110.3(B)

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Carultch

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Assuming copper, and assuming a 4 wire system with a current carrying neutral, make sure you install a 90? rated conductor to have sufficient ampacity.

One parallel run of 75? rated 600MCM copper does not get you there.

How common are conductors that are exclusively rated at 75C? Any practical examples today?

Most wire types I see, are usually dual-rated at 90C as well. For instance, THWN is usually dual listed as THWN/THHN/THWN-2.
 

Carultch

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Does 310.15(A)(2) Exception apply?

Cheers, Wayne


No. The "stub rule" applies only to conductor ampacities. Not to termination ampacities.
Termination ampacity should always be your starting point. Then you apply conditions of use as needed. Even though terminations are obviously less than 10 ft, and less than 10% of the overall length. Terminations are simply too critical of a location, and too dissimilar from the rest of the circuit, to take credit for neglecting their reduced ampacity.

One place you might see this exception, is when you have an HVAC unit on the roof, and there is 50 ft of wiring inside the building, and only a small 2 ft stub on the rooftop to connect to the device. The stub on the rooftop is in direct sunlight, but the remaining circuit still has enough mass to dominate the conditions of heating inside the conductor, that it draws the heat away from the small portion in direct sunlight.

Another place you might see this, is in photovoltaic applications of a group of PV inverters and a common wireway for the AC output (suppose DC is separate). In wireways, you are allowed up to 30 wires without derating. The instant you add the 31st wire, you drop down to the derated ampacity of 31 wires in a raceway. So in a group of 11 three-phase string inverters, the first 10 string inverters don't require ampacity adjustment until their wires are added with the 11th inverter. So for the last (perhaps 3) inverters, you might need to upsize because the higher ampacity is more than 10% of the overall length. But for the majority of them, you can ignore ampacity adjustments.
 
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roger

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How common are conductors that are exclusively rated at 75C? Any practical examples today?

Most wire types I see, are usually dual-rated at 90C as well. For instance, THWN is usually dual listed as THWN/THHN/THWN-2.
They may be dual rated but, if you're using THHN/THWN in a wet location [(note I did not say THWN-2) it is only 75 C THWN to start with and you can not start your adjustment based on 90 C THHN.

Roger
 

Smart $

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Location
Ohio
Assuming copper, and assuming a 4 wire system with a current carrying neutral, make sure you install a 90? rated conductor to have sufficient ampacity.

One parallel run of 75? rated 600MCM copper does not get you there.
??? In electrical speak, that's an oxymoron.

75?C 600kcmil copper is rated 420A. Times two is 840A. Derated can go as low as 701A, provided not less than the calculated load, continuous portion factored 125%
 

Smart $

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Location
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How are you getting out of derating for 4 current carrying?
On initial read I didn't take your statement to mean the neutral counted as a CCC, only that it is current carrying (i.e. automatically assumed wherever it is a circuit conductor). I can see now that you meant where the neutral actually counts as a CCC. Of course as a counted CCC (CCCC? :D), the conductors must be derated 80% just for the number of CCC and looking at 672A ampacity. However, temperature correction for 21-25?C (1.05; entirely feasible for an underground run) would take the ampacity back up to 706A and compliantly protected with an 800A breaker as long as the factored load is not more than 706A.

How's that for a recovery?... :p
 

Carultch

Senior Member
Location
Massachusetts
How are you getting out of derating for 4 current carrying?

It is a tricky concept to understand whether or not a neutral should be considered a CCC. Sometimes one might count it as a CCC, when in doubt.

The way I understand it, is that neutrals guaranteed to carry the full current as a required part of the return path count as current carrying. Neutrals that only carry the imbalance do not count no matter what the balancing, provided that loads are linear and the phase conductors can carry the return current among themselves of a balanced load.

The more difficult rule is the one regarding harmonic currents and nonlinear loads. Even though I know what these terms mean, I don't know how to identify this case, given a particular device in question.
 

Carultch

Senior Member
Location
Massachusetts
Never heard it called that... but the exception could come into play if the derated ampacity would otherwise be less than permitted by termination temperature limit.

I call it the "stub rule", because it reminds me when to expect to use it.

That's correct. One could have 50A of terminals, and 55A of 90 C wiring inside the building, even though the final foot on the rooftop is otherwise derated to 41A in sunlight. The circuit ampacity as a whole is 50A.
 
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