11.1 Motor Nameplate vs. Table FLC & Branch Circuit Conductor Sizing
Key Takeaways
- Under NEC 430.6(A)(1), conductors, switches, branch-circuit short-circuit and ground-fault protection, and feeder overcurrent protection MUST be sized using the Full-Load Current (FLC) values published in NEC Tables (430.247, 430.248, 430.249, 430.250), NEVER the motor nameplate rating.
- Motor nameplate Full-Load Amperes (FLA) is strictly reserved for sizing separate motor overload protection devices under NEC 430.32.
- NEC 430.22 requires single-motor branch-circuit conductors supplying a continuous-duty motor to have an allowable ampacity of not less than 125% (1.25) of the motor table FLC.
- Conductor sizing for non-continuous duty motors (short-time, intermittent, periodic, varying) must be calculated using the duty-cycle percentages from NEC Table 430.22(E).
- Conductor selection must comply with NEC 110.14(C) terminal temperature ratings (60°C for equipment rated 100A or less unless marked 75°C; 75°C for equipment rated over 100A).
11.1 Motor Nameplate vs. Table FLC & Branch Circuit Conductor Sizing
Electric motors represent one of the most dynamic and heavily tested subjects on the Idaho Journeyman Electrician Examination. Unlike standard resistive or lighting loads, motors exhibit substantial inductive characteristics, severe inrush currents during startup (often 600% or more of normal operating current), and varying operational duty cycles. Consequently, NEC Article 430 establishes a specialized set of engineering and installation rules that diverge significantly from standard branch-circuit calculations.
To successfully design, install, and troubleshoot motor circuits—and to score 100% on motor exam questions—electricians must master the fundamental distinction between Table Full-Load Current (FLC) and Nameplate Full-Load Amperes (FLA), the mandatory conductor sizing multipliers, duty-cycle adjustments, and terminal temperature ratings.
1. The Fundamental Mandate: NEC 430.6(A)(1) Golden Rule
The most pervasive trap on electrical licensing exams involves selecting the incorrect current value when performing circuit calculations. NEC 430.6(A)(1) establishes an uncompromising divide between when to use the NEC Tables versus when to read the physical motor nameplate.
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| NEC 430.6(A)(1) CURRENT SELECTION MATRIX |
| |
| [USE NEC TABLES: 430.247, 430.248, 430.249, 430.250] |
| ---------------------------------------------------- |
| * Branch-Circuit Conductor Sizing (NEC 430.22) |
| * Multimotor Feeder Conductor Sizing (NEC 430.24) |
| * Branch Short-Circuit & Ground-Fault Protection (NEC 430.52) |
| * Feeder Short-Circuit Protection (NEC 430.62) |
| * Motor Disconnecting Means Sizing (NEC 430.110) |
| * Motor Controller Ratings (NEC 430.83) |
| |
| [USE MOTOR NAMEPLATE (FLA)]: |
| ---------------------------- |
| * Motor Overload Protection Sizing ONLY (NEC 430.32 & 430.34) |
| * Thermal Overload Relays / Heater Elements |
| * Solid-State Electronic Overload Trip Settings |
+-----------------------------------------------------------------------------+
Why Does the NEC Mandate Table FLC for Conductors and Switches?
Motor manufacturers build motors with varying efficiencies and power factors. A high-efficiency 10 HP motor might draw only 12 amperes at 460V, whereas a standard or older motor of the same horsepower might draw 14 amperes.
If the branch circuit conductors and disconnect switch were sized strictly for the 12-ampere nameplate motor, replacing that motor in the future with a standard 14-ampere motor would create an undersized, hazardous installation. Therefore, the NEC mandates using conservative, standardized Table FLC values for all upstream conductors, switches, and short-circuit protective devices. Conversely, overload protection is designed to protect the specific motor winding from overheating and burning out; hence, it must be matched to the exact nameplate FLA of that specific motor.
