11.1 Motor Full-Load Current (FLC), Branch Circuit Conductor Sizing & Disconnects (430.6, 430.22 & 430.102)
Key Takeaways
- Under NEC 430.6(A)(1), motor branch-circuit conductors, disconnect switches, and short-circuit/ground-fault protective devices MUST be sized using the Full-Load Current (FLC) values published in NEC Tables 430.247–430.250, rather than motor nameplate FLA.
- Motor nameplate FLA is used strictly for sizing separate motor overload protection devices under NEC 430.6(A)(2) and 430.32, as well as for torque and special-duty motors under 430.6(B).
- Branch-circuit conductors supplying a single continuous-duty motor must have an allowable ampacity of not less than 125% of motor table FLC (NEC 430.22), with duty-cycle motor percentages ranging from 85% to 200% per Table 430.22(E).
- NEC 430.102 mandates a disconnecting means located 'in sight from' (visible and ≤50 ft) the controller and motor, rated not less than 115% of motor table FLC (NEC 430.110(A)) and horsepower-rated per NEC 430.109.
- Under the NEC 430.102(B) Exception, the local motor disconnect may be omitted if the controller disconnect has a permanent lockout provision complying with NEC 110.25 where local placement is impractical or introduces increased hazard.
Motor Full-Load Current (FLC), Branch Circuit Conductor Sizing & Disconnects (NEC 430.6, 430.22 & 430.102)
Electric motors represent inductive dynamic loads characterized by massive initial starting currents (inrush locked-rotor currents ranging from 4 to 8 times normal running current) and continuous mechanical duty. Because standard branch circuit rules from Article 210 cannot safely accommodate these high inrush currents without causing persistent nuisance tripping or conductor overheating, the National Electrical Code dedicates Article 430 to motor circuits. On the Oklahoma Journeyman Electrician examination, navigating Article 430 requires precise execution of table lookups, sizing formulas, and disconnect placement rules.
1. NEC Table FLC vs. Motor Nameplate FLA (NEC 430.6)
A primary source of calculation errors on licensing examinations is confusing Table Full-Load Current (FLC) with Nameplate Full-Load Amperes (FLA). NEC 430.6 establishes strict legal boundaries governing when each value must be applied:
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| TABLE FLC VS. NAMEPLATE FLA (NEC 430.6) |
| |
| APPLICATION MANDATORY VALUE SOURCE |
| ----------------------------------------------------------------------- |
| - Branch-Circuit Conductor Sizing NEC TABLES (430.248, 430.250) |
| - Feeder Conductor Sizing NEC TABLES (430.248, 430.250) |
| - Disconnect Ampere / HP Rating NEC TABLES (430.248, 430.250) |
| - Short-Circuit & Ground-Fault OCPD NEC TABLES (430.248, 430.250) |
| - Motor Controller Rating NEC TABLES (430.248, 430.250) |
| ----------------------------------------------------------------------- |
| - Motor Overload Protection (Heaters) MOTOR NAMEPLATE FLA (430.6(A)(2)) |
| - Separate Thermal Protection MOTOR NAMEPLATE FLA (430.6(A)(2)) |
| - Torque & Multi-Speed Motors MOTOR NAMEPLATE FLA (430.6(B)) |
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Why the NEC Mandates Table Values for Conductor and Switch Sizing
Motor nameplates reflect the specific electrical characteristics (efficiency, power factor, winding resistance) of a single proprietary motor design. If branch circuit conductors, disconnects, and circuit breakers were sized strictly to a high-efficiency nameplate rating, replacing that motor in the future with a standard-efficiency motor of identical horsepower would dangerously overload the conductors and cause overcurrent protective devices to trip. The values in NEC Tables represent industry-standard conservative averages, ensuring that the supply infrastructure safely supports any standard motor of that horsepower rating.
NEC Motor Full-Load Current Tables Summary:
- NEC Table 430.247: Direct-Current (DC) Motors (120V, 240V).
- NEC Table 430.248: Single-Phase AC Motors (115V, 200V, 208V, 230V).
- NEC Table 430.249: Two-Phase AC Motors (4-Wire).
