40 amp breaker for 30 amp central a/c


Old 05-25-04, 09:18 AM
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40 amp breaker for 30 amp central a/c

I own a home that was built in the mid 50s and has 100 amp service. I am adding a 3 ton central a/c unit that has a 30 amp requirement. The house used to have an electric stove, which was connected to a 40 amp breaker. Can I use this line to power the a/c unit or do I need to replace the 40 amp breaker with a 30 amp breaker?
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Old 05-25-04, 09:51 AM
Join Date: Sep 2003
Location: Central New York State
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No. And now I am padding my answer so that it will be accepted.
Old 05-25-04, 10:20 AM
Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
"30 amp requirement"------ this could apply to two distinct ratings-- the rating of the Banch-Circuit breaker and the Branch-Circuit condutors.

If you have an existing 40-amp breaker, it MAY be connected to 40amp conductors--- #8 copper. To detemine the ampacity of the Branch-Circuit conductors needed, multiply the Full-Load current of the compressor-motor by125%.The NEC requires that full-load current-value be marked on the compressor-motor name-plate, the equiptment name-plate, or both. The FLC X 125% value may appear on the name-plate as "minimum circuit (conductor)ampacity-- 26 amps." This value requires #10 condutors.

To determine the rating of the Branch-Circuit breaker,multiply the FLC X 175%.The minimum rating of the C-B may appear on the name-plate. It's important to know that the rating of the C-B will exceed the rating of the B-C conductors---- ex.;A 40-amp C-B protecting 30 amp conductors.

Good Luck & Enjoy the Experience!!!

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