NEC Article 430 — motor branch circuit & feeder sizing

Reference notes

NEC Article 430 — Motors, Motor Circuits, and Controllers — is the governing code section in the National Electrical Code (NFPA 70) for sizing every component of a motor branch circuit and feeder. The most frequently mis-applied rule: use the TABLE FLC (from NEC Tables 430.247-430.250) for conductors and short-circuit OCPD; use the NAMEPLATE FLA only for overload-relay sizing.

FLC vs FLA — the critical distinction

Reasoning: nameplate FLA varies with manufacturer and motor design; table FLC provides a conservative standardized value ensuring the branch circuit handles ANY motor of that horsepower rating. Overload protection, conversely, matches the actual motor's thermal characteristics, so it uses nameplate FLA.

Motor branch-circuit components (in sequence)

1. Feeder bus
2. Motor branch-circuit short-circuit / ground-fault OCPD (430.52) — fuse or circuit breaker
3. Branch-circuit conductors (430.22, sized 125% × FLC)
4. Motor disconnect (430.102, within sight or lockable)
5. Motor controller (430.83, the contactor/starter)
6. Motor overload protection (430.32, thermal or electronic relay)
7. Motor

NEC 430.22 — Conductor sizing for a single continuous motor

• The 125% multiplier comes from NEC 210.19 continuous-load rule (motors operate > 3 hours = continuous).
• Example: 50-hp 460-V 3-phase motor. Table 430.250 → FLC = 65 A. Required ampacity = 1.25 × 65 = 81.25 A. From Table 310.16, 75°C column: #4 AWG copper THWN at 85 A.
• NEC 110.14(C) terminal-temp limit: use 60°C column for circuits ≤ 100 A unless equipment listed for 75°C.
• Apply ambient temperature (310.15(B)(2)(a)) and conduit-fill (310.15(B)(3)(a)) derating to the table ampacity BEFORE comparing to 125% load.
• Voltage drop (FPN): recommended ≤ 3% on branch, ≤ 5% total branch + feeder. Long runs require oversizing.

NEC 430.24 — Conductors for two or more motors (feeder)

• The 25% continuous-load allowance applies only to the largest motor.
• Example: feeder serving 50-hp (FLC 65 A), 30-hp (FLC 40 A), and 10-hp (FLC 14 A) motors:
  • Ampacity = 1.25 × 65 + 40 + 14 = 81.25 + 54 = 135.25 A.
  • Standard wire: 1/0 AWG THWN at 150 A (75°C terminations).

NEC 430.52 — Branch-circuit short-circuit / ground-fault protection (OCPD)

Sized as a MULTIPLE of table FLC. Per NEC Table 430.52:

• Round the calculated value UP to next standard size per NEC 240.6.
• If the next standard size cannot start the motor (trips during inrush), increase to the next standard up to the allowed maximum.
• The OCPD is INTENTIONALLY OVERSIZED to permit motor inrush (5-7× FLC for several seconds during starting) without nuisance tripping.
• 430.52 protects the BRANCH CIRCUIT from short circuits and ground faults — NOT the motor from overload. Overload is a separate device per 430.32.
• Example for 50-hp motor (FLC = 65 A):
  • Inverse-time CB: 250% × 65 = 162.5 A → 175 A standard frame.
  • NTD fuse: 300% × 65 = 195 A → 200 A standard.
  • DE fuse: 175% × 65 = 113.75 A → 125 A standard.

NEC 430.32 — Motor overload protection

Protects the MOTOR ITSELF from sustained overload — different from 430.52 which protects the BRANCH CIRCUIT. Uses NAMEPLATE FLA, NOT table FLC.

• If the calculated setting cannot start the motor: per 430.32(C), trip can be increased to 130% (SF≥1.15) or 140% (SF<1.15) of nameplate.
• Overload class (modern relays): Class 10 (tight protection, 10 s to trip at 6× FLA — small motors); Class 20 (general industrial); Class 30 (hard-starting loads with high inertia — pumps, fans).
• Example: 50-hp motor nameplate FLA = 62 A, SF = 1.15: max overload trip = 1.25 × 62 = 77.5 A. Tighter setting: 1.15 × 62 = 71.3 A.

NEC 430.102 — Motor disconnect

• Disconnecting means within sight (≤ 50 ft, line-of-sight) of the motor or its driven machinery, OR capable of being locked in the open position.
• Rated for at least 115% of motor table FLC (430.110).
• Types: knife switch, motor-circuit switch (HP-rated, most common), molded-case switch (MCS, disconnect-only), MCCB combined with branch OCPD.
• COMBINATION STARTER = disconnect + OCPD + contactor + overload relay in one enclosure — standard for industrial motors.
• Allows visible LOTO (Lock-Out / Tag-Out) per OSHA — maintenance worker verifies motor de-energized before working.

NEC 430.83 — Motor controller

• The contactor / starter that opens and closes the motor circuit during normal operation.
• Rated by horsepower at the motor voltage:
  • NEMA: sizes 00 through 9 (small to large).
  • IEC: equivalent classification, smaller for similar HP.
• Combination starter pairs the controller with disconnect and OCPD.

NEC 430.62 — Feeder OCPD for multi-motor circuits

• Example: 50-hp branch OCPD (175 A inverse-time CB) + 30-hp FLC (40 A) + 10-hp FLC (14 A) = 175 + 40 + 14 = 229 A.
• Standard size per 240.6: 225 A (next down) or 250 A (next up — allowed in some interpretations).

NEC 430 Part X — VFD (adjustable-speed drive) special rules (430.120-430.130)

• Conductors supplying the VFD (input side): 125% × VFD's RATED INPUT CURRENT (often higher than motor FLC due to rectifier sizing margins and harmonic content).
• Conductors from VFD output to motor: 125% × motor FLC per 430.22 — standard motor branch rule.
• Branch OCPD: per VFD manufacturer's listing in addition to 430.52. Most VFD manufacturers provide a chart of acceptable upstream OCPD types and sizes (Type 1 or Type 2 SCCR coordination). Listed combination drives must use the OCPD specified on the unit labeling.
• Overload: typically provided by the VFD's electronic-overload function (programmable trip class and current setting in the drive keypad). Code-compliant when the drive is listed as motor-overload protection per UL 508C or UL 61800-5-1.
• Disconnect: still required within sight of the motor per 430.102 — usually a separate disconnect downstream of the VFD, since the VFD's input disconnect doesn't satisfy line-of-sight to the motor unless the VFD is physically near the motor.

Common compliance errors

• Using nameplate FLA for conductor sizing instead of table FLC.
• Forgetting the 125% multiplier on conductors (sizing only to FLC).
• Selecting overload trip from table FLC instead of nameplate FLA.
• Missing or undersized disconnect within sight of the motor.
• Failing to apply ambient temperature or conduit-fill derating before sizing.
• Selecting OCPD frame below the 250% minimum and tripping during starting.
• VFD-fed motor protected only by upstream branch OCPD — must ALSO have overload per 430.32 (typically built into the VFD).

PE exam tips

• Memorize the four key percentages: 125% (conductors, 430.22), 250% (inverse-time CB, 430.52), 300% (NTD fuse, 430.52), 175% (dual-element fuse, 430.52).
• For overload: 125% (SF≥1.15) or 115% (SF<1.15) of NAMEPLATE FLA.
• For feeder: 125% LARGEST + 100% OTHERS (430.24).
• Always round to the next standard size per NEC 240.6.
• Apply terminal-temperature rating per 110.14(C) BEFORE sizing.