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Old 07-11-2013, 03:26 AM
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djy
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Default central air wire size

I,m installing a 4 ton compressor.manufactures specs are as follows
supply voltage 208-230
Min circuit ampacity 29.2 amps
min overcurrent device 30 amps
max overcurrent device 50 amps

fan motor 1/3 HP
fan motor amps 2.8
rated load 21.2 amps
locked rotor 96 amps

the run to the disconect is just shy of forty feet.
what gauge wire and how many conductors do I need including the ground?
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Old 07-11-2013, 03:31 AM
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No need for you to worry about that. Your hvac contractor knows what to use.
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Old 07-11-2013, 03:41 AM
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hire a pro, #8 awg is what is needed BUT wiring is not a hobby!
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Old 07-11-2013, 05:09 AM
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2conductors plus ground. No neutral for 220.
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Old 07-11-2013, 05:27 AM
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wiring is not a hobby.I've been pulling wire in homes for thirty years but this is a first time ac install in my home after hurricane sandy knocked dwn half my home. but thanks for your concern about getting a pro.I have hired pros for this home build and I am amazed of the AO that are in buissness today.overpriced and underqualified. from the foundation,framers,siding every trade hired and fired have to go through 3 to get one keeper.
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Old 07-11-2013, 05:27 AM
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Wire is sized to you're over current device, not far enought to have to upsize based on voltage drop... Size it for 50 FLA, but many factors are involved in sizing wire..
For example #8 CU THHN is good for 50 amps but not the 8/3 romex..
If you're you're planning on using romex use 6/3...
THHN use #8.
Over sizing the wire by 1 gauge isn't that big of a deal as long as the over current device is correct...
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Old 07-11-2013, 06:55 AM
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The specs call for a 30 amp minimum circuit, so 40 amp would be the appropriate size, which for that run is 8 gauge wire.
The building code on Long Island requires both a ground and neutral (plus hot legs) for any circuit, so you are looking at running 8/3 romex (which is two hots, plus a neutral and unshielded ground), which must be in conduit where exposed.
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Old 07-11-2013, 08:20 AM
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The specs call for a 30 amp minimum circuit, so 40 amp would be the appropriate size, which for that run is 8 gauge wire.
The building code on Long Island requires both a ground and neutral (plus hot legs) for any circuit, so you are looking at running 8/3 romex (which is two hots, plus a neutral and unshielded ground), which must be in conduit where exposed.
I agree that a 40A breaker needs either 8/3 romex or #8 THHN / THWN. If you go to a 50A breaker, you need to upgrade to 6/3 romex or could use #8 THHN / THWN. That takes care of the AC compressor. Any questions for wiring the air handler? Does it have a built in heat strip?
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Old 07-11-2013, 08:28 AM
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I'd go with 50A breaker and wire size that appropriately. The last thing you want is nuisance trips on your main breaker with the high inrush of current to start the compressor. You would just have to redo the whole deal. There are breakers with manually settable time constants for inrush currents that may be useful for high starting loads just to avoid that problem altogether. I'd go bigger.

Also be mindful that putting romex into a conduit will derate the ampacity of the cable by some amount. In Ca. it's not advisabe and maybe not legal to put romex inside a conduit just for that reason.
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Old 07-11-2013, 10:21 AM
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Name plate states 50 amp max... Use 50 it's not much more in cost and if you change out later down the road you got a 50... And won't have to up size wire..
Don't you watch the sprint commercials... More is better
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Old 07-11-2013, 11:41 AM
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The minimum circuit ampacity is what you size your wire to. So in this case it would be #10, 10/2 romex as there's no need for a neutral. It wouldn't hurt to upsize to #8, but not necessary.
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Old 07-11-2013, 12:04 PM
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The minimum circuit ampacity is what you size your wire to. So in this case it would be #10, 10/2 romex as there's no need for a neutral. It wouldn't hurt to upsize to #8, but not necessary.
Once again, in theory you could use a 30A breaker and #10 wire. The compressor load is balanced so the nuetral is not needed, but as others mentioned above, local code requires a neutral. The MCA is 29.2A and using a 30A breaker could result in nuisance tripping. Not likely with a 40 or 50A breaker. You cannot use a #10 wire with 40A or 50A breaker regardless of the nameplate rating of a device.
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Old 07-11-2013, 12:29 PM
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This is funny. No need for a 50 or #6 wire on a condenser. Ahu yes with electric heat .
You put a 50 on a 30 here you fail.
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Old 07-11-2013, 12:48 PM
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NEC states that you can only load a circuit to 80 pct., which is 36.5 amps. 8/2 w/g for a direct connection, and a 40A 2 pole breaker should suffice. Good luck.
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Old 07-11-2013, 02:47 PM
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thank you all for replies
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Old 07-11-2013, 03:00 PM
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minimum circuit ampacity is 29.2 amps 10/2 romex is good for 30 amps 10 thhn stranded is good for 35 amps minimum overcurrent is 30 amps maximum is 50 amps this is only for start up I would run 10/2 romex or 10 thhn with a 40 amp breaker for no nuisance trips .
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Old 07-11-2013, 03:29 PM
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Quote:
Originally Posted by louiefl View Post
Once again, in theory you could use a 30A breaker and #10 wire. The compressor load is balanced so the nuetral is not needed, but as others mentioned above, local code requires a neutral. The MCA is 29.2A and using a 30A breaker could result in nuisance tripping. Not likely with a 40 or 50A breaker. You cannot use a #10 wire with 40A or 50A breaker regardless of the nameplate rating of a device.

For a motor load you can certainly use a 40 or 50 amp circuit breaker as per the nameplate as long as the ampacity of the conductor is greater than the minimum circuit ampacity required by the unit.
The 29.2 amp figure has already been upsized from the RLA by 25% as per NEC 440-32.
In 440-22 it's stated that the circuit breaker shall be sized at no more than 175% of the RLA in order to allow for carrying of the starting current. So in this case a 40 or 50 amp breaker is permissible.

Note-The article #'s above are from a book a few years old. My 2011 NEC is in my office, but the principle is the same.
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