Max ampere rating of multi mc cables

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brian klamik

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If you have a bundle of seven 12 awg thhn copper, 3wire mc cables thar are installed in bridle rings for a length of 50 feet. each 208/120v single phase multiwire circuit in the cable serves office space receptacle outlets. what would be the ampacity of the conductor and also the max ampere branch circiut rating. Thanks in advance.
 
Your question needs to provide more information. What are the loads that you're feeding? You did say MWBC so we're assuming two hot and a neutral?
 
Brian,

I would say the ampacity would be reduced to 50%, per table 310.15(B)(2).

#12 thwn ( if the mc cable is in fact thwn) use the 90deg. column for derating= 30amps,

15a OCPD. Others may have more to add.
 
If the System in the building is 3 phase, then you would have 21 CCC, and the multipler is 45%.
The amperage after ampacity adjustment is 13.5 amps.

If this is supplying receptacle loads , you would not be able to round up as per 240.3(B)(1).
A 10 amp rated circuit breaker would be required.

If it is feeding lighting circuits, then you can round up to a 15 amp CBer.
 
Yes, it could possibly be 60% due to 310.15(B)(2)Ex5. That would depend on whether or not you had neutrals in the 3-wire cables which would be CCC's. Or it may have no affect at all. If these cables are not feeding multi-receptacle circuits then your adjusted ampacity of 60% would give you 18 amps (30 * 60% = 18 amps). The next higher OCPD could be used so you're back to a 20 amp OCPD on #12 conductors. Derating has no real affect.
 
brian klamik said:
If you have a bundle of seven 12 awg thhn copper, 3wire mc cables thar are installed in bridle rings for a length of 50 feet. each 208/120v single phase multiwire circuit in the cable serves office space receptacle outlets. what would be the ampacity of the conductor and also the max ampere branch circiut rating. Thanks in advance.
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This sounds like a test question. :smile: Is it ? The reason I ask is in the field I never hear anyone call them "bridle rings".
 
infinity said:
If these cables are not feeding multi-receptacle circuits then your adjusted ampacity of 60% would give you 18 amps (30 * 60% = 18 amps). The next higher OCPD could be used so you're back to a 20 amp OCPD on #12 conductors.
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M. D. said:
They serve office receptacles so if the grounded conductor is a current carrying one ,..
it is 18 amps so looks like 15 amp breaker.
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Well, both of you can't be right. :rolleyes:
 
M. D. said:
I don't know,... But I've been thinking ,..what if only one of the 7 cables cable was feeding mutiple receptacle outlets used for portable loads???
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Then the circuits within that cable can only be protected at 15 amps. The other six can protected at 20 amps.
 
that was my take on it as well... thanks for responding. I wish the O.P. hadn't run off so fast... It leads one to believe that we helped with homework as opposed to real work:roll:
 
Actually I am a student, but also have been working in the field for several years for a small company and never got the chance to further my education. I am now trying to do both and want to be able to speak correctly when giving my input. This stuff can get confusing quickly and there are things I dont think of. Thanks to all that helped and hopefully I can return the favor some day. I dont have the internet at my apartment and that would be the delay in responding back to some of the questions asked. I was looking at 310.15 (2) but wanted to make sure I was on the right track.

Respectfully,

Brian
 
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