- NEC Table 430.250: Three-Phase AC Induction and Synchronous Motors (115V, 200V, 208V, 230V, 460V, 575V, 2300V).
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| REPRESENTATIVE VALUES: NEC TABLE 430.248 (1-PHASE AC) |
| |
| HORSEPOWER (HP) 115 VOLTS 208 VOLTS 230 VOLTS |
| ----------------------------------------------------------------------- |
| 1/2 HP 9.8 A 5.4 A 4.9 A |
| 3/4 HP 13.8 A 7.6 A 6.9 A |
| 1 HP 16.0 A 8.8 A 8.0 A |
| 1.5 HP 20.0 A 11.0 A 10.0 A |
| 2 HP 24.0 A 13.2 A 12.0 A |
| 3 HP 34.0 A 18.7 A 17.0 A |
| 5 HP 56.0 A 30.8 A 28.0 A |
| 7.5 HP 80.0 A 44.0 A 40.0 A |
| 10 HP 100.0 A 55.0 A 50.0 A |
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+-----------------------------------------------------------------------------+
| REPRESENTATIVE VALUES: NEC TABLE 430.250 (3-PHASE AC) |
| |
| 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 |
| 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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Voltage Interpolation Rule (Table 430.250 Footnote): To obtain full-load current ratings for 200-volt and 208-volt motors, increase the 230-volt table ratings by 15% and 10%, respectively, or reference the explicit 200V/208V columns provided in modern NEC editions.
2. Single-Motor Branch Circuit Conductor Sizing (NEC 430.22)
Branch circuit conductors supplying a single continuous-duty motor must have an allowable ampacity of not less than 125% of the motor full-load current (FLC) determined from NEC Tables 430.247 through 430.250:
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| MOTOR BRANCH CIRCUIT CONDUCTOR SIZING |
| |
| [Branch OCPD] ---> [Conductor (>= 125% FLC)] ---> [Controller] ---> [Motor]|
| |
| 1. Find FLC in Table 430.248 (1-ph) or Table 430.250 (3-ph). |
| 2. Multiply Table FLC by 1.25 (125%). |
| 3. Select conductor from NEC Table 310.16 at 75°C terminal rating. |
| 4. Apply temperature/conduit adjustment factors to 90°C base if needed. |
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Duty-Cycle and Non-Continuous Motors (NEC 430.22(E))
Motors that do not operate continuously (such as valves, cranes, hoists, elevators, and rolling mills) generate significantly less cumulative thermal heat. NEC Table 430.22(E) permits conductor ampacity percentages based on the duty classification and operating duration:
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| DUTY CYCLE CONDUCTOR SIZING PERCENTAGES (NEC TABLE 430.22(E)) |
| |
| DUTY CLASSIFICATION 5-MIN RATING 15-MIN RATING 30/60-MIN CONTINUOUS|
| ----------------------------------------------------------------------- |
| Short-Time Duty 110% 120% 150% --- |
| Intermittent Duty 85% 85% 90% 140% |
| Periodic Duty 85% 90% 95% 140% |
| Varying Duty 110% 120% 150% 200% |
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Step-by-Step Calculation: Continuous Duty 3-Phase Motor
Scenario: Calculate the minimum branch circuit conductor size (75°C THHN Copper) for a 15 HP, 460-volt, 3-phase squirrel-cage induction motor operating continuously. The motor nameplate reads: FLA = 18.5A, 460V, 3-phase, SF = 1.15.
- Determine Motor FLC: Look up 15 HP at 460V in NEC Table 430.250. Table FLC = 21.0 Amperes (ignore the 18.5A nameplate FLA).
- Apply 125% Conductor Multiplier (NEC 430.22):
- Select Conductor from NEC Table 310.16 (75°C Copper):
- #12 AWG THHN Cu has a 75°C rating of 25 Amperes (insufficient: $25\text{ A} < 26.25\text{ A}$).
- #10 AWG THHN Cu has a 75°C rating of 35 Amperes (compliant: $35\text{ A} \ge 26.25\text{ A}$).
