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06-13-2021, 06:27 PM
(This post was last modified: 06-13-2021, 08:16 PM by bottd.)
My day job is commercial HVAC, zero installs...all repair and maintenance. I have been ask to help with a charity install of a Mitsubishi VRF system this coming weekend. The equipment is already bought and sitting on site, and I do not know if permits are being pulled or not. But I want to do it right...
The MCA on the system is 22.1 amps, its a 208/230 machine and MOB is 25 amps. Not sure if we will be using romex or bx cable from the breaker panel to the disconnect.
Do I need 12 gauge or 10 gauge wire.
Don't know the length of the run, but it is a small house and we will be going through the crawl space. Thanks for any help provided.
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Someone asked this exact question on Mike Holt forums last year and I don't understand the answers.
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(06-13-2021, 09:21 PM)EricU Wrote: Someone asked this exact question on Mike Holt forums last year and I don't understand the answers.
I know generally speaking 12 gauge for 20 amps, 10 gauge for 30 amps. With a minimum circuit ampacity of 22.1 amps that would put me into the 10 gauge but I also know there are some exceptions for HVAC equipment but not being an installer, nor familiar with residential installs hoping someone here would be able to shed some light on it for me.
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How long is the run? 10 gauge isn't "that" much more for a single run, and is not the wrong answer... right?
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The a/c manufacturers whine a lot about the ampacity they are required to list. Except then they justify the overload protection, which is more. I certainly would use 10awg, all copper has gone up quite a bit recently, but I don't think the savings is enough to cut corners. But I'm not paying for it in this instance.
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I agree if I was buying the wire from scratch it would make sense to just go with the 10 gauge but I have at least 100 feet of 12/2 in both romex and bx sitting on a shelf in my shop and was hoping to use it but at this point will probably buy the 10 gauge. Thanks.
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(06-14-2021, 03:50 PM)bottd Wrote: I agree if I was buying the wire from scratch it would make sense to just go with the 10 gauge but I have at least 100 feet of 12/2 in both romex and bx sitting on a shelf in my shop and was hoping to use it but at this point will probably buy the 10 gauge. Thanks.
For what its worth - 10/2 is what we went with, really no choice. Only needed 25 feet and I got it at Lowes by the foot at a little over 2.00 a foot. Mitsubishi mini split with 2 indoor units, 265 microns in about an hour on the pump. Released the gas and working great. Electrical inspection was on Monday, no problems.
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06-22-2021, 02:22 PM
(This post was last modified: 06-22-2021, 02:23 PM by TDKPE.)
(06-13-2021, 09:41 PM)bottd Wrote: I know generally speaking 12 gauge for 20 amps, 10 gauge for 30 amps. With a minimum circuit ampacity of 22.1 amps that would put me into the 10 gauge but I also know there are some exceptions for HVAC equipment but not being an installer, nor familiar with residential installs hoping someone here would be able to shed some light on it for me.
I guess I'm too late, but the exceptions you're probably referring to are in Article 440, and allow an "oversized" breaker or fuse value than the wire gauge would normally indicate. It's allowed to be larger so that if it short-cycles (for instance) or has an extended start time (with massive current draw) due to low voltage conditons, the breaker won't pop. The unit's internal overload will open instead, and then reset itself. If the breaker popped, the unit would never try to restart. There should be a maximum CB or fuse size on its data sheet, too.
That often gets people who see a breaker that's 'too big' for the wiring, but it's safe and legal for most installations.
Induction motor circuits with proper motor controls with O/L protection similarly protect the wiring via the O/L, and the breaker or fuse only protects against short-circuits and ground-faults.
Tom
“This place smells like that odd combination of flop sweat, hopelessness, aaaand feet"
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(06-22-2021, 02:22 PM)TDKPE Wrote: I guess I'm too late, but the exceptions you're probably referring to are in Article 440, and allow an "oversized" breaker or fuse value than the wire gauge would normally indicate. It's allowed to be larger so that if it short-cycles (for instance) or has an extended start time (with massive current draw) due to low voltage conditons, the breaker won't pop. The unit's internal overload will open instead, and then reset itself. If the breaker popped, the unit would never try to restart. There should be a maximum CB or fuse size on its data sheet, too.
That often gets people who see a breaker that's 'too big' for the wiring, but it's safe and legal for most installations.
Induction motor circuits with proper motor controls with O/L protection similarly protect the wiring via the O/L, and the breaker or fuse only protects against short-circuits and ground-faults. You are right, the maximum over current was 25...it was a challenge to find a GE 25 amp circuit breaker.
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(06-22-2021, 03:10 PM)bottd Wrote: You are right, the maximum over current was 25...it was a challenge to find a GE 25 amp circuit breaker.
Oh, missed it (MOP perhaps?). Sorry.
Tom
“This place smells like that odd combination of flop sweat, hopelessness, aaaand feet"
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