MOP, Branch Circuit vs. Feeder

[xguard]

I have a panel on a rooftop. Circuit in question has a 35 amp breaker, #8's (XHHW-2).

Two scenarios:

1. A breaker in the panel is supplying a roof top unit with a MCA of 35 and MOP of 50. The circuit breaker in the panel serves as the disconnect and over current protection for the equipment. So I can use #8's.

2. Now, same as above but I add a fusible disconnect between the panel and unit fused at 50 amps with #8's running between the disconnect and the RTU. Now the wires between the panel and disconnect need to be #6. Is this correct?

Practically I don't see a difference between the 2 situations but code wise it seems like there is a difference because the conductors supplying the disconnect become a feeder as opposed to a branch circuit?

Thoughts?

 

[LarryFine]

1. A breaker in the panel is supplying a roof top unit with a MCA of 35 and MOP of 50. The circuit breaker in the panel serves as the disconnect and over current protection for the equipment. So I can use #8's.

You could even use a 50a breaker.

2. Now, same as above but I add a fusible disconnect between the panel and unit fused at 50 amps with #8's running between the disconnect and the RTU. Now the wires between the panel and disconnect need to be #6. Is this correct?

Not if you still use the 35a breaker.

Practically I don't see a difference between the 2 situations but code wise it seems like there is a difference because the conductors supplying the disconnect become a feeder as opposed to a branch circuit?

You're correct about the first part being a feeder vs a branch circuit.

The breaker must protect the feeder from overload, but not the branch circuit.

 

[xguard]

You could even use a 50a breaker.


Not if you still use the 35a breaker.


You're correct about the first part being a feeder vs a branch circuit.

The breaker must protect the feeder from overload, but not the branch circuit.

Sorry that was a mistake in my original post. I plan to replace the 35 amp breaker in the panel with a 50 amp breaker.

 

[david luchini]

1. A breaker in the panel is supplying a roof top unit with a MCA of 35 and MOP of 50. The circuit breaker in the panel serves as the disconnect and over current protection for the equipment. So I can use #8's.

With and MCA of 35 and MOP of 50, you can use a 50A c/b and run #10 (XHHW-2) to the RTU

2. Now, same as above but I add a fusible disconnect between the panel and unit fused at 50 amps with #8's running between the disconnect and the RTU. Now the wires between the panel and disconnect need to be #6. Is this correct?

Add a fused disconnect (50A fuses) between the panel and RTU, and you can run #10 from the panel to the fused disconnect, and #10 from the fused disconnect to the RTU.

 

[xguard]

With and MCA of 35 and MOP of 50, you can use a 50A c/b and run #10 (XHHW-2) to the RTU

Add a fused disconnect (50A fuses) between the panel and RTU, and you can run #10 from the panel to the fused disconnect, and #10 from the fused disconnect to the RTU.

I'm not limited to a 30 amp load with #10?

 

[david luchini]

I'm not limited to a 30 amp load with #10?

No, not for a motor or compressor load.

 

[augie47]

I've seen it enforced 2 different ways. As a general rule, the circuit from the panel (50 amp) and the disconnect (35 amp) have been required tom meet the MCA (#10 THWN-2 in this case) but. on occasion I have seen inspectors require the feeder to the disconnect to be sized by the 50 amp panel breaker.
Keep in mind, IF your disconnect was no fuse then the 50 amp and #10 should not even be questioned,

 

[infinity]

Why is this a feeder when installed with a fused disconnect? Is the OCPD in the disconnect a Supplementary Overcurrent Protective Device making the entire circuit back to the panel a branch circuit?

Overcurrent Protective Device, Supplementary. A device intended to provide limited overcurrent protection for specific applications and utilization equipment such as luminaires and appliances. This limited protection is in addition to the protection provided in the required branch circuit by the branch-circuit overcurrent protective device.

240.10 Supplementary Overcurrent Protection.
Where supplementary overcurrent protection is used for luminaires, appliances, and other equipment or for internal circuits and components of equipment, it shall not be used as a substitute for required branch-circuit overcurrent devices or in place of the required branch-circuit protection. Supplementary over‐current devices shall not be required to be readily accessible.

