Motor Circuit Design with NEC Article 430

Why NEC Uses Table FLA, Not Nameplate FLA

This is the single most important concept in motor circuit design, and it confuses electricians and engineers more than any other NEC rule. NEC 430.6(A)(1) requires that conductor ampacity and branch circuit protection be based on the values in Tables 430.247 through 430.250, not the nameplate full-load current. The only exception is overload protection, which uses nameplate current per 430.6(A)(2).

The reason is interchangeability. NEC table values represent the highest current that any motor of a given HP and voltage rating would draw, regardless of manufacturer. A 10 HP, 460V motor from Manufacturer A might have a nameplate FLA of 12.8A, while the same rating from Manufacturer B shows 13.6A. The NEC table lists 14A. If you size the conductors and protection for the 12.8A nameplate value and the motor is later replaced with the 13.6A model, the circuit is undersized. Using the table value ensures the circuit works for any compliant motor of that rating.

Overload protection is different because it protects the specific motor installed. A motor with a 1.15 service factor can handle 115% of its nameplate current continuously, so the overload is set at 125% of nameplate FLA per 430.32(A)(1). A motor with a 1.0 service factor gets overloads set at 115% of nameplate FLA. These percentages are based on the actual motor's thermal characteristics, which is why the nameplate value is correct here.

The practical effect: conductors and breakers are sized from the table (larger value), while overload heaters or electronic overloads are sized from the nameplate (specific value). Mixing these up is a code violation and a common inspection failure.

The 208V vs 200V Question

NEC Table 430.250 has separate columns for 200V and 230V motors. A 208V system does not appear in the table, and this creates confusion. Which column do you use?

The answer depends on the motor nameplate. Motors rated "200V" use the 200V column. Motors rated "230V" or "208-230V" use the 230V column. The 200V column exists because some motors are specifically designed and wound for 200V systems, and they draw higher current than 230V motors of the same HP because the lower voltage requires more current to produce the same power.

Here is where it gets tricky. A 208V power system delivers 208V line-to-line, which is 120 x sqrt(3). This is not the same as 240V (which comes from a 120/240V single-phase center-tapped transformer). A motor rated "230V" running on a 208V system operates at 208/230 = 90.4% of its rated voltage. NEMA MG 1-2016 Section 12.47 allows motors to operate at plus or minus 10% of rated voltage, so 208V on a 230V motor is within tolerance but at the low edge.

The practical impact: at 208V, a 230V-rated motor draws approximately 10% more current than at 230V for the same mechanical load, runs slightly hotter, and has reduced starting torque. For a 25 HP motor, the NEC table lists 34A at 230V but does not have a 208V column. Some designers use the 200V column (40A) for 208V applications as a conservative approach. Others use the 230V column (34A) and verify that the actual nameplate current at 208V does not exceed the conductor ampacity.

The safest approach: if the motor is specifically rated for 208V or 200V, use the 200V column. If the motor is rated 230V or "208-230V," use the 230V column but verify voltage drop does not push the motor terminal voltage below 90% of nameplate rating. On long runs where voltage drop is significant, the higher current draw at reduced voltage creates a compounding problem.

Conductor Sizing per NEC 430.22

NEC 430.22 is concise: branch circuit conductors for a single motor must have an ampacity of at least 125% of the motor FLA from Table 430.250. This 25% margin accounts for the motor's continuous duty rating and ensures conductors do not overheat during sustained full-load operation.

For a 25 HP, 460V, 3-phase motor: Table 430.250 lists 34A. Required ampacity: 34 x 1.25 = 42.5A. From NEC Table 310.16, 8 AWG copper THHN is rated 50A at 75 degrees C. This is adequate for the ampacity requirement.

However, you must also check voltage drop. NEC does not mandate a maximum voltage drop for branch circuits, but Informational Note No. 4 to 210.19(A)(1) recommends a maximum of 3% on the branch circuit, with 5% total (feeder plus branch circuit). For motor circuits, voltage drop is especially critical because reduced voltage increases current draw and reduces starting torque.

Voltage drop for a three-phase circuit: VD = (sqrt(3) x I x R x L) / 1000, where I is the full-load current, R is the resistance per 1000 feet from NEC Chapter 9 Table 9, and L is the one-way distance in feet. For 8 AWG copper at 75 degrees C, R = 0.764 ohms per 1000 feet (AC resistance). At 34A and 150 feet: VD = 1.732 x 34 x 0.764 x 150 / 1000 = 6.75V. Percentage: 6.75 / 460 = 1.5%. This is within the 3% recommendation.

If the voltage drop exceeds 3%, increase the conductor size until it meets the recommendation. This may mean using a conductor larger than the minimum required by 430.22. In the example above, if the run were 400 feet instead of 150, the voltage drop with 8 AWG would be 18V or 3.9%, requiring an increase to 6 AWG (VD = 2.8%) or even 4 AWG for conservative design.