Primary NEC Motor Tables
- Table 430.247: Full-Load Current in Amperes, Direct-Current Motors (DC)
- Table 430.248: Full-Load Current in Amperes, Single-Phase Alternating-Current Motors
- Table 430.249: Full-Load Current in Amperes, Two-Phase Alternating-Current Motors (4-Wire)
- Table 430.250: Full-Load Current in Amperes, Three-Phase Alternating-Current Motors
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| EXCERPT: NEC TABLE 430.248 (SINGLE-PHASE AC MOTORS) |
| |
| Horsepower (HP) | 115 Volts | 200 Volts | 208 Volts | 230 Volts |
| :---------------- | :------------ | :------------ | :---------- | :-------- |
| 1/2 HP | 9.8 A | 5.6 A | 5.4 A | 4.9 A |
| 3/4 HP | 13.8 A | 7.9 A | 7.6 A | 6.9 A |
| 1 HP | 16.0 A | 9.2 A | 8.8 A | 8.0 A |
| 1.5 HP | 20.0 A | 11.5 A | 11.0 A | 10.0 A |
| 2 HP | 24.0 A | 13.8 A | 13.2 A | 12.0 A |
| 3 HP | 34.0 A | 19.6 A | 18.7 A | 17.0 A |
| 5 HP | 56.0 A | 32.2 A | 30.8 A | 28.0 A |
| 7.5 HP | 80.0 A | 46.0 A | 44.0 A | 40.0 A |
| 10 HP | 100.0 A | 57.5 A | 55.0 A | 50.0 A |
+-----------------------------------------------------------------------------+
+-----------------------------------------------------------------------------+
| EXCERPT: NEC TABLE 430.250 (THREE-PHASE INDUCTION AC MOTORS) |
| |
| Horsepower (HP) | 208 Volts | 230 Volts | 460 Volts | 575 Volts |
| :---------------- | :------------ | :------------ | :---------- | :-------- |
| 3 HP | 10.6 A | 9.6 A | 4.8 A | 3.9 A |
| 5 HP | 16.7 A | 15.2 A | 7.6 A | 6.1 A |
| 7.5 HP | 24.2 A | 22.0 A | 11.0 A | 9.0 A |
| 10 HP | 30.8 A | 28.0 A | 14.0 A | 11.0 A |
| 15 HP | 46.2 A | 42.0 A | 21.0 A | 17.0 A |
| 20 HP | 59.4 A | 54.0 A | 27.0 A | 22.0 A |
| 25 HP | 74.8 A | 68.0 A | 34.0 A | 27.0 A |
| 30 HP | 88.0 A | 80.0 A | 40.0 A | 32.0 A |
| 40 HP | 114.0 A | 104.0 A | 52.0 A | 41.0 A |
| 50 HP | 143.0 A | 130.0 A | 65.0 A | 52.0 A |
| 75 HP | 211.0 A | 192.0 A | 96.0 A | 77.0 A |
| 100 HP | 273.0 A | 248.0 A | 124.0 A | 99.0 A |
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[!NOTE] Voltage Interpolation Rule for Table 430.250: When a motor operates at 200 volts nominal (rather than 208V or 230V), or at 115 volts 3-phase, the NEC instructs users to increase the 230V rating by 15% for 200V systems, or multiply the 230V rating by 2 for 115V systems.
2. Single-Motor Branch Circuit Conductor Sizing (NEC 430.22)
Under NEC 430.22, branch-circuit conductors that supply a single motor used in a continuous-duty application must have an allowable ampacity of not less than 125% (1.25) of the motor full-load current rating as determined by NEC 430.6(A)(1).
Minimum Conductor Ampacity = I_Table_FLC * 1.25
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| SINGLE-MOTOR BRANCH CIRCUIT SIZING FLOW |
| |
| [STEP 1] Identify Motor Phase, HP, and System Operating Voltage. |
| [STEP 2] Look up Table FLC in Table 430.248 (1-Ph) or Table 430.250 (3-Ph).|
| [STEP 3] Multiply Table FLC by 125% (1.25) for Continuous Duty. |
| [STEP 4] Apply 110.14(C) Terminal Temperature Ratings to Table 310.16. |
| [STEP 5] Apply Ambient Temperature & Bundling Deratings (if applicable). |
| [STEP 6] Select Final AWG / kcmil Conductor Size. |
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Why 125%?
