Motor Circuit Calculations: FLC Tables, Overloads and Branch Circuits

Motor circuits follow their own rules in Article 430, and most of the exam traps come from mixing those rules up with general branch-circuit rules. The key idea: different parts of the motor circuit are sized from different current values, and the code tells you which one to use for each part. This guide covers the full-load current tables, conductor sizing, overload protection, the short-circuit and ground-fault device, and a feeder with several motors.

Two different currents

A motor has two current values that matter, and using the wrong one is the most common motor mistake:

• FLC from the tables: the full-load current from Table 430.248 (single-phase AC) or Table 430.250 (three-phase AC). Under 430.6(A)(1), you use these table values to size conductors, the short-circuit and ground-fault protection, and the disconnect.
• FLA from the nameplate: the motor's actual rated current. You use the nameplate value to size the overload protection under 430.32.

So the rule of thumb is: tables for wire and breaker, nameplate for overloads.

Values used in the examples

These are values commonly listed in Tables 430.248 and 430.250. Confirm them in your own code book.

Step 1: branch-circuit conductors (125%)

430.22 requires the conductors for a single continuous-duty motor to have an ampacity of at least 125% of the motor's table FLC.

Note that 240.4(D), the small-conductor rule, does not limit motor circuits the same way. 240.4(G) points motor circuits to Article 430, which is why a motor branch circuit can have a breaker larger than the conductor's ampacity.

Step 2: overload protection (nameplate)

430.32(A)(1) sets the maximum overload rating as a percentage of the nameplate full-load current:

• 125% for motors with a marked service factor of 1.15 or more,
• 125% for motors with a marked temperature rise of 40 °C or less,
• 115% for all other motors.

If the overload sized this way is not enough to start the motor, 430.32(C) allows a higher setting within stated limits. Use that only if a question tells you the motor will not start.

Step 3: short-circuit and ground-fault protection

Table 430.52 gives the maximum rating of the branch-circuit short-circuit and ground-fault protective device as a percentage of the table FLC. For common motor types:

When the percentage does not land on a standard size from 240.6(A), 430.52(C)(1) allows the next higher standard rating. If that still will not let the motor start, the code allows a further increase within limits listed in the same section.

Single-phase example

A 70 A breaker on 10 AWG copper looks wrong if you think in general branch-circuit terms. In a motor circuit it is correct, because the overloads protect the conductors from overload and the breaker only handles short circuits and ground faults.

Feeder for several motors

A feeder supplies three 460 V three-phase motors: 10 hp (14 A), 7-1/2 hp (11 A) and 5 hp (7.6 A).

Feeder conductors (430.24)

125% of the largest motor FLC, plus 100% of the others.

Feeder protection (430.62)

The feeder device may not be larger than the largest branch-circuit short-circuit device in the group, plus the sum of the FLCs of the other motors.

Here you round down. Unlike the branch-circuit device, 430.62 does not have a next-size-up allowance, so the feeder device cannot exceed 53.6 A and 50 A is the largest standard size under that.

Common motor mistakes

• Using nameplate FLA to size conductors or the breaker. Use the table FLC.
• Using table FLC to size overloads. Use the nameplate.
• Rounding the feeder device up under 430.62.
• Applying the 240.4(D) small-conductor limits to a motor branch circuit.
• Calculating current from horsepower with P = E × I. The code wants the table value, not a current you work out from horsepower.

Long motor runs can also fail on voltage drop, which hurts starting torque. Check a run with the voltage drop calculator, and see transformer sizing for the supply side of the same equipment.