A master electrician is installing a new escalator in a shopping mall. According to NEC Article 620, what is the maximum permissible overcurrent protective device (OCPD) rating for the power and control circuits of the escalator, relative to the motor nameplate current rating?
- ADetermined by the manufacturer's recommendations and listed equipment ratings, not exceeding 400% for instantaneous trip breakers.
- B200% of the motor nameplate current rating.
- C125% of the motor nameplate current rating.
- D150% of the motor nameplate current rating.
Show answer & explanationAnswer & explanation
Correct answer: A. Determined by the manufacturer's recommendations and listed equipment ratings, not exceeding 400% for instantaneous trip breakers.
NEC 620.61(A) states that the overcurrent protective devices for the motor branch-circuit conductors and for the motors shall be sized in accordance with 430.52 and 430.53. However, 620.61(C) permits the use of instantaneous trip circuit breakers or motor short-circuit protectors rated or set to not more than 400 percent of the motor nameplate current rating, or more if required by the manufacturer and listed. This allows for the high inrush currents often associated with motor starting for elevators and escalators, while still providing short-circuit protection.
Why the other options are wrong
- B. 200% is also a common limit for some motor types but not the maximum for instantaneous trip breakers in this application.
- C. 125% is typically for continuous motor loads, but starting currents for escalators often exceed this for short-circuit protection.
- D. 150% is a common overcurrent protection limit for certain motors, but escalators have specific allowances.
Escalator OCPD Sizing (NEC)
For escalators and moving walks, the NEC allows for higher overcurrent protective device (OCPD) ratings, especially for instantaneous trip breakers (up to 400% or more if listed), to accommodate the high inrush currents of large motors while still ensuring short-circuit protection.
- OCPD for motors per Article 430, but with specific allowances.
- Instantaneous trip breakers can be up to 400% of motor FLA, or more if listed.
- Accommodates high starting currents of escalator motors.
- Ensures short-circuit protection without nuisance tripping.
Memory trick: Escalator Power: Motors surge, so the breaker can be big (400%), but only if it's instant.