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Old 05-27-2012, 10:34 AM   #31
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What size wire to use


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Originally Posted by Marc Frederick View Post
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
Here's how I read it...a #10 wire is fine for a 25 amp circuit. Since it is a motor circuit, you can use a 40 amp breaker. # 12 wire would be close because the RLA on the compressor is what it could draw under some circumstances. Obviously a locked rotor would trip a 40 amp breaker before it could damage the wire, but running a 20 amp load on a #12 wire is not a good idea. Again, this is my opinion, and how I would wire this circuit.

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Old 05-27-2012, 10:36 AM   #32
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What size wire to use


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Originally Posted by Missouri Bound View Post
Here's how I read it...a #10 wire is fine for a 25 amp circuit. Since it is a motor circuit, you can use a 40 amp breaker. # 12 wire would be close because the RLA on the compressor is what it could draw under some circumstances. Obviously a locked rotor would trip a 40 amp breaker before it could damage the wire, but running a 20 amp load on a #12 wire is not a good idea. Again, this is my opinion, and how I would wire this circuit.
you guys are not getting it, the manufacture ALREADY added 125% to that minimum circuit ampacity...nothing more needs to be done, #12 is perfectly fine for a 25 amp load...

And again, #12awg is actually rated for 25 amps... don't be scared of this fact.

And I doubt a locked rotor would trip the 40 amp breaker, because the thermal overload in the motor would kick in WAY before the thermal part of the 40 amp breaker...

Last edited by stickboy1375; 05-27-2012 at 10:40 AM.
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Old 05-27-2012, 10:43 AM   #33
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What size wire to use


Guys it's Sunday I've got to get out of here.

I guess we could go on and on, but please give this some thought.

12 ga copper wire like Romex needs a 20 amp breaker. The device, appliance,etc. may have other wires inside it that uses different wiring size tables.
Electrician's wire these appliances, units, etc. per application.

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Old 05-27-2012, 10:45 AM   #34
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What size wire to use


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Originally Posted by Kapriel View Post
Guys it's Sunday I've got to get out of here.

I guess we could go on and on, but please give this some thought.

12 ga copper wire like Romex needs a 20 amp breaker. The device, appliance,etc. may have other wires inside it that uses different wiring size tables.
Electrician's wire these appliances, units, etc. per application.

Hope every one has a safe Holiday !
In certain applications, I agree, in others, I disagree.

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Old 05-27-2012, 10:46 AM   #35
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What size wire to use


Later guys, Got to go.
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Old 05-27-2012, 10:50 AM   #36
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What size wire to use


Pretty picture to help see the light.

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Old 05-27-2012, 11:27 AM   #37
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What size wire to use


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Originally Posted by Kapriel View Post
I guess we could go on and on, but please give this some thought.

12 ga copper wire like Romex needs a 20 amp breaker.
No, #12 wire requires whatever breaker size the NEC requires for the specific application. For this application, a 40A breaker is allowed. Read Article 440, or any of the numerous prior threads on here where this issue has been beaten to death. The summary is this: the "typical" wire/breaker combinations do not apply strictly to HVAC, motor, and welder circuits. For those circuits, the wire can be sized to the LOAD, which has internal overload protection, and a larger breaker can be used to handle momentary surges of higher current.

This is a 20A load according to the nameplate, so #12 wire is just fine. A 40A breaker is allowed for this #12 wire, both according to the nameplate and also according to Article 440.
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Old 05-27-2012, 12:33 PM   #38
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What size wire to use


Article 440 in the code covers Air Conditioning And Refrigeration Equipment.

Part III of Article 400 is entitled "Branch-Circuit Short-Circuit and Ground-Fault Protection.

440.21 "The provisions of part III specify devices intended to protect the branch-circuit conductors, control apparatus, and motors in circuits supplying motor-compressors against overcurrent due to short circuits and grounds. They are in addition to or amendatory to the provisions of Article 240.

Article 240 is entitled "Overcurrent Protection".

