Calculating Electrical Usage in a Workshop - Popular Woodworking Magazine

Calculating Electrical Usage in a Workshop

 In Shop Blog

Editor’s note: Doing research for turning my detached garage into a shop space, I came across this section on electrical in the Woodworking Essentials book. It’s full of great info on setting up a shop (and everything else you should know about woodworking). I’ll be running a new subpanel and new outlets and lighting, so I was glad to find all of this electrical info in one place, and I’m glad to be able to share it with you.

Calculating Electrical Usage in the Workshop

When electrical circuits or subpanels are added to an electrical service, the total load must not exceed the service rating. Gener- ally, older homes have 100-amp service and newer homes have 200-amp service. If you are not certain about the service, look at the main circuit breaker in the service panel.

If you have 100-amp service, consult with both the utility company and a licensed electrician regarding upgrading to a 200- amp service.

Generally, home woodshops will have one woodworker using no more than two machines at one time (table saw and dust collector, drill press and vacuum). The advantage of this is that the electrical system isn’t going to need to support the simultaneous operation of all the woodshop’s machinery. As you plan the electrical layout of your shop, make a best guess as to the frequency of use of tools and machines.

Not only will this aid in determining circuit requirements, but it will also aid in planning the placement of outlets. It’s often easier to install separate outlets on separate circuits than to have one circuit with multiple outlets. For example, in my woodshop I have three machines requiring 220v service: the table saw, jointer/planer and band saw. Rarely, if ever, are two machines running at the same time. So it’s possible for the three machines to have their outlets wired to the same circuit.

However, these machines are located in different areas of the woodshop and it was much easier to install outlets at each of the machine locations and route wires through one or more conduits. Since there was adequate space in the subpanel, it was a straightforward addition of circuit breakers and wire. The exception to this is the dust collector, which is also 220v. Because it’s operated simultaneously with each of the stationary machines, there was no choice – it required its own circuit.

The National Electrical Code sets minimum capacities for circuits regarding use and amperage:

  • Small appliances: 20 amperes
  • General lighting: 15 or 20 amperes
  • Stationary tools: multiply the machine’s amperage by 125 percent. The 125 percent factors the electrical surge that occurs when a machine is first switched on. For example, a 12″ planer rated at 15 amps (1.25 X 15 = 18.75) will require a 20-amp circuit.

Outlets, Switches & Plugs

When designing a new electrical layout for your workshop, placement of outlets and switches requires planning, guesswork and a bit of luck. Work projects, new machines, relocation of cabinets, stacks of lumber and other fluctuating events will block existing outlets and switches from access. Often, well-thought-out locations aren’t that handy once the woodshop is actually used. The ideal situation is never having to use extension cords because you have outlets wherever you work. This can be accomplished simply by locating outlets 3′ to 5′ apart throughout the woodshop, including the ceiling. is may seem excessive, but consider the many work conditions that occur away from the workbench area: Using a vacuum, sander, plate joiner, rotary carving tool or heat gun are but a few.

If the workshop area is a new construction, cables should be installed within the wall framework. If wall coverings are already in place, outlets can be installed on the outside of the wall. Always check your local electrical codes concerning external installations. External conduit adds flexibility to designing and locating outlets because conduit can be routed just about anywhere. Metal conduit pipe can be bent to a variety of shapes and angles and conduit pipe can be cut to length wherever necessary.

Outlets

There are four acceptable plugs for home woodshop use:

  • Grounded three-prong, 120v, 15 amp
  • Grounded three-prong, 120v, 20 amp
  • Ground fault circuit interrupter, 120v, 15 amp and 20 amp
  • Grounded three-prong 220v/240v

Ungrounded two-prong, 120v receptacles are unacceptable in workshops. If an existing workshop has ungrounded two-prong outlets, return to the main power and replace them with grounded outlets. If there isn’t a ground wire to the outlet, attach one from the outlet to the receptacle box or the nearest cold water pipe. Check that the ground is functional by using a ground tester. To accommodate most tools, use grounded three-prong 20-amp outlets.

Ground fault circuit interrupter outlets (GFI) are designed to protect you from shock. GFI outlets monitor current; if the incoming and outgoing currents aren’t the same, the GFI instantly cuts o the electricity (in 1⁄40 second). A GFI outlet will trip if there is a ground fault of 0.005 amps. These outlets are generally in bathrooms and outdoor locations where someone may have wet hands and feet. Install GFI units outlets in damp basements or around sinks.

Switches

There are four basic types of switches:

  • Single-pole switches have two terminals: one for the incoming hot wire and one for the outgoing hot wire. e switch toggle is imprinted with ON/OFF.
  • Double-pole switches have four terminals and are used primarily for 240v circuits. The switch toggle is imprinted with ON/OFF.
  • Three-way switches have three terminals. One terminal is labeled COM (common), and the hot wire is connected to this terminal; the other two terminals are switch leads. Two three-way switches are used to control a circuit from two different locations. The toggle has no ON/OFF imprint.
  • Four-way switches have four terminals and are used with two three-way switches to control a circuit from more than two locations. The toggle has no ON/OFF imprint.

Switches are rated according to amperage and voltage, so choose the correct switch for compatibility with circuits, wire and outlets.

Plugs

Despite the proliferation of battery-powered tools, there are still many tools and machines that have AC plugs. Usually these plugs receive quite a lot of use and wear, often because of the neglectful act of pull- ing the cord and bending the prongs. When plugs need replacing, replace worn plugs with dead-front plugs. is type has no exposed wires or screws, and the prongs are surrounded by smooth plastic. If there are screws, they are recessed and are only for securing the plug body together.

When attaching wires to a three-prong 120v plug, connect the black wire to the brass terminal, the white wire to the silver terminal and the green wire to the green or gray terminal.

Polarized plugs have one prong with a wider tip. This type of plug is designed to fit into an outlet in only one direction. You’ll commonly encounter this kind of plug on smaller appliances and woodworking tools.

 


 

Woodworking EssentialsBased on a well-loved column from Popular Woodworking Magazine, “Woodworking Essentials” is a treasure trove of timeless woodworking instruction and advice for woodworkers of all skill levels. From shop set-up to techniques for using the most widely used woodworking tools and machines, this book is a reference you’ll turn to again and again.

Inside this woodworking book you’ll find:

  • Shop set-up ideas from location to lighting and power to organization to dust collection
  • Techniques, ideas and advice for using table saws, routers, miter saws, and many other widely-used woodworking machines
  • Best practices for casework construction
  • Safety guidelines and ideas for improving the way you work

With 200+ pages of fundamental woodworking information, Woodworking Essentials: Timeless Techniques for Woodworkers is the idea-packed reference you’ve been looking for.

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Showing 3 comments
  • C. Stanley Plane

    Leave the GFI in the kitchen or bathroom, no need for one in a shop. Also, if you ever want to incorporate a VFD into your workshop, the gfi will trip if it’s on the same circuit.

    • Andrew Zoellner
      Andrew Zoellner

      In my area, code requires GFCI outlets in the garage, which is where my shop will be. Good advice on the VFD – I didn’t know that a VFD will trip a GFCI.

      • Chip Sawdust

        Our local code called for a GFI in my garage as well. I did my own wiring so put one in. They’re cheap insurance either way. I have no 220 equipment but had an unused 30A 220V circuit in the garage. Gave me a lot of 110 outlets! Otherwise I had limited electricity (lighting and a few outlets) that wouldn’t run a table saw and DC at the same time.

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