NEC 2023 Electrical Exam: Motors & Controllers (Article 430)

Article 430 governs the sizing of motor branch-circuit conductors, overload protection, short-circuit and ground-fault protection, and disconnecting means requirements for motors, controllers, and adjustable-speed drives.

Key Points

• Motor conductor ampacity: always based on NEC Table FLC values, NOT motor nameplate current.
• Single motor branch-circuit conductors: minimum 125% of motor FLC (430.22).
• Multiple motor feeder: 125% of highest-rated motor FLC + 100% of all others (430.24).
• Overload protection trips at 125% FLC (SF ≥1.15 or temp rise ≤40°C); 115% for all others (430.32).
• Short-circuit protection is sized by Table 430.52 — dual-element fuse max 175%, inverse time breaker max 250% (single-phase).
• If Table 430.52 size won't allow motor to start, next higher standard size is permitted (430.52(C)(1)(a)).
• Disconnecting means must be in sight from the controller AND in sight from motor/driven machinery (430.102).
• Motor-circuit switch disconnecting means must be rated in horsepower, not amperes (430.109(A)(1)).
• Control circuit conductors extending beyond enclosure: max OCPD = 300% of Table 310.16 60°C value (430.72).
• Use NEC FLC tables for conductor sizing; use motor nameplate current for overload sizing.

Why It Matters: Motors are among the most complex NEC calculations — they involve four separate protection layers (branch-circuit conductors, overload, short-circuit/ground-fault, and feeder), each with different sizing rules and table references. Exam writers exploit the fact that FLC tables are used for conductors and OCPD sizing while nameplate current is used for overload — candidates who mix these up fail every motor question. In the field, incorrect motor protection causes nuisance tripping, motor burnout, and fire.

Terms To Remember

FLC (Full-Load Current)

NEC table value used for conductor and OCPD sizing — NOT nameplate.

Nameplate Current

Motor's actual rated current; used ONLY for overload device sizing.

Service Factor (SF)

Multiplier on nameplate indicating safe overload capacity; SF ≥1.15 allows 125% overload trip.

Overload Device

Protects motor from sustained overheating; trips at 115%–125% of nameplate FLC.

Branch-Circuit OCPD

Protects conductors from short-circuit and ground-fault; sized per Table 430.52.

Inverse Time Breaker

Breaker with purposeful trip delay that decreases as current increases.

Dual-Element Fuse

Time-delay fuse allowing motor inrush without nuisance opening; max 175% for single-phase.

LRA (Locked-Rotor Amperes)

Starting inrush current; much higher than FLC — why OCPD must be oversized.

In Sight

Visible and not more than 50 ft away; required for disconnect placement.

Motor-Circuit Switch

HP-rated disconnect switch; must be rated in horsepower, not amperes.

Controller

Device to start/stop a motor; HP rating must be ≥ motor HP rating.

Adjustable-Speed Drive (ASD)

Variable frequency drive (VFD); output conductors subject to waveform stress.

Step Process Formula

Motor Circuit Design: Single Motor vs. Multiple Motor Feeder — Side-by-Side

Fla Table Reference

Key FLC Values from NEC Tables 430.247–430.250 (Memorize These)

Single Phase 115V

Item 1

Hp: 1

Fla: 16A

Single Phase 230V

Item 1

Hp: 1.5

Fla: 10A

Item 2

Hp: 2

Fla: 12A

Three Phase 208V

Item 1

Hp: 2

Fla: 7.5A

Item 2

Hp: 7.5

Fla: 24.2A

Three Phase 230V

Item 1

Hp: 10

Fla: 28A

Three Phase 460V

Item 1

Hp: 15

Fla: 21A

Item 2

Hp: 25

Fla: 34A

Item 3

Hp: 50

Fla: 65A

Scenario A

Single Motor Branch Circuit

Given: 10 hp, 3-phase, 460V motor. Service factor = 1.15. Using dual-element fuse and copper conductors (60°C).

