100 amp subpanel 100' from main panel

Old 02-12-12, 03:35 PM
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Lightbulb 100 amp subpanel 100' from main panel

I majored in sociology, so please bear with me...

I am building a 400 sq ft out building with 2 1500 watt heaters, a small kitchen including a small range, small dishwasher and fan hood. There will also be a 30 gal hot water tank, and a .1/2 hp sewage ejection pump. My calculations so far indicate a 100A subpanel would be a good choice. Does this sound correct?

My 100A sub will be approx 100 ft from the main. I believe that #3 cu or #1 au would be ok for the feeder. I calculate the voltage drop on the copper at 3.22v or 2.7% at an 80 amp load. Does that seem right? Do I need to spend more $ and go up to #2 cu or 1/0 au to be safe? (I haven't done voltage drop on the #1 au but I assume it would be about the same as the #3 cu.)

The first 35' of the total 100' run will be in the unfinished portion of the main house basement. I assume the most logical choice for wire in the basement would be either a jacketed #3/3 run through joists (I am NOT running the main grnd to the sub) or 3 separate # 3s in a 1 1/4" pvc or metallic flex conduit placed at bottom of joists. Does this sound correct? Would it be cheaper and equally practical to go with au in the house? Should the neutral be the same gauge as the hots?

Once I get outside, I am planning to use au in buried pvc conduit. I'll do a splice in a box either inside or outside the basement.

For ground in the out building, I'll tie to a water pipe which happens to also to be tied to the main service. I'll also use a separate grounding rod and cut the jumper between the neutral bus and the ground bus.

Any suggestions or comments would be greatly appreciated. I would like to get my ducks really lined up before talking with the permit people.

Thank you.
Old 02-12-12, 03:50 PM
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I am NOT running the main grnd to the sub
That would be a code violation under modern code and even under previous code because you will probably have a phone line and definitely because of the metallic water line mentioned. You need 4 wires. Therefore yes 3-3 cable but 3-3 cable is four wires not three, red, black, white, bare. You will need #8 copper or #6 aluminum for your ground.
Old 02-12-12, 05:07 PM
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You're planning on running GOLD wire? It can't be a typo since you typed it several times. The symbol for aluminum is Al, not Au which is Aurum, the Latin word for gold.

Anyway, you MUST run an equipment grounding conductor along with your power conductors, no ifs, ands or buts. I personally would use type THHN/THWN copper in PVC conduit the entire run and you could probably get by quite easily with a 60 ampere panel and a #4 feeder. Keep the neutral the same size as the hot conductors and as much as possible balance the 120 loads on the sub-panel.
Old 02-12-12, 08:31 PM
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2-2-2-4 USE cable may be your best bet, on a 90A breaker.
Old 02-13-12, 05:33 AM
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A fourth conductor needs to be run in the feeder. The NEC has dropped the allowance to run 3 wire feeders and re-bond the neutrals.

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