Air Conditioning Electrical/Breaker Sizing

Hvac11

Member
Location
Iowa
I had a question regarding derating NM or UF wire for being located up in an attic as I have an application where around 5 foot is in the attic. I thought per one of Mikes videos that I could derate the NM wire starting at 90 C column but got the below email from Southwire support (blue text). Maybe this was only for derating THHN type wire as I would have to go back and watch the video. Also, do you have to derate if less than 10' or 10% of total length is in the attic so you can use the higher ampacity. See below for Southwire email on derating NM-B.


NM-B Cables are limited to the 60°C ampacity - period. The individual conductors cannot be used independently of the completed NM-B Cable. The conductor insulation is rated for 90°C max temp however that simply allows the conductors to be terminated in light fixtures that require 90°C conductors. You cannot de-rate the ampacity for NM-B Cable starting with the 90°C ampacity like you can for individual conductors. If you need to de-rate the ampacity for ambient temperature, you start with the 60°C ampacity.

I also got the following response below regarding NM vs UF to be ran from inside home breaker panel to outside Air Conditioning Disconnect. I see quite a bit of NM ran straight into the AC Disconnect where I live, but maybe UF should always be ran. I wasn't sure if you just go straight thru the wall right into disconnect there would only be a few inches or less behind the outside disconnect. I guess as they state that anything outside, even a few inches.

NM-B Cable is a dry location only product and cannot be used outdoors (even in conduit). Use UF-B Cable instead.


Also, Can you run NM-B or UF in liquid seal tight for a short distance( 6') along with should you use non metallic liquid tight vs metallic outside.


Please see the attachment for the new A/C /Heat pump that is being installed. In my mind I would use 10-2 with ground with a 40 Amp circuit breaker. But if I have to derate the wire since some is in the attic ( 5') and I go off the 90 C column, which I am not sure if that is correct I would have 40A for 10 awg at 90C X .71 factor for 140 F attic temp= 28.4 Amps. Would I then still keep my 40 Amp cirucit breaker as I know this is required per manufacturer due to startup up inrush amps.

The last thing what if I had an electric wall heater rated at 16.7 Amps and was using NM-B wiring and it was up in attic space. Per 310.15 chart 12 awg is good for 20 Amps or I could go 10 for 30 Amps with 10 awg. On this wall heaters alot of time they just list the amps and no mca or mop.

Sorry for so many questions as I appreciate the help. Thanks!



Heat Pump Electrical Requirements.jpg
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
"You cannot de-rate the ampacity for NM-B Cable starting with the 90°C ampacity like you can for individual conductors"

I disagree with that statement

I agree with the op-- you can de-rate nm cable from the 90C column but the final ampacity cannot be larger than the 60C column
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Electrical Contractor
If you lived in NC they allow nm leeved to an ac unit a long a it i not longer than 6' however the nec does not allow nm cable outide at all.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
334.80 Ampacity ... The 90°C (194°F) rating shall be permitted to be used for ampacity adjustment and correction calculations, provided the final calculated ampacity does not exceed that of a 60°C (140°F) rated conductor. ...
 

infinity

Moderator
Staff member
Location
New Jersey
Depending on the length of the run your derating for the 5' in the attic can probably be ignored. {310.15(A)(2)Exception}
 

Hvac11

Member
Location
Iowa
So from what you guys are saying I can derate NM-B wiring using the 90 C column. Not sure why a rep with Southwire told me I could not do that and if I did derate that I had to start the derate at 60C. If I can truly derate starting at 90C than for #10 wire we would get an ampacity of 40A for 10 awg in the 90C column. So I would take 40A X .71 factor for 140 F attic temp= 28.4 Amps for my new ampacity using NM-B 10 awg wire. With that being said that still meets my MCA of 23.5 per attachment showing the requirements for the condensing unit. I can't believe it says 12 awg wire is ok per the manufacturer and states romex for 23.5 amps. If you open attachment maybe I am missing something.

Also someone had noted that if I have only 5' of the wire in the attic that I may not need to derate anyway. Is that true being that it is less than 10' or less than 10% of the total wire length?

Also, I would obviously still use my 40 MOP required per label of condensing unit. Is that correct? So I would have 10/2 with grd, with a 40 A breaker to meet code requirements? Does most of you when running new wire truly run the 10 awg with a 40 amp breaker for instance if you were building a new house or would most people run #8 awg NM or UF with the 40 Amp breaker to cover themselves?

I also got some feedback from you guys saying that NM should not go outside even though it only goes straight into AC disconnect (only 6 inch in the electrical box). If that is the case I will use UF wiring the complete distance from the inside electrical box to the outside disconnect.

Thanks again for the help.







Heat Pump Electrical Requirements.jpg
 
I also got some feedback from you guys saying that NM should not go outside even though it only goes straight into AC disconnect (only 6 inch in the electrical box). If that is the case I will use UF wiring the complete distance from the inside electrical box to the outside disconnect.
You can run NM into the back of an exterior mounted disconnect. What you cant do is run it in conduit outside
 

LarryFine

Master Electrician Electric Contractor Richmond VA
With that being said that still meets my MCA of 23.5 per attachment showing the requirements for the condensing unit. I can't believe it says 12 awg wire is ok per the manufacturer and states romex for 23.5 amps. If you open attachment maybe I am missing something.
See NEC 240.4(D), then 240.4(G), then the appropriate parts of 440.

Also, I would obviously still use my 40 MOP required per label of condensing unit. Is that correct? So I would have 10/2 with grd, with a 40 A breaker to meet code requirements? Does most of you when running new wire truly run the 10 awg with a 40 amp breaker for instance if you were building a new house or would most people run #8 awg NM or UF with the 40 Amp breaker to cover themselves?
You can still use #12; some might use #10; none of us would use #8.

I also got some feedback from you guys saying that NM should not go outside even though it only goes straight into AC disconnect (only 6 inch in the electrical box). If that is the case I will use UF wiring the complete distance from the inside electrical box to the outside disconnect.
We normally extend NM into wall-mounted disconnects with no issues.
 

Hvac11

Member
Location
Iowa
The last thing that I had listed in my original thread, was what if I had an electric wall heater rated at 16.7 Amps and was using NM-B wiring. Per 310.15 chart 12 awg is good for 20 Amps or I could go 10 awg for 30 Amps. On these wall heaters I get alot of the time they just list the amps and no MCA or MOP like on air conditioning system. Since this is a continuous load ( Resistance load) do you take the 16.7 amps X 1.25 to size the breaker. which comes up to 20.9 Amps. Is this correct and what breaker size. Thanks!
 
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