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Overload Protection Rating
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Short Circuit Protection (Max)
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Full Load Current
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Overload Protection (Min)
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NEC Article 430.32 requires overload protection to be sized at 115% to 125% of motor nameplate full-load current rating for motors with service factor 1.15 or higher, and 125% for motors with service factor below 1.15. The protection must prevent dangerous overheating but allow normal starting.
Short-circuit protection per NEC 430.52 is sized based on motor type and starting method. For standard motors with full voltage starting, use 250% of full-load current for inverse time breakers or 300% for instantaneous trip breakers. This protection handles fault currents while overload protection handles thermal protection.
Overload protection prevents motor damage from sustained overcurrent conditions (115-125% of FLA) while short-circuit protection handles fault currents (typically 250-300% of FLA). Overload protection has longer time delays for starting, while short-circuit protection trips quickly for dangerous fault conditions.
Use NEC Table 430.248 (single-phase) and 430.250 (three-phase) for full-load current values rather than nameplate amperage. These tables provide standardized values that account for motor variations and ensure consistent protection sizing across different manufacturers.
VFDs typically provide built-in overload protection, but you still need short-circuit protection upstream. The VFD overload settings should be based on motor nameplate FLA. External overload protection may be required if the VFD doesn't provide adequate thermal protection per NEC 430.130.
Higher ambient temperatures reduce motor current-carrying capacity. For temperatures above 40°C, apply derating factors to both motor current and protection device ratings. This ensures the motor operates safely within its thermal limits and protection devices function correctly.
Reduced voltage starting methods like wye-delta or soft starters allow higher short-circuit protection percentages since starting current is reduced. Full voltage starting requires more conservative protection sizing due to high inrush currents that could cause nuisance tripping.
Motor protection must be coordinated with feeder and service protection to ensure selective operation. The motor short-circuit protection should be smaller than upstream devices, and time-current curves should be analyzed to prevent upstream devices from tripping during motor starting.