ampacities

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gunny7

Member
I have a question about Table 310-15(b)(2)(a).Correct me if I'm wrong folks! If I run 4 #14AWG THHN conductors (2 hot 2 neutral) in a 1/2 emt from a panel to say a bedroom for 2 circuits 30 feet away, these conductors SHALL be derated by 20%. Which means that each conductor now has the ampacity of 16amps correct? If I ran 10 #14 AWG THHN (to feed 2 bedrooms with lighting and several receptacles)I would derate by 50%, which each one is now rated at 10 amps. What is the correct overcurrent protection for both? First example 15 amp breaker? Second example 10 amp? Keeping in mind Art 240-3(b)etc.
Thanks in advance to all. Gunny
 

charlie

Senior Member
Location
Indianapolis
Re: ampacities

For derating purposes, you start with the ampacity shown in table 310.16 which is 25 amperes. Multiply for the adjustment factor which is 80% or 20 amperes. The overcurrent device can not be larger than either the answer or 15 amperes. In this case after the derating, you may still use a 15 ampere circuit breaker.

I edited this to add the previous paragraph to my answer for #14 wire. The following was for #12 wire because I didn't read the original question . . . DUH.

:eek: :eek: :eek: :eek: :eek: :eek: :eek:

For derating purposes, you start with the ampacity shown in table 310.16 which is 30 amperes. Multiply for the adjustment factor which is 80% or 24 amperes. The overcurrent device can not be larger than either the answer or 20 amperes. In this case after the derating, you may still use a 20 ampere circuit breaker. :D

[ January 07, 2004, 02:06 PM: Message edited by: charlie ]
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: ampacities

Gunny first it looks like you are using the wrong column for 14 AWG THHN.

14 AWG THHN is rated 25 amps, not 20 amps.

Four 14 AWGs THHNs in EMT the correct breaker is still a 15 amp 240.4(D)

Ten 14 AWGs THHNs in EMT the correct breaker could be a 15 if you meet the requirements of 240.4(B) but more likely the breaker would need to be a 10 amp.

240.4(B) can not be used on multioutlet branch circuits supplying receptacles for cord-and-plug-connected portable loads.
 

gunny7

Member
Re: ampacities

Bob; Thanks1 While trying to come up with an example I went back and forth soo many times I screwed up on the rating of the first example . I meant for it to be 20amps after the derating. You're right on as far as I'm concerned about the 2nd example. It MUST be broke at 10amps considering I have at least 4 receptales down stream. The fix. Run a couple of # 10s. Thanks.

Then WHY do soo many people run more than 3 wires in the raceway? I'm pretty sure that they either don't derate them or size the breaker correctly.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: ampacities

With 14 or 12 AWG THHN you can put up to 9 current carrying conductors together before the derating becomes an issue.

12 AWG, 30 x .7 = 21 amps.

14 AWG, 25 x .7 = 17.5 amps.

The jobs I do we often use 10 THHN on 20 amp circuits for voltage drop, this also helps with derating.

You can put 20 current carrying 10 AWG THHN conductors in a raceway and still use a 20 amp breaker per NEC.
 

gunny7

Member
Re: ampacities

Bob: I agree again, but keeping in mind the temp. the terminals are rated at. The reason behind this topic is that a contractor out here keeps putting 12, 13 and sometimes 15 wires THHN in a pipe and he thiks it's fine. Thanks for your help.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: ampacities

You are right many electricians do not pay any attention to the adjustment tables. :(

One more thing though, the terminal temp ratings will not factor in the derating.

The important thing is after the derating is done you use the lower figure, between the adjusted ampacity and the terminal, conductor temp.
 

bennie

Esteemed Member
Re: ampacities

This is an area that I do not agree. When the load is reduced to the derated ampacity, and the breaker is also dropped in size, to reflect the lower ampacity, the breaker is now at 100% of its rating.

10 ampacity X 1.25 will require a 15 amp breaker.

20 ampacity X 1.25 will require a 25 amp breaker.
 

gunny7

Member
Re: ampacities

Bennie: I think I know what you're saying about 125%. Do you receive the NEC Digest? If so there is an article in there exactly what we are talking about. It's under ask the Experts and the issue is the new one, winter 2003. Try it on-line. www.necdigest.org and then let me know what you think. Thanks Bennie
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: ampacities

Bennie I do not know anyone that really drops the breaker size.

Most times the load remains unchanged and you must increase the conductor size to cover the loads requirements.

The 100% loading is OK for 179 minutes. ;)
 

wyedelta

Member
Re: ampacities

i believe we derate conductor to reduce heat inside the conduit.Although the table specified already the ambient temperatrue but heat is generated while the conductor is in load/full load.If we reduce the ampacity we reduce also the heat thereby giving extra safety for insulation breakdown.If we will not derate conductor and we base only in ampacity the maximum heat is generated and when short circuit occur there's the problem lies conductor insulation will be easily damage which might produce another short circuit inside the conduit.

Temperature inside the conduit will be ambient temp + temp of the conductor.


15A should be use for #14 for lighting and 20A for #12 C.O./small appliance circuit even you applied the derated value you will still come up with that value.
 
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