Circuit breakers vs supplementary protection?

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TwoBlocked

TwoBlocked

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Bradford County, PA
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Industrial Electrician
I am embarrassed that I have probably installed supplementary protection breakers where I should not have. Been reading up on it and think I have it sorted out. Let me ask a few simple, direct questions to see if I got it right.

Scenario : A single pole 30 amp OCPD MCB is installed to provide protection to a branch circuit with #10 awg wire. Among a number of devices fed by this branch circuit, a 120VAC/24VDC power supply is fed by a 20A supplementary breaker.

Question 1: Does the wire from the supplementary breaker to the PS have to be #10 awg, because #12 awg wire would not be protected? I think this is TRUE.

Question 2: Would feeding another supplemental breaker, 15 amp, downstream of the 20 amp supplementary breaker to provide power to a panel heater, not be allowed even if #10 awg wire were used because this would make the 20 amp supplementary breaker a branch OCPD, which is not allowed? I think this is TRUE.
 
When you use the supplemental cut breaker, how is it installed? In a small load center? Is so that is fine. But in a separate enclosure, seems like that would have to be a listed assembly.
 
I got curious and looked up UL 508A. It's "teeth" come from NEC article 409:

409.1 Scope. This article covers industrial control panels intended for general use and operating at 1000 volts or less.

I would not consider gas field control panels to be intended for general use.
 
Ok, by “supplementary circuit breakers”, let’s assume you are talking about those that are UL1077 listed, as opposed to UL489 listed. A UL1077 listed supplementary circuit breaker is NOT listed for branch circuit protection, that’s what differentiates them. So in your scenario, the 30A breaker might be a feeder, or it might be a branch (branch being defined as the LAST circuit protective device before the load). So being that the 1077 listed breakers cannot be branch protective devices, you would need to have 10ga wire all the way to the load, even though you have a smaller 1077 listed breaker down stream. So per your question 1, the answer is Yes.

Per question 2, the 30A is STILL the only branch protective device, so you still have the 10ga wire all the way. It does not matter that you have another supplementary device down stream if the first one, they are still supplementary. So can you do that? Yes, so long as you run the #10 all the way you can have as many supplementary devices as you can afford.

The idea behind the 1077 listed devices is that you are simply CHOOSING to supplement your branch protection with something smaller because you want to protect the DEVICE, not the circuit.
 
Jraef said:
Ok, by “supplementary circuit breakers”, let’s assume you are talking about those that are UL1077 listed, as opposed to UL489 listed. A UL1077 listed supplementary circuit breaker is NOT listed for branch circuit protection, that’s what differentiates them. So in your scenario, the 30A breaker might be a feeder, or it might be a branch (branch being defined as the LAST circuit protective device before the load). So being that the 1077 listed breakers cannot be branch protective devices, you would need to have 10ga wire all the way to the load, even though you have a smaller 1077 listed breaker down stream. So per your question 1, the answer is Yes.

Per question 2, the 30A is STILL the only branch protective device, so you still have the 10ga wire all the way. It does not matter that you have another supplementary device down stream if the first one, they are still supplementary. So can you do that? Yes, so long as you run the #10 all the way you can have as many supplementary devices as you can afford.

The idea behind the 1077 listed devices is that you are simply CHOOSING to supplement your branch protection with something smaller because you want to protect the DEVICE, not the circuit.
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Thanks, that is what I basically understood. Let me be a little more clear about the second question. If a supplemental breaker feeds two separate circuits, (the power supply and another supplemental breaker feeding a panel heater) some might consider the first supplemental breaker to be acting as a branch OCPD.
 
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