Critical questions about workshop wiring

Sizing circuits and subpanels

A: Start by examining the list of your tools' amperage needs you jotted down earlier. Most small portable power tools can operate with 15 amps, but big routers and circular saws often require more. Plus, electrical codes dictate that the total load on a circuit can't exceed 80 percent of its capacity--that's 16 amps on a 20-amp circuit. Dedicate two 20-amp circuits for benchtop and portable-tool outlets.

Larger 120-volt machines (tablesaw, planer, dust-collector, etc.) require a 20- or 30-amp circuit. If you run two machines at once, such as your tablesaw and dust collector, then each needs a separate circuit.

This is where the ability to rewire to 240 volts is a bonus. Remember, power equals voltage multiplied by current. Because the power delivered by a motor doesn't change, the current it draws at 240 volts is half what it would require at 120 volts. Convert your 18-amp tablesaw and 14-amp dust-collector, and they'll consume 16 total amps instead of 32. That means both could run on one 20-amp, 240-volt circuit.

Always keep lighting on a separate circuit. That way, if a tool trips a breaker, you won't be left in the dark. You might get by with a 15-amp circuit for lights, but using a 20-amp circuit adds extra capacity.

Knowing this information, you can size your shop's total service requirement using the guidelines on the following page. As you can see, you don't need to add up the amp requirements of every tool. But don't forget such nontool items as lights, heaters, and chargers.

If you add up all of your circuits, you'll likely end up with a total higher than the subpanel rating. Don't worry. Having one 30-amp and five 20-amp circuits (130 amps total) in an 80-amp subpanel is common.