Derating wire in conduit

[reeseguy]

First post - want to make sure I get this derating correct.

Have a 3phase 120/208volt service. I have a 3/4" EMT raceway with 6 circuits and 2 neutrals and 1 common ground. Total of 9 - #12thhn wires. (1 neutral per 3 circuits). Using 310-16, #12 thhn rating @30amps, then using 310-15(B)(2)(a) > 30amps x 70% = 21amps. Overcurrent protection would be a 20amp breaker. Is this allowable per the NEC code in this conduit? Do I have to further derate for ambient temperature? If so, exactly how is that done? Which ambient factors am I supposed to use?

 

[infinity]

You likely only have 6 CCC's which would require a derating of 80%. Doesn't change your calculation but in most instances (in your example) the neutral does not count as a CCC. What's the ambient temperature that the conductors will be run in?

 

[reeseguy]

Thanks for responding: Since each set of three conductors shares a neutral (Boat), are you saying that the neutral does not count in the derating? And the circuits run from the MDP into the attic of the building I working on. Does the ambient temperature mean the temperature of the attic, 120-125 degrees?

 

[Volta]

Thanks for responding: Since each set of three conductors shares a neutral (Boat), are you saying that the neutral does not count in the derating? And the circuits run from the MDP into the attic of the building I working on. Does the ambient temperature mean the temperature of the attic, 120-125 degrees?

True, you will not need to count the neutral as a CCC unless the 'major portion of the load consists of nonlinear loads'. 310.15(B)(4)(c).

And yes, that temperature the wires are installed in is the temp to use. If the 90 deg C THHN is subjected to 125 deg F, then your previous ampacity (30 amps x 80% = 24) is reduced another 76%, and good for 18.24 amps.

Welcome to the forum.

 

[Little Bill]

True, you will not need to count the neutral as a CCC unless the 'major portion of the load consists of nonlinear loads'. 310.15(B)(4)(c).

And yes, that temperature the wires are installed in is the temp to use. If the 90 deg C THHN is subjected to 125 deg F, then your previous ampacity (30 amps x 80% = 24) is reduced another 76%, and good for 18.24 amps.

Welcome to the forum.

This sounds like commercial or industrial with the 3 PH. I don't know the practice of derating in an attic in these settings but I don't know of anyone that derates for an attic in a residential setting. I'm not saying it is right or wrong, just saying I don't see it done. Also never heard an inspector say anything about it. Maybe because that temperature is not constant.

 

[augie47]

You picked an interesting item for your first post
for, at this point, my friend, the fly is in the ointment

First let me say there are countless thousands of installations as you describe operating on 20 amp breakers. Attic ambient, IMHO, is one of the most ignored items in a conduit installation as Bill mentions.

As Volta pointed out your ampacity is now 18.24 amps. If you look closely at 240.4(B)(1) you may find your installation does not qualify for the "next higher ampacity" rule.

(In a few decades of installing/enforcing, I can count on one hand the number of times this install would be rejected.)

 

[Volta]

This sounds like commercial or industrial with the 3 PH. I don't know the practice of derating in an attic in these settings but I don't know of anyone that derates for an attic in a residential setting. I'm not saying it is right or wrong, just saying I don't see it done. Also never heard an inspector say anything about it. Maybe because that temperature is not constant.

Or maybe just that 'out of sight is out of mind'. I've heard persons argue that ambient is average, but I call bull on that simple a statement.

On the other hand, I wouldn't consider the one highest peak temperature to be ambient either. Seems like it should be the highest average over a period of _x_. I would guess that 'x' should equal something like 6 hours, rather than a month or year.