Motor Circuit Sizing: Wire, Overload, and Branch Protection

Why Motor Circuits Are Different

Motors behave unlike resistive loads. They draw high inrush current on startup (6–8 times normal running current), have significant impedance characteristics, and require specialized protection. Treating a motor circuit like a standard branch circuit leads to nuisance breaker trips, burnt wire, and failed equipment. NEC Article 430 exists because motors demand it.

NEC Reference: NEC Article 430 covers motors, motor circuits, and motor controllers.

Key Terminology: FLA vs. FLC vs. Service Factor

Before calculating, understand these critical terms:

Full Load Amperage (FLA)

The steady-state current a motor draws at its rated horsepower under normal conditions. Listed on the motor nameplate and in NEC Tables 430.247 through 430.250.

Example: A 5 hp, 240V, three-phase motor typically has an FLA of 15.2A (from NEC Table 430.250).

Full Load Current (FLC)

Often used interchangeably with FLA, FLC is the same concept—the continuous running current. Some sources distinguish slightly, but for practical purposes they’re identical.

Service Factor (SF)

A multiplier listed on the nameplate (typically 1.0, 1.15, or 1.25) indicating how much above rated load the motor can handle. SF 1.15 means the motor can safely run at 115% of nameplate rating.

This is NOT used in NEC calculations. The service factor is for equipment designers, not for sizing branch circuits.

Nameplate vs. Code Tables

Always use the nameplate current rating if it’s available. If nameplate is missing or illegible, use NEC Tables 430.247–430.250.

Motor Type	Table Reference
Single-phase, AC induction	NEC Table 430.248
Polyphase squirrel-cage induction	NEC Table 430.250
Wound-rotor induction	NEC Table 430.249
Direct-current motors	NEC Table 430.247

Step 1: Determine Motor Current (FLA)

Let’s work through a practical example: a 10 hp, 240V, three-phase motor.

From NEC Table 430.250 (three-phase AC induction motors):

10 hp at 240V = 27.6A FLA

This is the baseline for all calculations.

Step 2: Size the Branch Circuit Conductor (NEC 430.22)

The branch circuit conductor must be sized at least 125% of motor FLA:

Formula: Conductor ampacity = FLA × 1.25

Calculation:

27.6A × 1.25 = 34.5A

Next standard wire size from NEC Table 310.16 (using 75°C column for copper):

#8 AWG copper @ 75°C = 40A (acceptable)
#10 AWG copper @ 75°C = 30A (too small—fails)

Result: Use #8 AWG copper

Why 125%? Motors have high inrush current on startup. The 125% rule allows the wire to handle inrush without overheating, yet still be protected by the branch circuit breaker (which trips at a higher threshold).

Step 3: Select Branch Circuit Protection (NEC 430.52)

This is where most electricians get confused. The branch circuit breaker or fuses must protect the wire from overheating due to short circuit or overload, but it must be sized larger than the 125% conductor ampacity to allow motor inrush current.

NEC 430.52(C) provides multipliers for different protection device types:

Protection Type	Multiplier of FLA
Instantaneous trip breaker	1.1×
Inverse-time breaker (standard)	2.5×
Inverse-time breaker (energy efficient motor)	2.25×
Fast-acting fuses	1.75×
Time-delay fuses	1.75×

For our 10 hp motor (27.6A FLA) with standard inverse-time breaker:

Breaker size = 27.6A × 2.5 = 69A

Next standard breaker size: 70A (acceptable)

Why higher? The breaker allows inrush current (6–8 times FLA) to flow during startup without tripping. Once running, the motor draws only the FLA. If actual current exceeds FLA for too long (overload condition), separate overload protection (thermal overload relay) trips first, before the breaker even acts.

Table 430.52(C)(1) Detailed Breaker Sizes:

If 27.6A × 2.5 = 69A (not standard), use the next larger size (70A). If the motor has a service factor ≥ 1.15, you can use Table 430.52(C)(1) alternative values, which may allow a smaller breaker—but this is rare.