240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) or 240.4(G), the overcurrent protection shall not exceed 15 amperes for #14 AWG, 20 amperes for #12, and 30 amperes for #10 copper.

Table 240.4(G) lists Article 440 parts III and IV.

Therefore, the 20 amp rule for #12s does not apply to HVAC equipment. This is stated further by 440.21

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Old 05-27-2012, 07:08 PM   #39
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What size wire to use


I stand corrected.

Thank you Stickboy for the pretty picture, I always like those.
Thanks also to Micromind and mpoulton.

I wouldn't of believed it if I didn't see the NEC photo.
So I'm guessing the NEC will allow a 40 amp breaker in a CB panel lets say 75' away in the basement to serve as protection for the branch circuit and then another 40 amp breaker at the HVAC unit.

Old habits are hard to break, but I learned something today.
Thank you all for the work to explain this to me.

I owe you all.
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Old 05-27-2012, 07:10 PM   #40
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What size wire to use


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Originally Posted by Kapriel View Post
I stand corrected.

Thank you Stickboy for the pretty picture, I always like those.
Thanks also to Micromind and mpoulton.

I wouldn't of believed it if I didn't see the NEC photo.
So I'm guessing the NEC will allow a 40 amp breaker in a CB panel lets say 75' away in the basement to serve as protection for the branch circuit and then another 40 amp breaker at the HVAC unit.

Old habits are hard to break, but I learned something today.
Thank you all for the work to explain this to me.

I owe you all.
I don't use fuses or circuit breakers at the disco for the AC unit,(unless required) I just install pullout style disconnects, but yes, you could have 40 amp breaker at the outdoor unit.
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Old 05-27-2012, 07:11 PM   #41
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What size wire to use


There would be no need for two breakers in the circuit. One at the panel would be enough. The unit only needs a means to disconnect the power for servicing.
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Old 05-27-2012, 07:13 PM   #42
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What size wire to use


Quote:
Originally Posted by Kapriel View Post
I stand corrected.

Thank you Stickboy for the pretty picture, I always like those.
Thanks also to Micromind and mpoulton.

I wouldn't of believed it if I didn't see the NEC photo.
So I'm guessing the NEC will allow a 40 amp breaker in a CB panel lets say 75' away in the basement to serve as protection for the branch circuit and then another 40 amp breaker at the HVAC unit.

Old habits are hard to break, but I learned something today.
Thank you all for the work to explain this to me.

I owe you all.
Don't worry about it, you are not alone in this territory, I work with guys all the time and they freak out when I tell them it's safe. I don't know what people learn in the trade, but its nothing to do with motors I can tell you that much.

Last edited by stickboy1375; 05-27-2012 at 07:40 PM.
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Old 05-27-2012, 07:26 PM   #43
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What size wire to use


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Don't worry about, you are not alone in this territory, I work with guys all the time and they freak out when I tell them its safe. I don't know what people learn in the trade, but its nothing to do with motors I can tell you that much.
And I'd bet you've seen more than one inspector who doesn't understand this as well.

I know I have!
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Old 05-27-2012, 07:34 PM   #44
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What size wire to use


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And I'd bet you've seen more than one inspector who doesn't understand this as well.

I know I have!
Yep, and its a pain to teach someone thats adamant on being correct because he has this title of "inspector" heaven forbid someone learn something, pride has to be the hardest thing to swallow ever!
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Old 05-28-2012, 10:37 AM   #45
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What size wire to use


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Smaller wire has more resistance. A motor at a standstill is more of a resistor than an inductor. Therefore its current draw will depend almost entirely on the voltage at its terminals.

Since there's more resistance in a smaller wire, there will be less voltage at the motors terminals, therefore less current.

Less current will result in less voltage drop at the panel.

The difference between #12s and #10s on a 75' run is pretty small, but it could easily make the lights dim noticeably less.

If you'd like, I can run some numbers and see what comes up, but without the locked-rotor current curve, I'd be guessing to some degree.

Rob
Even with connecting wire of zero resistance, at 40A the 240 volts at my panel would drop 40 x .02 = 0.8V. Not much dimming for the rest of the house.

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