Derating discussion

[MBerner]

I have been discussing the derating factor for 12 gagewire. The way I read the NEC is, 3 or more current carrying conductors usetable 316. Well I see #12 wire is rated for 30 amps at 90'. And the asteriskfor 20 amps is for over current protection only. So at 70% of 30 amps which is21 amps I can run 7 to 9 # 12s in a conduit, right???

 

[kwired]

I have been discussing the derating factor for 12 gagewire. The way I read the NEC is, 3 or more current carrying conductors usetable 316. Well I see #12 wire is rated for 30 amps at 90'. And the asteriskfor 20 amps is for over current protection only. So at 70% of 30 amps which is21 amps I can run 7 to 9 # 12s in a conduit, right???

As long as no additional derating is necessary (like for ambient temperature) then yes.

 

[mwm1752]

Agree. Derating starts at the conductors highest temp rating it is design/listed for.

 

[charlie b]

And to clarify one step further, you only get to start at the 90C column if the conductor's insulation system is shown at the top of that column. For example, a #12 type THHN would be derated from the 90C value of 30 amps. A #12 type THWN would be derated from the 75C value of 25 amps. The code article that allows this is 110.14(C).

 

[Strathead]

And to clarify one step further, you only get to start at the 90C column if the conductor's insulation system is shown at the top of that column. For example, a #12 type THHN would be derated from the 90C value of 30 amps. A #12 type THWN would be derated from the 75C value of 25 amps. The code article that allows this is 110.14(C).

That is very correct and an a$$ biter. If the conductor runs through any wet location and the inside of conduit in a wet location is a wet location then you have to use the THWN conductor rating. Bet this has been violated a few times! (by mehmy:!)

 

[cowboyjwc]

And don't forget the terminal ratings too. If the terminals are rated 75 degrees then that's the highest table you can use.

 

[MBerner]

Thanks, Strathead, charlie b , mwm1752, kwired and cowboyjwc. And cowboyjwc, "And don't forget the terminal ratings too. If the terminals are rated 75 degrees then that's the highest table you can use." I did not know that that was a factor. Thanks.

 

[infinity]

And don't forget the terminal ratings too. If the terminals are rated 75 degrees then that's the highest table you can use.

I assume that you don't mean you would use the 75? C rating for derating.

 

[Strathead]

Thanks, Strathead, charlie b , mwm1752, kwired and cowboyjwc. And cowboyjwc, "And don't forget the terminal ratings too. If the terminals are rated 75 degrees then that's the highest table you can use." I did not know that that was a factor. Thanks.

To expand on Rob's statement, if the terminals are rated for 75 degrees, that is the highest amount of current you can allow the wire to carry. It doesn't have anything to do with the temperature rating of the conductor's insulation. The number of current carrying conductors and the ambient temperature considerations are used to determine the derating, based on the ampacity of the conductor, and then, if the allowable ampacity is still higher than that of the terminations, then you must select the column that matches the terminations for the allowable temperature.

 

[sandsnow]

That is very correct and an a$$ biter. If the conductor runs through any wet location and the inside of conduit in a wet location is a wet location then you have to use the THWN conductor rating. Bet this has been violated a few times! (by mehmy:!)

I thought all the manufacturers were producing THWN-2. The "-2" gives you the extra kick up to 90 degree rating for wet locations. I'll take a peek when I'm out today at what's being used around here.

 

[kwired]

I thought all the manufacturers were producing THWN-2. The "-2" gives you the extra kick up to 90 degree rating for wet locations. I'll take a peek when I'm out today at what's being used around here.

I think that is what you will mostly find, but will not promise you will never see any without the -2 rating.

 

[Fliz]

Big question. Say you have a 60 degree terminal (they exist) and you were running a 60 amp circuit. #6 75 degree wire is good for 60 amps right? Right. Of course you cannot run a #6 wire off a 60 degree terminal for a 60 amp circuit, because the terminal is not good for it.

Now, here's my question. You run a #4 wire off the 60 degree terminal to a junction box and it is spliced to a #6 75 degree wire and the rest of the circuit run on #6 (to presumably another terminal that is rated at 75 degrees). If no other derating factors are needed, is this code compliant?



This falls under the "derating discussion" topic but not specific to the OP.

 

[ActionDave]

Big question. Say you have a 60 degree terminal (they exist) and you were running a 60 amp circuit. #6 75 degree wire is good for 60 amps right? Right. Of course you cannot run a #6 wire off a 60 degree terminal for a 60 amp circuit, because the terminal is not good for it.

Now, here's my question. You run a #4 wire off the 60 degree terminal to a junction box and it is spliced to a #6 75 degree wire and the rest of the circuit run on #6 (to presumably another terminal that is rated at 75 degrees). If no other derating factors are needed, is this code compliant?

Yes it is.

 

[infinity]

Big question. Say you have a 60 degree terminal (they exist) and you were running a 60 amp circuit. #6 75 degree wire is good for 60 amps right? Right. Of course you cannot run a #6 wire off a 60 degree terminal for a 60 amp circuit, because the terminal is not good for it.

Now, here's my question. You run a #4 wire off the 60 degree terminal to a junction box and it is spliced to a #6 75 degree wire and the rest of the circuit run on #6 (to presumably another terminal that is rated at 75 degrees). If no other derating factors are needed, is this code compliant?



This falls under the "derating discussion" topic but not specific to the OP.

To muddy the waters a little more a #6 AWG at 60? C is rated at 55 amps. If your connected load is 55 amps or less you can go up to the next standard size OCPD which is 60 amps so you would even need to splice on the #4.

But yes you can do what you've suggested when trying to work with a 60? C terminal.