Derating?

1467 Views 3 Replies 4 Participants Last post by Bugz11B, Apr 29, 2014

travis13

Discussion Starter · Apr 29, 2014

Can someone tell me the process for calculating conduit fill with the derating factor? Example would be how many 6 AWG THHN conductors can fit into 1" EMT? Ambient temperature would be a factor I imagine?

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480sparky

· #2 · Apr 29, 2014

Raceway fill is a purely mathematical function..... how many little circles fit into a big circle.

Derating is a function of number of conductors in a raceway and the environment they are in. It has nothing to do with fill.

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wildleg

· #3 · Apr 29, 2014 (Edited)

this is one way to do it

Step 1 allowable ampacity
- find the allowable ampacity for your conductors according to the correct table (310.15 B 16 or thereabouts)

Step 2 temperature correction
- multiply the allowable ampacity by the correction factors from table 310.15 B 3 c.

Step 3 ampacity adjustment
- adjust the ampacity you got in the last step by the number of current carrying conductors in the pipe
according to table 310.B 3 a

you now have the allowable ampacity of your conductors, which you cannot exceed, and the conductors must
be protected at this ampacity, or meet the applicable rules for this ampacity. the result you came to may
have rules which further limit the ampacity depending on what section applies (article 210, 240 are especially
good to read, but a whole bunch of articles apply depending on what the use is)

As mikeman said, how many conductors that will fit in the pipe is a physical limit, and
you can determin that from the tables in chapter 9 or use the mils to figure it out (see
examples in the back of the codebook)

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Bugz11B

· #4 · Apr 29, 2014 (Edited)

travis13 said:
Can someone tell me the process for calculating conduit fill with the derating factor? Example would be how many 6 AWG THHN conductors can fit into 1" EMT? Ambient temperature would be a factor I imagine?

You could put 7 THHN into a 1" EMT pipe (Table C.1)
If you had 6 current carrying conductors in *any size pipe* you would need to derate the wire to 80% of the allowable ampacity (table 310.15 b 2 a), #6 THHN is good for 75 Amps in the 90 deg column (table 310.16) so 75*.80=60A

!!Note im in Florida and we use 08 NEC!! this info may be adjusted I believe some of the allowable amps have changed in later codes. I know the actual table #'s have changed i.e 310.16 I believe is different now.

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