Fla: From Table 430.250: 10 hp, 460V, 3-phase → FLC = 14A

Steps

• Step 1 — CONDUCTOR SIZING (430.22): Min ampacity = FLC × 125% = 14A × 1.25 = 17.5A → use 12 AWG copper (20A, Table 310.16) — next size up.
• Step 2 — OVERLOAD PROTECTION (430.32): Based on nameplate current. If nameplate = 13.6A with SF ≥1.15 → max trip = 13.6A × 125% = 17A. Select overload relay rated ≤17A.
• Step 3 — SHORT-CIRCUIT / GROUND-FAULT PROTECTION (430.52, Table 430.52): Dual-element fuse max = FLC × 175% = 14A × 1.75 = 24.5A → use next standard size = 25A fuse.
• Step 4 — VERIFY: If 25A fuse won't allow motor to start due to inrush, 430.52(C)(1)(a) permits next higher standard size → 30A fuse.
• Step 5 — DISCONNECT (430.102, 430.109): Must be HP-rated motor-circuit switch (≥10 hp), located in sight from controller AND in sight from motor.

Summary Table

Conductors: 12 AWG Cu (min 17.5A ampacity → 20A rated)

Overload Trip: ≤17A (125% × nameplate)

Dual Element Fuse: 25A (175% × 14A FLC, rounded up)

Inverse Time Breaker Alternative: 35A (250% × 14A = 35A)

Disconnect: HP-rated switch ≥10 hp, in sight from controller and motor

Scenario B

Multiple Motor Feeder

Given: Three motors on one feeder: Motor 1 = 15 hp, 460V, 3-phase (highest rated). Motor 2 = 10 hp, 460V, 3-phase. Motor 3 = 5 hp, 460V, 3-phase. Using inverse time breaker.

Fla Values

Motor 1 15Hp 460V: 21A (Table 430.250)

Motor 2 10Hp 460V: 14A (Table 430.250)

Motor 3 5Hp 460V: 7.6A (Table 430.250)

Steps

• Step 1 — FEEDER CONDUCTOR SIZING (430.24): 125% of highest motor + 100% of all others. (21A × 1.25) + 14A + 7.6A = 26.25 + 14 + 7.6 = 47.85A → minimum 47.85A ampacity → use 8 AWG Cu (50A, Table 310.16).
• Step 2 — FEEDER OCPD (430.62): Max rating = largest individual branch-circuit OCPD + FLC of all other motors. Largest OCPD (Motor 1, inverse time breaker) = 21A × 250% = 52.5A → use 50A breaker. Other motors: 14A + 7.6A = 21.6A. Max feeder OCPD = 50A + 21.6A = 71.6A → next standard size down = 70A breaker.
• Step 3 — INDIVIDUAL BRANCH-CIRCUIT OCPDs: Size each per Table 430.52 for its own motor. Motor 1: 21A × 250% = 52.5A → 50A breaker. Motor 2: 14A × 250% = 35A breaker. Motor 3: 7.6A × 250% = 19A → 20A breaker.
• Step 4 — DISCONNECTS: Each motor requires its own disconnect, in sight from each controller, and in sight from each motor location.

Summary Table

Feeder Conductor: 8 AWG Cu (min 47.85A → 50A rated)

Feeder Ocpd: 70A inverse time breaker

Motor 1 Branch Ocpd: 50A inverse time breaker

Motor 2 Branch Ocpd: 35A inverse time breaker

Motor 3 Branch Ocpd: 20A inverse time breaker

Table 430 52 Quick Reference

Max Branch-Circuit OCPD as % of FLC (Table 430.52(C)(1))

Note: Single-phase values shown. Three-phase values are the same percentages.

Next Size Up Rule: If calculated OCPD size doesn't match a standard rating AND the motor won't start reliably, the next higher standard rating is permitted (430.52(C)(1)(a)).

Rows

Item 1

Device Type: Dual-element (time-delay) fuse

Max Percent Of Flc: 175%

Item 2

Device Type: Non-time-delay fuse

Max Percent Of Flc: 300%

Item 3

Device Type: Inverse time (standard) circuit breaker

Max Percent Of Flc: 250%

Item 4

Device Type: Instantaneous trip breaker (ITB)

Max Percent Of Flc: 800%

Overload Trip Percentages

Overload Device Trip Thresholds (430.32) — Based on NAMEPLATE Current

Rows

SF ≥ 1.15 OR temp rise ≤ 40°C

Condition: SF ≥ 1.15 OR temp rise ≤ 40°C

Max Trip Percent: 125% of nameplate FLC

All other motors

Condition: All other motors

Max Trip Percent: 115% of nameplate FLC

Motors 1 hp or less, non-automatically started

Condition: Motors 1 hp or less, non-automatically started

Max Trip Percent: No separate overload required

Control Circuit Conductor Protection

Motor Control Circuit OCPD — 430.72(B)(2)

Example: 14 AWG control conductors, extending beyond enclosure: 14 AWG @ 60°C = 15A. Max OCPD = 15A × 300% = 45A.

