3-phase motor branch calculator (NEC 430)

Use the calculator

Pick the motor HP from the dropdown and choose the standard line-to-line voltage (200, 208, 230, 460, or 575 V). The calculator returns FLA from NEC Table 430.250, branch conductor size, inverse-time breaker rating, time-delay fuse rating, and overload setting. Switch to other tabs for general 3-phase calculations.

The four NEC 430 sizing rules

The 1.25 factor accounts for continuous operation at full load (NEC 210.19(A)). Look up the smallest 75 °C copper wire in NEC Table 310.16 with ampacity ≥ I_conductor. Apply NEC 240.4(D) caps for 14/12/10 AWG Cu if the breaker exceeds 15/20/30 A.

250 % is the maximum permitted by NEC 430.52(C)(1) for an inverse-time breaker on a typical 3-phase induction motor. For high-inertia loads (compressors, large fans) that won\'t start at this multiplier, NEC permits 400 % per NEC 430.52(C)(1) Exception 1. The breaker is sized to permit motor starting, not to protect the motor — the overload relay does that.

Time-delay fuses (Class RK1, RK5, J) sit at 175 % of FLA — significantly lower than the 250 % breaker because the fuse\'s slow-blow time-current curve allows it to ride through the starting inrush without sizing up. Used in motor combination starters where a separate overload relay handles steady-state protection.

The overload relay (typically NEMA Class 10, 20, or 30 trip class) protects the motor from sustained overcurrent that would damage the windings. Set at 115 % of FLA for severe-duty motors (SF = 1.0) or 125 % for standard NEMA SF = 1.15 motors. Modern electronic overloads (NEMA Class C) include phase-loss and asymmetry detection beyond the basic thermal model.

How to size a motor branch, step by step

1. Get the motor HP and voltage from the nameplate. Read the nameplate HP and the labelled voltage (200, 208, 230, 460, or 575 V — the standard NEC voltages). Service factor (SF) is also on the nameplate; standard NEMA motors are 1.15.
2. Look up FLA from NEC Table 430.250. NEC §430.6(A)(1) requires using the table value, not the motor nameplate FLA, for sizing wire and overcurrent protection. The table FLA is conservative; nameplate values vary by manufacturer. The calculator looks this up automatically.
3. Size the branch conductor — NEC 430.22. Conductor ampacity must be ≥ 125 % × FLA for a single motor on its own branch. Look up the smallest 75 °C copper wire in NEC Table 310.16 that meets this. For continuous operation (most industrial), the same 1.25 factor applies.
4. Pick the inverse-time breaker — NEC Table 430.52. For an inverse-time circuit breaker (most common): 250 % × FLA, rounded UP to the next standard breaker. For time-delay fuses: 175 %. For instantaneous trip breakers: 800–1100 %, used in motor controllers with separate overload protection.
5. Set the overload protection — NEC 430.32. For SF ≥ 1.15: overload trip = 125 % × FLA. For SF < 1.15: 115 %. The overload is sized to protect the motor from sustained overcurrent (slow), while the breaker protects the conductor from short circuit (fast).
6. Confirm everything matches. Wire ampacity ≥ MCA, breaker ≥ MCA but ≤ MOP, overload at 115–125 % FLA. The calculator returns all four values in one pass; cross-check on the nameplate that the actual motor FLA matches the table within ±10 %.

NEC Table 430.250 — quick reference (Cu, 75 °C, common HPs)

FLA values for the most common 3-phase motor sizes. Always confirm against the published table for unusual sizes.

Worked example: 30 hp pump at 460 V

A 30 hp 3-phase induction motor at 460 V drives a centrifugal pump. SF 1.15 standard. NEC 430 sizing:

Variants and special cases

VFD-driven motor (NEC 430.122)

The line-side conductor is sized at 1.25 × the VFD\'s rated input current — not the motor FLA. The VFD\'s built-in electronic overload replaces the separate relay. Output motor cable still follows standard motor wiring practice (often shielded VFD cable with separate ground).

Multi-motor branch (NEC 430.24)

Multiple motors on one feeder: ampacity = 1.25 × largest motor FLA + sum of other motor FLAs. The largest motor gets the 1.25 multiplier; smaller motors add at 100 %. Same logic as MCA per NEC 440 for HVAC equipment.

High-inertia loads (NEC 430.52(C)(1) Exception)

Compressors, large pumps, fans with significant rotating mass take longer to reach speed. The standard 250 % breaker may trip during start. NEC permits inverse-time breaker up to 400 % × FLA (or instantaneous trip up to 1300 %) for these loads. Verify the motor manufacturer\'s starting time and the breaker\'s long-time delay curve.

