Derating

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wirenut46

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I have Eight current carry conductors in a raceway flexible metal conduit. 4 # 8 THHN 4# 10 THHNth 1 # 12 grnd. I am derating at 70% based on 310.15 (b) (3) (a) using table 4 and table 5 in chapter 9 what is the minimum size flex i can use. My over current protection in the panel is 20 amp. The wire size has been increased due to voltage drop.
 

david luchini

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You can't use a #12 gnd in your raceway, because of 250.122(B). You'd need a #8 gnd.

That doesn't change the required fmc size, though. I come up with 1" fmc.
 

augie47

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I also came up with 1" for 5 #8 and 4 #10 (required area .2674)
 

MasterTheNEC

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I have Eight current carry conductors in a raceway flexible metal conduit. 4 # 8 THHN 4# 10 THHNth 1 # 12 grnd. I am derating at 70% based on 310.15 (b) (3) (a) using table 4 and table 5 in chapter 9 what is the minimum size flex i can use. My over current protection in the panel is 20 amp. The wire size has been increased due to voltage drop.
Assuming that CCC's is your only adjustment.

So you are telling us you have 8 CCC's (limited additional information)...

8 AWG THHN is 0.0366 x 4 = 0.1464 in2
10 AWG THHN is 0.0211 x 4 = 0.0844 in2
12 AWG THHN is 0.0133 X 1 = 0.0133 in2

Total is shown = 0.2441 in2

Total size of the FMC should be no less than Trade Size 1

Now....did you need to increase the EGC...and would it make a difference in the sizing........?.....That I have not considered as it was not asked.

If an 8 AWG EGC is considered it would change the in2 totals to 0.2674 in2 resulting in a Trade Size 1 - No Change.
 

Dennis Alwon

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Would 9 ccc be the amount that would require an up size in wire, or is it being do because of another factor
Look at the table. 9 current carrying conductor must be derated at 70%. So if you use 90C conductors it may be fine but if you have 10 current carrying conductor then you have a 50% derating.

Look at 12 THH wire-- rated 30 amps at 90C but with 9 current carrying conductor we would have to derate 70%. 30 * .7= 21 amps, thus we are still good at using a 20 amp overcurrent protective device. At 50% we need a 15 amp breaker
 
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