Critical questions about workshop wiring

Running on 240 volts

 

A: Contrary to common misconception, running tool motors on 240 volts instead of 120 doesn't make the motor more powerful. A motor's construction limits how much power it can take in, which is the amp rating on the nameplate.

To understand this better, think in terms of a motor's truest power measure: wattage, which equals amps multiplied by voltage. A motor rated for 14 amps at 110 volts draws 1,680 watts (14x120=1,680). Double the voltage, and the amp need gets cut in half, but output remains the same (7x240=1,680).

You may notice a "power" difference, though, if you've been running your 18-amp tablesaw on a 20-amp circuit. Because this motor, at maximum load, draws almost every amp the circuit can spare, it may slow. Wire the same motor for 240 volts, and at full load it only draws 9 amps of the 20 available.

Also, at least some tools are exceptions to the rules. We discovered a contractor's saw that has a motor with an extra set of windings that only come into play when wired for 240 volts. The horsepower rating on the nameplate, above right, was our first clue.