Motor horsepower and wire size

[Anthony Whitesell]

Hello All,

...This increase in wire gauge seems like it might be unnecessary since none of my tools are anywhere near the 100' max distance for the first wire gauge category, and since woodworking equipment does not typically have a pronounced startup load like a well pump might have...

What others have said about wire gauge, size, amps, is all correct. I'm going to make two comments to the quote above.

(1) 3% voltage loss occurs at (nearly) 50' for each max amperage-min wire gauge specification. Ie., 15A in a 14g, 20A in 12g, etc. each reach the 3% voltage at or just past 50 feet (IIRC, 50-65'). I'm not sure what the 100' refers to, but the fine print note in the actual NEC refers to 3% voltage drop.

(2) I'm not sure what is leading you to conclude that WW tools (motors) do not have the same startup loads/impact that a well pump does. My well pump is actually much better (less load and/or less noticeable impact) than most of my WW tools. I use only one tool at a time with DC on a separate circuit. When I fire up the bandsaw, the light dim. That doesn't happen with the well pump. Both well pumps and WW tools contain large motors and the startup current calculations are based on factors with no differentiation between the motor for a well pump and a motor for anything else. I write this is sincerely as possible and am curious as to what has lead to your conclusion.

[Chris Friesen]

Breakers protect the wiring - so choice of breaker drives the minimum awg.

Just a quibble...this doesn't apply to motor circuits.

In a motor circuit the motor overload controls protect against long-term but minor overcurrent. Thus the FLA of the motor (or the HP, depending on what rules you're using) drive the conductor size. The breaker protects against short-term shorts, which allows the breaker to be oversized by up to 400%.

In practice the oversized breaker is rarely necessary for home woodworking equipment, so it's simpler to treat it as a general purpose circuit and put a 20A breaker on #12 wiring, and a 30A breaker on #10 wiring.


Note that as a general rule of thumb a typical 3HP motor will need a 20A 240V circuit, and a 5HP motor will need a 30A circuit. I've never had to upsize the conductor to the next higher size, but I'm in a double garage rather than a large shop with long wiring runs.