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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.
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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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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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Joined: Jun 2004
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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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Please note that the 2014 edition will have changes to this requirement in air conditioning article.
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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.
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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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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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John
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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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