Wire sizing for HVAC equipment

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steve66

steve66

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Illinois
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Engineer
I have a piece of HVAC equipment with a Minimum circuit ampacity of 43 amps, and a Maximum Overcurrent protection of 60A. If I feed this unit directly from a 60A circuit breaker, I can use #8 wire (even though #8 is only rated for 50A). Right?

Here is the question: If I add a fusible disconnect at the equipment, can I still run #8 wire from the circuit breaker to the fusible disconnect? Or does this run change from a branch circuit to a feeder? If so, I would need #6 wire rated from 60A.

Thanks:
Steve
 
It depends. If the equipment you are referring to falls on the scope of Article 440, then you can use the #8. If it does not, the conductors will have to match the OCD.

Per definition, the circuit between the panel and disco would be a feeder, however I personally don't feel it would require the #8 to be increased to #6 due to the fact the feeder is not serving more than one load and not terminating at a panelboard.
 
Bryan,
Per definition, the circuit between the panel and disco would be a feeder, however I personally don't feel it would require the #8 to be increased to #6 due to the fact the feeder is not serving more than one load and not terminating at a panelboard.
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If the conductors are feeders then they are not covered by the overcurrent protection rules in Article 440, they are covered by 240 and 240.4(G) does not apply. In my opinion they are not feeders. The fuse at the equipment is not there to provide protection for the conductors, it is there to provide protection for the equipment and the circuit feeding the fuses is a branch circuit.
Don
 
I have never considered wires supplying a disconnect to be feeders.
Very Interesting!?
But, Feeders are permitted to supply multiple loads.

I would only consider the supply wires to be "feeders" if the disconnect supplied multiple circuits.

I don't think it's a violation to install "Additional" Overcurrent protection (Fuses in this case) within the branch circuit.


Dave
 
busman said:
Are we assuming the terminations are 75 deg C? How did we get #8 good for 50 amps?
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I would be very surprised if the terminations where not 75C rated.

However if this was an NM job we would be stuck at 60 C. (334.80)
 
The question of if a set of conductors is a feeder or not has nothing to do with the number of loads served.
Feeder. All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.
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In this case the question is, "are the fuses in the unit disconenct the final branch circuit OCPD"?
Don
 
don_resqcapt19 said:
In this case the question is, "are the fuses in the unit disconenct the final branch circuit OCPD"?
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I would say "no", that these fuses are the equipment OCPD; the branch-circuit OCPD is the breaker at the origin of this circuit.
 
If this continues as long as the same argument has among inspectors in this area, it's going to be along thread. IMHO, if you take Don's post literally, the conductors to the disconnect would be feeders and have to be sized according to the breaker. Some here play word games and call the fusible at the unit "supplementary" and still consider the conductors to the breaker as "branch circuit condcutors". It does seem a bit strange to say if you put a non-fuse unit as an disconnecting means at the unit you can use #8, but if you choose a fusible one you must use #6.
 
Thanks for the replies. I am going to assume the addition of a fusible disconnect should not require the wires to be a larger size. I don't see any real safety issues with the smaller wire.

Oh, and my understanding is that almost all equipment has 75 deg. connections. I think the 60 deg ratings are almost obsolete. (Someone will probably prove me wrong there.)

Steve
 
I was under the impression that we sized conductors for the ACTUAL running of the unit and the OCPD was sized for the start-up. For example you could have lets say a #10 to disconnect on a 60amp breaker at the panel.

Once we set a fused disconnect at the unit this would change and the line from the panel would need to be sized for the OCPD in the panel rated for the fuse at the disconnect.

I'm sure I will hear about this if I'm wrong.
 
Feeder. All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.

Branch Circuit, Individual. A branch circuit that supplies only one utilization equipment.

Using article 100 (which we must when applying the provisions of the NEC as I understand it) what will we call the conductors between the load side of the fuses in the disconnect and the final termination to the HVAC equipment? :?:
 
pete m. said:
. . . what will we call the conductors between the load side of the fuses in the disconnect and the final termination to the HVAC equipment? :?:
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A "whip?"
 
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