7.3 Motor Overcurrent and Short-Circuit Protection

Why Motor Overcurrent Matters for the TX Journeyman Exam

Motors are Content Outline area (7) — Motors & Generators — and they generate a disproportionate number of Calculations questions because the sizing rules are multi-step. The Texas exam expects you to know that motor overcurrent is two separate protections, sized from two different FLC sources, using two different sets of percentages. Mixing them up is the most common reason candidates miss motor questions.

The Two Motor Protections

The short-circuit/ground-fault device must be capable of clearing a bolted fault at the motor terminals. The overload device must protect the motor from sustained overcurrent (stall, single-phasing, excessive load). A single device can rarely perform both functions at the currents a motor needs, so Article 430 splits them.

NEC 430.52 and Table 430.52 — Short-Circuit and Ground-Fault Protection

430.52 requires the branch-circuit short-circuit and ground-fault protective device to be sized from the motor full-load current listed in Table 430.250 (for three-phase AC motors) — not from the motor nameplate. The nameplate is used for overload protection only.

Table 430.250 is organized by horsepower and voltage. For example, a 5 hp, 230 V three-phase motor has a Table 430.250 FLC of 15.2 A.

Table 430.52 — Maximum Rating or Setting as a Percentage of FLC

The percentages are the maximum rating or setting. Where the calculated value does not match a standard 240.6(A) rating, 430.52(C)(1) Exception 1 permits the next higher standard size (up to 600 A). Above 600 A, the next lower standard size is used. Where the maximum still will not carry the starting current, Exception 2 allows further increases for specific device types — for example, an inverse-time breaker rated 800 A or less may be raised up to 400% for high-inrush loads.

NEC 430.58 — Motor Controllers

NEC 430.58 requires the motor controller to have a horsepower rating not less than that of the motor, and to be capable of carrying the motor's full-load current and interrupting stalled-rotor current. A controller rated in horsepower is tested for the duty; a general-purpose switch or breaker is not automatically a motor controller. For the exam, remember that the controller (contactor) is sized in horsepower, while the short-circuit device is sized from Table 430.250 FLC.

NEC 430.32 — Overload Protection

430.32 requires overload protection for each motor based on the motor nameplate full-load current (not Table 430.250). The maximum overload is set by Table 430.32:

For motors with a 1.0 service factor and 50 °C rise, the overload is set at 115% of nameplate FLC. For a 1.15 SF motor, 125% is permitted. Where the overload setting is not standard, the next lower size is used for overload relays (the opposite of the 430.52 next-size-up).

A separate overload device is not required where the motor has integral thermal protection approved for the application, or where the motor is part of listed equipment with its own overload protection.

Worked Example — 5 hp, 230 V Three-Phase Motor

Size the inverse-time circuit breaker (short-circuit/ground-fault) and the overload for a 5 hp, 230 V three-phase motor with a 1.15 service factor. The motor nameplate FLC is 15.0 A.

Short-Circuit and Ground-Fault Protection (430.52)

Step 1 — Get the FLC from Table 430.250. A 5 hp, 230 V three-phase motor = 15.2 A (Table 430.250, not the 15.0 A nameplate).

Step 2 — Apply Table 430.52. For an inverse-time circuit breaker on a standard polyphase motor, the percentage is 250%.

15.2 A × 2.50 = 38.0 A

Step 3 — Round to a standard rating (430.52(C)(1) Exception 1). 38.0 A is not a standard 240.6(A) rating. The next higher standard rating above 38 A is 40 A.

Result — short-circuit/ground-fault device: 40 A inverse-time circuit breaker.

Overload Protection (430.32)

Step 1 — Use the motor NAMEPLATE FLC. Nameplate = 15.0 A (not the 15.2 A Table value).

Step 2 — Apply Table 430.32. The motor has a 1.15 service factor, so the maximum overload is 125% of nameplate FLC.

15.0 A × 1.25 = 18.75 A

For a motor with a 1.0 service factor, the 115% value would be:

15.0 A × 1.15 = 17.25 A

Result — overload setting: 18.75 A (1.15 SF motor) or 17.25 A (1.0 SF motor). The next lower standard overload relay rating is selected.

Arithmetic Check

• 15.2 × 2.50 = 38.0 → next standard up is 40 A. Correct.
• 15.0 × 1.25 = 18.75 A. Correct.
• 15.0 × 1.15 = 17.25 A. Correct.

Note that the short-circuit calculation used the Table 430.250 value (15.2 A), and the overload calculation used the nameplate value (15.0 A). The exam will give you both numbers and try to bait you into using the wrong one for each calculation.

Common Exam Traps

• Using the nameplate for 430.52 sizing. Wrong. Use Table 430.250.
• Using Table 430.250 for 430.32 overload sizing. Wrong. Use the nameplate.
• Forgetting the 430.52(C)(1) next-higher-standard-size exception. 38 A is not a standard rating; the answer is 40 A, not 35 A.
• Applying 240.4(D) to motor branch circuits. 240.4(D) does not apply — Article 430 controls.
• Combining the two protections into one device. A standard inverse-time breaker sized at 250% of FLC is not the overload; a separate overload relay is required at 115%/125% of nameplate FLC.