WHY ?

[vanwalker]

310.15(B)(2)EX 5
No matter how much you derate #12 MC it still ends up on 20 amp breaker.
Meaning WHY waste the time on this in(2002 HB)as it does not change anything?
Was trying to explain this to someone and ??
Please help as i do not see it.THANKS GEO

 

[infinity]

Re: WHY ?

You can stack or bundle up to 20 current carrying conductors and still use a 20 amp OCPD for #12 AC or MC cable. After 20 you would need to derate to 60%. Over 20 CCC's using cable with THHN would leave you with 30amp X 60% or 18 amps. The question is could this still be used on a 20 amp OCPD or would it only be allowable on a 15 amp OCPD?

 

[charlie b]

Re: WHY ?

Originally posted by vanwalker: No matter how much you derate #12 MC it still ends up on 20 amp breaker.

I think the point is that you don't have to derate #12 MC. Or at least you don't have to derate it until you go beyond 20 conductors.

Originally posted by infinity: The question is could this still be used on a 20 amp OCPD or would it only be allowable on a 15 amp OCPD?

The answer depends on the amount of load that you calculate for the circuit. If you have 17 amps of load, then you can use a conductor with an ampacity of 18 amps, and you can protect it with an 20 amp breaker. But if you have a load of 19 amps, then you cannot use a conductor with an ampacity of 18 amps. Whatever conductor you pick, you will be allowed to protect it with an 20 amp breaker. If for example you pick a conductor with an ampacity of 30 amps, you could use either a 20, a 25, or a 30 amp breaker.

 

[steve66]

Re: WHY ?

Never mind. I should have read the example first.

Steve

[ October 10, 2005, 01:26 PM: Message edited by: steve66 ]

 

[infinity]

Re: WHY ?

The answer depends on the amount of load that you calculate for the circuit. If you have 17 amps of load, then you can use a conductor with an ampacity of 18 amps, and you can protect it with an 20 amp breaker.

But if these conductors were feeding receptacle circuits than there ampacity would be limited to 15 amps. According to 240.4(B)(1) the next higher standard overcurrent device can not be used for multioutlet branch circuits.