# NM to AC unit



## Awg-Dawg (Jan 23, 2007)

Does anyone know why NM can’t be used in this article?

It’s from the EC magazine. 




Conductor protection
We are having an issue with the AHJ over a residential A/C compressor. The inspector is saying we cannot use a 40-amp (A) OCPD on No. 10 (NM) Romex. The compressor nameplate states the minimum circuit ampacity is 21.6A, with a maximum fuse or breaker size of 40A. We believe we are well within the NEC. Can you please help?
The electrical inspector is correct. In general, a 10 AWG copper conductor requires overcurrent protection that does not exceed 30A. As seen in your question, the nameplate on the compressor permits overcurrent protection not to exceed 40A. Using type NM cable, this would require an 8 AWG copper conductor that is rated at 40A. The general rule for the protection of conductors in Section 240.4 requires protection in accordance with the conductor ampacity in Section 310.15, unless otherwise permitted in 240.4(A) through (G). Section 240.4(D) provides requirements for small conductors. The maximum overcurrent protection for 10 AWG copper is 30A unless 240.4(E) or (G) apply. 240.4(E) provides a list of six tap conductor requirements that do not apply in this scenario. Section 240.4(G) provides a table for the Code user to reference requirements in other articles for specific conductor applications.

Requirements in Article 440 are referenced but do not apply in this scenario. Residential air conditioning compressors are located outdoors. Type NM cable is not permitted to be installed outdoors. Section 334.12(B)(4) prohibits type NM cable installation in damp or wet locations. See Section 300.9, which clarifies that raceways installed outdoors above grade are considered to be a wet location. This means that type NM cable cannot be installed in a raceway to a compressor installed outdoors.


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## Roger123 (Sep 23, 2007)

I think the exceptions of Article 440 to allow the #10 awg on a 40 amp breaker were not addressed because the NM conductors were outside.


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## HackWork (Oct 2, 2009)

This has been a silly argument for a long time. Basically, some people will say that you can’t bring romex into any type of box or enclosure mounted outside, even if you’re coming through the back. I understand why they would say that if they want to be super technical and nerdy, but realistically it’s not an issue and the code should make a provision allowing it. But the idiots who make the code would never allow something to be clarified like that.


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## Roger123 (Sep 23, 2007)

HackWork said:


> This has been a silly argument for a long time. Basically, some people will say that you can’t bring romex into any type of box or enclosure mounted outside, even if you’re coming through the back. I understand why they would say that if they want to be super technical and nerdy, but realistically it’s not an issue and the code should make a provision allowing it.




I agree, I also think it is not an issue.


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## Awg-Dawg (Jan 23, 2007)

I get the wet location part, it’s just the authormakes it sound as though you can’t use it all.


At least that’s how I read it.


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## Roger123 (Sep 23, 2007)

Awg-Dawg said:


> I get the wet location part, it’s just the authormakes it sound as though you can’t use it all.
> 
> 
> At least that’s how I read it.


Yes, I agree, the author never addresses the wiring size and breaker because he only addresses the wet location.


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## nrp3 (Jan 24, 2009)

I had read that too and scratched my head afterwards.


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## telsa (May 22, 2015)

The first part is a waste of ink.

Romex can't be used inside of Greenfield to reach an A/C compressor.

It has to stop at the NEMA3R disconnect.

From that point you must whip out with THWN or THWN-2 inside Greenfield or its equivalent.

What a waste of ink.


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## HertzHound (Jan 22, 2019)

Bringing up the Romex in a wet location just muddies the water. Granted the OP said it was residential, but he never mentioned the wiring method. Let’s say it’s conduit with THHN back to the panel, and we don’t care about the disconnect to the condenser. The point I think that was trying to be made, was that 440 does not exempt 240.4(D) when it comes to small conductors. That’s the part I’ve never heard before. I can’t believe I made it this far with all the books, apprenticeship, magazines and message boards, and never heard this before.


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## HertzHound (Jan 22, 2019)

So which one is it? In addition to or amended by? Do we get to pick and choose?



> Part III.Branch-Circuit Short-Circuit and Ground-Fault Protection
> 440.21 General.
> The provisions of Part III specify devices intended to protect the branch-circuit conductors, control apparatus, and motors in circuits supplying hermetic refrigerant motor-compressors against overcurrent due to short circuits and ground faults. They are in addition to or amendatory of the provisions of Article 240.


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## MTW (Aug 28, 2013)

The sad part is that they printed that article. It goes to show who is doing the proofreading over there.


