Sizes the three protection elements around a motor starter \u2014 overload relay setting and IEC trip class, contactor utilization-category rating, and ground-fault pickup \u2014 from a single set of motor and system parameters. Companion to the standalone Motor Calculator (which sizes the branch circuit/OCPD) and the Circuit Breaker & Fuse Sizer's motor path.
Standards followed
NEC 430.32(A)(1)IEC 60947-4-1 (trip classes, AC-3/AC-4)IEC 60034-1 (duty classes S1-S8)
What it can do
- Overload relay setting at 115% or 125% of FLA per NEC 430.32(A)(1), based on service factor/temperature rise.
- IEC 60947-4-1 trip class (10A/10/20/30) selected from the motor's accelerating time.
- AC-3 vs. AC-4 contactor sizing against a standard frame-size table.
- Ground-fault pickup guidance for solidly grounded, HRG and ungrounded systems.
- Flags cyclic/frequent-starting duty that calls for a thermal-memory relay.
Example problem
A motor with FLA=34A, service factor 1.0 (use 115%), starts in 8 seconds, is solidly grounded, and switched by a standard AC-3 contactor. Size its overload relay, trip class and ground-fault pickup.
- Overload setting = 115% × 34A = 39.1A (NEC 430.32(A)(1), since SF < 1.15).
- 8s starting time falls in the IEC 60947-4-1 Class 10 band (≤10s) → Trip Class 10.
- AC-3 contactor sized to FLA → recommended 40A standard frame.
- Ground-fault pickup (solidly grounded) = 20% × FLA = 6.8A, above the 5A minimum floor.
39.1A overload, Class 10, 40A AC-3 contactor, 6.8A ground-fault pickup.
Want the full step-by-step version?
AC-4 duty roughly doubles the required contactor rating versus AC-3 for the same motor \u2014 a conservative rule-of-thumb, since there's no single universal AC-4:AC-3 ratio in the standard. The ground-fault guidance is typical practice, not a substitute for a protective-device coordination study.