder and all my planned branch-circuits, I cut a couple of 5” slots by first boring 2, 1” holes and joining them with a reciprocating saw. I left a few inches between the slots and I used a lot of wire pulling lube on them, which I freshened for each new branch-circuit installation. I once tore a cable sheath while pulling it through a borehole, so I always use pulling lube.

    Running the Feeder

    AWG 2 cable is thick and heavy. The coil you bring home from the store has a memory and wants to remain coiled, like a garden hose, so consider these hints for dealing with it, especially if you'll be working alone. It's important not to damage the outer sheath when wrestling with the feeder so consider a helper to be important. Before running any cable, you need to lay out the length you need so it’s flat and straight. If you can support the coil as though it were still on the spool at the store and then pull it out, unwinding it, you’ll have one long cable with no twists.You can do this outside in the yard. Pass a bar of something, a large dowel or piece of pipe, through the center of the coil and support and secure the ends on sawhorses. Grab the end of the cable and walk away until the entire length is laying flat on the ground. Now you can drag the straight cable into the attic or crawlspace by one end. Use a vent or the access opening. Alternatively, you could set up a horizontally oriented spool and unwind the cable while in the attic. To do this, cut a square piece of plywood the width of which is the diameter of your spool and mark the center, where you will attach the end of a bar, resulting in something like a one legged table laying on it’s top. Attach a hook to the free end of the table leg from which the table, or spool, can be hung. Screw a swivel to a roof rafter, place your cable coil on the plywood so the leg is sticking through the center and hook your horizontal spool onto the swivel. The coils lays flat on the platform which when suspended will spin as you uncoil the cable. I imagine a few posts mounted to the plywood just inside the cable coil will keep it in place when unwinding. Actually, this second idea sounds a bit nutty because the coil will be too rigid to unwind without a helper to spin the spool. You might try it.

    I uncoiled my feeder outside, tied one end of a rope to one end of the feeder and threw the other end of the rope up on the roof so it dangled near a gable vent. I removed the vent, went inside the attic to the opening, reached for the rope and pulled up the cable, laying it across the length of the attic from where the main panel was to where the subpanel was, always being careful not to damage the outer sheath.

    Again, being careful not to rip the cable jacket, I threaded one end of the cable through the hole in the top plate on the main panel end of the run and down to the panel making sure there was enough length to reach inside the main panel with a couple feet to play with. I used cable lubricant to reduce friction in the borehole. For the other end, again I needed to go through the top-plate and into the space between 2 wall studs into the subpanel, a situation just like at the main panel end.

    I attached nail plates over the top plates in the inside room and garage where the feeder passed through and stapled the feeder to the studs at both ends near where it entered the panels. I used appropriately sized NM cable clamps in the panel cutouts.

    Appendix

    Alternative method to calculate voltage drop

    E = K x I x L x 2 / cross sectional area of conductor in circular mils.

    • E = voltage drop (NEC recommends 3% of 240 volts or 7.2 volts)
    • K = resistance in ohms per circular-mil foot at 75 degrees C. This is 12 ohms for copper. You can find these values in a Table in the NEC or various other books or on the Internet.
    • I = current in amps flowing through the conductor – I’m assuming my panel will be fully utilized, so this is 100.
    • L = length of conductor in feet – It’s 80 feet from my main panel to the subpanel
    • Cross-section for # 2 conductor - 66,360 circular mils. You can find these values in a Table in the NEC or various other books or on the Internet.
    • 2 is for a round trip to the subpanel from the main panel; there and back

    E = 12 x 100 x 80 x 2 / 66360 = 2.89 volts, which is within the NEC recommendation of 3% or 7.2 volts.

    I can re-run the calculation for a #4AWG feeder and I get:

    E = 12 x 100 x 80 x 2 / 41740 = 4.59 volts.