Central AC wire sizing

peterbuJune 15, 2009

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.

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petey_racer

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.

    Bookmark   June 15, 2009 at 9:11PM
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joefixit2

Why couldn't he use 12 for the condenser?

    Bookmark   June 15, 2009 at 11:28PM
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wayne440

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

    Bookmark   June 15, 2009 at 11:35PM
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joefixit2

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

    Bookmark   June 15, 2009 at 11:41PM
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wayne440

You must enjoy resetting circuit breakers.

    Bookmark   June 16, 2009 at 7:13AM
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normel

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.

    Bookmark   June 16, 2009 at 7:24AM
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joefixit2

"You must enjoy resetting circuit breakers."

How would smaller wire make the circuit breaker trip??

    Bookmark   June 16, 2009 at 9:04AM
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brickeyee

"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.

    Bookmark   June 16, 2009 at 9:29AM
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jpdime_yahoo_com

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.

    Bookmark   September 14, 2011 at 3:24PM
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bus_driver

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.

    Bookmark   September 14, 2011 at 6:44PM
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petey_racer

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.

    Bookmark   September 14, 2011 at 10:05PM
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petey_racer

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

    Bookmark   September 14, 2011 at 10:07PM
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jmorrow

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.

    Bookmark   September 27, 2011 at 7:32PM
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