I was taught to calculate my motor feeders from the 75 degree column. My question is, if I'm using 90 degree wire, why cant I use the 90 degree column? I can decrease my feeder size sometimes that way, and all of the wire I use is THWN-2 rated at 90 degrees? But, the former is what I was taught many years back. Might have been taught wrong? Or, I missed a valuable piece when I was supposed to be listening? who knows? Thanks
Motor Feeders? Do they have to be calculated from the 75 degree column?
- Thread starter Huntxtrm
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weressl
Esteemed Member
The breaking point usually is the end terminations, both on the motor end and the controller end. The terminating component, be they lugs to be attached to the wire or componet terminations on the motor controller, thye are usually rated 75*C, so that is why you need to use the 75*C column.
Now there ARE some 90*C rated terminating components, but they are uncommon, yet.
Don't forget the ambient adjustment either if one end happen to be in a different, climatized room. It may help you out.
So, my conductor ampacity de-rate for heat is based on the lowest rated in the system. Be it the wire, or the terminals? I was just contemplating, that because it is a conductor ampacity de-rate, it involved only the conductor.
On the other note, Are you saying for instance. If I am running underground, then I should use the ambient temp of the underground, or the ambient temp of where the conduit emerges? Which ever is higher? I've always used the ambient temp, that was highest in the system. Is that not correct?
thanks for the reply.
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- Chapel Hill, NC
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- Retired Electrical Contractor
You can de-rate from the 90C column if your wire is rated 90C. The final ampacity however is only as large as the weakest link- be it the terminals, wire etc.
For ex. If you have a piece of 12 thhn running in a conduit with 7 other current carrying conductor's then you must de-rate at 70% per 310.15(B)(3)(a). 12 thhn at 90C is rated 30 amps so 70 * 30 = 21. Thus you can use 20 amp overcurrent protective device and amp for the wire.
For ex. If you have a piece of 12 thhn running in a conduit with 7 other current carrying conductor's then you must de-rate at 70% per 310.15(B)(3)(a). 12 thhn at 90C is rated 30 amps so 70 * 30 = 21. Thus you can use 20 amp overcurrent protective device and amp for the wire.
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- Connecticut
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- Engineer
No, your ampacity derate can be based on the insulation temperature of the conductor, but the starting ampacity (before derating) has to be based on the lowest temperature rating of any connected termination, conductor or device...See 110.14(C).
So for example, if you had a 100HP, 460V motor (124 FLA), you would need a conductor with an ampacity of 124*1.25 = 155A. #1/0 90deg has an ampacity of 170, but 110.14(C) prevents you from using the 90 deg ampacity, so you would need a minimum #2/0 (175A @ 75deg.)
But if the circuit needed to be derated for a 100degF ambient, you would see that #2/0 THWN would have a corrected ampacity of 154. This is too small for the required circuit ampacity. However, using a #2/0 THWN-2 would give you a corrected ampacity of 177A. Using the 90 deg conductor for the derating would prevent you from having to increase your conductor size.
So let me see if I understand correctly. I need to initially get my conductor from the 75 degree column of 310.15b16. But, to de-rate fro 100degree ambient, I would use thwn-2 from the 90 degree column of 310.15b16 and de-rate by .91 from the 90degree column of 310.15b2a. but if not de-rating, I just use the 75 degree column of 310.15b16? Seems beneficial to de-rate? Or, am I not understanding the line? It seems to me, that just using the 90 degree column in the first place, has the same end result? What am I missing?
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- Connecticut
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- Engineer
I don't quite understand what you see a being "beneficial" to derate. Derating would generally involve using a larger conductor. In the example, we avoid using a larger conductor by using a 90deg rated conductor.
Using the 90 degree column in the first place would only require a #1/0 conductor for the required circuit ampacity. Using the 75 degree column requires a #2/0 conductor for the required circuit ampacity. These are certainly different results.
Separate requirements that all have to be met. You have to choose a wire whose ampacity works within termination temperature limits first. Most likely, that will put you into the 75C column and in some cases the 60C column. For ambient temperature or bundling, use that same wire from the 90C column (assuming it is 90C rated) and calculate the deratings. If it still works, you're good to go.
But you need a wire that satisfies both the termination temp rules and the bundling/derating rules. If that 90C conductor was bundled with a lot of other conductors and in a high ambient, it may not be large enough even though it meets the termination temp ampacity limit.
But you need a wire that satisfies both the termination temp rules and the bundling/derating rules. If that 90C conductor was bundled with a lot of other conductors and in a high ambient, it may not be large enough even though it meets the termination temp ampacity limit.
I understand now. Thanks for clarifying. All of you.
weressl
Esteemed Member
Or to put it even more simple terms: you can rate each 'segment' separately under the different rules, eg. start with 30C indoor ambient and 75C rise termination restriction, continue withe 90C rise column because your motor terminator is suitable for that, but derate the conductor to the 40C ambient and so on. You must choose the largest resulting conductor.
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