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Old 01-24-2010, 07:01 AM   #1
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What size wire for 100 amp sub panel


I am trying to help a friend out by running a 100 amp sub panel off of 200 amp service to an attached garage. The problem is we are going to have to run the wire about 125 feet under ground from one side of the house to the other side because it is a moduler and this is the only way. I have run a 100 amp sub panel before to a detached garage back when copper was affordable and used #2 but I am not for sure what size aluminum we should use for the distance and since this will be ran undergound. Also, since this is an attached garage do we need to run a seperate ground? I was looking at stubbies diagram and it doesn't look like I need to. Any help would be greatly appreciated. Thanks

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Old 01-24-2010, 10:56 AM   #2
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What size wire for 100 amp sub panel


From what I see in NEC table 310.16 you need #1 Alum for 100a
4 wires are needed for subs

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Old 01-24-2010, 11:03 AM   #3
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What size wire for 100 amp sub panel


Hi

Before we get into wire sizes a few questions will be helpful.

1.) Are you sure about 100 amps ? This is huge for a garage unless the garage is a shop. Typical would be 60 amps or less.
2.) Along the lines with question 1 ...what electrical needs will this garage require? Will there be power equipment like table saws , welder , air compressor ....etc..?
3.) Check with your friend and get an accurate list of what he is going to be operating in the garage now and in the future so we can get a better idea for the sub-panel wire size.
4.) Are you wanting a direct burial wire or or you going to use conduit like pvc with individual wires inside it?

To answer your question about the ground wire....YES you need a ground ... you will need to run 4 wires (H-H-N-Grd). The correct drawing is below ..

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Old 01-24-2010, 11:44 AM   #4
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What size wire for 100 amp sub panel


Quote:
Originally Posted by Stubbie View Post
Hi

Before we get into wire sizes a few questions will be helpful.

1.) Are you sure about 100 amps ? This is huge for a garage unless the garage is a shop. Typical would be 60 amps or less.
2.) Along the lines with question 1 ...what electrical needs will this garage require? Will there be power equipment like table saws , welder , air compressor ....etc..?
3.) Check with your friend and get an accurate list of what he is going to be operating in the garage now and in the future so we can get a better idea for the sub-panel wire size.
4.) Are you wanting a direct burial wire or or you going to use conduit like pvc with individual wires inside it?

To answer your question about the ground wire....YES you need a ground ... you will need to run 4 wires (H-H-N-Grd). The correct drawing is below ..

We did an addition on his house and added a garage. He had a heating and air conditoner man out and they are going to add another system. So the sub panel will be for the heating and air and for some outlets in the garage.

I was wanting to use direct burial.

What I mean't to ask originally about the ground was, does the sub panel need its own ground rod?

Thanks for your help.
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Old 01-24-2010, 11:59 AM   #5
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What size wire for 100 amp sub panel


Attached garage does not need a ground rod
Instead the 4th wire - ground - goes back to the main panel
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Old 01-24-2010, 01:28 PM   #6
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What size wire for 100 amp sub panel


Quote:
Originally Posted by Bean View Post
We did an addition on his house and added a garage. He had a heating and air conditoner man out and they are going to add another system. So the sub panel will be for the heating and air and for some outlets in the garage.

I was wanting to use direct burial.

What I mean't to ask originally about the ground was, does the sub panel need its own ground rod?

Thanks for your help.
Ok

As Dave said no ground rod needed. Remember the equipment ground is for human safety. The grounding electrode system (ground rods etc..) are for property protection.

If your wanting direct burial I would suggest 2-2-2-4 mobile home feeder which is USE-2 rated. You will need to protect it in conduit where it comes out of the ground as with any direct burial method. Since this is a panelboard downstream of the service equipment you can only use it a 90 amps. So install a 90 amp breaker in the service equipment panel. A main breaker panel is not necessary in the attached garage but may be preferred by the homeowner so that he has a means of disconnect at the panel itself. Cost savings is not great either way. Don't be confused by the 90 amp in the service equipment and a 100 amp breaker in the garage panel. The 90 amp breaker protects the feeder and the 100 amp breaker will simply serve as a disconnect at the panel in the garage. You may have to go to an electrical supply to get the 90 amp breaker. The feeder should be readily available at the big box store.
A typical installation for direct bury in this drawing below .... it may not be exactly how you will do yours but is a good guide in general. The drawing shows a 3R Service equipment panel board located on outside wall of dwelling and a underground direct bury feeder to an attached garage/shop.
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What size wire for 100 amp sub panel-feeder-sub-panel-attached.jpg  
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Old 01-24-2010, 05:44 PM   #7
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What size wire for 100 amp sub panel


Quote:
Originally Posted by Stubbie View Post
Ok

As Dave said no ground rod needed. Remember the equipment ground is for human safety. The grounding electrode system (ground rods etc..) are for property protection.

If your wanting direct burial I would suggest 2-2-2-4 mobile home feeder which is USE-2 rated. You will need to protect it in conduit where it comes out of the ground as with any direct burial method. Since this is a panelboard downstream of the service equipment you can only use it a 90 amps. So install a 90 amp breaker in the service equipment panel. A main breaker panel is not necessary in the attached garage but may be preferred by the homeowner so that he has a means of disconnect at the panel itself. Cost savings is not great either way. Don't be confused by the 90 amp in the service equipment and a 100 amp breaker in the garage panel. The 90 amp breaker protects the feeder and the 100 amp breaker will simply serve as a disconnect at the panel in the garage. You may have to go to an electrical supply to get the 90 amp breaker. The feeder should be readily available at the big box store.
A typical installation for direct bury in this drawing below .... it may not be exactly how you will do yours but is a good guide in general. The drawing shows a 3R Service equipment panel board located on outside wall of dwelling and a underground direct bury feeder to an attached garage/shop.
Thank you for your help.
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Old 01-24-2010, 07:31 PM   #8
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What size wire for 100 amp sub panel


I would suggest to consider to do wires in conduit rather than direct bury cable.
  • Electrical conduit is cheap.
  • Wire for conduit vs UF bury cable may be cheaper.
  • Conduit is 18" burial vs. 24" burial for UF cable - another 6" at the bottom of a long trench is a lot of extra work.
  • If you ever want/need to add to or change with conduit you can readily do so.
After much consideration, I recently did 120' of THWN #6 in conduit to my garage/workshop. Glad I did it this way.

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