Because electric motors have a tremendous amperage draw during the starting phase, wire and fuse sizes must be calculated very carefully. Even during the run cycle of a motor, if the voltage is low it will cause the motor to overheat and may cause failure.

Electrical flow in a wire is similar to water flow in a pipe. If a short pipe is connected to an abundant water source with a constant pressure and a faucet and pressure gauge connected to the end, the gauge would hardly drop when the faucet is opened. But extend the pipe a considerable length and an extreme drop occurs during the test. The pressure drop is caused by the fric- tion of the water moving against the stationary sides of the pipe. To achieve an adequate supply of water from the faucet, you would have to increase the size of the pipe so when the faucet was turned on only a slight variation in pressure occurs. The same principle applies to current in a wire.

When we connect a wire to a fuse or circuit breaker of proper size to handle the starting load requirements of a particular motor, we can test for voltage at the wire’s end and will have the same voltage as at the fusing device. Hook the motor to the wire ends and turn it on. If the voltage drops considerably, the wire size is too small. Low voltage can cause the motor to fail.

Motors should be fused with a time-delay fuse rated 175% of the motor’s full load operating amperage, 250% of the full load operating amperage if a circuit breaker is used. The wire to a motor must be rated to carry 25% more amperage than the motor draws when operating at full load. This amperage rating is listed on most motors. As stated earlier, it may be necessary to increase the wire by several sizes to compensate for voltage drop caused by excessive distances of the feeder run.


V = 2(R) x (L) x (I) 1000

Where “V” is voltage drop, “L” is the length of the run in feet, “R” is the resistance of the wire in ohms per thousand feet, and “I” the amount of current drawn in amperes. Voltage drop is based on 3%. Horsepower amperage from Table 430-248 and resistance of wires from Table 8, Chapter 9 of NFPA National Electrical Code. Fuse and breaker sizes are derived from motor data calculators published by major electrical manufacturers.