ampacity

[jctj88]

article 330 says that MC ampacity is going to be determined by 310.15. so 310.15 (B)(3) says that multicolor cables (Mc would be multicolor right?) installed without maintaining spacing for a continuous length longer than 24in. ( so a bundle of mc 12/3-12/2 as homeruns.) the allowable ampacity shall be reduced.

is there a exception about mc that allows me to run them in bundles without having to adjust the ampacity or do I just have to make sure they are separated. because if so then I wouldn't even be able to run two 12/2 together much less two or three or more 12/3 with out breaking this rule.

just want to make sure im interpreting this right and that im not missing something. i'm sure a lot of you have run bundles of mc in different projects.. .let me know what you guys end up doing.
thank you so much.

 

[LarryFine]

When calculating ampacity reduction, you don't have to start at the circuit rating. For #12, you can start at 30a, and at a 70% ampacity, still exceeds 20a.

 

[infinity]

If you're running only #12 AWG MC cable there is an exception where you can bundle up to 20 CCC's together without any derating.

310.15(B)(3)(a)
(4) Adjustment factors shall not apply to Type AC cable
or to Type MC cable under the following conditions:
a. The cables do not have an overall outer jacket.
b. Each cable has not more than three current-carrying
conductors.
c. The conductors are 12 AWG copper.
d. Not more than 20 current-carrying conductors are
installed without maintaining spacing, are stacked, or are
supported on “bridle rings.”

 

[jctj88]

aaaaaaaaaaah.. there it is.. i knew I was missing something.... thank you soooo much.... very helpful.

 

[infinity]

aaaaaaaaaaah.. there it is.. i knew I was missing something.... thank you soooo much.... very helpful.

You're welcome and when you go beyond the 20 CCC's you only need to derate to 60% so if you have 50-12/2 MC cables (100 CCC's) feeding something other than multi-receptacle circuits you could still possibly use them on 20 amp OCPD with the next size up rule. 30*60%=18 amps next size up 20 amps.