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Old 05-26-2012, 04:30 PM   #1
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What size wire to use


I have to run a new circuit for a central air unit. The nameplate on the unit reads: 200/230 volts, minimum circuit amp 25, max fuse/ breaker 40, comp mot 18.6 RLA, o.d. Mot 1.4 FLA. any suggestions would be great. I'm thinking 8/2 wire but wanted to get other opinion. Thanks

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Old 05-26-2012, 04:31 PM   #2
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What size wire to use


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I have to run a new circuit for a central air unit. The nameplate on the unit reads: 200/230 volts, minimum circuit amp 25, max fuse/ breaker 40, comp mot 18.6 RLA, o.d. Mot 1.4 FLA. any suggestions would be great. I'm thinking 8/2 wire but wanted to get other opinion. Thanks
You could run #12 and feed it with a 40 amp breaker...

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Old 05-26-2012, 04:42 PM   #3
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What size wire to use


What distance back to the panel?
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Old 05-26-2012, 04:43 PM   #4
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What size wire to use


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What distance back to the panel?
You really think voltage drop is an issue?
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Old 05-26-2012, 04:45 PM   #5
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What size wire to use


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You really think voltage drop is an issue?
Won't know till I run the numbers.

People who do this for a living can depend on their instincts - not me, not yet.

Plus, running the numbers shows Due Diligence and Duty of Care.
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Old 05-26-2012, 04:49 PM   #6
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What size wire to use


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Won't know till I run the numbers.

People who do this for a living can depend on their instincts - not me, not yet.

Plus, running the numbers shows Due Diligence and Duty of Care.
Do you know how I came up with # 12 AWG? This relates to the other thread with motors, so just being educational, not rude or anything... and just to clarify, I almost added a disclaimer that it was bare minimum...
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Old 05-26-2012, 04:50 PM   #7
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What size wire to use


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Do you know how I came up with # 12 AWG? This relates to the other thread with motors, so just being educational, not rude or anything...
These motors are killing me. . .how can they ignore nameplate data, even if it's just in some cases?

I'll have to fetch my '99 edition of the code and pick through some of these cites, not so much for the exact numbers but for the rationale behind the NEC.
This is worse than doing income tax.

Last edited by Yoyizit; 05-26-2012 at 04:53 PM.
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Old 05-26-2012, 04:55 PM   #8
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What size wire to use


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These motors are killing me. . .how can they ignore nameplate data, even if it's just in some cases?

I'll have to fetch my '99 edition of the code and pick through some of these cites, not so much for the exact numbers but for the rationale behind the NEC.
This is worse than doing income tax.
You just have tunnel vision, the NEC is not ignoring anything... #12 AWG is really good for 25 amps, BUT the NEC limits it to 20 in MOST situations... thats all there is to it.

Last edited by stickboy1375; 05-27-2012 at 08:56 AM.
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Old 05-26-2012, 05:25 PM   #9
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What size wire to use


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What distance back to the panel?
75 feet
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Old 05-26-2012, 05:27 PM   #10
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What size wire to use


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I have to run a new circuit for a central air unit. The nameplate on the unit reads: 200/230 volts, minimum circuit amp 25, max fuse/ breaker 40, comp mot 18.6 RLA, o.d. Mot 1.4 FLA. any suggestions would be great. I'm thinking 8/2 wire but wanted to get other opinion. Thanks
The distance to breaker is 75 ft. I would rather error on the side of caution to make sure the install is safe. I'd I have to use 10/2 or 8/2, I will.
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Old 05-26-2012, 05:29 PM   #11
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What size wire to use


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The distance to breaker is 75 ft. I would rather error on the side of caution to make sure the install is safe. I'd I have to use 10/2 or 8/2, I will.
I'd run 12 in a heart beat, but your coin, so run 10, # 8 is just being wasteful...

just to be clear, every wire gauge suggested is a safe and code compliant installation, your lights are most likely still going to dim regardless of which wire you select...

Last edited by stickboy1375; 05-26-2012 at 05:35 PM.
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Old 05-26-2012, 07:20 PM   #12
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What size wire to use


Thank you for the input. Much appreciated

Last edited by Marc Frederick; 05-26-2012 at 07:26 PM.
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Old 05-26-2012, 08:04 PM   #13
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What size wire to use


Less than 3% voltage drop for a 75 foot run of #12 at 20A (which is the actual load). That's definitely OK.
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Old 05-26-2012, 09:54 PM   #14
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What size wire to use


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I'd run 12 in a heart beat, but your coin, so run 10, # 8 is just being wasteful...

just to be clear, every wire gauge suggested is a safe and code compliant installation, your lights are most likely still going to dim regardless of which wire you select...
Correct.

And the larger wire you run, the more the lights will dim.

This is because smaller wire causes more voltage drop, especially during starting. The less voltage a motor (or HVAC compressor) has during starting, the less current it draws.

The less current drawn, the less voltage drop on the rest of the system.

I would have no problem with running 12/2, but if you want to run larger wire, I understand.

Also, some inspectors don't understand that with motors and HVAC it's perfectly code-compliant to have small wire under a large breaker.

Rob

P.S. If needed, one of us can easily lead you through the various code sections for these installations. You have to go to 4 or 5 different articles to get it all.
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Old 05-26-2012, 09:58 PM   #15
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What size wire to use


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Correct.

And the larger wire you run, the more the lights will dim.

This is because smaller wire causes more voltage drop, especially during starting. The less voltage a motor (or HVAC compressor) has during starting, the less current it draws.

The less current drawn, the less voltage drop on the rest of the system.
I dont understand this reasoning at all.... you'll never compensate for voltage drop of that magnitude because the power company is usually at fault for it to begin with.... but increased wire size compensates for voltage drop, not smaller.

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