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## telsa (May 22, 2015)

And how. 

The writer got totally off track.

The last bit sounds like someone tipped him off.


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## Roger123 (Sep 23, 2007)

HertzHound said:


> The point I think that was trying to be made, was that 440 does not exempt 240.4(D) when it comes to small conductors.


I agree, 440 does not exempt 240.4(D), but there are exceptions in 240.4(D) and the exceptions are listed in Table 240.4(G). One of the first listed exceptions is air -conditioning equip. 

So, in the OP case #10 AWG is good for the max current and a 40 amp breaker can be use for starting the unit.


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## Dennis Alwon (May 9, 2009)

The answer is that nm can be used to feed the disconnect. Type nm cable is not allowed to be used in a raceway in wet locations. A ac disconnect is not a raceway so that rule does not apply. If that were true then you would never be allowed to have an exterior panel with nm cable.

The long and short of it is as many have stated. The first part would apply if the nm was installed in a raceway. If you used a method that was rated 75C then #12 wire could be used for this install but with nm then #10 has to be installed


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## sbrn33 (Mar 15, 2007)

I have probably done a thousand AC whips with romex. The only failure I have ever seen is when the sealtite rusts out and even then it was still working. 
Some of these codes are just plain stupid as is the writer of that article. If it call for a 40 it gets a 40. at 22 amps he could have actually used 12.


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## Jlarson (Jun 28, 2009)

I don't see why, legally, the #10 NM couldn't be used to th disco for that unit.





sbrn33 said:


> I have probably done a thousand AC whips with romex. The only failure I have ever seen is when the sealtite rusts out and even then it was still working.
> Some of these codes are just plain stupid as is the writer of that article. If it call for a 40 it gets a 40. at 22 amps he could have actually used 12.



Yeah every RTU out here on the houses is done like that, SE or NM up a piece of EMT and Sealtight to the unit. 



I just redid mine a few weeks ago since the Sealtight was completely rotted and it was becoming a major roof leak :vs_laugh:


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## scotch (Oct 17, 2013)

Only comment I remember was when on one job romex type NMD was used in seatlite on rooftop to the AC unit ; and the inspector didn't like that the individual wires had no markings for approval when the jacket was peeled back to terminate on the blocks .


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## Easy (Oct 18, 2017)

HackWork said:


> This has been a silly argument for a long time. Basically, some people will say that you can’t bring romex into any type of box or enclosure mounted outside, even if you’re coming through the back. I understand why they would say that if they want to be super technical and nerdy, but realistically it’s not an issue and the code should make a provision allowing it. But the idiots who make the code would never allow something to be clarified like that.


I agree it is "nerdy". I have not wired any new homes lately but every residential AC disconnect I have come across has NM cable penetrating the outside wall and into the surface mounted disco. It is acceptable to have recessed boxes on outside walls for switches and plugs fed with nm cable. 
This is crazy logic that you can't come into the back of a surface mounted box or rain-tight enclosure. The last AC I fed with a flush mounted plastic box and a bell extension outside then EMT with rain tight connectors up into the bottom of the disco and the inspector was fine with that.


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## Easy (Oct 18, 2017)

Awg-Dawg said:


> I get the wet location part, it’s just the authormakes it sound as though you can’t use it all.
> 
> 
> At least that’s how I read it.


Perhaps it has to do with the fact that the disco has fusses. It seems to me that you mentioned about this in another post. 
"If you use a fused disconnect, the conductors from the breaker to the disconnect have to be sized to the breaker."


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## sbrn33 (Mar 15, 2007)

Honestly if every outside conduit is considered a wet location and need wet location rated wire, why do we need wet location conduit and raintite fittings? Why not just use flex?


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## Awg-Dawg (Jan 23, 2007)

Easy said:


> Perhaps it has to do with the fact that the disco has fusses. It seems to me that you mentioned about this in another post.
> "If you use a fused disconnect, the conductors from the breaker to the disconnect have to be sized to the breaker."


 There’s a discussion over at MH. the jist of it is that the small conductor part isn’t mentioned in 440 for nameplates. ( I think that's the convo)

I haven’t read any of it, I just noticed it while skimming thru today.


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## Easy (Oct 18, 2017)

sbrn33 said:


> Honestly if every outside conduit is considered a wet location and need wet location rated wire, why do we need wet location conduit and raintite fittings? Why not just use flex?