Step 4: Install Overload Protection (NEC 430.32)

This is the critical third layer. While the branch circuit breaker protects the wire, overload protection protects the motor itself from burning up under sustained overload.

Where it goes: Inside the motor starter/contactor, as a thermal overload relay.

Overload Sizing

For motors with service factor ≥ 1.15 or temperature rise ≤ 40°C:

Overload setting = FLA × 1.15

For motors with service factor < 1.15 or temperature rise > 40°C:

Overload setting = FLA × 1.25

Example (assuming SF ≥ 1.15):

Overload setting = 27.6A × 1.15 = 31.74A (round to nearest standard size: 32A)

Install a thermal overload relay rated for 32A in the starter.

Overload Protection Details

The thermal overload relay has a time-delay characteristic. It allows the inrush current to pass (a few seconds), then trips if current remains high. This protects the motor winding from heat damage without nuisance tripping during normal startup.

Types of overload protection:

Thermal overload relay — Inside contactor, most common
Inherent motor protection — PTC thermistor or internal winding sensor (some motors)
Separate overload relay — In the control circuit

Complete Motor Circuit Example: 10 hp, 240V, 3-Phase

Step 1: FLA (from NEC Table 430.250)

10 hp, 240V, 3-phase = 27.6A

Step 2: Branch Circuit Conductor (125% rule, NEC 430.22)

27.6A × 1.25 = 34.5A
Use #8 AWG copper (40A ampacity)

Step 3: Branch Circuit Breaker (2.5× for inverse-time breaker, NEC 430.52(C))

27.6A × 2.5 = 69A
Use 70A breaker

Step 4: Overload Relay (1.15× for SF ≥ 1.15, NEC 430.32)

27.6A × 1.15 = 31.74A
Use 32A thermal overload relay in the starter

Final BOM:

Component	Size	Reason
Wire	#8 AWG	125% FLA = 34.5A; #8 = 40A
Branch breaker	70A	2.5× FLA = 69A; next standard size
Overload relay	32A	1.15× FLA = 31.74A in starter
Disconnect	70A minimum	Per NEC 430.102; usually 100A+
Contactor	10 hp rated	Size per manufacturer

Single-Phase Motor Example: 5 hp, 115V

From NEC Table 430.248:

5 hp, 115V, single-phase = 48A FLA

Branch circuit conductor (125%):

48A × 1.25 = 60A
NEC Table 310.16: Use #6 AWG copper (65A @ 75°C)

Branch circuit breaker (2.5×):

48A × 2.5 = 120A
Use 150A breaker (next standard size)

Overload relay (1.15×):

48A × 1.15 = 55.2A
Use 60A thermal overload

Fractional Horsepower Motors (≤ 1 hp)

Fractional motors (1/4, 1/3, 1/2, 3/4 hp) use different rules:

If nameplate current is visible: Use 125% nameplate current for wire sizing.

If nameplate is missing: Use NEC Tables for FLA, then apply same rules.

Example: 1/2 hp, 115V, single-phase

From NEC Table 430.248 = 9.8A FLA
Conductor @ 125% = 9.8 × 1.25 = 12.25A; use #12 AWG
Breaker @ 2.5× = 9.8 × 2.5 = 24.5A; use 25A breaker
Overload @ 1.15× = 9.8 × 1.15 = 11.27A

Fractional motors under certain conditions may not require separate overload protection (if built-in thermal protection is adequate per NEC 430.32(C)), but explicit overload is safer and more reliable.

Soft Starters and VFDs (Adjustable Speed Drives)

When a motor is controlled by a soft starter or VFD, the rules change slightly:

Soft Starter:

Still use the same FLA and 125% rule for conductors
Breaker sizing is affected by the soft starter’s current profile—check manufacturer specs
Overload protection may be built into the soft starter

VFD:

Conductors between VFD and motor need special consideration because harmonics increase heating
Upsize the motor conductor by one size (e.g., #8 → #6)
Check VFD manual for overload settings (usually set lower than a standard starter)
Ground cables may need increased bonding to manage harmonics

NEC Reference: NEC 430.251(A)–(C) covers adjustable speed drives.

