Temperature of Lugs 2

[waltalltype]

Is ther a formula for determining what the temperature of the lugs on the load side of a main breaker should be?

Thanks

 

[DanDel]

The lug rating is based on what size cable you want to use. Typically, 75°C lugs are used, and the 75°C column in the NEC 310-16 is used to size the cable. Other lug ratings can be used, but the capacity of the cables connected is limited to the lug rating.

 

[busbar]

For ANSI regions, a genaral rule is 99 NEC 110-14 is unless marked otherwise, 100 ampere or less terminations are rated 60°C; above 100 amperes—75°C.

 

[waltalltype]

What I really need to know is how hot is too hot on a lug, for the amount of current flowing through it.

 

[jghrist]

US standards rate connectors at a 30°C rise over a 40°C ambient, indoors, in still but unconfined air. If the temperature is higher than this with rated current then it is too hot.

 

[dpc]

You might check with circuit breaker manufacturer. They probably have test data on lug temp vs amperage for larger circuit breakers. UL 489 may provide some additional data.

 

[wilber]

During operation (and testing for approvals),
lugs should always run "cooler" than the conductor.
If a lug is at the same temperature or a higher temperature then a failure is bound to occur.

 

[birddogger]

This probably doesn't answer your original question about the proper equation to use, but it should give you some additional insight into the subject.

http://www.bussman.com/library/docs/spd02/SPDSection04.pdf

 

[jbartos]

Suggestion: Traditionally, circuit breakers used lower ampacity rating at higher rated conductor insulation temperatures, e.g. 75degC ampacity rating of 90degC conductor insulation thermal rating for circuit breakers over 100A rating and 60degC ampacity rating for 75degC conductor insulation rating. Up to 100A C/B 60degC conductor insulation is often used. Some c/b manufacturers offer 75degC circuit breakers for any frame. The reason for the lower temperature of conductor insulation or ampacity rating is that the elevated conductor temperature might affect the thermal magnetic circuit breaker tripping.

Depending on the circuit breaker terminal thermal rating, the corresponding cable lug thermal rating should be used.

 

[CJFlatters]

What do you need to calculate the temperature for ?

The limiting factor on cables and therefore on the lugs attached thereto is the maximum insulation temperature, which determines the maximum conductor operating temperature (eg 90C for XLPE/EPR, 70C for PVC).

If there is a loose / poor connection somewhere (ie a badly crimped connection or a loose bolt) the the lug and cable will become much hotter due to I2R losses, this will cause the temperature to spiral upwards with a danger of fire.


_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager