Feeders for a group of motors, and the part everybody over-applies
Only one motor in the group gets the 125 percent. Applying it to all of them produces a number that looks careful and is wrong.
On this page
The rule is a sum of four things
When a feeder supplies several motors, or motors mixed with other loads, its conductors are sized from a sum with four parts (NEC 430.24).
- 125 percent of the full-load current of the highest rated motor.
- 100 percent of the full-load currents of every other motor in the group.
- 100 percent of the non-motor load that is not continuous.
- 125 percent of the non-motor load that is continuous.
All the motor currents come from the Article 430 table, not from nameplates.
Two of the four parts carry a 125 percent and two carry 100 percent, and they interleave. That is what makes this easy to misremember.
Why only one motor gets the bump
The 125 percent exists to cover starting current. In a group, the realistic worst case is that everything is already running and one motor starts.
The code assumes that motor is the largest one, sizes for its start, and takes the rest at their running current. Adding starting margin to every motor would assume they all start at the same instant, which is not the condition the feeder is built for.
Understanding that makes the rule stick. The 125 percent is not a safety factor sprinkled over the whole group. It is one motor starting.
Highest rated does not mean highest horsepower
The rule says the highest rated motor as determined from the full-load current table, which means the motor with the largest table current.
On a group that is all the same voltage and phase, that is the same motor as the biggest horsepower. On a mixed group it can be a different motor. A single-phase motor and a three-phase motor of the same horsepower draw very different currents, and the smaller horsepower can be the larger current.
Sort the group by table current, not by the horsepower printed in the question. The question will sometimes list them in horsepower order to encourage the shortcut.
Continuous and noncontinuous, on the non-motor side only
The continuous versus noncontinuous split applies to the non-motor load, not to the motors. Motors are already handled by the first two parts.
So a feeder carrying three motors and a bank of lighting gets: the largest motor at 125 percent, the other two motors at 100 percent, and the lighting at 125 percent because lighting on a commercial feeder is normally continuous.
Candidates who apply the continuous rule to the motors as well end up adding the same margin twice.
The feeder device is a different sum
The conductor and the device protecting it are not sized the same way, which is unusual and is exactly why it gets tested.
The conductor takes 125 percent of the largest motor current plus the others. The feeder protective device is built from the largest branch-circuit protective device in the group plus the full-load currents of the other motors.
One sum starts from a current. The other starts from a device rating. Mixing them is the standard wrong answer, and because both produce a plausible number, neither one looks obviously incorrect on the page.
Read 430.62 in your own book before you rely on the exact wording. It is short, and the difference between the two sums is the entire point of the section.
Four exceptions worth knowing exist
The several-motors rule has exceptions that narrow it, covering interlocked circuits where motors cannot run at the same time, service other than continuous duty, conductors already protected at their own ampacity, and equipment marked by the manufacturer with a minimum circuit ampacity.
That last one matters in the field more than on the exam. A packaged rooftop unit or a multimotor machine arrives with a minimum circuit ampacity on its nameplate, and that number replaces the calculation entirely. You do not add up the internal motors of a listed assembly.
Read the exceptions in your code book rather than working from this summary. Their conditions are specific and a paraphrase is not good enough to answer on.
What this page cites
- NEC 430.24 Conductors supplying several motors or a motor and other loads, including the four exceptions.
- NEC 430.62 Feeder short-circuit and ground-fault protection. Read the wording directly.
- NEC 430.6(A)(1) Full-load currents come from the table, not the nameplate.
- NEC 430.52 The branch-circuit device rating that feeds into the feeder device sum.