I have two residential AC units to wire. A single feeder to a outdoor sub-panel,two breakers and whips to the unit. The unit ratings (name plate on unit) are 27min 40 max, and 34min 60 max. The wiring and breaker for the feeder is in question. Is it 34 x 1.25 + 27 = 69.5 or 60 x 1.25 + 27 =102. There is a existing #2 AL SER in place on a 100amp breaker.
AC Condensing units
- Thread starter mbsi
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mdshunk
Senior Member
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- Right here.
Re: AC Condensing units
The 34 x 1.25 + 27 = 69.5 would be the math to use.
The 34 x 1.25 + 27 = 69.5 would be the math to use.
- Location
- Massachusetts
Re: AC Condensing units
This is not the same as figuring individual motor loads.
This type of HVAC equipment the NEC calls Multimotor and Combination-Load Equipment.
Take a look at 440.4(B)
You can forget about the 1.25 when feeding equipment like this.
The MCA (minimum circuit ampacity) stated on the units already have the 1.25 figured in.
27 + 34 = 61 amps minimum feeder capacity needed.
Unit 1 with a MCA of 27 could be wired with 10 AWG and a 40 amp breaker
Unit 2 with a MCA of 34 could also be wired with 10 AWG and a 60 amp breaker.
The 100 amp breaker and the 2 AL feeding the subpanel will be fine unless there are other loads you have not spoken of.
This is not the same as figuring individual motor loads.
This type of HVAC equipment the NEC calls Multimotor and Combination-Load Equipment.
Take a look at 440.4(B)
You can forget about the 1.25 when feeding equipment like this.
The MCA (minimum circuit ampacity) stated on the units already have the 1.25 figured in.
27 + 34 = 61 amps minimum feeder capacity needed.
Unit 1 with a MCA of 27 could be wired with 10 AWG and a 40 amp breaker
Unit 2 with a MCA of 34 could also be wired with 10 AWG and a 60 amp breaker.
The 100 amp breaker and the 2 AL feeding the subpanel will be fine unless there are other loads you have not spoken of.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: AC Condensing units
If the unit were wired with romex could you still use the #10 AWG, or would you need to use the 60 degree column in table 310.16?
- Location
- Massachusetts
Re: AC Condensing units
as I seldom wire HVAC equipment with NM.
Good point, one that I often forget
as I seldom wire HVAC equipment with NM.stew
Senior Member
- Location
- federal way,washington
Re: AC Condensing units
We use nm all the time for this and if in a home run it as normal and when it emerges we run it out a pvc lb and pc to the disc. then a flex non metallic sealtite to the unit less than 6 ft in lengdth.works well for us and eleiminates the need for pipe inside the house. We run everything in 3/4. Where in the code does it say that the 25% is already figured in or is this just what an experienced guru such as an i wire just knows?
[ June 17, 2005, 10:10 PM: Message edited by: stew ]
We use nm all the time for this and if in a home run it as normal and when it emerges we run it out a pvc lb and pc to the disc. then a flex non metallic sealtite to the unit less than 6 ft in lengdth.works well for us and eleiminates the need for pipe inside the house. We run everything in 3/4. Where in the code does it say that the 25% is already figured in or is this just what an experienced guru such as an i wire just knows?
[ June 17, 2005, 10:10 PM: Message edited by: stew ]
john m. caloggero
Senior Member
Re: AC Condensing units
The nameplate minimum ampacity for the compressor branch circuit includes the 25% required by 2005 NEC section 440.32. The 27A and the 34A are the minimum ampacity of the conductors needed to supply the compressors, No. 10awg, cu for the 27A compressor and No. 8awg, cu for the 34A compressor. However if the OCD and controller terminations are rated 75 degrees C, No. 10awg, 75 or 90 degree conductors can be used for the 34A compressor.
It seems that the 100A breaker is used as a feeder, in that case, section 440.32 is applicable and results in 27A + 34A + (.24x34A) = 69.5A which allows a minimum of No. 4awg, cu as the feeder provided the terminations are rated for 75 degrees C.
The No. 2 Al conductors have sufficient ampacity to carry the load in accordance with the NEC. however, the 100 amp breaker suppling the No. 2 Al is a violation, it must not be greater than 90 amperes for 75 degree terminations.
The nameplate minimum ampacity for the compressor branch circuit includes the 25% required by 2005 NEC section 440.32. The 27A and the 34A are the minimum ampacity of the conductors needed to supply the compressors, No. 10awg, cu for the 27A compressor and No. 8awg, cu for the 34A compressor. However if the OCD and controller terminations are rated 75 degrees C, No. 10awg, 75 or 90 degree conductors can be used for the 34A compressor.
It seems that the 100A breaker is used as a feeder, in that case, section 440.32 is applicable and results in 27A + 34A + (.24x34A) = 69.5A which allows a minimum of No. 4awg, cu as the feeder provided the terminations are rated for 75 degrees C.
The No. 2 Al conductors have sufficient ampacity to carry the load in accordance with the NEC. however, the 100 amp breaker suppling the No. 2 Al is a violation, it must not be greater than 90 amperes for 75 degree terminations.
pierre
Senior Member
- Location
- Westchester County, New York
Re: AC Condensing units
Up to 10 AWG, the NM cable ampacity is not affected by the 60 degree column because of 240.4(D).
