HVAC circuits and upgrade

[electro7]

We are doing a project changing multiple rooftop HVAC units out but installing new disconnects, fuses, and utilizing the existing wiring. We are having to extend the wiring in most cases. There are some existing breakers that are larger than the fuses we are using for the new units. As an example, there may be a unit with existing #8 originating from a 50A breaker that we are replacing with a unit that requires a max fuse of 30A. From the load side of these disconnects, we are wired in with #10, but when we splice and extend the existing conductors, can we use #10?

Is the HVAC circuit considered a feeder since it lands at a fused disconnect? If so, I would assume we could change to #10 as long as we follow the feeder tap rules in 240. If it's not considered a feeder, then I would assume we would need to keep #8 to the line side of the disconnect.

Thanks again!

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[Little Bill]

You need to size the conductors for the MCA on the HVAC units.
You size the fuse, or breaker, to the MOCP (Max breaker/fuse).
You would use that sizing from the disconnect to the unit.
You can treat the run from the panel to disconnect as a feeder and size accordingly.

 

[wwhitney]

You can treat the run from the panel to disconnect as a feeder and size accordingly.

For a feeder supplying just an individual branch circuit for one motor or one piece of HVAC equipment, the sizing and OCPD rules are identical to the rules for the branch circuit.

For a feeder supplying multiple such branch circuits, the rules are more complicated but will also result in the OCPD size being allowed to exceed the minimum ampacity of the conductors in a similar way.

Cheers, Wayne

 

[augie47]

The HVAC feeder vs branch circuit is getting to be as common as 'ground up-ground down"
Seemingly a grey area but Art 440 defers back to Art 430 for sizing and the conductor, being a feeder or branch circuit, would be sized the same, based on minimum branch circuit requirement.