Rows

Conductors DO NOT extend beyond enclosure (Column B, Table 310.16)

Condition: Conductors DO NOT extend beyond enclosure (Column B, Table 310.16)

Max Ocpd: 100% of Table 310.16 ampacity for 60°C

Conductors EXTEND beyond enclosure (Column C, Table 310.16)

Condition: Conductors EXTEND beyond enclosure (Column C, Table 310.16)

Max Ocpd: 300% of Table 310.16 ampacity for 60°C

Common Confusions

• Students confuse FLC table values with nameplate current because both express motor current — FLC table values (Tables 430.247–430.250) are used for conductor sizing and OCPD sizing; nameplate current is used ONLY for overload device sizing per 430.32.
• Students confuse overload protection with short-circuit protection because both are 'overcurrent' devices — overload devices protect against sustained heating from motor overloads and failure to start (slow, thermal events); short-circuit/ground-fault OCPDs protect against sudden high-current faults (fast events); they are sized completely differently.
• Students confuse the 125% conductor rule with the 125% overload rule because the same percentage appears in both — 430.22 requires conductors at 125% of FLC table value; 430.32 requires the overload to trip at no more than 125% of nameplate current; one sizes wire, the other limits the overload trip point.
• Students confuse disconnect location requirements because two separate 'in sight' rules exist — 430.102(A) requires the disconnect to be in sight FROM the controller; 430.102(B) requires a disconnect in sight FROM the motor and driven machinery; both must be satisfied.

Quick Questions

A 3-phase, 460V, 15 hp motor has a nameplate current of 20A and SF = 1.15. What is the maximum overload trip setting?

Question: A 3-phase, 460V, 15 hp motor has a nameplate current of 20A and SF = 1.15. What is the maximum overload trip setting?

Correct Answer: 20A × 125% = 25A. Because SF ≥ 1.15, the overload device may be set up to 125% of the motor's nameplate current per NEC 430.32(A)(1). The FLC table value (21A) is NOT used for overload sizing.

What is the maximum dual-element time-delay fuse size for a single-phase, 230V, 1.5 hp motor?

Question: What is the maximum dual-element time-delay fuse size for a single-phase, 230V, 1.5 hp motor?

Correct Answer: From Table 430.248: FLC for 1.5 hp, 230V single-phase = 10A. Table 430.52 allows dual-element fuse at max 175% of FLC: 10A × 175% = 17.5A → next standard fuse size = 20A. (If 17.5A is a standard size it would be used; otherwise next up = 20A.)

Question 3

Question: Three motors share a feeder: 25 hp (34A), 10 hp (14A), and 5 hp (7.6A), all 460V 3-phase. What is the minimum feeder conductor ampacity?

Correct Answer: Per 430.24: 125% of highest motor + 100% of others. (34A × 1.25) + 14A + 7.6A = 42.5 + 14 + 7.6 = 64.1A minimum ampacity. Select conductor with ≥64.1A ampacity from Table 310.16 — 4 AWG copper (70A).

Exam Answer Frame

Style: 5-mark

Question: Describe the four protective elements required for a single motor branch circuit and state the NEC section and sizing basis for each.

Model Answer: A complete motor branch circuit requires four separate protection layers.

(1) Branch-circuit conductors (430.22): sized at minimum 125% of the motor FLC from NEC Tables 430.247–430.250 — not nameplate. (2) Overload protection (430.32): a separate device responsive to motor current, set to trip at no more than 125% of nameplate current if the motor has SF ≥1.15 or temperature rise ≤40°C; otherwise 115% of nameplate. (3) Short-circuit and ground-fault protection (430.52): sized per Table 430.52 using FLC table values — dual-element fuse max 175%, inverse time breaker max 250%. If the calculated size doesn't correspond to a standard rating and the motor won't start, the next higher standard rating is permitted. (4) Disconnecting means (430.102, 430.109): an HP-rated motor-circuit switch or listed device, located in sight from the controller and separately in sight from the motor and driven machinery. Each layer has a distinct sizing basis — mixing nameplate with table values is the most common exam error.