 

[LarryFine]

Keep in mind, IF your disconnect was no fuse then the 50 amp and #10 should not even be questioned,

I wonder if one could argue that, since the actual overcurrent protection is part of the compressor, a fused disconnect is not actually "the final overcurrent device protecting the circuit" and is instead part of "a branch circuit that supplies only one utilization equipment" thus making the entire circuit a branch circuit?

 

[LarryFine]

Why is this a feeder when installed with a fused disconnect? Is the OCPD in the disconnect a Supplementary Overcurrent Protective Device making the entire circuit back to the panel a branch circuit?

Kinda what I'm getting at. Maybe we've been getting this one wrong all this time.

 

[david luchini]

Why is this a feeder when installed with a fused disconnect? Is the OCPD in the disconnect a Supplementary Overcurrent Protective Device making the entire circuit back to the panel a branch circuit?

How would the 50A fuses provide limited protection as compared the 50A c/b?

I would imagine the the RTU probably has supplemental overcurrent protection within it, but the fused disconnect would be the final OCPD before the outlet making it the branch OCPD.

 

[infinity]

I would imagine the the RTU probably has supplemental overcurrent protection within it, but the fused disconnect would be the final OCPD before the outlet making it the branch OCPD.

So you're saying that the supplementary overcurrent protection cannot be fuses in a fused disconnect? The definition in post #8 doesn't seem to imply that the supplementary overcurrent protection has to be integral to the equipment.

 

[david luchini]

So you're saying that the supplementary overcurrent protection cannot be fuses in a fused disconnect? The definition in post #8 doesn't seem to imply that the supplementary overcurrent protection has to be integral to the equipment.

How is the protection limited? It's providing the same protection that the c/b provides. There's nothing limited about it.

 

[LarryFine]

I would imagine the the RTU probably has supplemental overcurrent protection within it, but the fused disconnect would be the final OCPD before the outlet making it the branch OCPD.

Why wouldn't the fused disco be only for fault and short-circuit protection just like the breaker in the panel?

 

[LarryFine]

So you're saying that the supplementary overcurrent protection cannot be fuses in a fused disconnect? The definition in post #8 doesn't seem to imply that the supplementary overcurrent protection has to be integral to the equipment.

Does it even imply that "unnecessary" fuses in a disco must be considered to be supplementary protection?

 

[david luchini]

Why wouldn't the fused disco be only for fault and short-circuit protection just like the breaker in the panel?

It is. So it's a branch circuit OCPD.

 

[infinity]

Does it even imply that "unnecessary" fuses in a disco must be considered to be supplementary protection?

The definition lends itself to calling it that if you so choose making the feeder/branch circuit argument moot when there is a fused disconnect.

 

[kwired]

JMO, but as long as you are only supplying one load (self contained HVAC unit with sub loads still is one load for this purpose) there is no feeder, just a branch circuit and a supplemental OCPD. If you make that disconnect into a small panelboard and add more branch circuits then the supply side becomes a feeder.

But even then there are provisions in art 430 for feeders supplying motor loads that might still allow higher OCPD on said feeder than if not supplying motor loads.

 

[infinity]

JMO, but as long as you are only supplying one load (self contained HVAC unit with sub loads still is one load for this purpose) there is no feeder, just a branch circuit and a supplemental OCPD. If you make that disconnect into a small panelboard and add more branch circuits then the supply side becomes a feeder

That's my opinion as well. It's a single branch circuit with an extra OCPD. The fact that it overlaps with the definition of a feeder doesn't make it one.

 

[david luchini]

The definition lends itself to calling it that if you so choose making the feeder/branch circuit argument moot when there is a fused disconnect.

If you add a fused disconnect, what would the fuses be protecting? The RTU has an MOCP of 50, do the fuses protect the equipment? Do the fuses protect the conductors between the disconnect and the RTU?