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hvac disconnects #78312 09/06/01 10:39 PM
Joined: Jul 2001
Posts: 5
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Barry Offline OP
Junior Member
Whenever I install a disconnect for a roof top hvac unit I'm always told to install a fused disconnect. Would it not be to code to install a nonfused disconnect as long as a hacr breaker is installed in the panel? 440-52(a) seems to tell me I only need one means of protection and 440-52(a)(3) says an inverse time breaker can be used as long as it matches the nameplate rating.Thanks for any thoughts and if possible refer to the 99 code if you can. Barry

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Re: hvac disconnects #78313 09/07/01 01:00 AM
Joined: Aug 2001
Posts: 599
N
Nick Offline
Member
You must provide short circuit/ground fault protection per the listing of the equipment. The manufacturer has to put on the name plate acceptable OC protection devices. If the unit you are wiring states on the name plate maximum fuse size, you must use fuses. If it gives you the choice IE: maximum HACR circuit breaker/ or fuse size, you have a choice and a NF disco with HACR breaker feeding it is fine.

Re: hvac disconnects #78314 09/07/01 01:00 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Online Happy
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Well, Don [resqcapt13] and Nick can explain this better, or more accurate than me, but here's my $.02:

The fuses at the disconnect make up failsafe [somewhat [Linked Image]] running protection for the complete assembly.
This would ward off the evil overload spirits [mythological barbecue gods] from reaking too much havoc if the power system experienced a single phasing problem, or if the monkey wrench committed suicide within the compressor's belts / other exposed rotating items, while the unit is being serviced or troubleshot.

I'm just so use to installing fusable EXOs [disconnects], that it seems like "it's always done that way"...
[Linked Image] DOH!!! [Linked Image]

The breaker is branch circuit protection [in case the forklift gods are angry and drive into the conduit for the AC] offering overcurrent protection [fault current protection].
It needs an HACR rating to be used on a circuit with the characteristics of an AC unit - high and extended Locked Rotor currents, LCL [continuous load which may increase in amperage, but seldom decrease under load], short off cycle durations, increased loads [multistage AC units], and all that baloney.

The fuses would be running protection for the entire machine, plus nearly always will open from a heavy fault or overload [EMT fuses are the exception [Linked Image]].

Nick has a better explanation than me, so go with that!

Scott SET


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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