We have a disagreement as to conductor derating for ambient temperature. We have 75/90 degree C cable in conduit and 75 degree rated terminations(terminated at equipment in high ambient temp. area). The conductors in conduit are derated for ambient temp. and # of conductors in the conduit from the 90 degree column of 310-16. I contend that since the terminations are also located in the high ambient area, the maximum ampacity must be derated from the 75 degree C column. Other say the 75 degree rating is ok without futher derating. Who is correct?
NEC overcurrent protection
- Thread starter wschel
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- Location
- Massachusetts
Re: NEC overcurrent protection
No you do not have to derate the terminals, buy derating the conductors you have assured the terminations will not exceed 75C.
No you do not have to derate the terminals, buy derating the conductors you have assured the terminations will not exceed 75C.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: NEC overcurrent protection
I agree with Bob. The terminations are rated for a 45 degree temperature rise from a starting point of 30C ambient. If the ambient is above 30C, then you need to prevent the temperature rise from getting as high as 45C. You do that by limiting the current in the conductors. So derating the conductors will protect the terminations.
I agree with Bob. The terminations are rated for a 45 degree temperature rise from a starting point of 30C ambient. If the ambient is above 30C, then you need to prevent the temperature rise from getting as high as 45C. You do that by limiting the current in the conductors. So derating the conductors will protect the terminations.
Re: NEC overcurrent protection
How can that be? Take the extreme example -- a 75C ambient. Your terminal now has an ampacity of 0, yet a 90C conductor still has 41% of its ampacity.
I'd say you're limited to the 75C column if the terminations are also in the high ambient area. But this is only for the portion of the wire near the termination. If you used 90C wire nuts on pigtails to the terminations, then the wire away from the 75C terminations could be derated from the 90C column.
Does this make sense?
How can that be? Take the extreme example -- a 75C ambient. Your terminal now has an ampacity of 0, yet a 90C conductor still has 41% of its ampacity.
I'd say you're limited to the 75C column if the terminations are also in the high ambient area. But this is only for the portion of the wire near the termination. If you used 90C wire nuts on pigtails to the terminations, then the wire away from the 75C terminations could be derated from the 90C column.
Does this make sense?
Re: NEC overcurrent protection
iwire & charlie b,
I meant derating the cable ampacity from the 75 degree C column because the termination is 75 degree rated rather than for the 90 degree column.
For example, #2 THHN/THWN ran & terminated in 40 degree ambient.
In conduit @ 40 degree C:
Ifl = 130A x .91 = 118.3A
1. With 75 degree termination:
Ifl = 115A
2. 75 degree termination in 40 degree ambient:
Ifl = 115A x .88 = 101.2A
Which is the maximum conductor ampacity, 115A or 101.2A?
I think it depends on the termination rating, is it max. 75 degree C or is it 45 degree C Delta T?
iwire & charlie b,
I meant derating the cable ampacity from the 75 degree C column because the termination is 75 degree rated rather than for the 90 degree column.
For example, #2 THHN/THWN ran & terminated in 40 degree ambient.
In conduit @ 40 degree C:
Ifl = 130A x .91 = 118.3A
1. With 75 degree termination:
Ifl = 115A
2. 75 degree termination in 40 degree ambient:
Ifl = 115A x .88 = 101.2A
Which is the maximum conductor ampacity, 115A or 101.2A?
I think it depends on the termination rating, is it max. 75 degree C or is it 45 degree C Delta T?
- Location
- Massachusetts
Re: NEC overcurrent protection
Derate the conductor based on the 90 C rating, then when you done make sure you are not above the 75 C rating.
Derate the conductor based on the 90 C rating, then when you done make sure you are not above the 75 C rating.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Re: NEC overcurrent protection
If the terminations are only rated for 60C (just as a counter-example), the ampacity would be 95.
115.
If the terminations are only rated for 60C (just as a counter-example), the ampacity would be 95.
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