For feeders supplying multiple motors, NEC 430.24 requires ampacity of at least 125% of the largest motor FLA plus 100% of all other motor FLAs on the feeder. This rule recognizes that while all motors may run simultaneously, only one needs the 125% continuous duty margin at the conductor level.

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). For motors with a service factor of 1.0 or a temperature rise not exceeding 40 degrees C, 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.

Branch Circuit Short-Circuit and Ground-Fault Protection

Branch circuit protection for motors is sized per NEC 430.52 and Table 430.52. This protection does not protect the motor from overload (that is the overload relay's job). It protects the conductors from short-circuit and ground-fault currents. Because motor starting current is 6 to 8 times the running current and lasts for several seconds, the branch circuit protection must be sized large enough to ride through the starting inrush without tripping.

Table 430.52 specifies maximum percentages of FLA for different protection types. For inverse-time circuit breakers (the most common type): 250% of FLA for standard motors. For dual-element (time-delay) fuses: 175% of FLA. For instantaneous-trip circuit breakers (motor circuit protectors): 800% to 1300% depending on motor type.

Example: 25 HP, 460V motor, FLA = 34A (from Table 430.250). Maximum breaker size: 34 x 2.50 = 85A. The next standard breaker size is 90A per 240.6(A). NEC 430.52(C)(1)(a) allows rounding up to the next standard size if the calculated value does not correspond to a standard rating.

If the motor does not start on a 90A breaker (trips during inrush), Exception No. 1 to 430.52(C)(1) permits increasing to the next standard size. For inverse-time breakers, the absolute maximum is 400% of FLA: 34 x 4.0 = 136A, rounded to a 150A breaker. If the motor still does not start on 150A, the problem is not the breaker size; investigate the motor, power supply, or mechanical load.

For feeders supplying multiple motors, NEC 430.62 requires the feeder protection to be no larger than the largest motor's branch circuit protection plus the sum of the FLAs of all other motors on the feeder. This ensures the feeder protection coordinates with the individual branch circuit protection devices.

The coordination between overloads, branch circuit protection, and feeder protection is the heart of Article 430 motor protection. Each device has a specific job: overloads protect the motor, branch circuit protection protects the conductors, and feeder protection protects the feeder conductors. Sizing any one of them incorrectly breaks the coordination and creates either a nuisance tripping problem or a safety hazard.

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

This section brings everything together for a complete motor circuit design. The motor is 25 HP, 460V, 3-phase, with a nameplate FLA of 31.8A, service factor 1.15, and code letter G. The circuit runs 175 feet from the MCC in an ambient temperature of 30 degrees C.

1. FLA from NEC Table 430.250: 34A for a 25 HP, 460V, 3-phase motor.

2. Conductor sizing (NEC 430.22): 34 x 1.25 = 42.5A minimum ampacity. NEC Table 310.16, 75 degrees C column: 8 AWG copper THHN = 50A. Check voltage drop: VD = 1.732 x 34 x 0.764 x 175 / 1000 = 7.88V = 1.7%. Acceptable. Use 8 AWG THHN copper.

3. Conduit sizing: Three 8 AWG THHN conductors plus one 10 AWG ground (per 250.122, based on 90A breaker = 10 AWG). NEC Chapter 9, Table 1: maximum fill for 3+ conductors = 40%. Chapter 9, Table 5: 8 AWG THHN = 0.0366 sq in, 10 AWG THHN = 0.0211 sq in. Total = 3(0.0366) + 0.0211 = 0.1309 sq in. Required conduit area at 40% fill: 0.1309 / 0.40 = 0.327 sq in. NEC Chapter 9, Table 4: 3/4-inch EMT has 0.213 sq in (too small), 1-inch EMT has 0.346 sq in (adequate). Use 1-inch EMT.

4. Overload relay (NEC 430.32): Nameplate FLA 31.8A x 1.25 (SF = 1.15) = 39.75A. Select overload heater or set electronic OL to 31.8A, Class 20.

5. Branch circuit protection (NEC 430.52): 34A x 2.50 = 85A. Next standard size = 90A inverse-time breaker.

6. Equipment grounding conductor (NEC 250.122): Based on the 90A breaker: 10 AWG copper.

7. Disconnect (NEC 430.110): Must be rated at least 115% of FLA: 34 x 1.15 = 39.1A. Use a 60A fusible or non-fusible disconnect switch (next standard size above 39.1A). If the disconnect contains fuses, they serve as the branch circuit protection and must be sized per Step 5.

This seven-step process produces a complete bill of materials for the motor circuit: conductor size and type, conduit size, overload device, breaker size, ground conductor, and disconnect switch. Every size traces directly to a specific NEC section, making the design defensible during plan review and inspection.

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. Next standard size = 125A 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.