Very confused about 90 deg wire and derating

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Twophase

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I've come across the statement many times, that one can use the 90 deg column of 310.15(B)(16) (2017) for derating.

I've also heard that one must use the 75 degee column even if one is using a 90 degree conductor, due to all normally available equipment bring 75 C rated (including the vast majority of lugs)

I have a 275 A load, which is 344 A at 125%. I wanted to use 400 MCM but have been told that I need to use 500 MCM. So I am a bit confused here. What's the correct wire here, and what's the point of even having a 90C column of I always have to refer to the corresponding 75C value?
 
So you are wiring a 100HP motor? 3 phase 208 volt? that is 3 current carrying conductors - so where is the derating taking place?

Derating is for more than 3 current carrying conductors in the same raceway. I am not sure you are barking up the right tree.

The 75 degree column would be the standard for such a large load, but you might have 90 degree terminations here. The temperature limits on each of your wiring terminations is the key. If the motor terminals, controller lugs, the breaker, any splices, or any other lug is a standard lug and listed at 75 degrees, you are stuck with 500MCM CU THHN or equivalent. No marking means it is 75 degrees.

If every single lug in your circuit is listed at 90 degrees, you can use the 90 column and use 350 MCM CU THHN or equivalent. These will be marked if you have them.

This is does not appear to be about derating, it appears to be about temperature limitations of conductor terminations.

Look at 110.14(C)

This entire conversation changes if you have more than 3 current carrying conductors in one raceway (derating).
 
250 HP at 460V. Nameplate says 275A. Only three current carrying conductors (THHN) in a single conduit. Terminals in controller just say 75 degree minimum (does that help me in any way?). I also have a ground running from the service to the controller. No ground from controller to motor.

I can go to dual 4/0, but then I won't be able to run my ground due to fill issues. Can I run the ground in a separate conduit per code?
 
I've come across the statement many times, that one can use the 90 deg column of 310.15(B)(16) (2017) for derating.

That is correct. Reference 110.14(C), second sentence.
I've also heard that one must use the 75 degee column even if one is using a 90 degree conductor, due to all normally available equipment bring 75 C rated (including the vast majority of lugs)

That is also correct. Reference 110.14(C), first sentence.
I have a 275 A load, which is 344 A at 125%. I wanted to use 400 MCM but have been told that I need to use 500 MCM.

Sadly, this is true. A motor’s branch circuit conductors must be rated for 125% of FLA. The 75C rating of 400 MCM is only 335 amps, and you need 344. The correct wire size is 500 MCM.
What's . . . the point of even having a 90C column of I always have to refer to the corresponding 75C value?
The ampacity tables are based on having no more than 3 current-carrying conductors (CCCs) in the same conduit. For example, consider two single phase, 40 amp, 120V circuits (4 CCCs) in a conduit, with #8 wire. The ampacities in the table must be reduced to 80% of the values in the table. But if you use 90C wire (e.g., type THHN), then you calculate the 80% from the value in the 90C column. Thus, the #8 would get derated from 55 amps to 44 amps. That is still above the circuit’s rating (which is based on the circuit breaker). So you could still use a 40 amp breaker on each of the two circuits.

In your case, you are not derating for too many CCCs in the same conduit. There is also a correction factor that would come into play if the ambient temperature is over 30C. That, however, also does not apply to your installation. So you are stuck with using the 75C rating, and with using 500 MCM wire.
 
Be cautious in applying the above post No. 4). The word "lug" could cause a misunderstanding. 110.14 discusses termination limitations at equipment.
If you happen to have a lug rated 90° but the breaker or equipment is only rated for 75° terminations, you are limited to the 75° rating of the conductor.
You might aslo note in 110.14(C)(1) it mentions termination limits for motors based onm markings.

It is rare that you can use the 90° rating of conductors for any purpose but de-rating for such as ambient and fill.


