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#210577 - 07/11/13 08:33 AM AC wiring and 250.122
BigB Offline
Member

Registered: 03/31/04
Posts: 725
Loc: 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?


Edited by BigB (07/11/13 08:34 AM)

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2014 / 2011 NEC & Related Books and Study Guides
#210579 - 07/11/13 12:43 PM Re: AC wiring and 250.122 [Re: BigB]
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6785
Loc: 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

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#210581 - 07/11/13 04:58 PM Re: AC wiring and 250.122 [Re: BigB]
shortcircuit Offline
Member

Registered: 06/27/04
Posts: 608
Loc: 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...

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#210582 - 07/11/13 05:01 PM Re: AC wiring and 250.122 [Re: HotLine1]
shortcircuit Offline
Member

Registered: 06/27/04
Posts: 608
Loc: 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...

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#210585 - 07/11/13 08:17 PM Re: AC wiring and 250.122 [Re: BigB]
renosteinke Offline
Cat Servant
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Registered: 01/22/05
Posts: 5299
Loc: Blue Collar Country
Please note that the 2014 edition will have changes to this requirement in air conditioning article.

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#210586 - 07/11/13 08:33 PM Re: AC wiring and 250.122 [Re: shortcircuit]
BigB Offline
Member

Registered: 03/31/04
Posts: 725
Loc: 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.


Edited by BigB (07/11/13 08:33 PM)

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#210587 - 07/11/13 08:52 PM Re: AC wiring and 250.122 [Re: BigB]
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6785
Loc: 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

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#210588 - 07/11/13 08:56 PM Re: AC wiring and 250.122 [Re: BigB]
shortcircuit Offline
Member

Registered: 06/27/04
Posts: 608
Loc: 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...

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#210590 - 07/11/13 09:01 PM Re: AC wiring and 250.122 [Re: BigB]
HotLine1 Offline

Member

Registered: 04/03/02
Posts: 6785
Loc: Brick, NJ USA
short:
Ditto!!
_________________________
John

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#210591 - 07/11/13 09:04 PM Re: AC wiring and 250.122 [Re: HotLine1]
shortcircuit Offline
Member

Registered: 06/27/04
Posts: 608
Loc: 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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