When to use 75º C wire rating vs 90º C?

[HX_Guy]

As far as the 85% goes, possibly the city is interpolating between table entries, which is not allowed by the NEC. There are ASHRAE tables of ambient temperatures for cities throughout the US that you can use for the ambient temperature part of the calculation.

Ha that word! The designer mentioned that ASHRAE word when talking to him, he said that's where the temp calculations come from. I'll have to read up on it.

[inetdog]

Hey guys, sorry for yet another post, but figured this could be helpful to others who may not go through the mile long post on my bad install.

For the wiring sizing, it seems everyone references 75º C and I'm trying to figure out why?

I have a friend who did had a solar install done last year and lives about a mile from me, and I have a copy of his permit, and on his, the installer used the 90º C figure which allows for a higher amp rating in the same wire. For example:



This would make a pretty big difference because my inverter AC output is 47.5A x 1.25 x .82 derate = 72.4A so my 6AWG wire rated at 65A @ 75º wouldn't work, but that same wire is rated for 75A @ 90º C.

I actually just commented on that in your megathread.

Each terminating device (combiner terminal strip, motor connection, screw on a receptacle, or lug on a circuit breaker) will also have a temperature rating. You cannot calculate ampacity using a higher wire temperature than both termination ratings, even if the wire insulation is rated for 90C.
The rationale is that if you load the wire up to 90C the heat conducted from the wire will overheat the termination. Almost all motors and most circuit breakers are rated for 75C, but you can find parts with higher ratings.
(BTW, if you use NM (Romex(TM)) you are always limited to the 60C rating regardless of the insulation type.)
If you are really desperate, you can get around the terminal limitation by using a short length (most inspectors are OK with one foot) of larger wire at the termination and smaller wire for the long run.
(BTW, some types of wire, notably "fixture wire", are rated as high as 150C.)

As far as the 85% goes, possibly the city is interpolating between table entries, which is not allowed by the NEC. There are ASHRAE tables of ambient temperatures for cities throughout the US that you can use for the ambient temperature part of the calculation.

[Sunking]

They use of 90 degree insulation is only beneficial when using 3 or current carrying conductors in a raceway for derating purposes only. However you cannot exceed the current rating of the terminal which is always rated at 75 degrees.

Example lets say you are running 10 120 volt 20 amp circuits in a conduit. Your terminals for each circuit are 20 amps @ 75 degrees. If I were to use 75 degree cable I would have to use 8 AWG conductors. However if I use 90 degree insulation I can use 10 AWG which is cheaper and use a smaller raceway.

[HX_Guy]

Nevermind, inetdog answered already in the other thread.

[HX_Guy]

Hey guys, sorry for yet another post, but figured this could be helpful to others who may not go through the mile long post on my bad install.

For the wiring sizing, it seems everyone references 75º C and I'm trying to figure out why?

I have a friend who did had a solar install done last year and lives about a mile from me, and I have a copy of his permit, and on his, the installer used the 90º C figure which allows for a higher amp rating in the same wire. For example:



This would make a pretty big difference because my inverter AC output is 47.5A x 1.25 x .82 derate = 72.4A so my 6AWG wire rated at 65A @ 75º wouldn't work, but that same wire is rated for 75A @ 90º C.