Tapping 110v from 220

The whole idea is used regularly by many appliances, often using the ground lead as a neutral to get 120 volts. A proviso here is that the appliances are 'UL' approved so acceptable to insurers. If you have a neutral in the box, that whole point is moot. Now, to advisability of the idea. Yes, you can do it, but. . . And No, it is illegal (not to code) and insurers will do whatever they can to not pay. Just what you needed, an ambivalent answer, nei.

The issue I have (as a retired electrician) with what you spoke of is the use of one switch. The switch should be a (fusable?) disconnect, not a lighting switch. And further, the circuit should be sized so that the compressor AND the fan running does not exceed 80% of the breaker capacity. By rights, even a 240 volt motor on the fan should have a separate circuit feeding it. That covers the legal aspects of what you're doing. A disclaimer on my part, if you will. As far as will it work, it has been done countless times across the country and usually not found out.

A comment was made above about making it part of the compressor. The up side of that is that the code essentially ends at the plug. The cord is part of the appliance (compressor) and not subject to the code. Just make sure to connect at the cord, ahead of the pressure switch.

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Thanx for all the replies. You’ve given me the solution.
 
You have not indicated if you have a neutral line at the compressor and/or plan to pull in a neutral. You cannot use a neutral form another circuit and if you do pull a neutral line for the compressor circuit the gauge needs to be sized for the circuit and not what it is running. You also have the issue that if you have the compressor on say a 30-50A circuit, all the wiring needs to sized rated for the breaker including wiring to the fan, the fan would need to be fused and/or have it own breaker/overload device. Alternative when a neutral is not available, what is commonly used is a step down transformer 240->120, they can be picked up for not much and the VA rating would be a minimum of fan V x Amps = VA plus some margin. Fans typically do not take much current. The transformer may have ratings for input/output current and require fusing/breaker. Even if you were to buy a 220V fan, which would seem to be the most cost effective solution, I would have local fusing breaker for it. I bring 4 wire power to my machines and breakout local circuits with MCCB breakers or fusing for the 120/240V subsystems. Wiring is rated for the overload device and thermal rating of the connections.

My compressor after-cooler is bolted to the belt cage and uses the air from the mechanical fan which then cools the compressor.
 
All oven receptacles now are 4 wire for new installation. IIRC, the only three wire receptacles left that meet NEC are for welders.
 
Using the ground wire as a current carrying conductor is a very bad idea, regardless of whether done in the building wiring, or in the appliance.

The whole point of a ground is a redundant safety, in case something goes bad, like a poor wiring connection. Say you are in a shop run on a sub-panel. If the ground wire between the subpanel and main panel fails, and you have the compressor wired with the fan to ground, you have now connected the ground wire to 120V. This means that EVERY GROUNDED DEVICE has it's frame at 120V when the compressor fan is "on". Touching any of those frames could result in a shock, including grounded metal outlet boxes, etc. Even things plugged in to the shop through a GFCI will have this issue. Nor would such a shock trip the GFCI, or a breaker. The current will be limited by the draw through the fan, but it doesn't take that much to kill someone if the current path goes through the wrong place in their body. Your only physical evidence of the problem, other than the issue with shocks, will be that the compressor fan doesn't run.

Yes, that is a contrived worst-case scenario. But if everything is operating correctly, a ground is totally unnecessary, which is why early wiring didn't use grounds. In the above scenario, having a ground rod wired to the shop subpanel would mostly mitigate the issue. (But the ground and neutral should not be bonded in that subpanel).
 
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Old-school 120V accessory circuits, like in my stove, had a 15A fuse, not simply wire connection, because
the stove breaker does NOT trip at the right current level to protect nominal 120V circuitry. Something
like this switch/fuseholdermight be appropriate. Not sure it's up to modern safety standards, though; my stove is old.
 
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