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AdamsBurr 05-31-2011 08:02 AM

How to install a 100 Amp subpanel
I am a Homeowner with some electrical experience. I have run many new circuits in my 1836 home and updated many outlets and switches. I am into a new project to update my attached barn with a 100 Amp service panel. I have the 100 amp panel, it was removed from my home when a 200 amp service was installed by an electrician. My question in how I do this safely. I would like a recomendation on the subpanel feeder double pole cuicuit breaker, what size? Also what size feeder cable to install to power the subpanel. I will need to run about 50 feet of wire to get to the subpanel. Not sure if I should go with a 60 amp subpanel? I thought that I would use the 100 amp panel that I currently have, which has the curcuit breakers already and is in pretty good shape. Any help or information would be most helpful. Thanks.

Jim Port 05-31-2011 08:40 AM

You need to determine your loads that you want to run before someone can tell you the size of the wires and breaker.

AdamsBurr 05-31-2011 09:06 AM

I plan to run several 15 amp cuiruits to run lights. I will need 20 amp cuicuits to run table saws, planners, joiners. There will also be a fridge, stereo, outside lights. Not a large load, probably 6-8 curcuits. Not sure if that helps or not? Thanks.

WillK 05-31-2011 09:10 AM

You can use the 100 amp panel off a 60 amp breaker if you determine that it is adequate, but I would suggest that in sizing your feeder wires and conduit it would be a good idea to size them to accomodate 100 amps. Your feeder wires are where most of the work for this project will be.

At the very least, plan to use large enough conduit so that if you use wire that is not large enough for more ampacity, at least you would be able to pull new wire through the conduit. 2" will make it "easy" to pull 1/0 conductors. (Just recently I've been through this. It was easy to pull 1/0 conductors through 2" conduit on a 30' run with my wife on the other end guiding it into the pipe so it went in straight)

For some reason, around here HD seems to have the belief that 2 gage aluminum is rated for 100 amps. This is incorrect and it seems to be found in a lot of places... In the NEC code there is a table 310.15(b)(6) which lists 2 gage aluminum for 100 amps, but this table applies to service entrance and is not correct to be applied to feeders. You need to go by table 310.16 and use the 75 degrees C column for typical installations (check your panel to make sure it is rated for 75 degrees C)

By the way, what brand and type of panel is yours? If it is Federal Pacific don't use it. If it is something where you can readily get new breakers you're probably okay (if it isn't Federal Pacific)

If you're running your feeder cables underground, you can't use SE cable or NM cable. You might be limitted if you're getting from big box stores to something that isn't really rated for 100A, and if you're using a 60A breaker that's fine. You can use individual conductors like XHWN or THWN - the key is the third letter needs to be W for rated for wet locations, because that is what your underground conduit is - a wet location. Or you can use underground cables like USE. HD might have mobile home feeder.

You need a ground conductor. If you use rigid metal conduit or intermediate metal conduit, you can use a ground clamp on the pipe and use the pipe for the conductor by connecting the clamp to the panel with a copper ground wire.

You'll need ground rods, two 5/8" rods 8' long that are at least 6' apart covers most applications. They'll have markings on the end which the inspector probably wants to see, so they get driven all the way into the ground and dig away where the markings are until inspection is passed.

You can use PVC conduit, schedule 40 is okay if it is burried 18" at least and that means 18" above the top of the pipe. You need to use schedule 80 conduit for where it comes up out of the ground because it is subject to physical damage. With PVC you must pull a ground conductor.

Your conduit needs to end at the service panel. (That was one I missed, get it right the first time, it's not a fun rework.)

WillK 05-31-2011 09:16 AM


Originally Posted by AdamsBurr (Post 658361)
I plan to run several 15 amp cuiruits to run lights. I will need 20 amp cuicuits to run table saws, planners, joiners. There will also be a fridge, stereo, outside lights. Not a large load, probably 6-8 curcuits. Not sure if that helps or not? Thanks.

By the way, find out what codes apply - detatched buildings have been evolving a little bit. GFCI has been required on accessible outlets for a while, but with the 2008 NEC it's become more strict. All receptacles require GFCI - including the ones that had previously been exempt, such as a receptacle for a garage door openner. And they have to be tamper resistant.

The outdoor outlets need to be weather resistant. That means the outlets. The correct outlets will have a WR on the face. If the outdoor outlet is the first on the circuit it needs to be a WR GFCI. And of course the approriate cover is required, there's something funny about what cover is required but it has an exception for areas subject to power washing - someone else would have to explain that better than I can.

Missouri Bound 05-31-2011 09:25 PM

#2 aluminum is allowed for a 100 amp service feeder, but not for a 100 amp circuit. And a seperate ground rod in not needed in an attached structure. And unless I'm mistaken, there won't be any digging either.

WillK 05-31-2011 09:32 PM

I guess I assumed detatched and didn't see attached... I saw words like 50' and barn, and it never occured to me that a barn would be attached and be 50' from a main service...

On the service feeder table, I've heard it both ways on inspectors accepting or not accepting... I will tell you though in no uncertain terms that my inspector does not accept 310.15(b)(6) for subpanel feeders, only acceptable to him for the main panel. I say err on the side of caution unless you find out before buying materials that your inspector will accept it... Buying a smaller 50' cable than he will accept then having to buy another 50' cable gets expensive, and if you don't find out until rough inspection it's also a lot of wasted effort to pull a cable twice.

