Question for the Pros on Feeder Size

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  #1  
Old 03-17-07, 11:36 AM
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Question for the Pros on Feeder Size

My neighbor is converting his old garage into a shop (he added a new garage to the front of the old one for the cars).

He plans to install a 125A sub panel, which will be placed about 15 feet from the main panel. The guy at the local Big Box said he could use NM-B 2/3 AWG cu with ground since the run was so short and it won't be exposed to outside temperature extremes.

I disagree. I think he should follow code and use size 2 AWG cu THHN which provides a rating of 130A, rather than the 95A of NM-B.

One motivating factor for him is that the Big Box guy apparently has a 20ft. piece of 2/3 NM-B that he will sell at a 10ft. price.

I got my neighbor to agree that if the pros in this forum said not to use the NM-B then he will go with the THHN.

What say the pros??

Thanks!
 
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  #2  
Old 03-17-07, 12:33 PM
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#2THHN is only good for 115 amps when used for a circuit. We do not use the 90 deg c column for figuring any circuit amperages since it is likely the only thing at 90 deg is the wire itself, and we have to use the temp of the lowest rated item. This is typically 75 deg c, and #2THHN @ 75 deg c is 115 amp.

In my area,and most others, we use Table 310.15(B)(6) for figuring feeders. This would be #4cu conductors in conduit. NM cable is not listed in this table.
To use NM cable he would need #1 @ 110 amps.

I assume he is using a 100 amp sub-feed breaker, or is he actually using a 125? Be sure the main panel will accept a 125 before he goes too far with this.
 
  #3  
Old 03-17-07, 12:39 PM
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When you say "125 A sub panel", it is unclear whether you are talking about a panel rated for up to 125 amps, or feeder protected by a 125-amp breaker.

You can use 2/3 NM-B off a 100-amp breaker to feed a panel rated at 125 amps.
 
  #4  
Old 03-17-07, 12:52 PM
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we are talking apples and oranges.

Thhn needs to be in conduit. NM-B is a cable

And this bit about listening to the big box store clerk has me concerned. Very few have any clue as to what is proper.
 
  #5  
Old 03-17-07, 12:53 PM
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Sorry for any confusion.

He has a 400A main panel. He plans to feed the sub panel with a 125A breaker which he as already purchased. The sub panel he has already purchased is rated for 125A. He has yet to buy the feeder conductor.

Does this help?

Thanks!
 
  #6  
Old 03-17-07, 01:04 PM
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use number 2 ser or a four conductor NM-B is fine.

As pete said. this is a feeder, not a branch circiut.
 
  #7  
Old 03-17-07, 01:13 PM
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He's planning to use RNC for conduit regardless of the cable/conductor type.

So JWhite, you are saying that he may use NM-B 2/3 w/ground cu cable for a feeder in this application?

One other question: why is there a size difference allowed between feeders and branch circuit conductors? Does it have to do with conditions of operation?

Thanks!

rn
 
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Old 03-17-07, 01:13 PM
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Speedy said that his area allows 310.15(B)(6) for feeders. But not all do. You need to check with your building department.
 
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Old 03-17-07, 01:23 PM
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Originally Posted by rdn2113 View Post
He's planning to use RNC for conduit regardless of the cable/conductor type.

So JWhite, you are saying that he may use NM-B 2/3 w/ground cu cable for a feeder in this application?

One other question: why is there a size difference allowed between feeders and branch circuit conductors? Does it have to do with conditions of operation?

Thanks!

rn
If he is using conduit then I advise against pulling a cable in a concuit. Use Thhn. Just a preference.

Feeders can be smaller because of use. It is not likely that every load will be on all of the time. The conductor is only likely to draw max amps for brief periods of time, not for long durations. If they ever draw max amps at all.
 
  #10  
Old 03-17-07, 01:25 PM
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Yes, they do allow 310.15(B)(6) for feeders.

So, John and Speedy, using 310.15(B)(6), he should use #2 AWG cu THHN to support a 125A breaker from the main panel.

Otherwise, if we use Table 310.16, he would have to use 1/0 AWG cu if we wanted to use NM-B cable to support a 125A breaker from the main panel. Is this correct?

Thanks!
 
  #11  
Old 03-17-07, 01:33 PM
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You can use table 310.15(B)(6) regardless of wiring method.
 
  #12  
Old 03-17-07, 01:44 PM
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... except that, as Speedy said, if you want to use 310.15(B)(6), you cannot use NM-B, because that table specifically does not apply to NM-B cable.
 
  #13  
Old 03-17-07, 02:05 PM
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And the Final Answer Is... ?

OK. I need to add load information I just learned about. He plans to run shop lights, large shop vac system, power tools (table saw, joiner, planer, etc.), and a heat pump system off of the sub panel.

So under the following conditions:

1. Breaker from main panel is 125A.
2. Sub panel rated at 125A.
3. Using RNC conduit.
4. Run from main panel is max 15 ft.
5. Load is for items described above.
5. Using Table 310.15(B)(6).

He would be better off using #2 AWG THHN cu conductors. He may use the #2/3 w/ground AWG NM-B cu cable if he prefers.

Is this correct? (come on guys, I'm trying to win a 12-pack of beer here!)

Thanks!
 
  #14  
Old 03-17-07, 02:26 PM
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No, he absolutely may not use the 2/3 NM-B. And there's no reason to either. Since he's already running the conduit, there's no reason not to use the THHN.

If he does want to use 2/3 NM-B, have him switch to a 100-amp breaker. He doesn't need 125 amps anyway--not even close.
 
  #15  
Old 03-17-07, 02:43 PM
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Absoultely agree. Since he is running the conduit the NM cable is a moot point.
Use the conductors.
 
  #16  
Old 03-17-07, 03:10 PM
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OK. So #2 AWG THHN cu it will be.

He's a good neighbor, so I guess I'll share the beer.

Thanks guys!
 
  #17  
Old 03-17-07, 03:16 PM
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Share the beer AFTER the job is done. For the grounding wire, you can use #6 copper, bare or green.
 
  #18  
Old 03-17-07, 05:25 PM
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Agreed - no beer until the job is done.

I really appreciate the professional help from all you guys. Thanks!!
 
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