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Old 09-30-2009, 01:37 PM   #1
 
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Subpanel Question


I am planning on adding a 60amp subpanel using a 60amp double pole breaker on my main panel. I will use a packaged 100amp main breaker kit from Lowes as my subpanel (understanding that the 100amp breaker is essentially just a disconnect).

I am trying to decide between using 6/3wG or #6THHN/THWN in conduit to feed the subpanel. If i used the THHN, what size conduit? And if I use conduit does my ground have to be insulated or can it be bare wire and what size?

Thanks!!

ETA: both panels will be located in my basement - anywhere from 2' to 25' apart.

Last edited by meuseman; 09-30-2009 at 01:39 PM. Reason: clarity
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Old 09-30-2009, 01:53 PM   #2
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Not an electrician, but I'd run 6-3 w/ground. Bring in the same building, you could possible get away with a main lug panel.
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Old 09-30-2009, 02:26 PM   #3
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What do you see as the advantage of installing conduit and pulling 4 wires through it vs. just pulling one 6/3G wire? To me the choice is obvious, use the 6/3G wire. And I believe you could go w/a main lug box as you'd have the breaker in the main box as the disconnect.
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Old 09-30-2009, 02:37 PM   #4
 
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I think that the reason that I am thinking about conduit is because I want to make sure that the 6/3g is rated for 60amp. I know that it is rated at 55amp and that you round up, but the conduit rating is higher, and I'm thinking about being on the safe side.

I'm using a GE Box and best I can get for a main lug box is 4 spaces at Lowes. There are no GE boxes at HD. Though, I have a very large regional electrical supply house about 5 minutes from me. I should probably try there...
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Old 09-30-2009, 02:54 PM   #5
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Do you need all of the 60 amp? If so, I'd up it.
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Old 09-30-2009, 02:58 PM   #6
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Quote:
Originally Posted by meuseman View Post
If i used the THHN, what size conduit?
3/4 Pvc you can have up to 4- #6
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Old 09-30-2009, 03:06 PM   #7
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Whether you can use 6/3 is somewhat debated. Obviously the current rating of 6/3 is 55A. However, NEC2008 240.4(b) states that you can upsize the breaker to the "next higher standard" if it's not a multi-outlet run, there is no "correct" rated breaker, and it's not over 800A. Since there's no standard 55A breaker, a subpanel run isn't multioutlet, and 60A is well less than 800A you can use 6/3.
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Old 09-30-2009, 04:04 PM   #8
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6/3 with ground, main lug only( go to the supply house) . The way it works is the actual load and you will probably not exceed 55 amps. Remember its 55 amps per leg. Romex is actually good for higher amperages, 90 degree column in 310.16, it gets derated( 60 degree) because of attics and insulation purposes. You will be fine.
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Old 09-30-2009, 04:34 PM   #9
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Quote:
Originally Posted by Code05 View Post
6/3 with ground, main lug only( go to the supply house) . The way it works is the actual load and you will probably not exceed 55 amps. Remember its 55 amps per leg. Romex is actually good for higher amperages, 90 degree column in 310.16, it gets derated( 60 degree) because of attics and insulation purposes. You will be fine.
So, you're saying you don't have to use the 60 degree column for NM?
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Old 09-30-2009, 04:47 PM   #10
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Quote:
Originally Posted by jerryh3 View Post
So, you're saying you don't have to use the 60 degree column for NM?
No. You use the 90 degree column for derating purposes, if needed, and as long as your final calculation does not exceed the 60 degree column you are okay The notes also tell you that 14,12.10 are limited to certain breakers in most cases. I can explain this better if you are interested, but for most DIYers it wont apply. If you have a code book look at 334.80
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Old 10-01-2009, 07:46 AM   #11
 
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Quote:
Originally Posted by Code05 View Post
6/3 with ground, main lug only( go to the supply house) . The way it works is the actual load and you will probably not exceed 55 amps. Remember its 55 amps per leg. Romex is actually good for higher amperages, 90 degree column in 310.16, it gets derated( 60 degree) because of attics and insulation purposes. You will be fine.
This has convinced me, especially since this subpanel will have no double poles - essentially just an expansion of my main panel for a new kitchen - 3 small appliance circuits, DW, Fridge, Lighting.

Thanks for your help!!
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Old 10-01-2009, 09:45 AM   #12
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While i agree that the OP will be fine with 6/3, I question that interpretation of 334.80. Isn't that saying that if you need to derate you derate off the 90* column, but in no case can you go higher than the 60* column rating. That is, if you started with 75A carrying and derated to 50A then the max is 50. But if you started w/75A and derated to 70, you'd still be limited to 55 per the 60* column. My argument for the 60A breaker isn't that the cable is rated for 60A, but rather that it's rated for 55, but you're allowed to use the next larger standard breaker in this case. And yes, this is a code argument, not an actual protection argument. I think we all agree that the cable is capable of carrying well more than it's rated amperage. The rating has a safety margin built into it. But that's a safety margin, and shouldn't be counted on.
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Old 10-01-2009, 12:20 PM   #13
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If you use #6 NM wire you can only use a 50 amp breaker.
If you use #4 NM wire you can use a 60 amp breaker.
If you use #6 THHN wire and install it in conduit you can use a 60 amp breaker.

There is no provision for derating in this application. This is a sub panel not a service.
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Old 10-02-2009, 04:13 PM   #14
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Quote:
Originally Posted by WaldenL View Post
While i agree that the OP will be fine with 6/3, I question that interpretation of 334.80. Isn't that saying that if you need to derate you derate off the 90* column, but in no case can you go higher than the 60* column rating. That is, if you started with 75A carrying and derated to 50A then the max is 50. But if you started w/75A and derated to 70, you'd still be limited to 55 per the 60* column. My argument for the 60A breaker isn't that the cable is rated for 60A, but rather that it's rated for 55, but you're allowed to use the next larger standard breaker in this case. And yes, this is a code argument, not an actual protection argument. I think we all agree that the cable is capable of carrying well more than it's rated amperage. The rating has a safety margin built into it. But that's a safety margin, and shouldn't be counted on.
I think your opinion is reasonable and you have a good and valid reason for it, I have no arguement. Rarely do I just do things to code, it a bare minamum: however, it is safe. This does not it is the best, just safe. The engineers and others who write the code build in a pretty good safety margin that is well beyond that they allow.
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Old 10-02-2009, 04:24 PM   #15
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Quote:
Originally Posted by J. V. View Post
If you use #6 NM wire you can only use a 50 amp breaker.
OK. Why? I too see the table that says the max on #6 NM is 55A. But then how do you reconcile that with 240.4b?
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