Motor Starting Methods and Current

Across-the-Line (Full Voltage) Starting

The most common method. Motor draws 6–8 times FLA for 1–3 seconds during startup. Requires a robust breaker and contactor rated for the inrush.

Reduced Voltage Starting (Soft Starter)

Gradually ramps voltage to the motor, reducing inrush to 3–4 times FLA. Allows smaller conductors and breakers, but add the cost of the soft starter.

Star-Delta (Wye-Delta) Starting

Three-phase motors can be started in wye configuration (lower voltage per phase), reducing inrush. Requires a special starter and adds complexity.

Autotransformer Starting

Uses a transformer to reduce voltage during startup, then switches to full voltage. Less common now due to soft starters.

Common Motor Circuit Mistakes

Mistake #1: Using Branch Breaker Size for Wire

Some electricians size the #8 wire for a 70A breaker and call it done. Wrong. The wire must be sized at 125% of FLA (34.5A in our example), not at the breaker rating. If you upsize the breaker, you must still upsize the wire proportionally.

Mistake #2: Forgetting the Overload Relay

A breaker alone is not adequate protection for a motor. The overload relay inside the starter is mandatory. Without it, the motor will burn out if mechanically stalled or overloaded. The breaker will eventually trip (when the motor draws enough current to overheat the wire), but the motor is already damaged.

Mistake #3: Using Service Factor in NEC Calculations

The nameplate service factor (1.15 or 1.25) is NOT used to upsize conductors or breakers per NEC. It’s used only to verify the motor can run at higher than rated conditions if needed. All NEC calculations use FLA as listed on the nameplate or in the table.

Mistake #4: Mixing AC and DC Motor Tables

An AC motor uses NEC Table 430.250 (polyphase squirrel-cage). A DC motor uses NEC Table 430.247. Using the wrong table gives incorrect FLA and leads to improper sizing.

Mistake #5: Not Accounting for Conduit Fill

Motor cables in conduit must also meet NEC 300.17 (derating for bundled conductors). If multiple conductors run in the same conduit, ampacity is reduced by 10–20%. Compensate by upsizing the wire.

Mistake #6: Undersizing the Disconnect Switch

The disconnect switch serving the motor controller must be rated for at least 115% of the motor’s FLA per NEC 430.109. Many electricians use a 60A disconnect for motors rated 54A—this is wrong. Use 100A minimum if in doubt, and check nameplate requirements.

Quick Motor Circuit Sizing Table

For common three-phase motors at 240V:

HP	FLA	Wire (125%)	Breaker (2.5×)	Overload (1.15×)
1	3.6	#14	15A	4.1A
2	6.8	#12	20A	7.8A
3	9.6	#10	25A	11A
5	15.2	#8	40A	17.5A
7.5	22	#8	60A	25.3A
10	27.6	#8	70A	31.7A
15	42	#4	110A	48.3A
20	54	#3	150A	62.1A

(Values from NEC Table 430.250; adjust for your voltage and phase)

NEC Code References

NEC Article 430: Motors, motor circuits, and controllers
NEC 430.22: Branch circuit conductors
NEC 430.32: Overload protection
NEC 430.52: Branch circuit short-circuit and ground-fault protection
NEC 430.102: Motor disconnect switches
NEC Tables 430.247–430.250: Full-load currents
NEC 430.251: Adjustable-speed drive systems

Motor Circuit Design Checklist

Takeaway

Motor circuit sizing is a three-layer approach: (1) Conductors sized at 125% FLA, (2) Branch breaker sized at 2.5× FLA (inverse-time), (3) Overload relay at 1.15× FLA inside the starter. Each layer serves a purpose—the wire doesn’t melt, the breaker allows inrush without nuisance tripping, and the motor is protected from sustained overload. Memorize the 125% rule and the 2.5× multiplier, use the FLA from the nameplate or NEC tables, and you’ll size motor circuits correctly every time.