Roof AC Unit Disconnect Sizing

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vpower1989

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Hello There,

I have been drug into a project in which several existing Roof top AC units are going replaced with new slightly smaller units. Not rocket science but I always manage to make things complicated.

For each unit:
* The existing breaker in the panelboard and the wiring from panelboard to roof will re-used.
* A new fused disconnect will be installed on roof.
* New wiring from fused disconnect to new unit.

AC Unit information: 460V, MCA = 22.5A, MROPD = 30A. I do not have any specific motor information. I do not have any LRC information.
- So I need 30A fuses in my disconnect.
- I need #10 conductors from disconnect to new unit.

Where I am getting a little hung up is the hp rating of the disconnect. From what I understand, I need an amp rating of 115% * MCA and a hp rating that is equal to whatever the equivalent horsepower is. Is this correct?

If so, I come up with an equivalent hp of 20 based on MCA of 22.5A. For a Heavy Duty GE Safety switch I would require a 60A rated switch. For 30A fuses, I would need fuse reducers which I try to avoid.

Any feedback appreciated. Thanks.
 
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Actually if no ampacity adjustments are needed you can use 12 AWG conductor, presuming you do have 75C terminals.

Also keep in mind MCA already has 125% of compressor RLA plus all other loads factored into it. so that compressor rating may be only about 16-17 amps x 1.15 plus amp or two for the other loads would probably allow or come real close to allowing a 20 amp switch, though you are only going to find 30 amp in fused switch type. I know that Square D's 600 volt safety switch is rated for 20 HP if non fused, fused switch needs to jump to 60 on that size motor just because you need more than a 30 amp fuse and not because the fuse can't handle it. You are closer to a 15 HP and probably more like equivalent of 12 or 13 HP vs a 20 though.
 
Your being an EE I would not challange your concern but I have never seen a situatuion where, if the disconenct was sized for the MCA/MOCP, the HP rating was a problem.
 
kwired said:
Actually if no ampacity adjustments are needed you can use 12 AWG conductor, presuming you do have 75C terminals.

Also keep in mind MCA already has 125% of compressor RLA plus all other loads factored into it. so that compressor rating may be only about 16-17 amps x 1.15 plus amp or two for the other loads would probably allow or come real close to allowing a 20 amp switch, though you are only going to find 30 amp in fused switch type. I know that Square D's 600 volt safety switch is rated for 20 HP if non fused, fused switch needs to jump to 60 on that size motor just because you need more than a 30 amp fuse and not because the fuse can't handle it. You are closer to a 15 HP and probably more like equivalent of 12 or 13 HP vs a 20 though.
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Yes you are correct on #12. This particular client has standard circuit sizes they use and #10 happens to be the smallest conductor size they use for power.

* Also good point on the FLA. I may have been looking at the wrong section ( I don't look at 440 very often). For "combination load" my switch should be 115% * FLA. My horsepower should be equivalent to the FLA, not MCA right? Digging through the submittal, I see that the FLA is 20.5A. That gets me closer to 15hp which is what a 30A GE Heavy Duty Fused Safety Switch is rated for.
 
augie47 said:
Your being an EE I would not challange your concern but I have never seen a situatuion where, if the disconenct was sized for the MCA/MOCP, the HP rating was a problem.
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Me either, plus like I said take his MCA and take out an amp or two for condenser motor and even another amp or two if it is a RTU with another motor for indoor blower and you are left with about equivalent of a 15 HP motor or even a little less than that, which should be fine on a 30 amp switch, especially a motor rated switch.
 
vpower1989 said:
Yes you are correct on #12. This particular client has standard circuit sizes they use and #10 happens to be the smallest conductor size they use for power.

* Also good point on the FLA. I may have been looking at the wrong section ( I don't look at 440 very often). For "combination load" my switch should be 115% * FLA. My horsepower should be equivalent to the FLA, not MCA right? Digging through the submittal, I see that the FLA is 20.5A. That gets me closer to 15hp which is what a 30A GE Heavy Duty Fused Safety Switch is rated for.
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MCA is the result of taking 125% of compressor RLA (rated load amps) and adding the full load amps of all other loads, in your case probably a condenser fan motor and probably an indoor blower motor if unit is a self contained RTU.

MCA is primarily for determining minimum conductor size needed - no adjustments necessary to that value, but conductor must have at least MCA value after adjusting the conductor for ambient temp or number of conductors in raceway.
 
augie47 said:
Your being an EE I would not challange your concern but I have never seen a situatuion where, if the disconenct was sized for the MCA/MOCP, the HP rating was a problem.
Click to expand...

I've been doing this for a while, but I have not been involved with many RTU installation so I have been reading though 440 and confusing myself.
 
infinity said:
I'm confused, if the MaxOCPD is 30 amps why would a disconnect switch need to be larger than 30 amps?
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I'm with you here, chances are a time delay fuse is at least around 150% the compressor RLA and a thermal mag breaker around 200%, if unit doesn't specify OCPD type then they have to have it high enough for the breaker to not trip during starting.

If a VFD driven compressor that is entirely different game, and the switch wouldn't really need to be HP rated in that situation either.
 
infinity said:
I'm confused, if the MaxOCPD is 30 amps why would a disconnect switch need to be larger than 30 amps?
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In hindsight a dumb question. Sometimes reading sections of the NEC that I am unfamiliar with does that to me, unfortunately.
 
vpower1989 said:
In hindsight a dumb question. Sometimes reading sections of the NEC that I am unfamiliar with does that to me, unfortunately.
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No problem, there are no dumb questions here. I love the fact that you've gone into the NEC to find the answer instead of just posting and waiting for someone else to do the heavy lifting. :cool:
 
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