Rating of busbar verses breaker and conductor

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epelectric33

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I am wondering why busbars in panelboards can be rated at 60 degrees C and have circuit breakers connected to them (or conductor lugs) that have a conductor lug rated at 75 degrees C and you only have to de-rate the conductors to 75 degrees C and not 60 degrees C. Is it because of a UL listing or a manufacturer saying that the circuit breaker is capable of this?
 
If your equipment is rated 60C then you cannot use a conductor ampacity that is higher than the 60C. You cannot use the 75C because the breakers are rated 75C.

The weakest link in a chain is the one that will break. Same for the electrical system, we use the weakest link in the circuit.
 
Where do you find a temperature rating of a bus bar in a panelboard? I don't think I have ever seen that.
 
don_resqcapt19 said:
Where do you find a temperature rating of a bus bar in a panelboard? I don't think I have ever seen that.
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I review submittals for all types of equipment and gear. It is found in the spec's of the gear/equipment/panelboards. I believe that UL 891 has something to do with it.

Conductor Temperature Ratings

A switchboard rated 110 amperes or less, or having any circuits for field wiring rated 110 amperes or less, is marked to indicate use of conductors sized for 60o C (140o F) ampacity for circuits rated 110 amperes or less, and conductors sized for 75o C (167o F) ampacity for circuits rated more than 110 amperes as specified in Table 310.15(B)(16) of the National Electrical Code®. The marking may specify conductors sized for 75o C ampacity for circuits rated 110 amperes or less if any circuit breaker involved is marked 75o C or 60/75o C. If the circuit breaker is to be installed in the field, the switchboard marking indicates that the circuit breaker is to be marked either 60/75o C (140/167o F) or 75o C (167o F) if conductors sized for 75o C ampacity are to be used. A marking is provided near a terminal, such as “Use AWG 90o C (194o F) copper wire,” to indicate that 90o C (194o F) copper wire is to be used. UL determines the size of the conductor on the basis of 75o C (167o F) ampacity.
 
Dennis Alwon said:
If your equipment is rated 60C then you cannot use a conductor ampacity that is higher than the 60C. You cannot use the 75C because the breakers are rated 75C.

The weakest link in a chain is the one that will break. Same for the electrical system, we use the weakest link in the circuit.
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I was in the same line of thought, but after digging into the UL listings of panels and such is what has brought this question to light. Check out UL 891.
 
I also found this as well:

The IEEE states a busbar should be rated at the highest amperage passing through any section of the busbar, with a maximum of a 50° C rise in temperature from an ambient temperature of 50° C.
 
I am a bit confused... How are you getting around 110.14.... If the panel or lugs etc are marked 60C are you saying I can use 75C rating for the panel?
 
Dennis Alwon said:
I am a bit confused... How are you getting around 110.14.... If the panel or lugs etc are marked 60C are you saying I can use 75C rating for the panel?
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No sir... what I am saying is that whatever you are landing your conductors on (ie... breaker/lugs/terminals) are rated at is what you have to rate your conductors at to be in compliance of NEC 110.14. I am just diving in a little further to understand why it is ok to place a 75C rated breaker on a 60C rated busbar and that is ok (UL/IEEE). The NEC doesn't supersede UL or Manufacturer, so I am looking for the engineering value that explains why this is ok.
 
wwhitney said:
Nothing you quoted refers to a 60C rated busbar.

Cheers, Wayne
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My apologies I was using an example of a spec I was looking at. I believe busbars by IEEE only have to be rated at 50C. 60C would be above the requirement but the panel can still accept 75C rated breakers
 
epelectric33 said:
I was in the same line of thought, but after digging into the UL listings of panels and such is what has brought this question to light. Check out UL 891.
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UL 891 applies to "switchboards" and not to "panelboards". They are not the same products and not covered by the same standards. UL 67 applies to panelboards.
 
epelectric33 said:
My apologies I was using an example of a spec I was looking at. I believe busbars by IEEE only have to be rated at 50C. 60C would be above the requirement but the panel can still accept 75C rated breakers
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Hard to tell as it is uncited, but isn't the 50C figure a temperature rise, while the 75C figure is a temperature? E.g. 40C ambient + 50C temperature rise = 90C temperature.

Cheers, Wayne
 
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