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#210577 - 07/11/13 10:33 AM AC wiring and 250.122  
BigB  Offline
Member
Joined: Mar 2004
Posts: 721
Tucson, AZ USA
We are seeing a lot of "mini split" AC units. As an example, let's say we have a mini split with an MCA of 11 amps, and a maximum fuse or circuit breaker of 25 amps per the nameplate. The wiring method is Romex. Wiring the unit with 14/2 would satisfy the MCA, however I believe it would violate 250.122 due to the 25 amp OCPD. To comply with 250.122, using Romex it seems we would need 10/2, no? Am I missing something?

Last edited by BigB; 07/11/13 10:34 AM.

2017 / 2014 NEC & Related Books and Study Guides

#210579 - 07/11/13 02:43 PM Re: AC wiring and 250.122 [Re: BigB]  
HotLine1  Offline


Member
Joined: Apr 2002
Posts: 6,885
Brick, NJ USA
You have to determine the OCP that you are installing to determine the EGC for the circuit.

Using the nameplate MAX of 25 OCP would require #12. This is based on 250.122 table in '11 NEC as I read it.



John

#210581 - 07/11/13 06:58 PM Re: AC wiring and 250.122 [Re: BigB]  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
Originally Posted by BigB
We are seeing a lot of "mini split" AC units. As an example, let's say we have a mini split with an MCA of 11 amps, and a maximum fuse or circuit breaker of 25 amps per the nameplate. The wiring method is Romex. Wiring the unit with 14/2 would satisfy the MCA, however I believe it would violate 250.122 due to the 25 amp OCPD. To comply with 250.122, using Romex it seems we would need 10/2, no? Am I missing something?


BigB...The first sentence of 250.122(A) says that in no case shall the EGC be required to be larger than the circuit conductors supplying the equipment...


#210582 - 07/11/13 07:01 PM Re: AC wiring and 250.122 [Re: HotLine1]  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
Originally Posted by HotLine1
You have to determine the OCP that you are installing to determine the EGC for the circuit.

Using the nameplate MAX of 25 OCP would require #12. This is based on 250.122 table in '11 NEC as I read it.



I use the Minimum Circuit Ampacity to size my branch circuit conductors for an AC unit...


#210585 - 07/11/13 10:17 PM Re: AC wiring and 250.122 [Re: BigB]  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Please note that the 2014 edition will have changes to this requirement in air conditioning article.


#210586 - 07/11/13 10:33 PM Re: AC wiring and 250.122 [Re: shortcircuit]  
BigB  Offline
Member
Joined: Mar 2004
Posts: 721
Tucson, AZ USA
Ah so I was missing something. The scary part is I am pretty sure I knew that at one time. Getting old is no fun.

Last edited by BigB; 07/11/13 10:33 PM.

#210587 - 07/11/13 10:52 PM Re: AC wiring and 250.122 [Re: BigB]  
HotLine1  Offline


Member
Joined: Apr 2002
Posts: 6,885
Brick, NJ USA
Perhaps a little clearer....

BigB example is 14/2NM, but he said 25 amp OCP, using the nameplate max figure.

Using the max OCP, a #12 EGC would be required, so using NM he has to use 12/2.

Yes, the EGC does not have to be larger than the circuit conductors, 'oversize' EGC to my knowledge is not available.

IMHO, I believe that he did not read Table 250.122 where the amperage col. jumps from 20 amps for #12, and 60 amps for #10.




John

#210588 - 07/11/13 10:56 PM Re: AC wiring and 250.122 [Re: BigB]  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
Originally Posted by BigB
Ah so I was missing something. The scary part is I am pretty sure I knew that at one time. Getting old is no fun.


I agree with the getting old and memory loss...


#210590 - 07/11/13 11:01 PM Re: AC wiring and 250.122 [Re: BigB]  
HotLine1  Offline


Member
Joined: Apr 2002
Posts: 6,885
Brick, NJ USA
short:
Ditto!!


John

#210591 - 07/11/13 11:04 PM Re: AC wiring and 250.122 [Re: HotLine1]  
shortcircuit  Offline
Member
Joined: Jun 2004
Posts: 608
massachusetts
Originally Posted by HotLine1
Perhaps a little clearer....

BigB example is 14/2NM, but he said 25 amp OCP, using the nameplate max figure.

Using the max OCP, a #12 EGC would be required, so using NM he has to use 12/2.



I'm not sure I follow you Hotline...

When doing AC units the nameplate has all the info for sizing the branch circuit wiring...and sizing the overcurrent device.

For example...if the nameplate gives a MINIMUM BRANCH CIRCUIT AMPACITY of 30amps and a MAXIMUM BRANCH CIRCUIT OVERCURRENT DEVICE of 50amps...

I will wire the unit with 10-2 romex and connect it to a 50amp breaker.

Follow me?

shortcircuit


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