Fused Disconnects for HVAC Loads

[birddogger]

Early in my career, I worked at a firm where the standard spec always called for a fused local disconnect for rooftop A/C units, in addition to the circuit breaker protecting it from the service panel. Any other HVAC equip. like air handlers and unit heaters only required NF disconnects. I never did understand why this double protection was needed, and why it only applied to units with condensors. Is it a matter of fuses being able to open faster? Does such equipment have low withstand ratings and needs the fast-clearing time?

Either way, I’ve worked several places since then, and that was the only place where I was required to spec local fused disconnects, for A/C or otherwise. In fact, I don’t think I’ve ever spec’ed a fused disconnect since. Assuming that the branch circuit has O.C. protection back at the distribution board, where would you ever use a fused disconnect?

 

[busbar]

IIRC, the fuse requirement generally applies to multi-motor HVAC assemblies—id est, ‘package units’ and ‘condensing units’.

A couple of reasons deal with small-conductor protection and contactor damage during downstream faults {within the appliance.}

Admittedly by a fuse manufacturer—see page 10 of:

http://www.bussmann.com/library/docs/spd02/SPDSection10.pdf

In the US, the fuse requirement has been around for decades. If the equipment label specifies, an “HACR-rated” molded-case breaker can be used in place of fuses. This does not obviate the provision of local mechanical-equipment disconnecting means—a different issue.

 

[dpc]

There are a couple of different issues.

One issue is the need for a local disconnecting means, particularly on rooftop units. This is a basic NEC requirement, although there are some exceptions. But in general, any rooftop unit requires a local disconnecting means in sight of the unit.

As for the fuses, this is generally required by the UL listing for the HVAC equipment. Since current-limiting fuses increase the ability of the HVAC gear in terms of available short circuit current, they are generally used for UL testing, and thus become required for the field installation in order to comply with UL requirements.

So, it is quite common to see both a circuit breaker and a fused disconnect in series on feeders to HVAC systems.

 

[busbar]

Unless the conductors upstream of the fuses ‘fit’ the NEC tap rules, then the breaker protects the branch-circuit conductors. The fuses protect the appliance.