Feeder Size?
- Thread starter blackdog2
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Would you care to give us your answer first, or at least tell us what you have tried to do to get the answer yourself?
ray cyr
Senior Member
- Location
- Yakima, Wash.
In a raceway or in free air?
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
blackdog2 said:
You may use the 90C for derating as long as the final ampacity rating is not greater than the 75C ampacity.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Since the insulation is rated for 90 degrees C start your adjustments from there.
430 amps * .91 = 391 amps. Your maximum ampacity for this conductor is 380 amps due to the 75 degree terminations.
430 amps * .91 = 391 amps. Your maximum ampacity for this conductor is 380 amps due to the 75 degree terminations.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Two things noteworthy about Trevor's reply. One is that you use the 90C column to get the 430 amps (instead of your 380). The other is that you use the 90C column to get the 91% (instead of your 88%).
The article that says you can do what we have described is 110.14(C). It's not written as clearly as one might wish, but take a look at the last sentence in that article.
The article that says you can do what we have described is 110.14(C). It's not written as clearly as one might wish, but take a look at the last sentence in that article.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
AlWired said:
It means that the conductor is dual rated both THHN and THWN. THHN is rated for 90 degrees C, THWN is rated for 75 degrees C. Since the OP never mentioned using this is in a wet location most assumed it would be dry therefore the 90 degree rating was used when applying adjustments.
Feeder size
Feeder size
For a 90C, 480v conductor in conduit in a 40C ambient attached to 75C terminations, it seems like we are saying the ampacity could be either the 90C rating derated for ambient temp, or the 75C rating with no derating. For a #6 wire, for example, we could have
(1) 75 A (the 90C rating) X 0.91 = 68 A., or
(2) 65 A (the 75C rating).
Since 65 is the lower of the two, we would say that the ampacity of this conductor is 65 A. Am I correct?
The thing that bothers me is that this would allow the operating temperature of the conductor to exceed the temperature rating of the termination. If this conductor carries 65 A in conduit (possibly just a few inches from the termination) in a 30C ambient, (according to Table 310.16) the conductor will operate at a temperature of about 75C. If the ambient temp is 40C instead of 30C, then the operating temp of the conductor will be higher than 75C, and this will exceed the rating of the termination. Can someone explain the logic that the NEC followed in apparently allowing the wire temp to exceed the termination rating.
Feeder size
For a 90C, 480v conductor in conduit in a 40C ambient attached to 75C terminations, it seems like we are saying the ampacity could be either the 90C rating derated for ambient temp, or the 75C rating with no derating. For a #6 wire, for example, we could have
(1) 75 A (the 90C rating) X 0.91 = 68 A., or
(2) 65 A (the 75C rating).
Since 65 is the lower of the two, we would say that the ampacity of this conductor is 65 A. Am I correct?
The thing that bothers me is that this would allow the operating temperature of the conductor to exceed the temperature rating of the termination. If this conductor carries 65 A in conduit (possibly just a few inches from the termination) in a 30C ambient, (according to Table 310.16) the conductor will operate at a temperature of about 75C. If the ambient temp is 40C instead of 30C, then the operating temp of the conductor will be higher than 75C, and this will exceed the rating of the termination. Can someone explain the logic that the NEC followed in apparently allowing the wire temp to exceed the termination rating.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
T = ThermoplasticAlWired said:
H = Heat rating
W = Wet location
N = Nylon clad
THHN/THWN refers to a single insulation that meets the requirements for both ratings. No H's means a 60?C rating, one a 75?C rating, and two a 90?C rating.
So, THHN/THWN means a conductor that is either rated for 90? dry or for 75? wet. Add a -2 and you get the 90? rating even in wet locations.
Added: Apologies to Rob; I answered before seeing you did.
I agree with your concerns.whitlow said:
IMO, the reference ampacity 60?C or75?C, as applicable, should also be adjusted for ambient temperature if?with emphasis?the ambient temperauture at the terminations and up to entry into a raceway is other than 26?30?C. However, it should not be construed to mean an ambient adjustment to the same temperature as the conductors in a raceway. For example, a panelboard has an internal ambient operating temperature of 35?C and a run of conductors passes through a 45?C environment somewhere after leaving the location of the panelboard. In this case there would be a different ambient adjustment factor applied to the reference 75?C ampacity as compared to the one applied to the conductors.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I have always wondered why the conductor is rated lower for wet location. It would seem the opposite to me.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
It may be because air can move due to convection, while the water, once heated, stays around the conductors. For the first few minutes, the water sinks heat, but then it just sits there. That's my theory, anyway.Dennis Alwon said:
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
LarryFine said:
Okay then why does the -2 make the difference. What inorganic addictives are they putting in my wires? :grin:
- Status