Why Does #4 CU Have Different Ampacities? - Electrical - DIY Chatroom Home Improvement Forum

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11-04-2010, 01:13 AM   #1

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## Why does #4 CU have different ampacities?

Per tables 310-15 and 310-16:

#4 copper is rated at 100 amps when used as a service entrance (SE) but only 70 amps (60 C) when in NM-B cable. The wire is the same gauge so why the restrictions on ampacity?

Also, NM-B is not even listed in table 310-16. The insulation is PVC per the manufacturer but are the individual conductors considered any specific type (i.e. THHN, etc.)?

11-04-2010, 01:21 AM   #2
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Quote:
 Originally Posted by sandersdon Per tables 310-15 and 310-16: #4 copper is rated at 100 amps when used as a service entrance (SE) but only 70 amps (60 C) when in NM-B cable. The wire is the same gauge so why the restrictions on ampacity? Also, NM-B is not even listed in table 310-16. The insulation is PVC per the manufacturer but are the individual conductors considered any specific type (i.e. THHN, etc.)?
the size of the conductor is only part of the determination. The insulation further restricts the ampacity. Different types of insulation allow different ampacities.

Then, the use places other restrictions or alllowances of the ampacity.

to determine NM ampacity, go to the chapter on NM. It will tell you the ampacity is that in the 60º column of 310.16 but when derating, you can use the next column (75º?) but the final figure cannot be any greater than the 60º column.
what the individual conductors are insulated with are irrelevant. It is rated as I stated and it doesn't matter what the insulation is. You cannot modify the rating or use based on the insulation of the individual conductors.

and if you were talkiing of 310.15(B)(6), that is a special exception basically due to the fact that residential services are not generally loaded to capacity so they allow a slightly undersized wire for such installations.

Last edited by nap; 11-04-2010 at 01:25 AM.

 11-04-2010, 01:40 AM #3 Knothead     Join Date: Oct 2010 Location: Houston, TX, USA Posts: 17 Rewards Points: 10 Let me rephrase the question: (humor me please) It seems to me a wire of a certain gauge will physically pass a certain amount of current regardless of the insulation type or thickness (as in a bare wire). Isn't the insulation an electrical insulation as opposed to a thermal insulation? Obviously any thickness of insulation will have thermal properties but that isn't its primary function, is it? I guess it's more of an electrical theory question rather than a code question. Also, what is the reasoning for rating all NM-B at 60 C?.

 11-04-2010, 01:42 AM #4 Knothead     Join Date: Oct 2010 Location: Houston, TX, USA Posts: 17 Rewards Points: 10 Also, it's late. Go to bed.
11-04-2010, 01:49 AM   #5
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Quote:
 Isn't the insulation an electrical insulation as opposed to a thermal insulation?
yes

Quote:
 Obviously any thickness of insulation will have thermal properties but that isn't its primary function, is it?
no and it isn't really a consideration.

the ability to withstand higher temps without degradation of the insulation is why one wire type will be rated a higher ampacity than another.

Quote:
 Also, what is the reasoning for rating all NM-B at 60 C?.
Not sure but I suspect it is because it is a cable where the outer sheath will retain heat. If so, it would be a derating by the code similar to the limitations that #10 wire be limited to 30 amp circuits, #12 to 20 amp circuits, and #14 to 15 amp circuits.

11-04-2010, 01:50 AM   #6
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Quote:
 Originally Posted by sandersdon Also, it's late. Go to bed.

It's only 3 am. I sleep until 8:30-9:00. That's 5 1/2 to 6 hours. Old people don't need as much sleep

either that or the fear of never waking up again keeps us awake

 11-04-2010, 05:33 AM #7 Member   Join Date: Nov 2007 Location: Nashua, NH, USA Posts: 7,971 Rewards Points: 1,548 There is also a greater number of amperes allowed when the wire is hung "in free air" such as between the house and the utility pole as opposed to in a cable or conduit in the wall. In the former case heat is dissipated more rapidly. For a given wire size and material and a given number of amperes, the same voltage drop occurs and the same number of calories/BTU of heat are generated per foot of wire. __________________ The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit. Last edited by AllanJ; 11-04-2010 at 05:39 AM.
 11-04-2010, 05:59 AM #8 Licensed Electrical Cont.     Join Date: Feb 2004 Location: NY State Posts: 7,820 Rewards Points: 1,990 The size of the conductor is only one part of several in the ampacity equation. Why do your think there are so many columns in 310.16 and differing numbers. The setting and intended use are two main factors. IMO this is why NM is confined to 60 deg C. NM is typically installed in walls and ceiling that are insulated and attics which get very hot. SE cable is typically installed exposed.
11-04-2010, 10:04 AM   #9

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Interesting. Thanks for all your replies. I hope y'all don't mind my curiosity - I always want to know why things are as they are.

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