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Old 11-11-2008, 01:11 AM   #1
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Sub Panel


I am getting ready to install a garage sub panel. I had a couple of questions.

I am adding a 60 amp sub panel off of my 200 amp panel. The question I have is about running the wire. I am running a #6 (or should it be #4) wire from the main panel (with a 60 amp breaker) to the sub panel. I had planned on running it in PVC conduit to the edge of the house and then going underground to the side of the garage, back in using PVC conduit up into the box. two questions:

1. I read that the depth of the underground conduit needed to be at least 18 inches for PVC. Is this correct? Does it matter?

2. If I am grounded at the main box on the side of the house, do I need to re-ground when I get to the garage? The main box is where I am running the power from.

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Old 11-11-2008, 11:41 AM   #2
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If the garage is attached then no ground rod. If it is detached then you need a ground rod. In both case you need a four wire feeder and the ground and neutral must be isolated in the sub panel.

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Old 11-11-2008, 11:46 AM   #3
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Originally Posted by BenJoeM View Post
I am getting ready to install a garage sub panel. I had a couple of questions.

I am adding a 60 amp sub panel off of my 200 amp panel. The question I have is about running the wire. I am running a #6 (or should it be #4) wire from the main panel (with a 60 amp breaker) to the sub panel. I had planned on running it in PVC conduit to the edge of the house and then going underground to the side of the garage, back in using PVC conduit up into the box. two questions:

1. I read that the depth of the underground conduit needed to be at least 18 inches for PVC. Is this correct? Does it matter?

2. If I am grounded at the main box on the side of the house, do I need to re-ground when I get to the garage? The main box is where I am running the power from.
#6 is fine for THHN/THWN. Not for UF (direct burial).
Using conduit is IMO the best way to go. Yes 18" is correct for the depth. You need four wires. 2-hots 1-neutral and one ground.
You need to drive at least one ground rod at the garage near the panel. You will install a bare #6 from the rod to the grounding bus terminal strip in the sub. You will keep the grounds and neutrals separate. You will use a disconnecting means at the garage. A main breaker sub panel will do the job for you.
Run a green ground wire with the other three wires. Terminating at the neutral/ground bar in the main panel and the ground terminal strip in the sub.
Make sure the wire is rated for wet locations. THHN and THWN are usually rated the same. There must be a "W" in the wire description.
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Old 11-11-2008, 12:32 PM   #4
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Originally Posted by J. V. View Post
#6 is fine for THHN/THWN. Not for UF (direct burial).
Using conduit is IMO the best way to go. Yes 18" is correct for the depth. You need four wires. 2-hots 1-neutral and one ground.
You need to drive at least one ground rod at the garage near the panel. You will install a bare #6 from the rod to the grounding bus terminal strip in the sub. You will keep the grounds and neutrals separate. You will use a disconnecting means at the garage. A main breaker sub panel will do the job for you.
Run a green ground wire with the other three wires. Terminating at the neutral/ground bar in the main panel and the ground terminal strip in the sub.
Make sure the wire is rated for wet locations. THHN and THWN are usually rated the same. There must be a "W" in the wire description.
Cool. That is what I needed to know. I just finished digging the trench and now I am headed to get the wire.
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Old 11-14-2008, 01:50 PM   #5
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Thanks everyone,

Sub-panel is now installed and working!
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Old 11-15-2008, 02:43 AM   #6
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#6 is fine for THHN/THWN. Not for UF (direct burial).
why not UF? I planned on doing the same with UF-B run through schedule 40.
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Old 11-15-2008, 07:41 AM   #7
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why not UF? I planned on doing the same with UF-B run through schedule 40.

That was a reference to ampacity, #6 THWN is good for 65 amps, and #6 UF is only good for 55 amps, the OP wanted to use a 60 amp breaker, so you can see why the UF was shot down.
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Old 11-15-2008, 10:19 AM   #8
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However, I thought that since a 55 amp breaker doesn't exist and since the ampacity ratings are scaled back that the use of a 60 amp breaker was approved. No?
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Old 11-15-2008, 10:38 AM   #9
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However, I thought that since a 55 amp breaker doesn't exist and since the ampacity ratings are scaled back that the use of a 60 amp breaker was approved. No?
No. The feeder breaker amp rating cannot exceed the amp rating of the conductors. (some exceptions exist) but not in this application.
If you have #6 UF cable then you change the breaker to 50 amp not go up to 60.
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Old 11-15-2008, 11:09 AM   #10
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Originally Posted by J. V. View Post
No. The feeder breaker amp rating cannot exceed the amp rating of the conductors. (some exceptions exist) but not in this application.
If you have #6 UF cable then you change the breaker to 50 amp not go up to 60.
Sure you can, check out 240.4 (B) If the calculated load is 55 amps, then I dont see a problem at all with running #6 UF backed with a 60 amp breaker. Since the OP never gave a load to begin with, I'm sure he just thought 60 was a good number.

Last edited by chris75; 11-15-2008 at 11:14 AM.
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Old 11-16-2008, 11:53 AM   #11
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Originally Posted by chris75 View Post
Sure you can, check out 240.4 (B) If the calculated load is 55 amps, then I dont see a problem at all with running #6 UF backed with a 60 amp breaker. Since the OP never gave a load to begin with, I'm sure he just thought 60 was a good number.
Good catch. Thanks Chris.
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Old 11-16-2008, 01:12 PM   #12
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Our city requires a minimum of 60 amps for a sub-panel for a garage. that is why I went with 60 amps.
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Old 11-16-2008, 01:37 PM   #13
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Our city requires a minimum of 60 amps for a sub-panel for a garage. that is why I went with 60 amps.
They are misreading the NEC, the only thing required to be 60 amps is the DISCONNECT, not the FEEDER. 225.39 (D) if you want a code reference.

Also, ask for a state amendment in writing if this is really the case.

Last edited by chris75; 11-16-2008 at 01:40 PM.
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Old 11-16-2008, 01:42 PM   #14
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Thanks,

But I already installed it and everything is done now. 60 amps will do me just fine.

Thanks again!
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Old 11-16-2008, 01:46 PM   #15
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Thanks,

But I already installed it and everything is done now. 60 amps will do me just fine.

Thanks again!

cool, I just think its funny to see a 5x8 shed with a 60 amp feeder for a receptacle and a light.

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