Also note the wording of 430.6(A)(1). For most applications you must use the Table value for motors when sizing conductors. In you case, that value would be 302 amps (Table 430.250) so you need a 377 amp conductor... still a 500 but that 430.6 ruel can catch you.
 
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250 HP at 460V. Nameplate says 275A.
That makes things worse for you. The nameplate information is not relevant. You have to use the FLA values in table 430.250. Reference 430.6(A)(1). That value is 302 amps, and 125% of that is 377 amps. So you still need a 500 MCM, as it has an ampacity of 380.

You could, however, use a pair of 3/0. That would give you an ampacity of 400 amps. But you would have to run them in two parallel conduits. Otherwise, there would be 6 CCCs in the same conduit, and the ampacity would be reduced to 360 amps (using the process I described in my earlier comment).
 
The point of the 90C rating is that the _wire itself_ is permitted to be used at 90C.

If you connect 90C wire to 75C terminations, then you are limited to 75C operation because the _terminations_ are not permitted to exceed 75C.

Where the 90C rating is commonly useful is if the wire passes through different regions with different conditions which affect its temperature.

For example the wire might be in a panelboard, with lots of space around it connected to a 75C termination. In this region we are forced to use the 75C rating because of the terminal limitation.

But then the wire goes into a conduit, crammed in with a bunch of other wires. In this region we have higher temperature because of the combined heating of all of the wires. This combined heating is what causes the requirement for derating. But there are no terminals in this region, so the temperature is limited only by the wire insulation. So in this region we derate from the 90C rating.

In the end we are limited by the weakest link anywhere along the wire.

-Jon
 
That makes things worse for you. The nameplate information is not relevant. You have to use the FLA values in table 430.250. Reference 430.6(A)(1). That value is 302 amps, and 125% of that is 377 amps. So you still need a 500 MCM, as it has an ampacity of 380.

You could, however, use a pair of 3/0. That would give you an ampacity of 400 amps. But you would have to run them in two parallel conduits. Otherwise, there would be 6 CCCs in the same conduit, and the ampacity would be reduced to 360 amps (using the process I described in my earlier comment).

Thanks. We aren't supposed to use the nameplate values? I was told to use NEC values only if I don't know the nameplate values. Isn't the nameplate value more accurate?
 
Is there any rule that prevents me from using a single multiconductor cable (like a G-GC, SHD-GC) in conduit for this scenario?
 
Thanks. We aren't supposed to use the nameplate values? I was told to use NEC values only if I don't know the nameplate values. Isn't the nameplate value more accurate?
It depends, generally I usually use the tables, if you have access to a codebook check out [2020] 430.6 'Ampacity and Motor Rating Determination' if you meet one of the exceptions listed there you can go with the nameplate. If the motor is not in the table they don't really address that.
 
Is there any rule that prevents me from using a single multiconductor cable (like a G-GC, SHD-GC) in conduit for this scenario?

This comes down to a question of 'who is in charge'. If the installation is dictated by NEC code, then you can use a multi-conductor cable in conduit if the cable is constructed from one of the _NEC_ recognized cable ratings, and used at NEC recognized ampacities.

I looked up SHD-GC cable, and it looks like it is normally used in mining situations at much higher than NEC recognized ampacity. You won't get the benefit of these capacities for NEC compliant use. (Presumably this cable is designed for harsh treatment under high load, but with regular inspection and replacement, rather different than an NEC installation in a conduit and then forgotten about for decades.)

-Jon
 
IIRC, the NEC does have G-GC, but free air ampacities are given. I'll verify in a bit, but this would mean I can't use the cable since the NEC ampacities are free air, and my example is conduit, right?
 
IIRC, the NEC does have G-GC, but free air ampacities are given. I'll verify in a bit, but this would mean I can't use the cable since the NEC ampacities are free air, and my example is conduit, right?
So I could use G-GC to wire the motor if it is for one of the reasons in 400.10(A). I guess this helps for high vibration scenarios....
 
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