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Old 01-04-2010, 02:08 PM   #1
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Subpanel


Ok i going to put a subpanel in my walkin attic up stairs. It's going to be a 100 amp panel. What size wire do I need to run from the main to the sub? There's going to be maybe 6 lights,12 outlets and 3 120v electric base board heaters.
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Old 01-04-2010, 02:17 PM   #2
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To fully answer this question we need to know the wattage for the baseboard heat.

If you want to go wthe max capacity for your panel you would use #3 copper for your conductors 2 hots 1 neutral and a #8 copper equipment grounding conductor with a 100 A breaker.

There are othe ways however we would need the info on the heaters as stated before.
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Old 01-04-2010, 03:53 PM   #3
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#8 is OK on the ground ?
I thought you were allowed 2nd size smaller then the hot, which would be a #6?



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Old 01-04-2010, 03:59 PM   #4
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I'm only going to need 2 heaters 1-750 watt 120v 1-1500 watt 120v.
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Old 01-04-2010, 04:15 PM   #5
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#8 for 70-100 amps. #6 aluminum
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Old 01-04-2010, 04:46 PM   #6
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Quote:
Originally Posted by Scuba_Dave View Post
#8 is OK on the ground ?
I thought you were allowed 2nd size smaller then the hot, which would be a #6?
This is for the equipment grounding conductor not the grounded conductor (neutral) taken from NEC Table 250.122
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Old 01-04-2010, 04:48 PM   #7
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OK, Thanks



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Old 01-04-2010, 04:57 PM   #8
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PS: Scuba Dave I personaly dont reduce the grounded conductor(neutral).
But you are right as a general rule two sizes smaller.

Old habits from being a commercial electrician in NY years ago at the plant I worked at. The policy we had.

Last edited by codeone; 01-04-2010 at 05:00 PM.
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Old 01-04-2010, 05:07 PM   #9
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Yes, I do not reduce the size of the neutral either, I knew you could...but prefer not to
I do oversize my grounds....my 100a sub I used #6
My 60a sub I used #8
I wasn't aware it was different for the Neutral VS the ground



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Old 01-04-2010, 07:05 PM   #10
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Quote:
Originally Posted by codeone View Post
To fully answer this question we need to know the wattage for the baseboard heat.

If you want to go wthe max capacity for your panel you would use #3 copper for your conductors 2 hots 1 neutral and a #8 copper equipment grounding conductor with a 100 A breaker.

There are othe ways however we would need the info on the heaters as stated before.
So, how did you get #3 for 100 amps? Do you know the termination provisions??

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Old 01-04-2010, 07:17 PM   #11
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Quote:
Originally Posted by codeone View Post
PS: Scuba Dave I personaly dont reduce the grounded conductor(neutral).
But you are right as a general rule two sizes smaller.

Old habits from being a commercial electrician in NY years ago at the plant I worked at. The policy we had.
So where'd you get the two sizes smaller rule for neutrals?? The only allowance for smaller neutral conductors is for service conductors (which this is NOT). In fact, neutral conductors are frequently oversized due to harmonic currents.

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Old 01-04-2010, 07:19 PM   #12
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Quote:
Originally Posted by busman View Post
So, how did you get #3 for 100 amps? Do you know the termination provisions??

Mark
Table 310.16 From the NEC.
From the 75 degree column.
The 90 degree column is only for derating purposes. Have never seen anyone get a UL listing for 90 degrees C.

Last edited by codeone; 01-04-2010 at 07:21 PM.
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Old 01-04-2010, 07:20 PM   #13
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Table 310.16 From the NEC.
Yeah? What column?
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Old 01-04-2010, 07:23 PM   #14
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Quote:
Originally Posted by busman View Post
So where'd you get the two sizes smaller rule for neutrals?? The only allowance for smaller neutral conductors is for service conductors (which this is NOT). In fact, neutral conductors are frequently oversized due to harmonic currents.

Mark
In my discussion with Scuba Dave I was answering his question not addressing the Subpanel in the original question.
Sorry to mislead you.
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Old 01-04-2010, 07:25 PM   #15
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Quote:
Originally Posted by busman View Post
Yeah? What column?
The 75 degree column. under copper. With the NEC Unless its stated its copper conductors.
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