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Old 02-12-2012, 05:21 PM   #1
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Fedder for 100A subpanel 100' from main


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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Old 02-12-2012, 05:46 PM   #2
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Fedder for 100A subpanel 100' from main


You got most of it right, but you will have to install 4th grounding conductor from the house to the outbuilding.

I'd install conduit all the way, and forget about trying to splice those wires. You can use type THWN or XHHW conductors with such a scenario.

#3 copper, with a #8 ground would be sufficient.

#1 Aluminum with a #6 ground if you choose to use AL conductors.

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Old 02-12-2012, 06:44 PM   #3
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Fedder for 100A subpanel 100' from main


I agree with KBsparky and will add some details

Quote:
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?
Yep that sounds good

Quote:
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?
That's fine

Quote:
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.
Nope but if your supply is one of the big box stores you may find certain wire awg not in stock so you might have to upsize.


Quote:
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?
No .. see below comments ... mine in bold

Quote:
Would it be cheaper and equally practical to go with au in the house? Should the neutral be the same gauge as the hots?
Yes and Yes

Quote:
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
.

I hate splices on feeders but yes if done correctly that is fine. What do you have in mind to make the splices? I'm against any splicing ...

Quote:
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.
Several comments with code sections.....

You cannot run 3 wires to the detached building if you have a metal waterline between the building that you are using for grounding purposes. You must run 4 wires.

If you could run 3 wires you would not remove/cut the jumper between neutral and ground.

Bold and underlining are mine

250.32 Buildings or structures supplied by a feeder or branch circuit


(B) Grounded Systems.

(1) Supplied by a Feeder or Branch Circuit. An equipment
grounding conductor, as described in 250.118, shall
be run with the supply conductors and be connected to the
building or structure disconnecting means and to the
grounding electrode(s). The equipment grounding conductor
shall be used for grounding or bonding of equipment,
structures, or frames required to be grounded or bonded.
The equipment grounding conductor shall be sized in accordance
with 250.122. Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).

This part above is talking about 4 wire feeders and is when you separate neutral from ground

In order to run three wires your jurisdiction must be enforcing 2005 or an earlier code cycle of the NEC. However it won't work for your situation

The below exception applies

Exception: For installations made in compliance with previous editions of this Code that permitted such connection,
the grounded conductor run with the supply to the building
or structure shall be permitted to serve as the ground-fault
return path if all of the following requirements continue to
be met:
(1) An equipment grounding conductor is not run with the
supply to the building or structure.
(2) There are no continuous metallic paths bonded to the
grounding system in each building or structure involved.

(3) Ground-fault protection of equipment has not been installed
on the supply side of the feeder(s).
If the grounded conductor is used for grounding in accordance
with the provision of this exception, the size of the
grounded conductor shall not be smaller than the larger of
either of the following:
(1) That required by 220.61
(2) That required by 250.122

What all this means is you need to run a 4 wire feeder ... no choice.

You will separate neutral from your equipment ground and earth grounding. You may have to install a grounding bar in the panel in the out building to do this. Your feeder equipment ground, branch circuit equipment grounds and earth grounding conductors will terminate to the grounding bar. All neutrals will terminate to the neutral bar.

Questions?
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Last edited by Stubbie; 02-12-2012 at 06:51 PM.
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Old 02-13-2012, 06:14 PM   #4
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Fedder for 100A subpanel 100' from main


Thank you kbsparky and stubbie for your extremely helpful answers. I am in the process of digesting all of this info and doing a little more research. I've gotten so far into this now, I am also very interested in understanding the reasoning and purpose behind some of these rules. So far, I only get the most general of cocepts: that the rules are designed to optimize the protection from fault currents and all this apparently has something to do with the conductivity and efficiency of the electrical path clear back to the xformer. I'll read some more for my own edification.

Meanwhile, I've learned that we use the '08 code here. I'll post later with any additional observations I come up with as well as additional questions prior to finalizing my design.

Thanks again for now.
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Old 02-13-2012, 06:29 PM   #5
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Fedder for 100A subpanel 100' from main


The other thing I will add along the way.,

For the subpanel you will denfitally need main breaker no question asked due the numbers of circuits.

The kitchen area you will required minuim of 2 X 20 amp circuits and it have to be GFCI all the way thru unless speced otherwise in your code.

Bathroom it have it own circuit 20 amp size also have to be GFCI protected as well.

For the heater what voltage are you plan to run them ?? that will make the differnce how to run the circuits ( I rather keep them on 240 volts if they are perament mounted heaters )

The range if you have a small one like 24 inch verison a 40 amp circuit* will handle good but keep in your mind it have to be 4 conductor set up you can not use the 3 conductor set up at all.

The water heater ( 30 gallon size ) typically use a 240 volt circuit and it will be 30 amp.

Dishwasher it own circuit ( typically 20 amp circuit unless stated otherwise per manufacter instruction )

I did noted that you mention not to run the ground from the main panel I am sorry but Stubbie and KBsparky did got right on the spot on the details on them.

Merci.
Marc
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Old 02-13-2012, 07:10 PM   #6
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Fedder for 100A subpanel 100' from main


I am currently fixing a sub panel that should have been fed by four wires and in conduit. It's best to make sure you go the right way and use four. It's not fun doing it all over again.

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