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Old 06-21-2009, 10:48 AM   #1
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Wire sizing to the condensor


Seems like you can get either 10 gauge or 8 gauge whips to go from the panel to the condenser. What gauge wire do people usually from the main breaker to the condenser panel? I won't need this in a conduit, and it looks like you can get 10 gauge romex style with 2 conductors and ground (3-wire) (flat sheathing), or you can get the 3 conductor and ground (4-wire) (round sheathing) in 8 gauge. Do people typically run the three or 4 conductor? Do people use the 10 gauge on smaller tonnage units? I'd like to have less IR drop so I'm leaning towards the 8 gauge.. But again, should I be considering something even thicker?

Any rules of thumb people can share with me? The condensors have a typical max current draw in the data sheet. It would be nice if there was a table that had 1. The length from the breaker to panel 2. The max current of the unit and 3. A resultant reccomended gauge of wiring..

If people don't have opinions on this, I'll just go with what the AWG guides suggest along these lines for a 30 amp breaker and then overdesign a little.

-Jeff

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Old 06-21-2009, 12:02 PM   #2
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Wire sizing to the condensor


The name plate on unit gives you max fuse amps size wire to the breaker accordingly. You can use standard romex 2 conductor with grnd inside but not outside you would need to add junction to seal tite then to outside . You could run uf from panel to outside

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Old 06-21-2009, 02:54 PM   #3
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Wire sizing to the condensor


Wire AWG depends on how long a run itis.
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Old 06-21-2009, 04:04 PM   #4
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Wire sizing to the condensor


The unit will have a 'minimum circuit ampacity' listing, or something like it. This determines the minimum wire size. You can go larger, but not smaller. If you're using NM cable (Romex), 14/2 is good for up to 15 amps, 12/2-20 amps, 10/2-30 amps, 8/2-40 amps, and 6/2-60 amps. You need 2 wires plus a ground, no neutral.

The unit will also have a 'maximum HACR circuit breaker or fuse' listing. This is the largest breaker that can be used. It's usually larger than the wire is rated for. This is one of the few times it's completely legal to use a larger breaker. Unfortunately, a lot of inspectors don't understand article 440 in the code.

If you guess at the wire and breaker size, you'll almost certainly get it wrong. Either look at the nameplate or get the spec's from the supplier.

Rob

P.S. The above wire sizes are good for about 150 feet. For longer runs, upsize.

Last edited by micromind; 06-21-2009 at 04:08 PM. Reason: Added P.S.
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Old 06-21-2009, 05:42 PM   #5
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Wire sizing to the condensor


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Originally Posted by micromind View Post


Rob

P.S. The above wire sizes are good for about 150 feet. For longer runs, upsize.
Non of those wire sizes were good for 150' at the posted amps.
When applied toa moptor starting circuit.
Will cause shortened comressor life.
And light dimming when the compressor tries to start.
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Old 06-21-2009, 09:26 PM   #6
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Wire sizing to the condensor


Here are some actual numbers;

A 14/2 150' long loaded to 15 amps will drop 13.5 volts. At 75 amps it'll drop 67 volts.

A 12/2 150' long loaded to 20 amps will drop 12 volts. At 100 amps it'll drop 60 volts.

A 10/2 150' long loaded to 30 amps will drop 11.3 volts. At 150 amps it'll drop 56 volts.

A 8/2 150' long loaded to 40 amps will drop 8.4 volts. At 200 amps it'll drop 42 volts.

A 6/2 150' long loaded to 60 amps will drop 9 volts. At 300 amps it'll drop 45 volts.

The nameplate on most, if not all, single phase A/C units states 207 to 253 volts as the operating range. If you start at 240 volts, you'll need to drop 33 volts to get to the bottom end of the range. The worst case of these wire sizes for running current results in 226.5 volts. Well within the operating range.

In my experience, having the voltage drop to 173 (240 - 67 = 173) during starting will still allow the unit to start. Reduced voltage starting is done on a regular basis on larger compressors (and other motors) to reduce the strain on the electrical system as well as the motor. A common type of reduced voltage starting is accomplished by an autotransformer. Usually, the 65% tap is used. This results in 156 volts (on a 240 volt system) delivered to the motor during starting. Compressor (or any motor) life is shortened by excessive current at starting. High surge current at starting results in mechanical shock to the windings. If this can be diminished (lower current), the compressor will actually last longer, provided it will still start.

Lights dimming is directly proportional to voltage drop. Voltage drop is directly proportional to current. If surge current can be reduced, lights will dim less. Basic physics. This is one of the reasons for using a reduced voltage starting system on larger motors.

Rob
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Old 06-21-2009, 09:33 PM   #7
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Wire sizing to the condensor


We not talking about commercial.

Allowing the voltage to drop like that will shorten the life of residential equipment.

So the few pennies a customer would save by using smaller wire at its max amp rating, are lost by early repair, and or replacement.

Not to mention the longer time it takes the motor to drop down from LRA because of the voltage drop, costing the customer more in elelctric.

If a customer needs to save 5 bucks by using smaller wire. They shouldn't be getting central A/C.
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Old 06-21-2009, 10:21 PM   #8
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Wire sizing to the condensor


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If a customer needs to save 5 bucks by using smaller wire. They shouldn't be getting central A/C.

That's what I was trying to get a general feel for. How much of an extra mile do you go to for your own stuff.. 10 or 8 gauge will work with my 30 amp breaker, I was planning on using 8, but curious if people are even more interested in reducing IR drop so that they might use 6 on a 30 amp circuit...

-Jeff
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Old 06-22-2009, 04:49 AM   #9
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Wire sizing to the condensor


If your run is under 100', 10AWG should be fine.

Over 100', then 8 is better.
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Old 06-22-2009, 08:02 AM   #10
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Wire sizing to the condensor


Quote:
Originally Posted by zootjeff View Post
That's what I was trying to get a general feel for. How much of an extra mile do you go to for your own stuff.. 10 or 8 gauge will work with my 30 amp breaker, I was planning on using 8, but curious if people are even more interested in reducing IR drop so that they might use 6 on a 30 amp circuit...

-Jeff
Usually the voltage drop at full load amps at the destination end of the wire is 1%, 3% or 5%. Using way oversized wire may give you diminishing returns.

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