MC Cable Derating

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WeisD

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Question: There are eight 10-3 MC cables grouped together without spacing run through a plenum space above a hard lid ceiling. The cables are exposed to an area where temperatures of 155° F are often encountered. The circuits are fed from a 120/208, 3 phase, 4 wire panel. What is the corrected ampacity of the conductors within the MC cables? 60% of 40 Amps per 310.15(B)(3)(4) exception to (4):, or, multiplier of .58 from 310.15(B)(2)(a)?
 
WeisD said:
Question: There are eight 10-3 MC cables grouped together without spacing run through a plenum space above a hard lid ceiling. The cables are exposed to an area where temperatures of 155° F are often encountered. The circuits are fed from a 120/208, 3 phase, 4 wire panel. What is the corrected ampacity of the conductors within the MC cables? 60% of 40 Amps per 310.15(B)(3)(4) exception to (4):, or, multiplier of .58 from 310.15(B)(2)(a)?
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I believe that these both apply. First the temperature correction then the bundling adjustment.
 
ActionDave said:
There's an exception for MC that bundling rules don't apply till 20 CCC I believe. I don't know the exact reference, but I know it's in 310.
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jumper said:
Cats are lazy.:D

310.15(B)(3)(a)(4)(d)

d. Not more than 20 current-carrying conductors are
installed without maintaining spacing, are stacked, or are
supported on “bridle rings.”
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310.15(B)(3)(a)(4) - Am I correct in thinking all the conditions must apply?

310.15(B)(3)(a)(4)(c) - condition: the conductors are 12 AWG copper - these are #10's.
 
Coppersmith said:
310.15(B)(3)(a)(4) - Am I correct in thinking all the conditions must apply?

310.15(B)(3)(a)(4)(c) - condition: the conductors are 12 AWG copper - these are #10's.
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I asked a similar question before and I believe the conclusion was that 12 does not get derated for bundling up to 20 ccc's but 10 does. Never made sense to me
 
mkgrady said:
I asked a similar question before and I believe the conclusion was that 12 does not get derated for bundling up to 20 ccc's but 10 does. Never made sense to me
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CMP dropped the ball on this one. But it says what it says, and no one is busting down doors to change it.
 
ron said:
Is it possible the #10s are upsized due to another derating, and they should really be #12s which would allow you to use this exception?
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luckylerado said:
I disagree with the premise. 10s are 10s, 12s are 12s.
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Assuming MWBC's, if both adjustment and correction factors apply
40A@90°C × 50%@16CCC × 0.58@155°F = 11.6A​

With just correction applying when using #12
30A@90°C × 0.58@155°F = 17.4A​

#12 would work at not greater than 17.4A calculated load on a 20A circuit. Running #10 would have to be on 15A breakers with the calculated load not greater than 11.6A.
 
Uh, if you have 8 10/3 fed from 3 phase, arent there 24 CCC, not 16?

and clearly a test question. show me an air-handling space that reaches 155*, or an EC who would eat such massive derating vs separating the cables a bit. dumb and totally not a real world install = worthless.
 
Last edited:
Thank You!

Thank You!

Thank you to all who replied to the question, and yes it is a practice test question for JW exam, not a real world installation for obvious reasons. I wanted to see if anyone else calculated it the same way I did. I figured 24 current carrying conductors because the neutral is considered current carrying, applying the exception for more than 20 CCC's at 60% and ambient temperature correction at .58 or 40 X .60 X .58 = 13.92 agree or disagree?
 
After reading Coppersmith's reply a second time, he is correct, all conditions must apply, question states #10's so only the derating for ambient temperature applies here? Again thank you all for the wisdom and expertise!
 
WeisD said:
After reading Coppersmith's reply a second time, he is correct, all conditions must apply, question states #10's so only the derating for ambient temperature applies here? Again thank you all for the wisdom and expertise!
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the 60% rule is indented under exception #5, which leads me to believe it applies only to that exception or the exceptions under 310.15(B)(2). I'm inclined to think that 24 CCC in bundled MC would get derated to 45% before any temp adjustments. 40 x .45 x .58 = 10.44A.

eta: re-reading it, I believe you are correct with the 60% and 58% temp adjustments.
 
WeisD said:
After reading Coppersmith's reply a second time, he is correct, all conditions must apply, question states #10's so only the derating for ambient temperature applies here? Again thank you all for the wisdom and expertise!
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With #10's both adjustment (45%) and correction (0.58) apply.

Additionally, there is nothing in the question which one can deduce the neutrals are CCC. However, there is also nothing which says there are any neutrals, or if there are, that the circuit originates from a split phase system. On this part, the question is poorly worded. Questions should not leave anything to assumption... but we all know the authors will continue to write 'em.
 
Smart $ said:
With #10's both adjustment (45%) and correction (0.58) apply.

Additionally, there is nothing in the question which one can deduce the neutrals are CCC. However, there is also nothing which says there are any neutrals, or if there are, that the circuit originates from a split phase system. On this part, the question is poorly worded. Questions should not leave anything to assumption... but we all know the authors will continue to write 'em.
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So you are agreeing with my initial assessment in the previous post?

There is nothing in the original post that mentions neutrals at all. Could be 208V 3ph equipment. ofc (of course), all three conductors would be current-carrying then, and they'd also all be CCC with 208V 1ph with neutral. At least that is the assumption we both made as 45% applies to 21-30 CCC. In lieu of other pertinent information, like 'super sparky ran 10/3 to 208V lighting or water heaters' (that don't use a third CCC, hence we can ignore it for calculations), don't we have to assume that all 3 conductors are current carrying? The circuits come from a 208Y/120V 3ph panel.

WeisD said:
The circuits are fed from a 120/208, 3 phase, 4 wire panel.
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eta: dunno about poorly worded, but I find it a terrible question nonetheless. Is there ANY real world application where the question would be applicable?
 
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