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Old 05-22-2010, 02:40 PM   #1
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Ampacity of #6 THWN feeder


Folks,

I can't seem to find a clear cut answer on this. For my garage, I'll be pulling #6 copper THWN about 60 feet through PVC for a sub-panel. I'm not sure whether the breaker feeding this run should a 50A or 60A. I've heard people say 40A, 50A, or 60A.

Anyone got the definitive answer?

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Old 05-22-2010, 02:55 PM   #2
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Ampacity of #6 THWN feeder


Maximum 65 amps.

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Old 05-22-2010, 03:37 PM   #3
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Ampacity of #6 THWN feeder


A 60-amp breaker or smaller!
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Old 05-22-2010, 06:17 PM   #4
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Ampacity of #6 THWN feeder


Here's the codebook answer; (to avoid having to type for hours, I'll include only the parts of the code articles that apply to your specific situation).

110.14 is entitled 'Electrical Connections'.

110.14 (C) states... Temperature limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as to not exceed the lowest temperature rating of any connected termination, conductor, or device.

110.14 (C) (1) (a) states... Termination provisions of equipment for circuits rated 100 amps or less, or marked for 14 AWG through 1 AWG conductors shall be used only for one of the following.

110.14 (C) (1) (a) (3) is one of the following and states... Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.

All of this means that you must look at the temperature limitations at the terminations at both ends of the wire, and the wire itself, and size according to the lowest. Almost all electrical equipment (breakers, panels etc.) will be rated at 75C. THWN is also rated at 75C.

Next, we look at table 310.16. This applies to not more than 3 current carrying wires in a conduit. (The neutral is not considered current carrying here). The 75C column lists #6 copper as having an ampacity of 65 amps.

Now (we're almost done!) we need to look at 240.4 'Protection of conductors'.

240.4 (B) is entitled 'Devices Rated 800 Amps Or Less'. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all the following conditions are met.

240.4 (B) (2) is the only one of those conditions that applies here, and it states... The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or circuit breaker.

240.6 lists the standard ampere ratings of fuses and circuit breakers, and it goes from 60 to 70; no 65.

Therefore, your #6 copper wires can be protected by a maximum 70 amp breaker. Smaller is OK, I'd likely use a 60.

Rob

Note; Since we deal with NM (Romex) a lot on this site, 334 is for NM cable, and 334.80 states that NM shall be sized using the 60C column of 310.16.

Last edited by micromind; 05-22-2010 at 06:20 PM.
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Old 05-22-2010, 06:37 PM   #5
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Ampacity of #6 THWN feeder


Quote:
Originally Posted by brric View Post
Maximum 65 amps.
1st answer is correct
You can use up to a 70a breaker, if your load will not exceed 65a
Big box stores only seem to have 60a breakers around here, so that is what I used
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Old 05-22-2010, 08:14 PM   #6
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Ampacity of #6 THWN feeder


According to 215.3, if the load is 65 amps, the wire size will need to be #4. #6 is good for 52 amps of calculated load.

Rob
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Old 05-22-2010, 08:28 PM   #7
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Ampacity of #6 THWN feeder


Quote:
Originally Posted by micromind View Post
According to 215.3, if the load is 65 amps, the wire size will need to be #4. #6 is good for 52 amps of calculated load.

Rob
Who said anything about a continous load ?
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Old 05-22-2010, 08:32 PM   #8
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Ampacity of #6 THWN feeder


Quote:
Originally Posted by micromind View Post
According to 215.3, if the load is 65 amps, the wire size will need to be #4. #6 is good for 52 amps of calculated load.

Rob
What are you talking about? How could you possibly know the load calculated or otherwise.
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Old 05-23-2010, 12:40 AM   #9
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Ampacity of #6 THWN feeder


Quote:
Originally Posted by McSteve View Post
Folks,

I can't seem to find a clear cut answer on this. For my garage, I'll be pulling #6 copper THWN about 60 feet through PVC for a sub-panel. I'm not sure whether the breaker feeding this run should a 50A or 60A. I've heard people say 40A, 50A, or 60A.

Anyone got the definitive answer?
Similar installation, and I put a 60a breaker in mine.
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Old 05-23-2010, 10:57 AM   #10
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Ampacity of #6 THWN feeder


60 amp breaker on #6 THWN wire. Four conductors. H-H-N-G. Use a green #10 for the fourth wire.
Please search "sub panels" on this forum for plenty information. It's no doubt the most commonly asked question around here.

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