9.2 Motor Overload Protection (Article 430 Part III)

Overload vs. Short-Circuit Protection — Two Different Jobs

A motor circuit has two protective zones, and confusing them is the classic exam trap:

• Branch-circuit short-circuit/ground-fault protection (430.52, Table 430.52) protects the conductors and the motor circuit against fault-level current. It is sized from the Table FLC (430.6(A)(1)) and can be 250% or more of FLC — high enough to ride out inrush.
• Motor running overload protection (430.32) protects the motor against sustained current slightly above its nameplate rating that would overheat the insulation over minutes or hours. It is sized from the motor nameplate FLA (430.6(A)(2)) and is set tightly — typically 115% or 125%.

A combination starter (contactor + overload block + OCPD in one listed enclosure) handles both jobs, but the calculations are independent.

The 125% vs. 115% Rule — 430.32(A)(1)

For continuous-duty motors rated more than 1 hp without integral thermal protection, 430.32(A)(1) sets the maximum separate overload device rating as a percentage of motor nameplate FLA:

The service factor is the safety margin printed on the nameplate (e.g., SF 1.15 means the motor can carry 15% above rated horsepower intermittantly). A motor with a 1.15 service factor can tolerate a slightly higher overload trip setting because its thermal headroom is larger.

Worked Example — SF 1.15 Motor, Nameplate 17 A

• Determine the multiplier: SF 1.15 ≥ 1.15 → use 125%.
• Overload trip rating = 17 A × 1.25 = 21.25 A.
• Select the next lower standard overload heater or selectable electronic trip rating that does not exceed 21.25 A (e.g., a 21 A overload block). If the motor will not start with that setting, 430.32(C) allows raising it toward 140% of nameplate (17 × 1.40 = 23.8 A) — but never above.

Worked Example — SF 1.0 Motor, Nameplate 26 A (Separate Overload Relay)

• Read the nameplate: SF 1.0, temperature rise > 40°C → use the 115% column of 430.32(A)(1).
• Maximum trip rating = 26 × 1.15 = 29.9 A.
• Selecting a separate overload relay. Overload relays come as bimetallic heater elements (sized from a manufacturer's chart) or electronic overloads with a dial or DIP-switch range. For a bimetallic heater, look up the element rated at or below 29.9 A — if the nearest available heaters are 28 A and 32 A, pick the 28 A heater (next lower size), because 430.32 caps the rating at 115% of nameplate and you may not exceed it. For an electronic overload with 0.1 A steps, set the dial at 29.9 A or the next lower setting (e.g., 29.5 A).
• Start-up bump (430.32(C)). If the motor will not start at that setting, raise the trip rating toward the 130% ceiling for SF < 1.15 motors: 26 × 1.30 = 33.8 A, and no higher. The 140% ceiling applies only to motors with SF ≥ 1.15 or temp rise ≤ 40°C.

Where the Nameplate Comes From

The motor nameplate FLA reflects the actual motor design at its rated voltage, frequency, and duty. Two 10 hp motors from different manufacturers can have slightly different nameplate FLA values — that is exactly why 430.6(A)(2) uses nameplate for overload (the protection must track the actual motor's heating) while 430.6(A)(1) uses the Table value for everything else (replacement motors must still have adequate conductor and short-circuit protection).

Continuous Duty vs. Intermittent Duty

430.32(A) covers continuous-duty motors — the default assumption on the exam. Intermittent-duty motors (short-time rated motors such as hoists, valve actuators, or crane motors) are covered by 430.32(B) and are sized to the duty-cycle nameplate current, with different — often higher — permitted percentages. Always confirm duty classification on the nameplate before applying the 125%/115% rule. If a question gives you only horsepower and voltage with no duty statement, assume continuous duty.

Integral Thermal Protection

Some motors carry internal thermal protectors (TP motors per 430.32(A)(2)–(A)(4)). When the motor is marked to indicate that integral protection complies with 430.32, separate overload protection is not required. The exam will usually ask whether separate overload protection is required — read the nameplate marking first.

Why Overload and Branch Protection Are Both Required

A frequent error is treating the overload relay and the branch-circuit OCPD as interchangeable — they are not. The overload relay (430.32) is a slow, low-magnitude device: it trips after seconds-to-minutes of current 115–125% above nameplate, protecting the motor's insulation from cumulative heating. The branch-circuit OCPD (430.52) is a fast, high-magnitude device: it clears in cycles when current reaches 250% (breaker) or 175% (fuse) of Table FLC, protecting conductors against bolted faults. A motor can run at 130% of nameplate indefinitely without tripping a 250% inverse-time breaker — which is why the Code requires both devices in series. The two occupy different parts of the time-current curve and neither substitutes for the other.

Common Exam Traps

1. Using Table FLC instead of nameplate FLA for overload sizing — wrong current.
2. Applying 125% to a motor with SF 1.0 — should be 115%.
3. Forgetting the 430.32(C) start-up bump and insisting the original setting is the only permitted value.
4. Treating overload protection as a substitute for branch-circuit short-circuit protection — they are independent and both are required.

Quick Check

If a question gives you a 15 hp, 460V, 3-phase motor with nameplate 19 A and SF 1.15:

• Conductor ampacity uses Table FLC 21 A × 1.25 = 26.25 A → #10 AWG.
• Breaker uses Table FLC 21 A × 2.50 = 52.5 A → 60 A (next standard up).
• Overload uses nameplate 19 A × 1.25 = 23.75 A → 23 A overload block.

Three different starting currents, three different devices — and the exam loves to mix them.