# Brain Tease: A/C unit branch circuit



## raider1 (Jan 13, 2011)

Situation;

Electrician has run an 8/2 NM cable to feed an A/C condenser unit on the exterior of a home.

The unit arrives and the nameplate states maximum overcurrent protective device 30 amps.

Can the electrician use the 8/2 NM cable with a 30 amp breaker?

Give code sections please.

Chris


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## globe trekker (Jan 13, 2011)

Table E3605.1 in the `06 IRC requires a minimum sized # 10, at 90 degrees celsius,

so a # 8 should be fine, if the NM cable is installed in an approved raceway to the

unit outdoors.  The # 8 cable is allowed an ampacity of 55 per this table.

.


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## chris kennedy (Jan 14, 2011)

raider1 said:
			
		

> Give code sections please.
> 
> Chris


I'll try 250.122(B) for $400 please Alex.


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## Bryan Holland (Jan 14, 2011)

Also don't forget to consider Section 300.9 of the 2008 NEC...


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## jar546 (Jan 14, 2011)

At home without my 2008 NEC or IRC:

Globetrekker- your ampacity for the NM is incorrect.  The 90 deg column is only for derating.  NM is limited to the 60 degree column

The trick part of this question is that 300.? (can't remember the exact #m might be 300.7) does not allow NM in conduit outside or underground.


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## globe trekker (Jan 14, 2011)

Since this teaser question is for a residence, the NEC will not apply ( in most jurisdictions ).  The

IRC is the governing code... yes?

Chris K.,

Article 250.122(B) ( in the `08 NEC ) is for the sizing of equipment grounding conductors.  Is

this what you intended?

Bryan,

Your Article 300.9 in the NEC would correspond to Table E3701.4, "allowable applications", the

column for NM.

.


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## Jobsaver (Jan 14, 2011)

NEC rules in Arkansas. I mention this because it is an applicable thread, and do not want it to deteriorate into an "IRC" only answer. Thanks.

2008NEC: 300.9, 310.8.

NM not rated for wet location.


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## raider1 (Jan 14, 2011)

chris kennedy said:
			
		

> I'll try 250.122(B) for $400 please Alex.


You would be correct, next category please.  



			
				Bryan Holland said:
			
		

> Also don't forget to consider Section 300.9 of the 2008 NEC...


Yes, this section would also be applicable unless you terminated the NM cable at the required disconnecting means at the building wall and transitioned to a suitable wiring method to the unit itself.



			
				globe trekker said:
			
		

> Since this teaser question is for a residence, the NEC will not apply ( in most jurisdictions ).  TheIRC is the governing code... yes?


I will give you the IRC sections.



> Chris K.,Article 250.122(B) ( in the `08 NEC ) is for the sizing of equipment grounding conductors.  Is
> 
> this what you intended?


Yes, the section that he post is correct.

IRC Section E3908.12 (2009 IRC)  last sentence before the Table states;

"Where ungrounded conductors are increased in size, equipment grounding conductors shall be increased proportionally according to the circular mil area of the ungrounded conductors."

In my example the ungrounded conductors have been increased in size therefore the equipment grounding conductor must be increased proportionally.

For a 30 amp overcurrent device the EGC is required to be a #10 the same size as the ungrounded conductors. So if I increase the ungrounded conductors to a #8 the EGC must also be a #8. 8/2 NM cable has a #10 EGC therefore it is a violation. (Albeit a very minor one in my opinion in this instance)



> Bryan,Your Article 300.9 in the NEC would correspond to Table E3701.4, "allowable applications", the
> 
> column for NM.
> 
> .


Also 2009 IRC Section E3802.7 Mirrors section 300.9 in the NEC.

Chris


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## jar546 (Jan 14, 2011)

> IRC Section E3908.12 (2009 IRC)  last sentence before the Table states;"Where ungrounded conductors are increased in size, equipment grounding  conductors shall be increased proportionally according to the circular  mil area of the ungrounded conductors."
> 
> In my example the ungrounded conductors have been increased in size  therefore the equipment grounding conductor must be increased  proportionally.
> 
> For a 30 amp overcurrent device the EGC is required to be a #10 the same  size as the ungrounded conductors. So if I increase the ungrounded  conductors to a #8 the EGC must also be a #8. 8/2 NM cable has a #10 EGC  therefore it is a violation. (Albeit a very minor one in my opinion in  this instance)


Excellent thread.  I overlooked the grounding size and went right to the NM in conduit outside problem per NEC 300.9 and IRC 3802.7


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## chris kennedy (Jan 14, 2011)

jar546 said:
			
		

> Excellent thread.


Agreed, I clicked on this thread at 4AM and knew it had nothing to do with NM ampacity. Coffee kicks in and I get it. That was fun first thing in the morning Chris, thanks, keep em coming.

So the question for you inspectors is, would you tag this install and have the EC turn this into a branch circuit protected at 40A and a fusible disco at the condensing unit or let it slide? I would have no problem with the install Chris poses as IMO the wording of 250.122(B) could use some tweaking.


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## Bryan Holland (Jan 14, 2011)

Part II.

Save for the 250.122(B) and 300.9 violations, would that same 8/2 NM cable be a suitable conductor size if the nameplate of the A/C unit stated the following:

Minimum Circuit Ampacity: 40A

Maximum Overcurrent Device: 60A

Assume a 60A circuit breaker is installed.  Is 8/2 NM cable rated correctly for this appliance?

Code sections?


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## peach (Jan 15, 2011)

NM outside even in conduit.. no.

30 amp with #8?  yeah.. it could be #10.. we see lots of #12 with 15 amp OCPD.

Is 8/2 ok?  I don't know that it wouldn't have to be 8/3 ... that's why I have a super electrical inspector


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## Jobsaver (Jan 15, 2011)

Bryan Holland said:
			
		

> Minimum Circuit Ampacity: 40A. Code sections?


Conducter must be sized for minimum ampacity & 20%. 8/2 is undersized in example (40 x1.2 = 48 amps). No code book handy.


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## chris kennedy (Jan 15, 2011)

Jobsaver said:
			
		

> No code book handy.


When you do get your hands on a book, take a look at 440.6.


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## chris kennedy (Jan 15, 2011)

peach said:
			
		

> that's why I have a super electrical inspector


Thats good to know, I thought you killed him.    I never see him around any of the forums anymore and called him at the holidays, no answer.


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## Jobsaver (Jan 15, 2011)

chris kennedy said:
			
		

> When you do get your hands on a book, take a look at 440.6.


2008 NEC: 440.6 (A) . . . the rated-load current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used to determine the rating or ampacity of the disconnecting means, _the branch-circuit conductors_, . . .

It seems I have been taught wrong again. Thanks Master Kennedy!

Isolating 440.6 (A), I believe the branch-circuit conductor acceptable for the appliance, but the circuit-breaker should be changed to a 40A, but, if the circuit breaker serves as the disconnecting means, it appears the circuit-breaker must be upsized to a 50A with consideration for *440.12(A)(1): Rating and Interrupting Capacity:*

Ampere Rating. The ampere rating shall be at least 115 percent of the nameplate rated-load current or branch-circuit selection current, whichever is greater.

???


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