75 degree rating OCPD w/ 90 degree conductors.

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Abe Stallcup

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An electrical engineer told me the other day that I should not be specifying 90 degree rated conductors connected to the OCPD because most are UL rated at 75. He said I can use 90 for all other wiring because most bus bars, lugs, etc. are rated for 90. Just not the wiring to the OCPD. True?
 
You can use 90? C conductors with 75? C terminals as long as you size the conductors at 75? C.
 
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This particular breaker requires 90 degree C conductors when the rating exceeds 125 amps. See the last two lines on the nameplate. The 90 degree C conductors are as stated before sized using the 75 degree column.
 
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In Addition, when applying 100% rated breakers at 100% of their rating as allowed by the listing 90degC rated wire must be used but applied at 75degC.

If I recall correctly, standard breakers are listed for use with60/75degC rated wire up to 100a, that is 60degC rated wire is allowed for use up to 100a. Above 100a 75degC rated wire must be used. That's where I believe that 125a breaker rating SG01 was refering to. I haven't looked in the NEC but I believe that there is a reference to that in there.
 
SG-1 said:
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This particular breaker requires 90 degree C conductors when the rating exceeds 125 amps. See the last two lines on the nameplate. The 90 degree C conductors are as stated before sized using the 75 degree column.
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According to the label, the terminals on that breaker ar 60?c and if the rating was 125 or lower then the terminals would be rated 75?c.
 
It doesn't seem to cost much to use 90C wiring today, as almost everything common is 90C. There may be reasons you need 90C downstream -- bundling, high ambients, or just additional design margin. Seems kind of silly to run 90C in those areas where needed and then splice on a 75C chunk where you don't. Don't see any value in doing that.
 
suemarkp said:
It doesn't seem to cost much to use 90C wiring today, as almost everything common is 90C. There may be reasons you need 90C downstream -- bundling, high ambients, or just additional design margin. Seems kind of silly to run 90C in those areas where needed and then splice on a 75C chunk where you don't. Don't see any value in doing that.
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Most people use 90?c c wire everywhere and derate the ampacity and upsize where needed for 75?c terminations.
 
Can anyone explain the theory/reason for this particular breaker why 60dec wire ratings must be used below 125A and 75deg wire be used above 125A?

Can anyone explain why when using 90deg wire it can be derated but must never be lower than the 75deg vaule due the the terminals being rated for 75deg. Does it have something to do that with using a smaller wire on the 75deg terminals there will be a larger heat buildup? What is the reason/theory behind having to use the 75deg ampacity for the terminals?

I have also seen cases where the 75deg rating was used at the breaker terminals and then was spliced onto 90deg wire running out into the field. In this case the 90 deg wire can have an ampacity of its 90deg rating.
 
mull982 said:
Can anyone explain the theory/reason for this particular breaker why 60dec wire ratings must be used below 125A and 75deg wire be used above 125A?
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You have that backwards.
60?c = above 125 amps
75?c = 125 amps and below
 
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