90C conductor terminated on 75C question.

[ggunn]

It is correct that for a continuous load, the NEC requires that 125% of the load not exceed the unadjusted termination ampacity.

However, the "that means" portion of your statement is not correct. To keep the conductor temperature at the termination from exceeding 75C, the continuous current could be up to the full 75C ampacity. See the definition of ampacity--it is already a continuous rating. As such, the NEC is being overly conservative here.

Those two statements seem contradictory to me.

 

[wwhitney]

Those two statements seem contradictory to me.

How so?

E.g. #8 Cu has a 75C ampacity of 50A at 30C ambient. So if I connect some pieces of it together with some 75C wirenuts, and I run 50A through the whole assembly forever at 30C ambient, the equilibrium temperature of the system should be less than 75C everywhere. [Or if the wirenuts create a hot spot because of the contact resistance of the connection, presumably the actual allowable temperature of its materials is higher than the 75C rating of the wirenut.]

And if I have a 100% rated 50A breaker in its special enclosure (if such a thing exists), then you could land your #8 Cu on it and run 50A through the circuit forever and the breaker shouldn't trip.

But if I have a regular 50A breaker in a panelboard full of other breakers, then even in a 30C ambient there's apparently a possibility the heat generated from that continuous 50A current (and conductor operating at up to 75C) will skew the breaker trip curve and cause it to trip below 50A. As a result, the NEC inserts various 125% continuous use factors as required (and I would argue in some cases where not required) to keep that from happening.

Cheers, Wayne

 

[ggunn]

How so?

E.g. #8 Cu has a 75C ampacity of 50A at 30C ambient. So if I connect some pieces of it together with some 75C wirenuts, and I run 50A through the whole assembly forever at 30C ambient, the equilibrium temperature of the system should be less than 75C everywhere. [Or if the wirenuts create a hot spot because of the contact resistance of the connection, presumably the actual allowable temperature of its materials is higher than the 75C rating of the wirenut.]

And if I have a 100% rated 50A breaker in its special enclosure (if such a thing exists), then you could land your #8 Cu on it and run 50A through the circuit forever and the breaker shouldn't trip.

But if I have a regular 50A breaker in a panelboard full of other breakers, then even in a 30C ambient there's apparently a possibility the heat generated from that continuous 50A current (and conductor operating at up to 75C) will skew the breaker trip curve and cause it to trip below 50A. As a result, the NEC inserts various 125% continuous use factors as required (and I would argue in some cases where not required) to keep that from happening.

Cheers, Wayne

I think this is semantics. When you apply the 125% factor to the continuous current and compare it to the 75 degree ampacity, you have indeed seen to it that the wire will not heat up to 75 degrees, but you haven't necessarily shown that it will get all the way to 75 degrees; that's what the 125% factor is about.

 

[wwhitney]

I think this is semantics. When you apply the 125% factor to the continuous current and compare it to the 75 degree ampacity, you have indeed seen to it that the wire will not heat up to 75 degrees, but you haven't necessarily shown that it will get all the way to 75 degrees; that's what the 125% factor is about.

For 30C ambient, limiting the current to 80% of the 75C ampacity should limit the worst case temperature rise to 64% (I²R heating) of the allowable, or a maximum temperature of 59C.

Cheers, Wayne

 

[wwhitney]

Those two statements seem contradictory to me.

Maybe "To keep the conductor temperature at the termination from exceeding 75C, the continuous current could be up to the full 75C ampacity." is poorly worded.

How about "The continuous current could be up to the full 75C ampacity, and the conductor temperature at the termination should be 75C or below--no 125% factor is required."

Cheers, Wayne

 

[ggunn]

Maybe "To keep the conductor temperature at the termination from exceeding 75C, the continuous current could be up to the full 75C ampacity." is poorly worded.

How about "The continuous current could be up to the full 75C ampacity, and the conductor temperature at the termination should be 75C or below--no 125% factor is required."

Cheers, Wayne

Well, years ago I studied at the feet (almost literally) of John Wiles, solar guru, and the method I was taught for sizing 90 degree conductors is what I use to this day. Starting with Imax (AC or DC) I choose a wire size for which 125% of Imax does not exceed the 75 degree ampacity (75 degree column). Next I choose another wire size for which the 90 degree ampacity derated for conditions of use is greater than Imax. I use the larger of the two wire sizes for my design.

 

[wwhitney]

Well, years ago I studied at the feet (almost literally) of John Wiles, solar guru, and the method I was taught for sizing 90 degree conductors is what I use to this day. Starting with Imax (AC or DC) I choose a wire size for which 125% of Imax does not exceed the 75 degree ampacity (75 degree column). Next I choose another wire size for which the 90 degree ampacity derated for conditions of use is greater than Imax. I use the larger of the two wire sizes for my design.

Since Imax is continuous, that's the same as everything I've been saying so far. The point being that in the 2nd step you use Imax, not 125% * Imax.

But as my correction earlier today shows, there is a 3rd check you need to do. Which is that the minimum breaker size (125% of Imax) is still allowable protection for the actual ampacity, the 2nd step.

Cheers, Wayne

PS 690.8(B)(1) Exception would sometimes let you omit the 125% factor on Imax for the termination (75C, no COU) check. How often depends on how you interpret "together with its overcurrent device(s)" in the case of no overcurrent devices. It either automatically fails, as there is no overcurrent device, or that part automatically complies, as there is no device to impose a listing requirement on.

 

[ggunn]

But as my correction earlier today shows, there is a 3rd check you need to do. Which is that the minimum breaker size (125% of Imax) is still allowable protection for the actual ampacity, the 2nd step.

Of course, and I always do, but since step 1 is so conservative, I've never seen it fail.