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

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.

Vic

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

Hi Hx,

There can often be the case of just what the cable attaches to. For example, most breakers and perhaps some disconnects have terminal temperature ratings of 75 C max. This CAN limit the temperature rating of the cable, and therefore, the maximum current rating of the cable ... I believe. But am not a Code Jock. Also, often cable that may be exposed to water is limited to 75 C, often, IIRC .... YMMV, so on ... How did your inspection go ?? Vic

HX_Guy

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

Thank you for the info, I believe my disconnects and such are rated at 75º so I guess the cable needs to also.

Install went horrible, as expected. See the other post starting at post #186.
http://forum.solar-electric.com/showthread.php?25538-Solar-install-going-very-bad-looking-for-some-help-advice-please&p=210455#post210455

Vic

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

Sorry HX, just now finished reading of your Inspection fiasco, this post was on top, and am working down.

Good Luck, agree that sometimes it is good to have an easy fix gimme for the Inspector to find ... However, you seem to really be in the home-stretch.

Almost there, Vic

Ethan Brush

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

Pretty much all modern NEC recognized conductors are are rated 90 degrees C wet or dry (the "-2" in say "THWN-2" means 90 degrees wet or dry). However just about all equipment has terminals rated 75 degrees so that is what you have to use. Note that pretty much all lugs are rated 90 degrees (the "AL9CU" you see on lugs means listed for aluminum or copper conductors and 90 degrees, in case you couldn't figure that out...), BUT even if you see a lug like this factory installed in some gear, you will find the manufacture still rates the terminal for 75 so you have to go by that. There are some cables that have 90 degree conductors in them, but still have to be sized at the 60 degree column, NM cable ("romex") and SER cable (depending on how installed and code cycle) come to mind. Note that you can still use the 90 degree column ampacity to start your adjustments for number of conductors, temperature, etc, derating. For example, most of the time 12 AWG goes on a 20 amp breaker, but say we have 9 current carrying conductor installed which requires a 70% derating per 310.15(b)(2)(a) (going off of memory here - some of the ampacity sections moved around a little in 2014 and the codebook is out in the truck). We can start our derating at the 90 degree rating which is 30, so 30*.7=21 so we can actually still use the 20 amp OCPD for up to 9 current carrying conductors.

Surfpath

HX,
This was an excellent resource I came across that addresses your question:

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Conductor_Ampacities.pdf