- Note on Small Conductor Rule: Under NEC 240.4(G), motor branch circuits are governed by Article 430 rather than the 30A limit of 240.4(D)(7).
3. Motor Disconnecting Means (NEC 430.102, 430.109 & 430.110)
The motor disconnecting means provides electrical isolation to ensure that maintenance personnel can de-energize the controller, motor, and driven machinery before servicing.
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| MOTOR DISCONNECT LOCATION ARCHITECTURE |
| |
| [PANELBOARD] |
| Branch OCPD |
| | |
| v (Feeder / Branch) |
| [CONTROLLER DISCONNECT] <==== "IN SIGHT FROM" ====> [CONTROLLER] |
| - Visible and <= 50 ft (430.102(A)) |
| | |
| v |
| [MOTOR DISCONNECT] <========= "IN SIGHT FROM" ====> [MOTOR & MACHINERY] |
| - Visible and <= 50 ft (430.102(B)) |
| - Exception: Single lockable disconnect at controller |
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Location Requirements (NEC 430.102)
- Controller Disconnect (NEC 430.102(A)): A disconnecting means shall be located in sight from the controller location.
- Motor Disconnect (NEC 430.102(B)): A disconnecting means shall be located in sight from the motor location and the driven machinery location.
- Definition of "In Sight From" (NEC Article 100): Where one piece of equipment must be "in sight from" another, the specified equipment must be visible and not more than 50 feet (15 meters) away. If a wall, partition, or distance exceeding 50 feet separates the disconnect from the motor, it is NOT in sight.
Lockable Disconnect Exception (NEC 430.102(B) Exception)
An individual disconnecting means is NOT required at the motor location if the controller disconnecting means is individually capable of being locked in the open position in accordance with NEC 110.25, provided one of the following conditions is met:
- Locating the disconnect in sight from the motor is impracticable or introduces increased hazards to persons or property (e.g., submerged pumps, chemical cleanrooms, high-temperature kilns, hazardous classified locations).
- In industrial installations with written safety procedures where conditions of maintenance and supervision ensure only qualified persons service the equipment.
NEC 110.25 Lockout Requirement: The provision for locking must remain in place with or without the lock installed. Portable lockout hasps or clip-on attachments that can be detached do NOT satisfy this rule unless permanently attached to the switch or enclosure.
Disconnect Type and Ratings (NEC 430.109 & 430.110)
| Code Section | Requirement | Engineering Standard |
|---|---|---|
| NEC 430.109(A) | Type of Disconnect | Must be a listed horsepower-rated switch, a listed molded-case circuit breaker, or a listed manual motor controller. General-use switches are prohibited unless permitted by specific exceptions. |
| NEC 430.110(A) | Ampere Rating | The disconnecting means for motor circuits rated 1000V or less must have an ampere rating of not less than 115% of the motor full-load current (FLC) from NEC Tables. |
| NEC 430.109(B) | Fractional HP Motors (≤ 1/8 HP) | A general-use switch with an ampere rating of at least twice the motor FLC is permitted for motors rated 1/8 HP or less. |
| NEC 430.109(C) | Stationary Motors (≤ 2 HP) | A general-use AC snap switch is permitted if its ampere rating is at least 125% of motor FLC, or an AC-DC general-use snap switch rated not less than twice motor FLC. |
An electrician is sizing branch circuit conductors for a 10 HP, 230-volt, single-phase continuous-duty induction motor. The motor nameplate lists a full-load current of 44.0 amperes. What is the minimum required conductor ampacity under the National Electrical Code?
When is it permissible under NEC 430.6(A)(2) to use the motor nameplate Full-Load Amperes (FLA) rating instead of the full-load current values listed in NEC Tables 430.247 through 430.250?
An industrial facility installs a 50 HP, 460-volt, 3-phase motor with a Table 430.250 full-load current of 65 amperes. The motor controller is located in an electrical room 120 feet away from the motor and out of sight. Which of the following installations complies with NEC 430.102(B)?
What is the minimum continuous ampere rating required by NEC 430.110(A) for a disconnecting means serving a 20 HP, 460-volt, 3-phase induction motor with a Table 430.250 full-load current rating of 27.0 amperes?