The 125% factor accounts for the continuous heating effect of motor operating currents, transient overloads, and minor voltage fluctuations, preventing thermal fatigue on conductor insulation and terminal terminations over decades of service.
3. Continuous Duty vs. Intermittent Duty Ratings (NEC Table 430.22(E))
Not all motors operate continuously. Applications such as passenger elevators, overhead bridge cranes, drawbridges, freight hoists, and valve actuators operate for brief intervals or with long cooling cycles between operations. For these specialized non-continuous applications, NEC 430.22(E) mandates the use of duty-cycle multiplying percentages from NEC Table 430.22(E) rather than the standard 125% continuous multiplier.
Master Summary of NEC Table 430.22(E) Percentages
| Classification of Service | 5-Minute Rated Motor | 15-Minute Rated Motor | 30- & 60-Minute Rated Motor | Continuous Rated Motor |
|---|---|---|---|---|
| Short-Time Duty (Valves, raising/lowering rolls) | 110% | 120% | 150% | — |
| Intermittent Duty (Pumps, elevators, toolheads) | 85% | 85% | 90% | 140% |
| Periodic Duty (Rolls, ore-handling machines) | 85% | 90% | 95% | 140% |
| Varying Duty (Shop tools, general hoists) | 110% | 120% | 150% | 200% |
[!WARNING] Default to Continuous Duty: Any motor application must be treated as continuous duty unless the nature of the apparatus it drives is inherently such that the motor cannot operate continuously under load. If an exam question does not explicitly state a short-time, intermittent, or periodic duty cycle, always apply the mandatory 125% continuous duty rule under NEC 430.22.
4. Terminal Temperature Ratings (NEC 110.14(C)) Applied to Motors
Selecting a conductor size requires coordinating the conductor ampacity from NEC Table 310.16 with the temperature ratings of the termination lugs under NEC 110.14(C).
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| TERMINAL TEMPERATURE RULES (NEC 110.14(C)) |
| |
| [CIRCUITS RATED 100A OR LESS] (or conductors 14 AWG through 1 AWG): |
| * Default rating is 60°C (Table 310.16 60°C column). |
| * 75°C column permitted ONLY if all equipment terminals (breaker, starter,|
| disconnect, motor junction box) are listed and marked for 75°C. |
| |
| [CIRCUITS RATED OVER 100A] (or conductors larger than 1 AWG): |
| * Default rating is 75°C (Table 310.16 75°C column). |
| |
| [USING 90°C CONDUCTORS (e.g., THHN, XHHW-2)] |
| * 90°C ampacity is used for derating (ambient temp & raceway fill). |
| * Final adjusted ampacity CANNOT exceed the terminal rating (usually 75°C)|
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Most modern magnetic starters, disconnect switches, and circuit breakers are listed with dual 60°C/75°C or straight 75°C terminals. Therefore, in commercial and industrial exam problems, the 75°C column of Table 310.16 is standard unless 60°C terminals are specifically identified.
5. Step-by-Step Worked Conductor Sizing Calculations
Let us work through three real-world calculation scenarios directly reflecting the Idaho Journeyman licensing exam format.
Calculation Problem 1: Single-Phase 230V Motor
Scenario: Calculate the minimum size 75°C copper THHN branch-circuit conductors required for a 5 HP, 230V, single-phase, continuous-duty squirrel-cage induction motor. The motor nameplate reads: FLA = 24.8 A, SF = 1.15, Temp Rise = 40°C.
