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jheiderich · Feb 12, 2012, 03:58 PM

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.

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