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#195598 - 08/12/10 09:31 PM calculating voltage drop for 2 RTUs
BigB Offline
Member

Registered: 03/31/04
Posts: 727
Loc: Tucson, AZ USA
I have 2 roof top units, a 2.5 ton and a 3.5 ton. Together the RLA is 35.2A. The run is 120 feet. Using #6 THHN to a double disconnect, I come up with a 1.7% voltage drop at 240V. Am I figuring this right? If I use a 70A breaker will it hold if they both kick on at the same time?

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#195601 - 08/12/10 10:21 PM Re: calculating voltage drop for 2 RTUs [Re: BigB]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Your math looks right on the V/D. You could probably bump the breaker up to 80 or maybe even 90a (using motor rules) but the condenser label is your guide.
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#195603 - 08/12/10 10:54 PM Re: calculating voltage drop for 2 RTUs [Re: gfretwell]
BigB Offline
Member

Registered: 03/31/04
Posts: 727
Loc: Tucson, AZ USA
Well I though about bumping up the breaker but I am not sure how to figure it for 2 condensers. The condensers will each be fused at the double disconnect as per their respective nameplates, so would't I be allowed to add the FLAs together for my 430.52 calculation?

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#195604 - 08/13/10 01:17 AM Re: calculating voltage drop for 2 RTUs [Re: BigB]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
That (430.52) was where I got the 87a or so. I think that is wrong
Actually we should have been using 440.22 (usually you just use the nameplate)
Quote:

440.22 Application and Selection.
(A) Rating or Setting for Individual Motor-Compressor. The motor-compressor branch-circuit short-circuit and ground-fault protective device shall be capable of carrying the starting current of the motor. A protective device having a rating or setting not exceeding 175 percent of the motor-compressor rated-load current or branch-circuit selection current, whichever is greater, shall be permitted, provided that, where the protection specified is not sufficient for the starting current of the motor, the rating or setting shall be permitted to be increased but shall not exceed 225 percent of the motor rated-load current or branch-circuit selection current, whichever is greater.

To apply it to a feeder you use 430.62

Quote:

430.62 Rating or Setting — Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch-circuit short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group.
Where the same rating or setting of the branch-circuit short-circuit and ground-fault protective device is used on two or more of the branch circuits supplied by the feeder, one of the protective devices shall be considered the largest for the above calculations.


That really still will not give us the relief you want for starting both at once since you only get a max of 225% of the largest one. (rounded up to the next breaker)

Since you are using #6 THHN/THWN you can still go with a 70a just by rounding up the 65a in 310.16 75c column so this may have all just been an exercise in code page flipping. wink

BTW don't forget to upsize the EGC when you upsize the feeder for the voltage drop.
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#195643 - 08/16/10 01:13 AM Re: calculating voltage drop for 2 RTUs [Re: gfretwell]
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
In all my days...

I've NEVER seen a RTU that didn't have its own specific breaker.

This is less of a Code question than practicality.

Stacking the loads onto one breaker requires quite a bump on the feeder.

It does not eliminate the need for RTU disconnects.

Who came up with this nightmare?
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Tesla

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#195648 - 08/16/10 11:45 AM Re: calculating voltage drop for 2 RTUs [Re: Tesla]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
I had the impression that the "double disconnect" is a pair of breakers sized to each RTU. I was just talking about a feeder to these breakers. If that is wrong, I agree he has big problems with the install.
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Greg Fretwell

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