Disconnects For HVAC, Fused or Non-Fused

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GamecockEE

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First of all, This is my first post on this site. Really enjoy doing searches on various subjects. My question is, when should the required disconnects for HVAC Applications be fused vs Non-fused. Is there any information out there as to the typical short circuit withstand of HVAC equipment? I have been typically specifying them on projects where I felt that there was a good probability of a greater than 10KAIC available at the service.
What are you folks seeing in the real world?
 
Re: Disconnects For HVAC, Fused or Non-Fused

If the nameplate on the unit specifies "Minimum Fuse Size," "Maximum Fuse Size," then you must use fuses.

Most units specify "Minimum OCPD size" which gives the option to use a breaker or fuses.
 
Re: Disconnects For HVAC, Fused or Non-Fused

The nonfused disconnect only has a withstand rating of xxxx amps when protedted by an upstream OCD. Many of the non fused disconnects require a fuse as that OCD. Some jurisdictions have decided that a non fuse disconnect does not have the working clearance requirements of 110.26 because it is not intended to be opened while energized.

All thinge considered, we always demand a fused disconnect!

Fred
 
Re: Disconnects For HVAC, Fused or Non-Fused

Fred, I'm very confused by your post. :confused:

Why do working space rules not apply to a non-fused disconnect? A disconnect by its very nature must remain accesible.

And why would a non-fused disconnect require a fuse for its OCPD?

[ October 19, 2005, 02:39 PM: Message edited by: peter d ]
 
Re: Disconnects For HVAC, Fused or Non-Fused

Originally posted by peter d:
Why do working space rules not apply to a non-fused disconnect? A disconnect by its very nature must remain accessible.
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Accessible is thankfully much different than the work space rules.

We violate the work space rules every time we install a disconnect for equipment above a suspended ceiling. :p

IMO fused or unfused disconnects do not require maintenance while energized, they can be shut off.

Perhaps that section could be worded stronger.
 
Re: Disconnects For HVAC, Fused or Non-Fused

I agree with Bob.

IMO, the disconnect is there to allow the equipment it is disconnecting to be maintained safely. That's the whole reason the disconnect is there. If the disconnect requires maintenance, why should it be left on? Chances are good a dropped screwdriver will shut the circuit off for you anyway, so why not just kill it in the first place? :D
 
Re: Disconnects For HVAC, Fused or Non-Fused

The withstand of a non-fused disconnect is typically 5kA. I have not found a non-fused disconnect that is series rated/tested with an upstream OCPD to offer a higher withstand.
If you calculate a short circuit available of >5kA, then I suggest (that's what I specify) a fused disconnect which often take on the kAIC rating of the fuse installed (check the listing of the particular unit you are using).
 
Re: Disconnects For HVAC, Fused or Non-Fused

Bob,
IMO fused or unfused disconnects do not require maintenance while energized, they can be shut off.
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While I will agree that they don't require maintenance while energized, they are "likely to require examination" while energized, and that means that 110.26(A) applies to disconnects.
Don
 
Re: Disconnects For HVAC, Fused or Non-Fused

Just had a walk-through with the AHJ yesterday to look at all our fan power boxes in the ceiling. All units are 277 volts and the grid is all 2x2. Hmmm...can't possibly get our required working clearance on any of them. We invited him out to see what we can do to best resolve the problem. Basically what was determined is that we want the whole tile space in front of the panel for our working clearance, since there really wasn't anything inside these panels.

Also discussed the fused v. non-fused and how some feel that non-fused doesn't require the working clearance. He chuckled and said that doesn't apply in his jurisdiction. They require the working clearance.

BTW...all of these units are getting a single pole 277v rated switch for their disconnecting means. No fuses.
 
Re: Disconnects For HVAC, Fused or Non-Fused

Originally posted by iwire:
Accessible is thankfully much different than the work space rules.
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Yup, you're right, I was getting working space and accesibility mixed up. :eek:
 
Re: Disconnects For HVAC, Fused or Non-Fused

We need to follow the nameplate on the AC unit. If the namplate specifies fuses, we must use fuses.

The code section is 110.3(B)
 
Re: Disconnects For HVAC, Fused or Non-Fused

Disconnects for the AC units is one of the most abused rules in the code.

For some reason people have come to believe that power to the AC disconnect can be turned off and this would relieve the working space rule of 110.26.

I can find nothing to sustain this thought. Let?s look at the rule. Being that we are only talking about the disconnect being behind the unit I will only address this.

110.26 Spaces About Electrical Equipment.
Sufficient access and working space shall be provided and maintained about all electric equipment to permit ready and safe operation and maintenance of such equipment. Enclosures housing electrical apparatus that are controlled by a lock(s) shall be considered accessible to qualified persons.

(A) Working Space. Working space for equipment operating at 600 volts, nominal, or less to ground and likely to require examination, adjustment, servicing, or maintenance while energized shall comply with the dimensions of 110.26(A)(1), (A)(2), and (A)(3) or as required or permitted elsewhere in this Code.
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Where the debate arises is that this disconnect ?can? be turned off before servicing.
The one questions that comes to mind is, has any one ever used this disconnect to turn the unit off with out going to the main panel? Has anyone ever checked the voltage in one of these disconnects? Couldn?t do that with the circuit turned off.

More that this, these disconnects are required to be readily accessible.
440.14 Location.
Disconnecting means shall be located within sight from and readily accessible from the air-conditioning or refrigerating equipment. The disconnecting means shall be permitted to be installed on or within the air-conditioning or refrigerating equipment.
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The definition of readily accessible is,
Capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, and so forth.
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There are exceptions to this rule but I can not find one that applies to the AC unit. Should someone here know of this exception please show it to me.

Now we have the ?excuse? that the heat man put the unit there. Does that relieve the rule? No. Now the rest is simple. Give me 30 inches side to side and 36 inches in front or get a red tag!

:)
 
Re: Disconnects For HVAC, Fused or Non-Fused

Originally posted by jwelectric:
Disconnects for the AC units is one of the most abused rules in the code.
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That's true, and who is doing the abusing depends on which side of the fence you're on. :eek:

[ October 20, 2005, 05:30 PM: Message edited by: georgestolz ]
 
Re: Disconnects For HVAC, Fused or Non-Fused

Now Geroge just how did you get up there to sit on that unit to check that voltage?

Accessible, Readily (Readily Accessible). Capable of being reached quickly for operation, renewal, or inspections without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, and so forth.
Click to expand...
 
Re: Disconnects For HVAC, Fused or Non-Fused

I like the fact that two disconnects can share the same 30" space, as in between two units.
 
Re: Disconnects For HVAC, Fused or Non-Fused

No I climb in it

32bigchair.jpg


Edited to add picture of my chair

[ October 20, 2005, 11:17 PM: Message edited by: jwelectric ]
 
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