Motor Circuit Source-Boundary Guide

Keep FLA Row Basis and Nameplate Basis Separate

Motor-circuit rules can use different current bases for different decisions. Some conductor and short-circuit/ground-fault protection checks point to code table values, while overload protection depends on selected motor nameplate data and device instructions. The exact rule, table, exception, and adopted-edition text must be checked in the controlling source.

ToolGrit local rows help keep that distinction visible, but they are not certified reproductions of current NEC tables. Treat each row as a prompt to verify the adopted code, selected motor, controller, overload device, conductor, terminal rating, and AHJ requirements.

The 208V vs 200V Question

208 V motor work is source-sensitive because the correct row basis can depend on the adopted NEC edition, motor nameplate, product instructions, voltage tolerance, and AHJ interpretation. ToolGrit may display local 208 V rows, but those rows must be checked against the controlling source before design use.

Also separate table lookup from field suitability. A motor operating near the low end of its voltage range may have different current, torque, heating, and starting behavior. Long runs, voltage drop, source impedance, VFD or soft-starter behavior, and driven-load acceleration should be reviewed with manufacturer data and qualified electrical judgment.

Conductor Ampacity Needs More Than One Multiplier

A common first pass is to multiply the selected FLA basis by 125 percent, then compare that ampacity to a conductor table. That is only a first pass. Final conductor selection also depends on terminal temperature, conductor insulation, copper or aluminum material, ambient correction, conductor-count adjustment, raceway or cable assembly, voltage drop, equipment listing, manufacturer instructions, grounding, disconnects, and local amendments.

The app wire row is therefore a planning value, not a conductor schedule. If the row appears acceptable, the next step is still to verify the adopted source text and the actual installation conditions. If voltage drop, temperature, fill, or equipment instructions push the design higher, the larger conductor or different wiring method may be required.

Overload Protection per NEC 430.32

Overload protection prevents the motor from overheating during a sustained mechanical overload (a jammed conveyor, a seized pump, a binding gearbox). It is sized based on the motor nameplate FLA, not the NEC table value, because the overload device protects this specific motor.

For motors with a service factor of 1.15 or greater (most general-purpose motors), the overload device must trip at no more than 125% of nameplate FLA per 430.32(A)(1). Motors with a marked temperature rise not exceeding 40 degrees C also get 125% per 430.32(A)(1). For all other motors (including most service factor 1.0 motors without a 40 degree C rise marking), the overload must trip at no more than 115% of nameplate FLA.

Example: A 25 HP, 460V motor with nameplate FLA of 31.8A and a service factor of 1.15. Overload trip setting: 31.8 x 1.25 = 39.75A. If using thermal overload heaters, select the heater that covers 39.75A from the manufacturer's heater selection table. If using an electronic overload relay, set the FLA dial to 31.8A and the trip class to Class 20 (standard for most applications).

Trip class defines how quickly the overload relay trips at a given overcurrent level. Class 10 trips faster (used for hermetic compressors and motors that can tolerate brief overloads). Class 20 is the default for most industrial motors. Class 30 trips slower (used for high-inertia loads like fans and flywheels that take longer to accelerate). Setting the wrong trip class can cause nuisance tripping on startup or inadequate protection during a genuine overload.

If the motor will not start with the overload sized at 125% (or 115%), NEC 430.32(C) allows increasing the overload to a maximum of 140% for motors with a service factor of 1.15, or 130% for motors with a service factor of 1.0. This should be a last resort after verifying that the starting problem is not caused by low voltage, incorrect wiring, or a mechanical issue.

Protection Percentages Are Not Final Device Selection

Motor branch-circuit short-circuit and ground-fault protection is not selected by multiplying FLA alone. The percentage estimate must be reconciled with the adopted code text, motor type, standard device sizes, time-current curves, starting current, controller and overload data, SCCR/AIC, selective coordination, equipment listing, manufacturer instructions, and AHJ requirements.

If a motor has starting or tripping problems, increasing a breaker or fuse size from an app output is not a diagnosis. Source impedance, voltage drop, low voltage, locked-rotor current, load inertia, mechanical binding, controller settings, overload class, and motor condition may be involved. A qualified electrical review is required before changing protection.

Review Example: 25 HP, 460V, 3-Phase Motor

A thorough review starts by recording the selected motor nameplate, the adopted code edition, voltage and phase, controller type, overload device, conductor and raceway method, ambient conditions, run length, available fault current, and AHJ constraints. A local FLA row can provide a first comparison point, but each downstream step must be checked against controlling source text and equipment data.

For a 25 HP, 460 V, three-phase motor, the app shows the NEC-table FLA row and simple percentage estimates. Those outputs are not a bill of materials. Before choosing wire, conduit, breaker, fuse, overload, disconnect, grounding conductor, or labels, verify terminal temperature, adjustment and correction factors, voltage drop, standard sizes, time-current curves, SCCR/AIC, coordination, manufacturer instructions, and safe-work requirements.

Multi-Motor Feeders and Group Installation

Most real installations have multiple motors on a single feeder or panel. NEC 430.24 and 430.62 govern these situations with rules that are straightforward but easy to misapply.

Feeder conductor sizing (430.24): Ampacity must be at least 125% of the largest motor FLA plus 100% of all other motor FLAs. Example: A feeder supplies three motors: 25 HP (34A), 10 HP (14A), and 5 HP (7.6A). Feeder ampacity = (34 x 1.25) + 14 + 7.6 = 42.5 + 14 + 7.6 = 64.1A. This requires 6 AWG copper THHN (65A at 75 degrees C) or 4 AWG for voltage drop margin on longer runs.

Feeder protection (430.62): The feeder overcurrent device must not exceed the largest motor's branch circuit protection plus the FLA of all other motors. Using the example above: largest motor branch circuit protection = 90A (from 34 x 2.5 = 85, rounded up). Feeder protection = 90 + 14 + 7.6 = 111.6A maximum. Because 430.62(A) sets a maximum (the next-size-up allowance of 240.4(B) does not apply), round DOWN to the next standard size: a 110A breaker.

Group installation (430.53): Under specific conditions, NEC allows multiple small motors (1 HP or less at 120V, or specific conditions for larger motors) to share a single branch circuit with one set of branch circuit protection. The motors must each have individual overload protection, and the branch circuit protection must not exceed the value permitted for the smallest motor in the group. This is common for HVAC unit heaters, exhaust fans, and other small motor loads on a shared 20A circuit.

When designing multi-motor panels (motor control centers), remember that each motor requires its own starter, overload, and disconnect. The panel bus must be rated for the sum of all motor FLAs plus any non-motor loads. Voltage drop calculations for the feeder use the total feeder current, while voltage drop for individual motor circuits uses only that motor's FLA from the panel to the motor.