Using 60 degree ampacity for less than 100A

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philly

Senior Member
If I recall there is a section in the NEC stating that for any breaker with an ampacity less than 100A you must use the 60deg C rating for determining cable ampacity for cables I believe up to size #1.

Is this always the case, that you must use the 60deg column or if you know that the breaker terminals are rated for 75deg C then can you use the 75deg rating?
 

danickstr

Senior Member
If it is a UL listed name brand and rated 75? I don't see why you couldn't get a 75? wire rating even if it is below 100 amps.
 

sparky519

Member
75 deg. terminals

75 deg. terminals

NEC states you must use 60 deg. rating if the terminals do not have a deg. rating. If the say 75 deg. than use size based on 75 deg.
 

tmason

Member
so if you have a 100 amp panel labled for 75 dagrees and you use no 4 thhn then your saying the ampacity is 85 amps and 95 amps at 95 dagrees
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
You have to look at a number of things.
Is your supply breaker rated for 75? ? (most likely yes)
Is you panel rated for 75? ? (most likely yes)
Is the conductor allowed to be used at 75? rating (Romex is not, SE is not in some conditions)
What is your load (you can not exceed the wires ampacity)

If you have a #4 THWN/THHN, RHW, etc and meet the above conditions, you may install it on a 90 amp breaker (85 amp load)

If it is a main feeder for residential and meets the requirements of 310.15(B)(6), you may install it on a 100 amp breaker.
 

tmason

Member
the panel and breakers are all 75 dagree rated their for the chart says 85 amps so it would be a 80 amp breaker not 90 or em i reading it wrong and i belive their is a code that if the building is detached it has to have a disconnect means if so do you know that article
 

roger

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Location
Fl
Occupation
Retired Electrician
the panel and breakers are all 75 dagree rated their for the chart says 85 amps so it would be a 80 amp breaker not 90 or em i reading it wrong
You may want to read 240.4(B)

and i belive their is a code that if the building is detached it has to have a disconnect means if so do you know that article
Read Part II of article 225

Roger
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
i dont have a book handy would i be required to put a disconnect in for the panel

240.4(B) states that if you are under 800 amps you are allowed to use the next size breaker if the amp. of the wire is not a standard size breaker. Also the calculated load must be less then amp of the wire not the breaker.

So a #4 can be protected by 90 amps if 75C all around and the calculated load is 85 or less. Make sense???
 

tmason

Member
thank you denis thats makes it clear would you happen to know what the code is about a disconnect for the service if its in a seperate building and the main is in the house.
 

tmason

Member
so i read 225 part two and i found that 225.30 says any ungrounded conductors that pass through a seperate structure need a disconnect is that correct
 

tmason

Member
what is that rule this is a single fam dwelling with a 200 amp service i put a 100 amp brk in the main panel and put a main lug with a 90 amp brk and a retention clip in the garage that is detached so would you say that would safice for the rule
 
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