Rating of busbar verses breaker and conductor

[epelectric33]

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?

 

[Dennis Alwon]

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.

 

[don_resqcapt19]

Where do you find a temperature rating of a bus bar in a panelboard? I don't think I have ever seen that.

 

[epelectric33]

Where do you find a temperature rating of a bus bar in a panelboard? I don't think I have ever seen that.

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.

 

[epelectric33]

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.

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.

 

[Dennis Alwon]

I don't have access to UL 891

 

[epelectric33]

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.

 

[Dennis Alwon]

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?

 

[epelectric33]

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?

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]

I don't have access to UL 891

You can make a free account at ul.com and get free online access.

Cheers, Wayne

 

[wwhitney]

a 60C rated busbar

Nothing you quoted refers to a 60C rated busbar.

Cheers, Wayne

 

[epelectric33]

Nothing you quoted refers to a 60C rated busbar.

Cheers, Wayne

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

 

[don_resqcapt19]

I don't have access to UL 891

Actually you do. UL has the same free viewing system that the NFPA uses.

 

[don_resqcapt19]

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.

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.

 

[wwhitney]

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

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