Derating THHN for conduit fill

[SCOURGE42]

I told my boss today that you can put 7-9 #12 AWG current carrying conductors into a 3/4" pipe each protected by a 20amp breaker.

I used table 310.15(b)(16) and 310.15(C)(1) to calculate that 7-9 #12s in a conduit can be rated at 21amps.

My boss claims that I can't use the 90 degree column to do my calculation because the circuit breaker terminal is only rated for 70 degrees.

Can anyone shed some insight on this disagreement?

 

[augie47]

You are correct. The 90° rating of the THHN (30 amps) X .7 (fill derating) is 21 amps which is higher than tye 75° termination limit of 25 amps or the 60° of 20 amps..

 

[SCOURGE42]

So does putting 9 20 amp circuits in a pipe mean that the 12# wire can reach temperatures close to 90 degrees?

If that is the case won't the heat equalize across the entire length of the wire and the termination points will also reach 90 degrees?

 

[LarryFine]

Welcome to the forum.

You're correct. Have him read 310.15(B), I think.

 

[infinity]

So does putting 9 20 amp circuits in a pipe mean that the 12# wire can reach temperatures close to 90 degrees?

If that is the case won't the heat equalize across the entire length of the wire and the termination points will also reach 90 degrees?

It's not 9 circuits it's 9 CCC's derated to 70%. You're correct that you can use the 90° ampacity in the derating calculation. The terminal value of 75° C is irrelevant.

 

[kwired]

I told my boss today that you can put 7-9 #12 AWG current carrying conductors into a 3/4" pipe each protected by a 20amp breaker.

I used table 310.15(b)(16) and 310.15(C)(1) to calculate that 7-9 #12s in a conduit can be rated at 21amps.

My boss claims that I can't use the 90 degree column to do my calculation because the circuit breaker terminal is only rated for 70 degrees.

Can anyone shed some insight on this disagreement?

Simple answer, the deration applies to the conductors in the raceway and you are not adjusting the terminal ampacity so the 12 AWG having 75 C or higher insulation is still fine on a 75 C terminal.

On the other side of things say you didn't have more than 3 CCC's, maybe even had low ambient temp and can adjust up instead of down you possibly could determine that you could have 14 AWG in the raceway but most cases you still must use 12 AWG but that usually because of 240.4(D) the "small conductor rule". Outside that rule 14 is still good for 20 amps and 12 is good for 25 amps at 75C.

 

[winnie]

Instead of the ampacity of the conductor, think of a chain of different ampacities.

At the terminals the conductors are not bundled together. But the temperature limit is the 75C terminal rating. Calculate the ampacity at 75C.

In the raceway the temperature limit is set by the 90C insulation, but you have multiple CCCs and have to derate, so you calculate based on the 90C ampacity as derated by the total number of conductors.

Perhaps somewhere else you have a conduit branch with only 3CCCs, but it is in a warm ambient space. Now you use the 90C ampacity, but adjusted for the high ampacity.

And so on down to the far end of the conductor.

Now you take the lowest calculated value for ampacity anywhere along that chain. That is the ampacity of the conductor that you are permitted to use.

Jon

 

[kwired]

Instead of the ampacity of the conductor, think of a chain of different ampacities.

At the terminals the conductors are not bundled together. But the temperature limit is the 75C terminal rating. Calculate the ampacity at 75C.

In the raceway the temperature limit is set by the 90C insulation, but you have multiple CCCs and have to derate, so you calculate based on the 90C ampacity as derated by the total number of conductors.

Perhaps somewhere else you have a conduit branch with only 3CCCs, but it is in a warm ambient space. Now you use the 90C ampacity, but adjusted for the high ampacity.

And so on down to the far end of the conductor.

Now you take the lowest calculated value for ampacity anywhere along that chain. That is the ampacity of the conductor that you are permitted to use.

Jon

Take smallest conductor required anywhere in that chain that is size you must use on the lower temp rated termination (nearly all the time) You possibly can have situation where you run a smaller conductor part the run, then must increase conductor size for a different portion of run becasue ampacity adjustment is required for that portion of the run. Such portions don't have lower ampacity, they have conductors with adjusted ampacity