Service-factor motors

SF 1.15 is the NEMA standard — the motor can run continuously at 115 % of nameplate HP without damage. SF 1.25 is rare and usually marked. SF < 1.0 motors are severe-duty and require 115 % overload, not 125 %. Always confirm SF from the nameplate before sizing the overload.

NEC Article 430 mandate

Each continuous-duty motor rated more than 1 horsepower shall be protected against overload by one of the following means: (1) A separate overload device that is responsive to motor current. This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load amps: (a) Motors with a marked service factor not less than 1.15: 125 percent. (b) Motors with a marked temperature rise not over 40 °C: 125 percent. (c) All other motors: 115 percent.

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Frequently asked questions

What size wire for a 5 hp motor at 480 V 3-phase?

NEC Table 430.250 FLA = 7.6 A. Branch conductor: 1.25 × 7.6 = 9.5 A → smallest is 14 AWG Cu (75 °C, 20 A capacity). Inverse-time breaker: 2.5 × 7.6 = 19 A → 20 A standard. Overload (SF 1.15): 1.25 × 7.6 = 9.5 A. The calculator above does this in one shot.

What size wire for a 50 hp 3-phase motor?

At 460 V: FLA = 65 A (NEC Table 430.250). Conductor ampacity: 1.25 × 65 = 81.25 A → 4 AWG Cu (75 °C, 85 A capacity). Inverse-time breaker: 2.5 × 65 = 162.5 A → 175 A standard. Overload (SF 1.15): 1.25 × 65 = 81 A. At 230 V: FLA = 130 A → 1/0 AWG, 350 A breaker.

Why use 250 % for the breaker instead of 125 %?

Because a 3-phase induction motor draws 5–8 × FLA at start (locked-rotor current). A breaker sized only for steady-state current would trip every time the motor starts. NEC 430.52 picks 250 % as the standard sweet spot — large enough to allow starting, small enough to still protect the conductor and the motor from short circuits.

What is FLA vs MCA for a motor?

FLA (Full-Load Amperes) — the current a motor draws at its rated load and rated voltage. From NEC Table 430.250 for sizing. MCA (Minimum Circuit Ampacity) — the minimum continuous current the supply conductor must carry, usually 1.25 × FLA. The MCA is what you look up against NEC 310.16 to pick wire size.

When do I need an instantaneous-trip breaker?

For motor controllers (combination starter) where a separate overload relay handles the slow protection. Instantaneous breakers (NEC 430.52(C)(3)) trip only on short circuits, not motor starting; allowed at 800–1100 % × FLA per NEC Table 430.52. They cannot protect the motor against overload — that's the overload relay's job.

Service factor — what does it mean?

A multiplier on the motor's rated horsepower indicating how much overload the motor can sustain without damage. SF 1.0: no overload margin (severe duty). SF 1.15: standard NEMA design — motor can run continuously at 115 % of rated HP. NEC 430.32 lets you size the overload at 125 % × FLA when SF ≥ 1.15, vs 115 % otherwise. Verify SF on the nameplate.

How do I size the conduit for a motor branch?

Use NEC Chapter 9 Tables 1, 4, and 5: pick the conductor cross-section, count conductors (3-phase = 3 phase + 1 ground), look up conduit fill. Most 3-phase motor branches above ~10 hp need ¾ in to 1 in EMT or rigid conduit. Type MC cable is also common and simplifies sizing — the cable itself is listed for the application.

Do VFDs change the FLA sizing?

Yes. NEC 430.122 sizes the conductor on the line side of a VFD at 1.25 × the VFD's rated input current (not the motor FLA). The motor side may use the FLA approach for the motor, but the VFD's built-in motor protection (electronic overload) replaces a separate overload relay. Check the manufacturer's installation manual for specifics.

Sources and methodology

1. NFPA. National Electrical Code (NEC) NFPA 70, 2023 Ed. Article 430 — Motors, Motor Circuits, and Controllers; Table 430.250.
2. IEEE. IEEE Std 141 — Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book), 1993 (R1999).
3. NEMA. NEMA MG 1 — Motors and Generators, 2021. Chapter 12 — induction motors, code letters, locked-rotor amps.
4. UL. UL 508 — Industrial Control Equipment; UL 489 — Molded-Case Circuit Breakers.
5. NEMA. NEMA ICS 2 — Industrial Control and Systems Controllers, Contactors, and Overload Relays.