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.

[HX_Guy]

Nevermind, inetdog answered already in the other thread.

[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.

[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.

[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]

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.

That may be where the temperatures come from, but the wire ampacity calculations come entirely from the NEC.
ASHRAE may also give calculations for the increase in attic temperature over ambient temperature. But that part depends strongly on the configuration of the attic. In particular, putting solar panels over the south part of the attic roof will decrease the attic temperature.

[HX_Guy]

Hmm, so then where did the city come up with the 85% derate? Seems odd, as the cooler the outside temps, the lower the derate right? If the 82% derate is for ambient temps of 114-122F, that seems pretty accurate for the climate here on the worst days of summer, which is what I assume you have to account for?

[HX_Guy]

Ok quick follow up, I looked at this site here: http://www.solarabcs.org/about/publi...map/index.html

For my location, the closest "station" shows...



So it looks like the 46ºC is what should be used to calculate the temperature correction derate? That's 114.8º F, which falls just inside the 82% derate correction.

[sensij]

Hmm, so then where did the city come up with the 85% derate? Seems odd, as the cooler the outside temps, the lower the derate right? If the 82% derate is for ambient temps of 114-122F, that seems pretty accurate for the climate here on the worst days of summer, which is what I assume you have to account for?

Attached is the applicable temp correction table from 2011 NEC. Maybe this will help steer the conversation more productively.

Temp correction.JPG

[sensij]

Ok quick follow up, I looked at this site here: http://www.solarabcs.org/about/publi...map/index.html

For my location, the closest "station" shows...


So it looks like the 46ºC is what should be used to calculate the temperature correction derate? That's 114.8º F, which falls just inside the 82% derate correction.

In this post, showing that 6 AWG THWN-2 is OK, I used the 0.82 correction. That was not a coincidence.

[sensij]

It may be academic at this point, but the temperatures used by your designer were listed on the permit. I can't read them because of the quality of the picture, but it is in the lower right of the image attached.

permit.JPG

Also, if you would, can you share the name of who did your permit? They should go on the "stay away" list too.

[HX_Guy]

Good eye! Here is a clearer picture...



What...did he use .65 adjustment for? I guess for the DC wire from the combiner to the inverter, which was supposed to be in conduit on the roof?
That would make sense because according to ASHREA, 3.5" about the roof is 60º C.

Seems the .85 is a typo though? But the city accepted it, so I don't know.

The designer has actually been super cool throughout all of this though and has tried to be as helpful as they can, they will provide new plans if I need them at no charge (which is expected seeing at it seems their calculation were wrong). This is their first time ever dealing with a SolarEdge system though.

EDIT: Hmm, if that's the case though, that the conductor has that big of a derate for temperature, then 6AWG wouldn't be good based on conduit on the roof, so if we resubmit the permit, we also need to specify that the conduit is not run outside on the roof?

If my calculation are correct, then 35.4ADC x 1.25 x 1.25 = 55.3A.
6AWG conductor x .65 derate = 48.75A which is below the above number. Seems we would need 4AWG in that case, if the conduit were on the roof.

[sensij]

Good eye! Here is a clearer picture...

The designer was looking at the wrong table.

Edit: Table 310.15(B)(2)(b) cited in the permit in 2011 NEC has those corrections with those temperatures. The problem is that isn't the right table to use with THWN-2 wire.

310.15(B)(2)(a) is the table I posted earlier, and the right one to have used.

[HX_Guy]

The designer was looking at the wrong table. 0.65 and 0.85 match up with those temperatures in table 310.20 in the 2008 NEC. Massive fail on that one.

Is it possible that my city is still going off of 2008 NEC?

Nevermind, according to this PDF they are using 2011 NEC. Guess the designer did not know that.

http://www.peoriaaz.gov/uploadedFiles/Peoriaaz/Departments/Community_Development/Building_Safety/Building_Forms/506_Detached_Bldg_Standards.pdf

[sensij]

Caught me in another edit. The mistake was simpler than that, and wrong table was even called out on the permit.