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.
 
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.
 
I also came up with 1" for 5 #8 and 4 #10 (required area .2674)
 
wirenut46 said:
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.
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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.
 
electricalist said:
Would 9 ccc be the amount that would require an up size in wire, or is it being do because of another factor
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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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