Critical questions about workshop wiring

Power tools have big appetites for electricity, and unless you built your shop from scratch, you've likely tripped circuit breakers trying to feed them.

A properly wired shop offers the only way to ensure an adequate food supply. So how do you determine your shopwiring needs? Take the time to answer the following questions, and you'll be well on your way.

Note: Unless you are skilled at electrical work and familiar with local codes, leave wiring jobs to a professional. Use the information gathered here to guide your conversations with an electrician.

A: Tools feed on amps. The nameplate, located on the tool body or motor housing, below, indicates just how many the tool will need (draw) under full load. The chart, below, shows average ranges for some common tools.

Jot down your major power tools' requirements, and keep the list for later planning. Note any tools that can be wired to run on 240 volts instead of 120. (The nameplate indicates this, as well.)

A: Look at the number printed on the main breaker in your service panel determine the total amperage available to your home from the power supply line. This tells you the maximum amperage that all electrical circuits can draw simultaneously. Most homes built in the past 40 years are equipped with 100- or 200-amp service, which should provide ample power to run your household and, in many cases, a shop. Plus, the service panel may have unused circuits available for your shop wiring needs.

Even if you have space for extra circuits, consider running a separate feeder to a subpanel in your shop. Advantages include not having to share circuits with the house, snaking just one large cable instead of multiple smaller ones, and being able to shut off shop power when it's not in use.

Adding a subpanel also allows shorter wiring runs in the shop, which decrease power loss and heat buildup. But, a subpanel won't increase your total capacity. In other words, if you have 200-amp service, and you split off 80 amps to a subpanel, you don't have 280 amps available.

If your home was built before the 1950s and hasn't been electrically updated, you may have only 60-amp service. If that's the case, if you still have a fuse box, or if you frequently trip breakers, you need increased service and a new panel.

Be aware, too, that if your shop is located in a garage or unfinished basement, electrical codes will likely require Ground-Fault Circuit Interrupter (GFCI) protection on all general-use outlets. These devices detect current leaks and shut down power instantly if a short occurs. GFCI outlets protect specific areas within a circuit, while a GFCI breaker serves the entire circuit.

To determine how much capacity your shop needs, first find your highest amp-draw tool (often a tablesaw or dust collector), and multiply the amperage by 125 percent

Highest amps x 1.25 = (A) _______

Now, total the amperage of the highest draw tools that run simultaneously, such as a tablesaw and dust collector, router and shop vac, etc.

Simultaneous tool amps = (B) _______

Total the amp draw of all other loads that run continuously, such as lighting, heat/air conditioning, air filter, radio, etc. (If amps unknown, such as with lights, divide watts by voltage to get amps.)

Continuous-draw amps = (C) _______

MINIMUM AMPS REQUIRED FOR SHOP (A+B+C) = _______

Highest draw (A):
(18-amp tablesaw) × 1.25 = 22.5

Highest simultaneous (B):
(18-amp saw+11-amp dust coll.) = 29

Continuous (C):
(lights, heat, television, air filter) = 24

Minimum Amps Required (A+B+C) = 75.5

Powering this shop adequately requires 80-amps of extra capacity in the existing service panel, or an 80-amp subpanel.

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.

A: The most common wiring for residential use is non-metallic sheathed cable, called type NM-B, shown in the photos. If you run your wiring inside walls, this is your likely choice. In surfacemounted conduit, individual insulated wires are acceptable. Underground feeder cable (type UF-B) looks similar, and gets used in damp areas or for underground burial.

In addition to the right type, you need the correct size, or American wire gauge (AWG), which is dictated by the amperage the wire must carry. The larger the wire number, the smaller the gauge. You can always use heavier-gauge wire than specified, but never use lighter gauge. It may get hot enough to melt the insulation and short out. Color coding used by most manufacturers these days simplifies identification.

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, below, was our first clue.

A: No matter how many outlets your shop contains, you may occasionally need an extension cord. Keep the following rules in mind:

1. The longer the cord, the fewer amps it can handle, and the greater the voltage drop will be across it's length.
2. The lighter the gauge (larger AWG number), the fewer amps the cord can handle.

For example, a 50'-long, 12-gauge cord can handle 15 amps. At 150', though, a 12-gauge cord can't handle more than 10 amps.

The lesson: For shop use, buy only 10- or 12-gauge extension cords that are no longer than necessary for the job.