Seal tight Flex or the pre-made "Junk" flex whips with plastic fittings are fine.
Save money and time.


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## MTW (Aug 28, 2013)

Awg-Dawg said:


> There’s a discussion *over at MH*. the jist of it is that the small conductor part isn’t mentioned in 440 for nameplates. ( I think that's the convo)
> 
> I haven’t read any of it, I just noticed it while skimming thru today.


In that case, the discussion is by a bunch of people who haven't done electrical work in years, or have never done it at all.


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## telsa (May 22, 2015)

sbrn33 said:


> Honestly* if every outside conduit is considered a wet location and need wet location rated wire*, why do we need wet location conduit and raintite fittings? *Why not just use flex*?


Out here that's the universal practice in Residential -- to include my own home and neighborhood. Romex inside Greenfield, if spotted, would bring a Correction Notice. 

However, Commercial and Industrial standards dictate Sealtite -- it's more robust.

Folks take care of their own home. 

Employees are not gentle around RTU and such.

&&&

I prep all of my whips back in the shop -- if at all possible. I use a fishing rod ( LSD ) to poke through Greenfield// Sealtite... leaving the necessary extra make-up lengths at both ends, of course. These are merely pulled back and taped so that they don't shift during their trip to the site -- usually the roof.

Roof work demands that you do as much prep as possible at ground level -- so you're not hauling tools up and down -- needlessly. 

This means that the terminating hardware is already installed back at the shop.

If my whip is too long -- looks ugly -- it's no problem to trim it back with a Roto-Split with the conductors still inside it. ( Sealtite or Greenfield ) 

Tesla tip of the week: Use Sealtite 45 instead of 90. BIG savings in labor -- and usually look great as installed. Many j-men have never seen one in their career, don't even know to order them... that includes the PM and the office.


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## Easy (Oct 18, 2017)

Dennis Alwon said:


> The answer is that nm can be used to feed the disconnect. Type nm cable is not allowed to be used in a raceway in wet locations. A ac disconnect is not a raceway so that rule does not apply. If that were true then you would never be allowed to have an exterior panel with nm cable.
> 
> The long and short of it is as many have stated. The first part would apply if the nm was installed in a raceway. If you used a method that was rated 75C then #12 wire could be used for this install but with nm then #10 has to be installed



Your correct.. according to what I found here. 
While the rating of the conductors is 90 C the overall ampacity is based on the cable jacket or 60 deg C" .. I think that is for older NM cable.

Non-metallic sheathed NM-B cable is used in normally dry installations in residential wiring, as branch circuits for outlets, lighting and other residential loads. Its applications are outlined in NEC 2008 and NEC 2011 Article 334. NM-B is rated at 600 volts and although it is rated at a conductor temperature of 90°C dry , its ampacity is limited to 60°C according to the NEC 2008 and NEC 2011. It may be fished through masonry or tile walls, where not exposed or subject to excessive moisture or dampness. NM-B cables are LEAD FREE and RoHS compliant.


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## Dennis Alwon (May 9, 2009)

Easy said:


> Your correct.. according to what I found here.
> While the rating of the conductors is 90 C the overall ampacity is based on the cable jacket or 60 deg C" .. I think that is for older NM cable.
> 
> Non-metallic sheathed NM-B cable is used in normally dry installations in residential wiring, as branch circuits for outlets, lighting and other residential loads. Its applications are outlined in NEC 2008 and NEC 2011 Article 334. NM-B is rated at 600 volts and although it is rated at a conductor temperature of 90°C dry , its ampacity is limited to 60°C according to the NEC 2008 and NEC 2011. It may be fished through masonry or tile walls, where not exposed or subject to excessive moisture or dampness. NM-B cables are LEAD FREE and RoHS compliant.


NM is still rated 60C in the 2020 NEC.


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## btharmy2 (Mar 11, 2017)

North Carolina allows a 6' whip of NM inside sealtite for condensers.


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## HertzHound (Jan 22, 2019)

So what happens in other parts of the code where small conductors are allowed to be used above their ampacity? Like class 1 taps off of motor circuits?

The explanation I just read in the Handbook was that 15,20 and 30a breakers have a short circuit rating that will not protect the insulation if the wire was sized higher than the sixty degree column. That was why they have the limitation. 

But it also say that this whole section is amended by other parts of the code. And specifically calls out 440. I don’t see how someone can enforce not simply sizing the wire by the minimum circuit amps and allowing #10 on a 40a breaker for a condenser.


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