Enter your Motor Voltage, Horsepower, Motor Type, and Phase Configuration into the Motor Branch Circuit Calculator, along with Overcurrent Protection, Conductor Count, and Circuit Length to get your Required Wire Size, Full Load Amperes, and Overcurrent Protection Size.
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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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.
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.
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.
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.
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.
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.
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.