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Central AC wire sizing

Posted by peterbu (My Page) on
Mon, Jun 15, 09 at 21:08

I am installing a 2.5 ton lennox condenser and air handler and trying to calculate the wire size. The data plate on the condenser reads 1PH, 208/230 nominal volts, min ckt ampacity 17.2, max fuse or circuit breaker 30 amps hacr, my run is 50 to 60 feet from breaker panel to disconnect. For the Air Handler the plate reads 208/230 nominal volts, min ckt ampacity 5, max fuse or circuit breaker 15 amps hacr, my run is 25 to 35 feet from breaker panel to disconnect. Most AC disconnects say 60 amps, that must be the max? Thank you very much.


Follow-Up Postings:

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RE: Central AC wire sizing

10/2 for the condenser.

14/2 or 12/2 for the air handler.

Yes, the 60a figure is the max rated amps. You an use it for anything up to 60 amps.


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RE: Central AC wire sizing

Why couldn't he use 12 for the condenser?


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RE: Central AC wire sizing

A 30a breaker will be used with the condenser, so #10 wire is appropriate.


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RE: Central AC wire sizing2

Maybe appropriate but I still say 12 meets code and mfgr instructions. Heck even 14 meets code and mfgr instructions.


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RE: Central AC wire sizing

You must enjoy resetting circuit breakers.


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RE: Central AC wire sizing

Wiring to the compressor could be 12/2 or 10/2. The requirement that 10/2 be used with a 30A breaker does not apply for a compressor/motor.


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RE: Central AC wire sizing

"You must enjoy resetting circuit breakers."

How would smaller wire make the circuit breaker trip??


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RE: Central AC wire sizing

"A 30a breaker will be used with the condenser, so #10 wire is appropriate."

This is a hermetic compressor load plus a motor load for the condenser fan.

Just like electric motors there is a separate code section covering the branch circuit requirements.
The motors contain thermal overload protection preventing them from drawing excessive current.
The wires are sized at 125% OF full load running current, and the breaker is chosen to allow for starting without tripping.

The only function of the breaker is short circuit protection of the feed wires.

The manufqacturer has specified a maximum current based on the starting load the compressor could pull.
It is not used to size the conductors.

Higher limits than a general purpose branch circuit are allowed and can be many time the full load running current.

The wire size and breaker are no longer sized to match each other like general purpose branch circuits.

The run is short enough that excessive drop is unlikely, even at the possible 30 amp starting current.


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RE: Central AC wire sizing

Good answer Brickeyee.

Take the minimum ampacity in this case is 17.2 amps and multiply this by 1.25 which is 21.5 amps. Number 10 Awg is what is called for.

And the manufacturer calls for a 30 amp breaker. And because this is a dedicated motor circuit meaning no other electrical devices are on the circuit we have to size accordingly.


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RE: Central AC wire sizing

Oversizing the conductors above the minimum to the outdoor unit of an AC or heat pump has some potential advantages. Voltage drop at the unit while starting is less than it is with smaller conductors. And running voltage drop is slightly reduced as well. It saves a little on the power bill.


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RE: Central AC wire sizing

Paul, WHY are you multiplying the MCA @ 1.25??

The MCA is the MCA. 17.2A is 17.2A MINIMUM CIRCUIT AMPACITY.

12/2 on a 30A breaker would be typical. 10/2 is fine, but for such a short distance I see NO need to oversize to #10.


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To add....

My point is the 125% is ALREADY taken into account.


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RE: Central AC wire sizing

if you are putting electric heat strips in the air handler (highly likely unless you live in Hawaii), the strip kit will come with new ampacities that nullify what's originally posted on the unit. so if you are, check those specs so you don't end up having to do it twice. 12 is adequate for the heat pump.


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