Why circuit breaker suitable for 75 deg but not 60 deg?

Status
Not open for further replies.
O

olc

Senior Member
If literature for a circuit breaker says it is suitable for 75 deg only - why would it not be suitable for 60 deg?
Maybe I am misunderstanding something but why would it not be suitable for the lower temp?

e.g.
15-20A tandem ... are suitable for use with 60 or 75. 25-50A tandem are suitable for use with 75 deg only. ????

e.g.
15-30A breakers are suitable for use with 60 or 75 conductors. 40-125 are suitable with 75 conductors. ????
 
olc said:
If literature for a circuit breaker says it is suitable for 75 deg only - why would it not be suitable for 60 deg?
Maybe I am misunderstanding something but why would it not be suitable for the lower temp?
Click to expand...

They are telling you that the breaker termination 'runs hot'. Effectively the breaker is dumping heat into the conductor, so the insulation needs to be rated high enough to handle it, plus the heat from the load.
 
jim dungar said:
They are telling you that the breaker termination 'runs hot'. Effectively the breaker is dumping heat into the conductor, so the insulation needs to be rated high enough to handle it, plus the heat from the load.
Click to expand...

So if you load a conductor to its 75C rating where is the extra "space" for the heat from the terminals?
 
Thanks.
I got it.

Follow up question. Say a 50A-2P circuit breaker (for a range receptacle):
Does the conductor (which is rated up to 90 deg) need to be sized for 50A at 60 deg or can it be sized for 50A at 75 deg?
Does the circuit breaker need to specifically say the terminations are OK at 75 deg. (Trying to work through this)
 
olc said:
Thanks.
I got it.

Follow up question. Say a 50A-2P circuit breaker (for a range receptacle):
Does the conductor (which is rated up to 90 deg) need to be sized for 50A at 60 deg or can it be sized for 50A at 75 deg?
Does the circuit breaker need to specifically say the terminations are OK at 75 deg. (Trying to work through this)
Click to expand...

NM would limit you to the 60C column.
 
olc said:
Thanks.
I got it.

Follow up question. Say a 50A-2P circuit breaker (for a range receptacle):
Does the conductor (which is rated up to 90 deg) need to be sized for 50A at 60 deg or can it be sized for 50A at 75 deg?
Does the circuit breaker need to specifically say the terminations are OK at 75 deg. (Trying to work through this)
Click to expand...
With 75?C rating specified, you can use a 75?C wiring method. However,being the breaker is likely installed in a panelboard, the panelboard must also be rated for 75?C wiring.

In summation, all the equipment at both ends must be rated for 75?C wiring.
 
That way you could put 50A continuously through that conductor and not get a temp above 75C so you wouldn't overheat the terminals.
 
As Db touched on in post #5 it depends on your wiring method. For NM cable you would need to use the 60? C ampacity. Switch over to the same size MC cable and then you can use the 75? C ampacity.
 
But even though you need to use the 60C ampacity, you are also relying on the NM to have a 75C terminal rating.
If you are within the 60C ampacity, there is no reason to use 75C rated terminations at the far end.

Tapatalk!
 
DBoone said:
So if you load a conductor to its 75C rating where is the extra "space" for the heat from the terminals?
Click to expand...
It was included when the termination got its listed temperature rating. There is no need to worry about it.

Loading conductors to their 'full rated ampacity' does not mean that the insulation reaches its ultimate temperature.
 
jim dungar said:
It was included when the termination got its listed temperature rating. There is no need to worry about it.

Loading conductors to their 'full rated ampacity' does not mean that the insulation reaches its ultimate temperature.
Click to expand...

Exactly. It just means the temperature will not exceed the rating of the insulation.
 
It's all about removing heat from the termination, most all failures of wiring are at the termination point, so it is very important to select the correct column in 310.16, or in the 2011, 310.15(B)(16) for the rating of the terminal the wiring is connected to, most terminals we will deal with will have a 75?c rating, and even if you have termination ratings at the other end of the wire you still have to select the amperage or wire size for the amperage for the 75?c rating, like was said the NEC requires us to use the 60?c column no matter what the rating of the terminal for NM, SE, SER and other cables which you will find in their respective articles.

If you ever look at a 90?c rated terminal you will notice that they have a lot more mass to dissipate the heat and doesn't depend upon the wire as much to help remove some of the heat away from the terminal, so if you have 90?c terminals at both ends of a conductor you may use it, except of course NM, SE, SER and others the the NEC limit to the 60?c column.

Also most wiring we get to day is rated at 90?c but you still have to go by the terminal rating for the ampacity rating of the conductor.
 
Last edited:
SE cable sized for 60 deg only when installed in thermal insulation? (SE can be sized for 75 deg when not installed in thermal insulation (assuming termination equipment is 75 deg)?)
 
My above question is based on NEC 2014.
NEC 2008 is the local adopted so I may need to adjust the question?
 
PetrosA said:
Not true. You have to take ambient temp into account and depending on that you could reach the rating of the insulation without a full load on the wire.
Click to expand...

I should have been clearer. I meant that based on 310.15(B)(16) with not more than 3 current carrying conductors in a raceway, cable etc and at an ambient temp of 78F-86F, you could load a 12 awg copper to 20A continuously and it's temperature may not reach 60C but it will definitely not exceed 60C.

Adjustment and correction factors definitely change the conductor ampacity.

I know you know this though. Just clarifying my post. :)
 
hurk27 said:
...so if you have 90?c terminals at both ends of a conductor you may use it, ....
Click to expand...

The limiting factor is the termination rating (conductor + lug + mounting pad + air flow + ?) not the rating of the terminal.

Simply replacing a lower rated lug with a 90?C one is not sufficient.
 
olc said:
My above question is based on NEC 2014.
NEC 2008 is the local adopted so I may need to adjust the question?
Click to expand...

338.10(B)(4)(a) requires us to follow section II of 334 in the 2008,
334.80 which is in section II reduces the rating of SE cable to the 60?c column for feeder and branch circuit use, not sure if the 2014 has changed this?
 
Status
Not open for further replies.
Top