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| PROBLEM 1 STEP-BY-STEP SOLUTION |
| |
| STEP 1: Select Current Value per NEC 430.6(A)(1). |
| - Ignore nameplate FLA (24.8 A). |
| - Go to Table 430.248: 5 HP at 230V = 28.0 A. |
| |
| STEP 2: Apply NEC 430.22 Continuous Duty Multiplier (125%). |
| - Required Ampacity = 28.0 A x 1.25 = 35.0 A. |
| |
| STEP 3: Select Conductor from Table 310.16 (75°C Copper). |
| - 12 AWG Cu (75°C) = 25 A (Too small) |
| - 10 AWG Cu (75°C) = 35 A (Exactly matches 35.0 A) |
| |
| FINAL RESULT: Minimum conductor size is 10 AWG Copper. |
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Calculation Problem 2: Three-Phase 460V Motor
Scenario: Determine the minimum size 75°C copper THWN-2 branch-circuit conductors supplying a 15 HP, 460V, three-phase, continuous-duty motor with 75°C-rated terminal terminations.
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| PROBLEM 2 STEP-BY-STEP SOLUTION |
| |
| STEP 1: Table Lookup per NEC 430.6(A)(1). |
| - Go to NEC Table 430.250: 15 HP at 460V = 21.0 A. |
| |
| STEP 2: Calculate Minimum Conductor Ampacity per NEC 430.22. |
| - Ampacity = 21.0 A x 1.25 = 26.25 A. |
| |
| STEP 3: Select Conductor from Table 310.16 (75°C Copper). |
| - 14 AWG Cu (75°C) = 20 A (Too small) |
| - 12 AWG Cu (75°C) = 25 A (Too small, 25 A < 26.25 A) |
| - 10 AWG Cu (75°C) = 35 A (Adequate, 35 A >= 26.25 A) |
| |
| FINAL RESULT: Minimum conductor size is 10 AWG Copper. |
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Calculation Problem 3: Three-Phase 208V Motor with Ambient Temperature Derating
Scenario: A 25 HP, 208V, 3-phase continuous-duty motor is installed in an industrial boiler room where the ambient temperature is 40°C (104°F). The circuit consists of three copper THHN (90°C) conductors in EMT connected to 75°C rated terminals. Determine the minimum wire size.
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| PROBLEM 3 STEP-BY-STEP SOLUTION |
| |
| STEP 1: Determine Table FLC (NEC Table 430.250). |
| - 25 HP at 208V 3-Phase = 74.8 A. |
| |
| STEP 2: Calculate Minimum Base Conductor Ampacity (NEC 430.22). |
| - Minimum Required Ampacity = 74.8 A x 1.25 = 93.5 A. |
| |
| STEP 3: Check Ambient Temperature Correction Factor (Table 310.15(B)(1)). |
| - For 90°C insulation at 40°C ambient, Correction Factor = 0.91. |
| |
| STEP 4: Evaluate Conductor Candidates: |
| - Try 3 AWG THHN (90°C Table 310.16 rating = 115 A): |
| Derated Ampacity = 115 A x 0.91 = 104.65 A. |
| Check 75°C Terminal Limit: 3 AWG (75°C) = 100 A. |
| Since 100 A >= 93.5 A and derated 104.65 A >= 93.5 A, |
| 3 AWG Cu satisfies all conditions! |
| |
| FINAL RESULT: Minimum conductor size is 3 AWG Copper. |
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[!TIP] Quick Math Verification: On the exam, always verify that your selected conductor's 75°C ampacity is greater than or equal to 1.25 * Table FLC, AND that the conductor's derated 90°C ampacity is also greater than or equal to 1.25 * Table FLC.
Under NEC 430.6(A)(1), which current rating must be used to size the branch-circuit conductors supplying a standard 3-phase induction motor?
What is the minimum branch-circuit conductor ampacity required under NEC 430.22 for a 3 HP, 230-volt, single-phase continuous-duty motor with a nameplate FLA of 14.8 A and a Table 430.248 FLC of 17.0 A?
When sizing branch-circuit conductors for a 15-minute rated short-time duty motor driving a freight valve mechanism, what code table determines the required conductor ampacity percentage multiplier?