Re: PVC Conduit Fill Rating?

pbs wrote:
Can #12 THHN be run through 1' gray PVC Electrical conduit?

If Yes, what is the code fill rate for 1" PVC? ( I believe that 1" EMT
can handle 18 #12 THHN maximum.)


Table C10 allows 25 THHN conductors in a 1 inch schedule 40 conduit.
That's how many THHN conductors that are PHYSICALLY permited. More
important is DERATING of those conductors. IOW one _CANNOT_ pull 25
THHN conductors in a 1 inch PVC conduit and expect that they will be
rated for the full allowed 20 amps.....they won't. One needs to look
at Table 310.15(B)(2)a for conductors with no load diversity (All
conductors in the raceway are loaded to their maximum rated load) or
Table B.310.11 in Annex B for conductors with 50% load diversity.

Both Tables are the same until one gets to over 10 current carrying
conductors. #12 THHN can be derated from the 90 degree column in Table
310.16 which is 30 amps, HOWEVER the #12 conductors _CANNOT_ be fused
at over 20 amps. For derating, an equipment grounding conductor is
not counted as a current carrying conductor Also, for Residential
Single Phase 120/240, a true neutral (shared between two hots from
opposite legs) is not counted as a current carrying conductor. A
"neutral from a two wire circuit _IS_ counted.

In your case (from the above Tables) for #12 THHN conductors:

4 to 6 conductors must be derated at 80% 30 amps x .8 = 24 amps
7 to 9 conductors must be derated at 70% 30 amps x .7 = 21 amps.

You can safely install 9 current carrying #12 THHN conductors in that

To install more conductors, the Tables differ.

With no diversity (fully loaded):
10 to 20 conductors must be derated at 50% 30 amps x .5 = 15 amps
Now there is a problem!!!!!! If you install more than 9 current
carrying conductors (up to and the physically allowed 20 conductors)
you MUST either use 15 amp breakers on the #12 THHN conductors or
increase the size of ALL wires to #10 THHN since, from the 90 degree
column in Table 310.16 one can derate from 40 amps for #10 THHN:
40 amps x .5 = 20 amps (you must still use a 20 amp breaker.)

HOWEVER, if load diversity is 50% (from Table B)
10 to 24 conductors must be derated at 70% 30 amps x .7 = 21 amps.

You must ALSO derate for ambient temperature, especially if more than
10% of the conduit is in an attic. Table 310.16 is only good for
ambient temperatures of 30 C (86 F)

To be safe, I would strongly recommend that you use #10 conductors in
the 1 inch conduit(s) if you are going to use 20 amp breakers. If you
set J-boxes in the basement and the attic, you can use the existing #12
Romex elsewhere, just use #10 in the conduit(s).

This is one reason why Romex is used in is inherently
derated for use in attics, basically being dummy proof as long as one
stays with the common #14 Romex = 15 amp breakers, #12 Romex = 20 amp
breakers and #10 Romex = 30 amp breakers. When one starts installing
conductors in conduit, ya gotta watch your P and Q's.


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