Plan for electrical demand when building a farm shop


MARSHFIELD, Mo. — An energy-efficient machinery maintenance shop is a necessity on the modern farm according to Bob Schultheis, a natural resource engineering specialist with University of Missouri Extension.

“A well-planned, well-equipped shop encourages a farmer to practice on-going preventive maintenance on his equipment, which extends its useful life and also reduces the chances of costly downtime during field operations,” said Schultheis.


A shop can also serve as a place for repairing or modifying farm equipment and for working on the family car, farm trucks, recreation vehicle or even hobby projects.

“Providing adequate electrical capacity for both current use and future growth is important from both a functional and safety perspective,” said Schultheis.

He recommends installing at least a 200-amp, 240-volt service to the building. “Locate the service entrance panel near a walk door for emergency shut-off access, and plan for expansion with enough branch circuits with generous wire sizes to avoid the expense of rewiring in the future,” said Schultheis.


Schultheis offers the following basic guidelines for electrical outlets:

Install one 20-amp duplex outlet for each 4 feet of work bench, and put it under the front edge to keep cords off the bench.

Use an outlet to serve each permanent motor-driven tool that is one-half HP or less.

Locate outlets at 10-foot intervals around the shops perimeter, 4 feet above the floor.

Install ground-fault circuit interrupters on all circuits where tools might be operated in damp locations, and on any outside outlets.

Individual branch circuits must be used for outlets serving motors larger than one-half HP. Plan for one circuit to serve no more than three one-third HP motors, two one-half HP motors, or one 1 HP motor or larger.

Use at least a 50-amp, 240-volt, 2-wire with ground circuit and outlet for a welder. Locate it just inside the door, or add a second weatherproof outlet on an outdoor yard pole, so larger machinery can be repaired outside.


Because ceiling heights vary, shop lighting is difficult to specify, but Schultheis offers the following general tips:

For general indoor lighting, provide at least 20 foot-candles of illumination at floor level. That’s equivalent to one double-tube, 4-foot fluorescent fixture for every 80-100 square feet of floor area, assuming an 8-foot mounting height. Light-colored ceilings and upper walls help.

For task lighting, use double-tube, 4-foot fluorescent fixtures mounted about 4 feet above the work bench and positioned toward the front half.

Use incandescent lamps over rotating tools like grinding wheels to avoid the strobe effect.

For general outdoor lighting, use 200-400 watt high-pressure sodium or metal halide lamps, mounted at a 15-25 foot height, with photocell control. Space lights no more than 125 feet apart for best lighting conditions. Place a 150-watt spotlight, with motion sensor activation, on one or both sides of the entrance door instead of over the door, to keep out flying insects.


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