At the highest level, we could break electrical circuits into 2 categories: branch circuits and feeder circuits. Branch circuits describe the conductors from the last overcurrent protection device to the final load/source. For example, the conductors from a 15A breaker in your house's main panel to a receptacle would be considered a branch circuit. The current flowing through a branch circuit is entirely decided by the a single piece of equipment, whether that is a receptacle, a solar module, a motor, or something else. Feeder circuits are the conductors upstream of branch circuits. A panel could have branch circuits supplying various loads: motors, heaters, lights, receptacles, etc. The conductors that supply the panel have to consider ALL of these loads operating together.
So how do we size feeder circuits and branch circuits? Branch circuits are easy, and the sizing requirements for conductors and overcurrent protection devices are discussed in a variety of other articles on Breaker & Fuse. Feeder circuits are more interesting. The load on a feeder circuit for conductor sizing is based on the sum of the following:
125% of the continuous load (loads that run for 3 hours or more continuously)
100% of the noncontinuous load
125% of the largest individual motor load
100% of the remaining motor load
The overcurrent protection is sized based on the sum of the following:
125% of the continuous load
100% of the noncontinuous load
The largest overcurrent protection device rating required by any individual motor
100% of the remaining motor load
Overload protection isn't required at the feeder level. Motors will be protected from overload at the branch circuit level. Why don't we size the feeder circuit to carry 125% of all motor loads, not just the largest? Motor overload protection is accomplished via a dedicated relay system and short circuit protection is via a device with a much higher long-time trip rating. The result is that the systems won't be prone to nuisance tripping like a typical continuous load. If several motors will be started simultaneously, additional consideration should be made for increased ampacity.
Comments