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Wood Magazine

Get Wired

Don't take your power needs for granted, or you may starve your tools

Save your tools, know your required amperage

Save your tools, know your required amperage

Shop wiring requirements can vary widely, depending on the type of projects you tackle. If your hobby is building radio-controlled airplanes, you may need a single convenience outlet for a soldering iron. But if you're into woodworking, you'll need some serious amperage to simultaneously run a tablesaw and dust-collection system. And if cars are your passion, you may have a large air compressor and welding rig that demand wiring on nearly an industrial scale.

But no matter your requirements, one thing's for certain: You can't starve a power tool and expect it to last. When you don't provide a motor with enough power, it runs inefficiently, tempting you to push it harder to get the job done. That further overheats the motor, and you're quickly way beyond the tool's designed operating range.

At that point, several things can happen, and none of them are good. If you blow a circuit breaker, that's a dead giveaway that the circuit is trying to deliver more amperage than it can handle. By resetting the breaker, you continue torturing the tool for a while, but it doesn't solve the problem. In severe overheating conditions, you may burn the insulation off the motor's windings or fry another component. Either way, you'll have a dead tool. If things really go wrong, you could start a fire.


Determine your power requirements
Electrical box
Enlarge Image
This electrical sub panel for a
home wood shop identifies each
tool's circuit. There's also room to
add more circuits later.

Determine your power requirements

Figure your power needs

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

DIY or hire an electrician?

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.


GFCIs guard against shock
Electrical box pulled out of wall
Enlarge Image
A ground-fault circuit interrupter
(GFCI) duplex outlet can provide
valuable protection against electrical

GFCIs guard against shock

Ground-fault protection

Electrical codes may require that your shop's outlets have protection called GFCI (ground-fault circuit interrupter), particularly for installations in a basement or garage. GFCI is designed to sense dangerous conditions and shut off the power. This could prevent a serious or even fatal electrical shock.

You can achieve GFCI protection with specially designed receptacles or breakers. Either way, carefully follow the manufacturer's installation instructions to ensure protection. Check the breaker or receptacle monthly by pushing the test button.


You get what you pay for
Extension cord held in hand
Enlarge Image
Buy high-quality extension cords,
and protect them from abrasion to
keep your tools running strong. This
pigtail model includes a GFCI.

You get what you pay for

Extension cord cautions

Short and fat may not describe the ideal body type, but it's a good rule of thumb for choosing an extension cord. "Short" means picking a cord that's no longer than necessary because long cords can cause a loss of voltage. "Fat" means wires with a large cross section, enabling the conductor to carry more power. The gauge numbering system for electrical wires is counterintuitive: the smaller the number, the larger the wire. For example, 10-gauge wire is substantially larger than 14-gauge.

If you're doing projects outdoors, make certain that you plug your extension cord into an outlet with GFCI protection. Some outdoor cords include GFCI protection plus a multiple outlet block that allows you to have several tools connected at once.


More on extension cords
Chart of cord recommendations
Enlarge Image
A fluorescent light on a retractable
reel gives your home shop a
professional look and eliminates
tangled extension cords.
Hand holding light attached to wall mounted extension cord
Enlarge Image

More on extension cords

See the extension cord chart right or refer to your tool's owner's manual for selection advice. Here are other cautions about extension cords:

- Never walk on a cord or roll wheeled objects over it. That can stress or break the insulation or even the wire itself.

- Never use an extension cord that's tightly coiled on a storage reel because that can create an undesired electrical field. (Think of the motor you made in school by winding copper wire around a nail.) Instead, fully unwind the cord before use.



Wood Magazine