Main purpose of overprotection in NEC or any other literature?

Sparky Bill PE · Nov 28, 2020

I can't find this in NEC or any other literature I was looking in. I could see this being a PE question, but I don't want to take this books "word for it" without verifying it.

DilutedAr18_PE · Nov 28, 2020

Not sure where an actual answer is in a book, but logically it is correct. When you size the overcurrent protection, you have to size the conductors to meet your load. The insulation has a temperature rating that determines how many amps the conductor is rated for. The overcurrent protection has to be sized so that if the ampacity is greater than the load it is sized to handle, the circuit breaker will trip and protect the insulation of the conductor. For thermal/magnetic circuit breakers, the magnetic portion protects against faults and the thermal portion protects against overloads; both to protect the conductors insulation.

Is answer A correct? Yes, but preventing fires comes from protecting the insulation. B, It does isolate the circuit, but only to save the insulation of the conductors. C, It can protect the device, but ground fault protection is what really protects devices.
This is one of those cases of what is “most” correct.

speakeelsy PE · Nov 28, 2020

I was taught that in a motor starter, overloads are to protect your motor, SCCP or OCPD (breaker or fuses) were to protect your wiring. When there is a fault, you get lots of amps which means lots of heat - this is bad.

Your breaker or fuses are meant to protect your cables -- which is why you size them based on the cable sizing!

Motor overload protection is meant to protect your motor -- that's why it's sized based off your motor.

Sparky Bill PE · Nov 28, 2020

DilutedAr18 said:
Not sure where an actual answer is in a book, but logically it is correct. When you size the overcurrent protection, you have to size the conductors to meet your load. The insulation has a temperature rating that determines how many amps the conductor is rated for. The overcurrent protection has to be sized so that if the ampacity is greater than the load it is sized to handle, the circuit breaker will trip and protect the insulation of the conductor. For thermal/magnetic circuit breakers, the magnetic portion protects against faults and the thermal portion protects against overloads; both to protect the conductors insulation.

Is answer A correct? Yes, but preventing fires comes from protecting the insulation. B, It does isolate the circuit, but only to save the insulation of the conductors. C, It can protect the device, but ground fault protection is what really protects devices.
This is one of those cases of what is “most” correct.

The answer is D.

Byk · Nov 28, 2020

SparkyBill said:
I can't find this in NEC or any other literature I was looking in. I could see this being a PE question, but I don't want to take this books "word for it" without verifying it.

View attachment 19640 View attachment 19639

I do not have code in front of me right now but I believe it's section 240 something that talks about overcurrent protection.

My understanding is that overcurrent protection protects both conductors and equipment. It will protect a circuit by opening when current reaches a value that would cause an excessive temperature rise in the conductor (i.e. protecting insulation and conductor).

On the other hand, it protects equipment by opening when it detects a short circuit or ground fault.

I can see how one can be debating between C and D for possible answer but I thing that "trick" here is device (for example circuit breaker) which does not make sense.