Motor Branch Circuit Calculator

The Motor Branch Circuit Calculator determines the correct electrical components needed to safely wire and protect an AC motor — following NEC guidelines for branch circuit design. Select your motor voltage, horsepower, motor type, and phase configuration, then choose your overcurrent protection device, installation conditions, and optional circuit length to get your required wire size, recommended breaker size, starter size, heater amperes, and conduit size.

HP
feet

Optional: For voltage drop calculation

%

NEC recommends 3% for branch circuits

Results

Required Wire Size

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Motor Full Load Amperes

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Recommended Breaker Size

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Starter Size

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Heater Amperes

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Recommended Conduit Size

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Frequently Asked Questions

What wire size do I need for my motor?

Wire size depends on motor amperage, voltage, and installation conditions. The calculator determines the minimum wire size based on NEC ampacity tables and applies derating factors for temperature and conductor bundling.

How is the breaker size calculated for motors?

Motor breaker sizing follows NEC Article 430, typically 125% of full load amperes for inverse time breakers. Different protection devices have different multipliers based on motor type and starting characteristics.

What is the difference between motor types in the calculator?

Different motor types have varying starting current characteristics. Design B energy-efficient motors are most common, while synchronous motors have different protection requirements. Single-phase motors typically require higher overcurrent protection.

Why does ambient temperature affect wire sizing?

Higher ambient temperatures reduce wire ampacity because conductors can't dissipate heat as effectively. NEC Table 310.15(B)(2)(a) provides temperature correction factors that must be applied to base ampacity ratings.

How do I account for voltage drop in motor circuits?

Voltage drop calculation considers circuit length, current, and conductor resistance. NEC recommends maximum 3% voltage drop for branch circuits. Longer runs may require larger conductors than ampacity alone would indicate.

What starter size corresponds to my motor horsepower?

Starter size is typically designated by NEMA ratings (Size 0, 1, 2, etc.) based on motor horsepower and voltage. Each size has specific current handling capabilities and physical dimensions.

How does conductor bundling affect ampacity?

When multiple current-carrying conductors are bundled together, heat dissipation is reduced. NEC requires derating factors: no derating for 1-3 conductors, but progressively lower ampacities for 4 or more conductors in the same raceway.