I have a customer who has a sub-panel fed by a 6/3 piece of AC. It's overcurrent protection is a double-pole 60AMP Homeline Square 'D' circuit breaker. One phase is operating at 53AMPS, the other at 43. On Tuesday, the circuit breaker tripped. I went there today and the breaker was RED-HOT. My question is, without running new #4 feeders to the sub-panel, can I upgrade to a double-pole 70 according to 310.16 using the 90º column?

First of the wire must be of a type listed in the 90 degree column. Second the terminals of the breaker must be rated for 90 degrees. Also make sure that ambient tempature doesn't play a role. IE: does the cable run through a hot attic?

[This message has been edited by bot540 (edited 06-22-2006).]

The cable DOES run through a suspended ceiling near the roof of the building, but not in or through any thermal insulation per 320.80 (A).

The "red hot" is alarming to me. I wonder if you just had a bad breaker.

No you can not use the 90 C column.

The reason is that none of your terminations are rated 90 C. The terminations will be 75 C at most.

However you could install a 2 pole 70 per 240.4(B) although the load on the conductors would have to remain under 65 amps.

I would install a new 70 amp breaker and look at balancing the load a little better.

With 53 and 43 amp readings you could try to balance it to about 48 amps each.

One other factor to consider is continuous loading.

With a 60 amp breaker the continuous load should not exceed 48 amps.

If you swap to a 70 amp breaker the continuous load could be 56 amps.

A 60A circuit breaker carrying 53A should not be 'red hot'. Quite warm, perhaps, but not red hot.

I would check very carefully for damage to the bus stabs, or damage to the end of the conductor, both of which could cause a high impedance connection and localized heating. Was the breaker correct for the panel, or one that 'just happens to fit'?

If you have a high impedance connection, then it will heat and trip a new 70A breaker just as it did the 60A breaker.

-Jon

I was there today farting around during business hours. I balanced the loads (50AMPS and 48AMPS), but the new 70 AMP circuit breaker still got warm. Not red hot, but warm. The terminal connections to the breaker are solid, but I'll have to check for damage on the buss when I go back there later tonight (it's a bar so I do most of my work there when it is closed). And yes, the circuit breaker is the same as the manufacturer of the panelboard. 110.3 (B) was not violated.

Thanks to everyone for all of your help, and timely advice.

It's very much appreciated.

--Ron

Sure thats possible.

However by replacing the breaker you usually take care of any high impedance connection that is only affecting one branch circuit.

If the panel bus is causing the problem it will be quite evident when the breaker is removed.