Up to 10 AWG, the NM cable ampacity is not affected by the 60 degree column because of 240.4(D).
stew
Senior Member
- Location
- federal way,washington
Re: AC Condensing units
pierre tell me what 240.4D has to do with this? 334.80 specifically says that the ampacities of nm,nmc and nms shal be determined in accordance with 310.15. The ampacity shal be in accordance with the 60 deg table. I cannot see where 240.4 d pertains and where does the 10 gage limitaion come from?
pierre tell me what 240.4D has to do with this? 334.80 specifically says that the ampacities of nm,nmc and nms shal be determined in accordance with 310.15. The ampacity shal be in accordance with the 60 deg table. I cannot see where 240.4 d pertains and where does the 10 gage limitaion come from?
pierre
Senior Member
- Location
- Westchester County, New York
Re: AC Condensing units
Stew
240.4(D) limits the overcurrent protection of small conductors, so the 60 degree column for those size conductors will not affect the size of the overcurrent device, as 240.4(D) already does that.
Stew
240.4(D) limits the overcurrent protection of small conductors, so the 60 degree column for those size conductors will not affect the size of the overcurrent device, as 240.4(D) already does that.
pierre
Senior Member
- Location
- Westchester County, New York
Re: AC Condensing units
Stew
240.4(D) limits the overcurrent protection of small conductors, so the 60 degree column for those size conductors will not affect the size of the overcurrent device, as 240.4(D) already does that.
Look at the 60 degree column ampacity values, then take a look at the ampacity values required in 240.4(D).
Stew
240.4(D) limits the overcurrent protection of small conductors, so the 60 degree column for those size conductors will not affect the size of the overcurrent device, as 240.4(D) already does that.
Look at the 60 degree column ampacity values, then take a look at the ampacity values required in 240.4(D).
- Location
- Massachusetts
Re: AC Condensing units
I think that we should start at where the 'extra' 25% is required for HVAC units.
One place would be 440.32
The same basic requirement is also in 440.33 Motor-Compressor(s) With or Without Additional Motor Loads.
Neither of those sections apply to the HVAC units that we normally install, the section that does apply is 440.35.
You will also notice it references 440.4(B) which basically says the manufacturer will be responsible for the calculations and mark the unit with the minimum supply circuit conductor ampacity, and the maximum rating of the branch-circuit short-circuit and ground-fault protective device.
A unit marked with an minimum supply circuit conductor ampacity of 20 amps and a maximum branch-circuit short-circuit and ground-fault protective device of 35 could be wired with 14/2 NM and a 35 amp breaker.
NM is not rated for wet locations, outdoor raceways are wet locations.
First off that is code geek, not guru.
I think that we should start at where the 'extra' 25% is required for HVAC units.
One place would be 440.32
Notice that section only applies to Single Motor-Compressor.
The same basic requirement is also in 440.33 Motor-Compressor(s) With or Without Additional Motor Loads.
Neither of those sections apply to the HVAC units that we normally install, the section that does apply is 440.35.
In this section you will notice the absence of the 25% extra required for single compressors or Motor-Compressor(s) With or Without Additional Motor Loads.
You will also notice it references 440.4(B) which basically says the manufacturer will be responsible for the calculations and mark the unit with the minimum supply circuit conductor ampacity, and the maximum rating of the branch-circuit short-circuit and ground-fault protective device.
A unit marked with an minimum supply circuit conductor ampacity of 20 amps and a maximum branch-circuit short-circuit and ground-fault protective device of 35 could be wired with 14/2 NM and a 35 amp breaker.
I have some bad news for you here, if you are running the NM in the PVC outside you have a violation.
NM is not rated for wet locations, outdoor raceways are wet locations.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Re: AC Condensing units
If we're talking about A/C's, then 240.4(D) dismisses itself from the discussion. It no longer exists.
So 334.80 is the code-mechanism behind following the 60? ampacity. Otherwise, you're back to a 90? rated cable, why use the 60? ampacities? Because 334.80 requires it.
Pierre, you've got the cart in front of the horse, IMO.
If we're talking about A/C's, then 240.4(D) dismisses itself from the discussion. It no longer exists.
So 334.80 is the code-mechanism behind following the 60? ampacity. Otherwise, you're back to a 90? rated cable, why use the 60? ampacities? Because 334.80 requires it.
stew
Senior Member
- Location
- federal way,washington
Re: AC Condensing units
ill be danged! Never thot about wet location requirement. Typically thes runs are very short and we havent been called on it yet. yet yet thx
ill be danged! Never thot about wet location requirement. Typically thes runs are very short and we havent been called on it yet. yet yet thx
- Location
- Massachusetts
Re: AC Condensing units
But it is IMO a NEC violation. :roll:
Stew I'll be honest here, I think this is a non-issue, I have not seen any problems from doing what you describe and it is common around here.
But it is IMO a NEC violation. :roll:
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Re: AC Condensing units
I'm with George on this one. 334.80 makes no exception for an AC unit to be wired with type NM cable using any thing other than the 60 degree column in 310.16 even if we need to derate it. 60 degree ampacity is the maximum permitted.
- Location
- Illinois
- Occupation
- retired electrician
Re: AC Condensing units
Pierre,
240.4(D) has nothing to do with the ampacity of a conductor, it just limits the size of the OCPD that can be used with the conductors that are covered by that section. While the size of the OCPD and the ampacity of the conductor are related, they are not the same. As George pointed out, 240.4(D) also does not apply in air conditioner installations.
Don
Pierre,
240.4(D) has nothing to do with the ampacity of a conductor, it just limits the size of the OCPD that can be used with the conductors that are covered by that section. While the size of the OCPD and the ampacity of the conductor are related, they are not the same. As George pointed out, 240.4(D) also does not apply in air conditioner installations.
Don
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