Providing Overload Protection for Cables

[CuriousElectron]

Hi Sparkies,

In a scenario where the breaker rating is lower than cable ampacity that the breaker is protecting, isn't the overload protection automatically applied by the virtue of the CB rating being lower than the cable ampacity? For example, 100A mains protecting 1/0 service conductors. If the breaker is equipped with thermal magnetic overload or long time delay function, it will eventually trip before the allowed amperage of the 1/0 cable is exceeded.

Am I right on this? I could see a scenario where a 30 Amp breaker is protecting a #10 conductor, you'd not want to use MCP, you'd need a long time delay element on the breaker.

Thank you for your feedback.
God Bless.
EE

 

[VEBill]

Yes, but...

Make sure that the maximum fault current is within the rating of the circuit breaker (e.g. "10 kA"). This would only be a concern in the most unusual circumstances.

 

[jraef]

just FYI, thee is no legal use for an "MCP" (Magnetic-Only circuit breaker) other than as part of a factory assembled and listed motor starter unit that has another Over Load Relay. So the scenario you described would be illegal from the outset.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[dpc]

What Jeff said. Magnetic-trip only MCCB, at least in the US, can only be used in a combination motor starter. A serious NEC violation to use them as a feeder breaker, although I've seen a few pressed into service as feeders.