Missouri Bound 05-31-2011 09:38 PM

I hear ya. Only after I re-read the post did I catch the "attached barn" (kind of strange). And you are right about the inspector, shoving the NEC in his face probably won't make him too happy or any easier to work with. Just thought I would put my 2 cents in and not confuse the OP too much.
I'm curious now....did the OP mean detached? The only attached barn I have ever seen was on TOH.

frenchelectrican 05-31-2011 09:42 PM


Originally Posted by Missouri Bound (Post 658847)
#2 aluminum is allowed for a 100 amp service feeder, but not for a 100 amp circuit. And a seperate ground rod in not needed in an attached structure. And unless I'm mistaken, there won't be any digging either.

The feeder some area may allow use full 100 amp breaker but most area AFAIK most will say 90 amps to meet the NEC code requirement which the cable or conductor tempture rating will affect this as well.

AFAIK most SE or SER cable are rated for 75C but any inside run it have to be at 60C ampcapcity.

For attached building no ground rods are needed but make sure you have full 4 conductor either in cable or conductors in the conduit both will meet the codes.


AdamsBurr 06-01-2011 08:17 AM

Thanks for all the responses, I appreciate it. Our barn IS attached through a woodshed that is attached to our 1836 home. Sort of what you will see on TOH:yes: My subpanel that I am planning to use is a Square D? I believe that is the Company. NO cables will need to be buried under ground. I plan to run the feeder line through the house basement on the floor joists. Under the woodshed I assume it will be good to run some sort of PVC conduit so that no animals try to eat the cable? This will also be attached to the floor joists. The subpanel will be mounted in the woodshed to shorten the feeder cable, for cost. All circuits will be run above head in a hidden run that will blend in with the old post and beam woodshed and barn. Even though the feeder cable will run 50 feet or so, it will be all above ground. Sorry for the confusion. This is a new project for me and I am also talking with friends that have done similar projects. I want to be safe:). Thanks again and I look forward to more info.

WillK 06-01-2011 09:28 AM

You might want to consider whether a shorter feeder run is really a cost savings with the longer branch circuit runs it will require... I'd go for getting the panel closer to the circuits myself. SE-R aluminum will probably work out to be cheaper per foot than copper NM cables x the number of circuits that have to have a longer run into the garage.

Plus it will presumably be more convenient to have the panel closer to where the circuits are, both while you're working on them and when you trip breakers or need to shut down while in use.

AdamsBurr 06-03-2011 11:02 AM

I have a few more questions? I just ran a wiring plan to see what I actually plan to have for plugs and lights in my barn and woodshed and to see what electrical load I actually have. I believe that I will need 10 circuits in total. For circuits that will have regular plugs and maybe a light I figured 1500 watts per circuit. I assumed that it was better to estimate higher instead of lower? A seperate circuit for the table saw, again 1500 watts. My bench plugs, 4 total, was figured at 4500 watts. I plan to use 20 amp circuits for the table saw, bench plugs, and plugs that will have a joiner or planner plugged into them. I will run 15 amp circuits to the regular plugs and lights. I came up with 18,200 total watts needed. I figured the first 10,000 watts at 100%. The remaining 8,200 watts were figured at 40%, a number I found in one of my wiring books. This was 13,280 watts which I divided by 230 to find total amps. This was 57.7 total amps needed. Also bear in mind that most of these circuits will only be occasional use. Most of the circuits will not have any power draw most of the time and sit idle. I also looked at my old Breaker Panel and it is a little more rusted than I remember. I priced a 100 amp Square D from Home Depot and it is only $48. I assume that I might be better off going new so that I do not have issues with the old panel or breakers? Can I run a 60 amp line from my main panel to a 100 amp subpanel? If so, what size Feeder Breaker will I need in the main panel, a double pole 60 amp? The Feeder Cable should be what size? Looks like a 6 gauge wire would be for 50 amps. Would I buy a 4-3 Feeder Cable, if one is available? Also I believe that I need 4 wires in the Feeder Cable, ground, white, red, and black? Sorry for all the questions, but I want to get this right and safe. Any info would be helpful and thanks for any information that you can supply.

Missouri Bound 06-03-2011 02:25 PM

You can feed a 100 amp panel with a 60 amp breaker, theres no problem there. Running smaller wire will save you quite a bit. Yes you will need 4 wires, and you can use 3#6 thhn and a ground which could be #8. Try to get the panel as close to where you will be adding the circuits as WillK suggested. If you intend to use conduit then individual wires are the way to go. The conduit offers protection and if you oversize it a bit it will make it easier to upgrade should you need a higher ampacity in the future. You are on the right track.

AdamsBurr 06-03-2011 09:56 PM

Thanks for the information, I appreciate it. I measured the distance to move the subpanel closer to where I need the power and it will now be like 125 feet. I agree that the closer I get the subpanel to the area I want, the less wire on circuits later. I found that Home Depot has a Cerrowire 6/3 NM-B for $251 for 125 feet, although the ground is # 10 instead of the recommended # 8. If I were to purchase THHN # 6 in the 500 foot roll, it will be $300. If I am thinking correctly, with the THHN I will need to run the wire 4 different runs to make the 4 wire connections, using the full 500 feet? Basically, it looks cheaper for me to buy the Cerrowire 6/3 cable. Also I will have the red, white, black, and ground colored instead of all black THHN. I assume that I would not need the conduit this way? Again saving a little cash. All wire will be above ground on floor joists and indoors, I just thought that I could provide a little more protection to the wire. Most of my wire run should be fine from animals with the new wire path. Looks like the 60 amp double pole breaker at the main panel will be able to power the 100 amp subpanel. Thanks again and looking forward to more information from everyone.

Missouri Bound 06-03-2011 10:24 PM

You have thought it out well. My consideration for conduit was protection and the ability to increase the circuit capacity if you so desired. If you don't have an issue with the wire protection and don't think that you will need to increase capacity in the future your plan is just fine.

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