Introduction

Motors comprise a significant portion of a building's electrical system loads. They are needed to power fans and pumps for basic mechanical building infrastructure, such as heating, ventilation, air-conditioning, plumbing, fire protection, elevators, and escalators. They are also needed to power equipment endemic to the occupancy, such as commercial kitchen equipment in an institutional facility, CT and MRI scanners in a hospital, and process equipment such as conveyors and machinery in an industrial plant or stone quarry Consequently, designing motor-circuit feeders is very much in the mainstream of the electrical design professional's daily work. To save time in this process, the following information is provided.

Sizing Motor-Circuit Feeders and Their Overcurrent Protection

1. For AC single-phase motors, polyphase motors other than woundrotor (synchronous and induction other than Code E): ^{[1] , [2]}

   1. Feeder wire size is 125 percent of motor full-load (FL) current minimum.

   2. Feeder breaker (thermal-magnetic fixed-trip type) is 250 percent of FL current maximum.

   3. Feeder breaker (instantaneous magnetic-only type) is 800 percent of FL current maximum.

   4. Feeder fuse (dual-element time-delay type) is 175 percent of FL current maximum.

   5. Feeder fuse (NEC non-time-delay type) is 300 percent of FL current maximum.

2. For wound-rotor motors:

   1. Feeder wire size is 125 percent of motor FL current minimum.

   2. Feeder breaker (thermal-magnetic fixed-trip type) is 150 percent of FL current maximum.

   3. Feeder breaker (instantaneous magnetic-only type) is 800 percent of FL current maximum