temperature rise in bus bar

[hogdawg]

Could anyone tell me why bus bar ampacity tables show a higher ampacity rate for higher a higher temperature rise? It doesn't make sense to me.

 

[rbulsara]

Do you mean to ask why the busbar ampacity is higher at higher temperature rise?

If so which part does not make sense? The temperature o of the bus will rise higher with higher curernt. May be you are confused in relationship of actual temperature of the bus bar, the ambient temperature and the temperature rise of the busbar.

Actual temperature= Ambient temp + Temperature Rise

Amapcity is altimately related to what maximum temperature is sustaibable witout over heating either the conductor material or its insulation or its enclosure. So if the assembly/insulation can withstand higher temperature you can use the busbar/conductor to carry higher current.

For example, say an assembly is rated for 100 deg C max at certain ampacity. Now if it is to be used in ambient of 45 degC , it will be said to have 55 deg temperature rise. The same assembly could be rated to have 70 deg temperature rise in 30 deg C ambient. Or in other words it will be able to carry more current than compared to a 50 deg. temperature rise. But the key there is to know ambient temperature.

Hope this helps.

 

[waross]

The Canadian Electrical Code uses 30 Deg.C for ambient temperature. If the ambient is above 30 deg. C conductors must be derated.
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