2p-40A breaker - is 10g wire sufficient?

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Old 12-05-20, 07:23 AM
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2p-40A breaker - is 10g wire sufficient?

I have a central air compressor/condensor unit that is connected to a 2p-40A breaker using 10/2 romex (no neutral) from the exterior AC disconnect to the main panel. It's a short run since the unit is just outside the wall on which the main panel is mounted. Is #10 sufficient or should this have been run using #8 romex?
 

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12-05-20, 09:52 AM
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Sizing rules for motors and HVAC equipment use different values than standard circuits. Check the nameplate on the unit for minimum circuit ampacity to size the wiring.
 
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Old 12-05-20, 08:44 AM
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Use 8 gauge for 40 amps, even for a short run.But if the name and specs plate on the unit says the continuous current draw is under 24 amps and the installation manual says use 10 gauge then you may use 10 gauge. (The 40 amp breaker may have been specified to allow for a brief higher current draw each time it kicks on.)
Very long runs may call for a yet heavier gauge cable but very short runs do not allow a lighter gauge cable than normal.
 
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Old 12-05-20, 09:52 AM
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Sizing rules for motors and HVAC equipment use different values than standard circuits. Check the nameplate on the unit for minimum circuit ampacity to size the wiring.
 
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Old 12-05-20, 10:08 AM
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Per electrical code it is unconditionally the wire size which dictates the breaker size. Electrical code says 10 gauge is safe up to 30 amps. So the breaker canít be larger than 30 amps for 10 gauge wire. A 40 amp breaker on 10 gauge wire is NOT correct.

A 30 amp breaker on 10 gauge wire is correct. A 40 amp breaker on 8 gauge wire is correct.

What the correct wire size is for the specific application is another story.

Mod Note : the above is based on standard circuits sizing and does not apply to HVAC equipment. HVAC wiring is sized based on the manufacturer nameplate data.
 
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Last edited by pcboss; 12-05-20 at 10:35 AM.
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Old 12-05-20, 12:26 PM
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Listen to pcboss, he's got it right. The equipment data plate should give MCA (Minimum Circuit Ampacity) and MOCP (Maximum Overcurrent Protection). It's not at all unusual to see the MCA spec'd as 26 amps (requiring #10 conductors) and the MOCP spec'd as 40 amps, could be 40 amp fuses or a 40 amp HACR rated breaker.

As a real life example, I have a 3 1/2 Ton Luxaire condensing unit with a MCA of 21.5 amps and MOCP of 35 amps.
 
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Old 12-05-20, 12:26 PM
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OK, so HVAC wire sizing is based on manufacturer nameplate. Let's assume (since any visible markings have long since faded away) that 10g was called for by the manufacturer.

Does NEC then require use of a 2p-30A breaker, or does NEC defer to HVAC manufacturer for breaker sizing?

Edit: @CasualJoe posted same time as me, and he answered the question I asked in this post. So NEC defers to HVAC equipment mfr for both wire and breaker sizing.

I'm going to assume original install (back when nameplate would have been legible) was done correctly. Only reason I asked was because I noticed the mismatch between wire size and breaker size (by "regular", as in non-HVAC, NEC rules).
 
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Old 12-05-20, 12:31 PM
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If the manufacturer's name and catalog number is available the MCA and MOCP can be looked up from their website.
 
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Old 12-05-20, 12:33 PM
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Manufacturer name is visible, but that's it. No model # or anything else.
 
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Old 12-05-20, 02:37 PM
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Follow NEC rules for motor circuit wiring. See NEC section 430. Wire size is based on motor HP from table 430.248. Provided motor has its own overload protection use table 430.52 to size the breaker. Under these rules a 40A breaker is okay on #10Cu.
The motor must be hardwired to use these rules, no cord and plug connection.
 
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Old 12-06-20, 12:51 PM
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Sorry guys, looks like I was just muddying up the waters in post #4 Ė itís wrong!!!. I should have read pcbossís post #3 a little better.

(I better get a new version of the NEC. I wondered why I couldnít find the very good information pattenp referenced by numbers in post #9, and then I realized - my NEC version is 20 years old Ė lol. )




 
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Old 12-07-20, 12:03 PM
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Follow NEC rules for motor circuit wiring. See NEC section 430. Wire size is based on motor HP from table 430.248.
Many people forget that in a typical A-C condensing units they have not only the condenser fan motor, but the compressor motor too.
 
 

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