Home
Back
Ampacity Formula:
| From: | To: |
Ampacity is the maximum current a conductor can carry continuously under the conditions of use without exceeding its temperature rating. For motor circuits, the National Electrical Code (NEC) requires conductors to be sized at 125% of the motor full-load current.
The calculator uses the simple formula:
Where:
Explanation: The 125% factor accounts for motor starting currents and provides a safety margin for continuous operation.
Details: Correct ampacity calculation prevents overheating, ensures safe operation, and complies with electrical codes. Undersized conductors can lead to insulation damage, fire hazards, and equipment failure.
Tips: Enter the motor's full-load current in amps (typically found on the motor nameplate). The calculator will determine the minimum required ampacity for the circuit conductors.
Q1: Why is the 125% factor used for motors?
A: Motors draw higher current during startup (locked-rotor current), and the 125% factor ensures conductors can handle these temporary overloads without damage.
Q2: Does this calculation include other derating factors?
A: No, this is the basic calculation. Additional derating may be needed for high ambient temperatures, multiple conductors in a raceway, or other special conditions.
Q3: What about motor overload protection?
A: Overload protection is separate from conductor sizing and is typically set at 115-125% of motor nameplate current.
Q4: How does this relate to circuit breaker sizing?
A: Circuit breakers for motors are sized differently (typically 250% of FLA) to allow for starting current while still providing short-circuit protection.
Q5: Are there exceptions to the 125% rule?
A: Some special motor applications may have different requirements, but 125% is the standard for most general-purpose motor circuits.