What is a motor branch circuit and why does NEC Article 430 apply?

A motor branch circuit is the wiring between the last overcurrent protective device and the motor terminals. NEC Article 430 applies because motors draw locked-rotor starting current far exceeding full-load current, so standard branch-circuit rules (NEC 240.4) would cause nuisance trips — Article 430 provides special sizing rules for conductors, protection, and overload devices calibrated for motor operating characteristics.

What are the NEC Article 430 motor circuit sizing rules?

NEC 430.22 requires branch conductors rated at least 125% of the motor full-load current (FLC). NEC 430.52 sets maximum protective device ratings as a percentage of FLC — 250% for inverse-time breakers, 175% for dual-element fuses. NEC 430.32 sizes overload relays at 125% of FLC (SF ≥ 1.15 motors) or 115% (all others). NEC 250.122 sizes the EGC based on the OCPD rating.

How do I use this motor branch circuit calculator?

Select HP Lookup or Enter FLC mode. In HP Lookup, choose motor horsepower, voltage, and phase — the calculator automatically pulls the full-load current from NEC Tables 430.248 or 430.250. Then select motor type and protective device. The tool instantly sizes all four motor circuit components: branch conductor, short-circuit protection, overload relay, and equipment grounding conductor.

What are the NEC 430.52 protection device percentages for motors?

NEC Table 430.52 sets maximum OCPD ratings as a percentage of motor FLC: inverse-time circuit breakers = 250%, dual-element time-delay fuses = 175%, non-time-delay fuses = 300%, instantaneous-trip breakers (MCP) = 800% (or 1100% for Design E). If the calculated value doesn't match a standard size, NEC 430.52(C)(1) permits rounding up to the next standard size.

How do I fix a motor circuit that trips or fails to start?

If the motor trips during starting, NEC 430.52(C)(1) Exception No. 1 permits increasing the protective device to the next standard size above the calculated maximum. For breakers, you can go up to 400% of FLC; for non-time-delay fuses, up to 400%. If overload relays trip during normal running, NEC 430.32(C) allows increasing to 140% (SF ≥ 1.15) or 130% (other motors) before the motor is considered adequately protected.

How is the motor branch circuit formula calculated step by step?

Step 1: Look up FLC from NEC Table 430.248 (single-phase) or Table 430.250 (three-phase) — never use nameplate amps for circuit sizing. Step 2: Conductor = FLC × 125% (NEC 430.22). Step 3: Protection = FLC × Table 430.52 percentage, rounded up to next standard size. Step 4: Overload relay = FLC × 125% (SF ≥ 1.15) or 115% (other). Step 5: EGC from NEC Table 250.122 based on OCPD size.

Frequently asked questions about motor branch circuit sizing

Always use NEC table FLC — not nameplate — for conductor and protection sizing per NEC 430.6(A)(1). A single motor circuit requires four separate protective components: a branch conductor, an OCPD, an overload relay, and an EGC. VFD installations have additional requirements. For multi-motor feeders, add 125% of the largest motor FLC plus 100% of all other motors per NEC 430.24.

⚡ Electrical Calculator

Motor Branch Circuit Calculator

NEC Article 430 · Conductors (430.22) · Protection (430.52) · Overloads (430.32) · EGC (250.122) · Tables 430.248 & 430.250

⚙️ Motor Specifications

Phase

Motor Rating (HP)

Voltage (V)

Motor Type (for NEC Table 430.52)

Service Factor / Temp Rise

Most general-purpose motors have SF ≥ 1.15. Check nameplate.

🔒 Protection Device Selection

Protective Device Type

Inverse-time breakers and dual-element fuses are most common for motor circuits.

Conductor Material

Please fill in all required fields.

🔌

—

Branch
Conductor

NEC 430.22

🔒

—

OCPD
Max Size

NEC 430.52

🌡️

—

Overload
Relay Max

NEC 430.32

🌍

—

EGC Min
Size

NEC 250.122

📊 Step-by-Step Calculation — NEC Article 430

🔒 Protection Device Comparison — NEC Table 430.52

Device Type % Calc (A) Std Size

💡

Need to look up NEC Article 430 on the job?

VoltageLab's AI Code Lookup has every NEC Article 430 section referenced and explained — motor circuits, conductor sizing, protection coordination, and overload requirements. Used by thousands of electricians in the field and studying for their journeyman and master exams.

Download on iOS Download on Android

Branch conductor: NEC 430.22 — ampacity ≥ 125% × FLC | Protection: NEC 430.52 — max rating per Table 430.52 % × FLC, next standard size up | Overload: NEC 430.32 — 125% (SF ≥ 1.15) or 115% (others) | EGC: NEC Table 250.122 based on OCPD rating
FLC from NEC Table 430.248 (single-phase) or Table 430.250 (three-phase) per NEC 430.6(A)(1) — nameplate current not used for circuit sizing.
Standard device sizes per NEC 240.6(A). Results are indicative. Verify with a licensed electrical engineer before installation.

Voltage Lab

Motor Branch Circuit Calculator

ABOUT US

Tools

FOLLOW US