12.2 Sizing Motor Branch-Circuit Conductors & Overload Protection

12.2 Sizing Motor Branch-Circuit Conductors & Overload Protection

Exam Fast Fact: When sizing branch-circuit conductors for a single continuous-duty motor under NEC 430.22, you must multiply the Table FLC by 125% (1.25). When sizing separate overload protection for the same motor under NEC 430.32(A)(1), you multiply the Nameplate FLA by either 125% (if Service Factor >= 1.15 or temperature rise <= 40°C) or 115% (all other motors). Never mix these two multipliers or their current bases!

A complete motor branch circuit requires two distinct levels of thermal and overcurrent protection operating in tandem. The conductors must be capable of carrying sustained operating currents plus starting transients without thermal degradation. Concurrently, the motor windings require a sensitive thermal protective device—the overload relay—calibrated to trip when mechanical friction, motor stalling, single-phasing, or sustained mechanical overload draws excessive current through the copper windings.

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Sizing Motor Branch-Circuit Conductors: NEC 430.22

Branch-circuit conductors supplying a single motor used in a continuous-duty application must have an ampacity of not less than 125 percent of the motor's full-load current rating as determined by NEC 430.6(A)(1) (NEC Tables 430.247–430.250).

Minimum Conductor Ampacity = Table FLC * 1.25

Why the 125% Multiplier is Mandated

Continuous-duty motors generate continuous heat in upstream distribution conductors. Because a motor circuit frequently operates for hours under full mechanical load, the conductors are treated identically to continuous loads under NEC 210.19(A) and 215.2(A). Furthermore, the 125% headroom ensures that normal starting currents and minor voltage variations do not cause conductor temperatures to exceed their insulation ratings.

Step-by-Step Conductor Sizing Example

Problem: Determine the minimum copper conductor size (THHN/THWN-2) for a 10 HP, 208-volt, 3-phase squirrel-cage induction motor operating under continuous duty. Assume terminations are rated for 75°C per NEC 110.14(C).

1. Step 1 — Find Table FLC: Open NEC Table 430.250. Look up 10 HP under the 208V column. Table FLC = 30.8 A
2. Step 2 — Apply NEC 430.22 Multiplier: Minimum Required Ampacity = 30.8 A * 1.25 = 38.5 A
3. Step 3 — Select Conductor from Table 310.16:
   • Check the 75°C column of Table 310.16 for copper conductors:
     • 10 AWG copper has an ampacity of 35 A (insufficient, 35 A < 38.5 A).
     • 8 AWG copper has an ampacity of 50 A (compliant, 50 A >= 38.5 A).
   • Conclusion: The minimum permissible branch-circuit conductor is 8 AWG Copper THHN/THWN-2.

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Terminal Temperature Limitations & Derating Integration

When sizing motor conductors, electricians must strictly adhere to NEC 110.14(C) termination provisions while factoring in ambient temperature correction and conduit fill adjustments:

• NEC 110.14(C)(1)(a): Circuits rated 100 amperes or less, or marked for 14 AWG through 1 AWG conductors, must be sized using the 60°C column of Table 310.16, unless the equipment and terminals are listed and marked for 75°C.
• Modern Motor Equipment: Standard industrial motor control centers, across-the-line starters, and molded-case circuit breakers are almost universally listed for 75°C terminations. Motor lead junction boxes (peckerheads) are typically rated for 75°C or 90°C. Therefore, unless an exam question specifies 60°C terminals, standard commercial/industrial practice coordinates with the 75°C column for final ampacity selection.
• Derating Rule: If derating factors apply (such as more than 3 current-carrying conductors in a raceway per Table 310.15(C)(1), or elevated ambient temperatures per Table 310.16), you begin derating from the 90°C column for THHN wire, but the final derated ampacity must not exceed the conductor's 75°C terminal rating or drop below the 125% FLC requirement.

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Non-Continuous & Duty-Cycle Motors: NEC Table 430.22(E)

Not all motors run continuously. Equipment such as freight elevators, cranes, hoists, drawbridges, and motorized gate valves operate intermittently. Sizing conductors for these applications using the continuous 125% rule would result in massive, unnecessary copper over-sizing.

Under NEC 430.22(E), conductors supplying motors operating on short-time, intermittent, periodic, or varying duty are sized using the percentage multipliers found in Table 430.22(E) applied to the motor nameplate current rating:

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Motor Overload Protection: NEC Article 430 Part III

Motor overload protection guards against excessive heating caused by sustained mechanical loads, operating over full load rating, electrical undervoltage, single-phasing on 3-phase systems, or bearing failure.

Critical Difference: An overload is an operating overcurrent condition that stays within normal conduction paths (not a short circuit or ground fault). Overloads develop heat slowly over seconds or minutes. Branch-circuit short-circuit devices (breakers and fuses) are sized much too high (175% to 300%) to sense low-grade overloads. Without a dedicated overload device, a motor drawing 135% of its rated current would overheat and catch fire long before the branch breaker tripped.

Overload Sizing Rules: NEC 430.32(A)(1)

For continuous-duty motors rated more than 1 horsepower, separate overload devices (such as thermal overload relays, heaters, or electronic overcurrent relays) are sized based on the nameplate full-load current (FLA) according to a strict two-tier percentage criteria:

Maximum Overload Trip Setting = Nameplate FLA * Percentage

Step-by-Step Overload Calculation Example

Problem: A 3-phase, 460V, 25 HP induction motor has a nameplate rating of 30.0 A, a marked Service Factor of 1.15, and a marked temperature rise of 40°C. Table 430.250 lists an FLC of 34.0 A. What is the maximum initial overload protection device rating permitted under NEC 430.32(A)(1)?

1. Identify the correct current base: Sizing overloads mandates using Nameplate FLA = 30.0 A. (Discard the 34.0 A table value).
2. Identify the motor's thermal rating: The motor has a Service Factor of 1.15 (and a temperature rise of 40°C). This qualifies for the 125% tier.
3. Calculate maximum overload rating: Overload Setting = 30.0 A * 1.25 = 37.5 A

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Maximum Modification of Overload Devices: NEC 430.34

In field installations, a motor driving a high-inertia load (such as a large centrifugal exhaust blower or heavy industrial flywheel) may take a long time to accelerate up to operating speed. The sustained starting current may cause the standard 115% or 125% overload device to trip before the motor reaches full running speed.

To address this nuisance tripping, NEC 430.34 permits selecting the next higher size overload relay or increasing the trip setting, subject to an absolute statutory ceiling:

Exam Trap Alert: The 140% and 130% values in NEC 430.34 are exceptions, not initial sizing values! You can never jump directly to 140% or 130% on a test question unless the question explicitly states that the standard overload device selected under 430.32 was insufficient to start the motor or carry the load.

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