Determine your power requirements
The unit of electrical power is called the ampere, usually shortened to amp. To figure the size required for a tool circuit, start by looking at the motor nameplate of your most power-hungry tool. For discussion purposes, let's say it's a tablesaw with a 14.2-amp motor (at 110/120 volts). Tools often draw a surge of power at startup, so we'll figure a 25 percent allowance for this by multiplying 14.2 times 1.25. The result is 17.75, so a 20-amp circuit is a good choice for this one tool.
The math remains easy even if you'll run more than one tool simultaneously on a circuit--say a router and dust collector. Look up the nameplate amperage for each tool, figure its surge allowance, then add the requirements of the two tools. If the total exceeds 20 amps, though, you'll usually find that the most practical solution is to run each tool on a separate circuit.
Some large motors have dual-voltage capability: They can be wired to run on a 110/120-volt circuit or on a 220/240 circuit. You or the motor won't notice a particular advantage either way. That's because doubling the voltage cuts the amperage in half. In the example of the tablesaw motor requiring 14.2 amps at 110 volts, wiring it for 220 power drops the amperage needed to 7.1.
Other power requirements in your shop may be relatively minor, such as a radio or fan, but adding an appliance, like as a refrigerator or space heater, can create a serious demand on a circuit.
The circuit for the lighting should always be separate from the power receptacles. Otherwise, if a tool trips a circuit breaker, you'll be in the dark. For more details about shop wiring, see woodmagazine.com/shopwiring.
If you're confident in your ability to plan and install circuits, you may want to take on the actual wiring yourself. But check first with the local governmental agency that issues permits and conducts inspections. Many locations allow the homeowner to complete wiring chores; some require that you prove your knowledge by first passing a test. Others absolutely prohibit the homeowner from doing wiring unless it's completed by a licensed electrician.
In almost all cases, electrical work requires that you obtain a permit and have the work inspected by a building official. If you proceed without a permit, you could have problems with your homeowner's insurance in the event that you need to submit a claim.
If you have doubts about your wiring skills, hire an electrician. Skilled tradesmen don't work cheaply, but the peace of mind that comes from having a pro handle the job is worth it.
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