electrical
I have a 30 amp breaker with a 10/2 feeding a 220 volt dryer outlet. Currently it is powering an air compresser in the garage. I want to put in a 220 wall switch so I don't have to go to the panel and turn on the breaker to power the compresser.
Can I use 12/2 from the 10/2 to switch and back to the compressor outlet?
NEC 630.11(A) may be of interest.
National Electric Code 2008
ARTICLE 630 Electric Welders
II. Arc Welders
630.11 Ampacity of Supply Conductors.
(A) Individual Welders. The ampacity of the supply conductors shall be not less than the I1eff value on the rating plate. Alternatively, if the I1eff is not given, the ampacity of the supply conductors shall not be less than the current value determined by multiplying the rated primary current in amperes given on the welder rating plate by the factor shown in Table 630.11(A) based on the duty cycle of the welder.
Also take a look at 630.12 Overcurrent Protection.
The Owners manual (PDF) seems to contain the answer.
If we look at 110.14(C) in 2008 NEC, it says:
National Electrical Code 2008
ARTICLE 110 Requirements for Electrical Installations
I. General
110.14 Electrical Connections.
(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.
Which means we'll have to check the breakers temperature rating, and the subpanel terminals temperature rating. Then we'll have to look at table 310.16 (Table 310.15(B)(16) NEC 2011), so we can determine the wire size we should use.
In this example image you can see if we're using copper, we'll use 60°C. Once we look at the table, we'll see that for 50A @ 60°C we need 6 AWG copper wire. (50A @ 75°C = 8 AWG).
110.14(C)(1)(a) basically says that if the temperature is not listed, use 60°C for circuits less than 100A. 110.14(C)(1)(b) says if the temperature is not listed, use 75°C for circuits over 100A. However, in the real world 75°C is often used for both.
Many devices are rated for either 110 or 220. There should be a tag on the device which says this. In some appliances its done intentionally so that one model can be sold and safely used in countries which use either voltage. In other cases its simply dumb luck that the component parts can tolerate either voltage.
I'm not sure if this fully answers your question. "The device is rated for it" doesn't really cover what aspects of the device make this possible.
Best Answer
No. 12 AWG wire is only rated for 20 amperes, and so cannot be protected by a 30 ampere breaker.
The switch also has to be rated 30 amperes. Which should solve your problem, as the 30 amp switch should accept the larger wire.