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Old 03-09-2007, 05:12 PM   #1
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12awg To 10awg, Why?


hello,

i have yet another question. i just replaced the cable (3 10awg wires) from the fuse box on the back of the house to the central ac compressure, because the cable was cracked and rusted in places. the romex from the service panel to this fuse box looks to be 12awg. why would they run 10awg from the fuse box to the compressor, when they have 12awg comming into the fuse box, with 30a fuses??? as long as the wire is heavy enough not to burn before it trips the breaker is ok, but why???

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Old 03-09-2007, 06:15 PM   #2
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12awg To 10awg, Why?


I'm not an electrician, but 220v off of a 30 amp breaker should have 10g all the way from the panel out, such as to an HVAC unit or water heater.
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Old 03-09-2007, 08:13 PM   #3
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12awg To 10awg, Why?


A/C units and motor circuit follow different rules than other circuits. They are exempt from the restrictions of 240.4(D).

It is completely feasible to have #12 from a 30 amp breaker feeding an A/C unit.

My guess is that they used one of those home center pre-packaged sealtite whips with the connectors and #10 conductors included.
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Old 03-09-2007, 08:21 PM   #4
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12awg To 10awg, Why?


See.
A good pro will tell it like it is, and we old DIYers will get it wrong almost every time. LOL.
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Old 03-11-2007, 08:03 AM   #5
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12awg To 10awg, Why?


Quote:
It is completely feasible to have #12 from a 30 amp breaker feeding an A/C unit
I never understood that. My thinking has been that a motor/compressor will pull more amps than they are rated when startingup. 12 g wire =20 amps. 10 g wire =30 amps. Right?
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Old 03-11-2007, 08:30 AM   #6
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12awg To 10awg, Why?


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Originally Posted by boman47k View Post
I never understood that. My thinking has been that a motor/compressor will pull more amps than they are rated when startingup. 12 g wire =20 amps. 10 g wire =30 amps. Right?
The compressors and motors will only draw the extra current at start up. The wire is sized or the continous running load, and the breaker is allowed to be larger so that it will not neusance trip on start up.

The confusion is what you are protecting against. On most circuits the breaker serves two functions. It protects against short circuits, and against overloads.

On motor circuits the breaker is only protecting against short circuit. The motor over loads (on hvac equipment these are usually built into the motor) protect against overload.

Since a motor is on a dedicated circuit, and the built in overloads serve that function, there is nothing else on the circuit that would cause the need for overload protection of the wire.

If you look at the trip curves for different size breakers you will see that the difference in the time it takes for the breaker to clear a fault on short circuit for two breakers that are close to eachother in amp rating (say for example: 20 or 30 amp breaker) is minimal. So allowing the breaker to be up to 200 percent of the actual motor FLA is not a bad thing at all.

It is important to understand that the available fault current on a short circuit is limited by the size of the transformer and the size and length of the wires between the transformer and the short, NOT the size of the breaker. The size of the breaker will only limit the lenght of time before the fault is cleared.
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Old 03-11-2007, 09:08 AM   #7
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12awg To 10awg, Why?


As usual Jeff, you said that better than I could have.



Quote:
Originally Posted by boman47k View Post
12 g wire =20 amps. 10 g wire =30 amps. Right?
No. Not always. That is what I meant about the exceptions to NEC 240.4(D).
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Old 03-12-2007, 10:34 PM   #8
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12awg To 10awg, Why?


speedy,

you are probably correct. the wire from the fuse box on the outside wall of the house to the compressor was a sealtite with two 10awg wires inside. i just replaced it with the a vinal sheething with 3 10awg conductors, red, black and green/ground. i wasnt aware that 12awg was ok for a 30 amp central ac circuit.

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