Relay Coordination Calculator

Calculate overcurrent relay settings per IEEE C37.112.

Primary:Secondary
Primary amps
Typical: 1.25 - 1.5
At relay location
End of zone
At maximum fault (0 if no downstream device)
Typical: 0.2 - 0.4s

IEEE C37.112 Curve Equations

t = TD × (A / (M^p - 1) + B)
Curve A B p
MI0.05150.1140.02
VI19.610.4912.0
EI28.20.12172.0

Coordination Guidelines

Coordination Time Interval (CTI)

  • Electromechanical relays: 0.3 - 0.4s
  • Static/Digital relays: 0.2 - 0.3s
  • Fuse-fuse: 0.1s minimum
  • Breaker-fuse: 0.2s minimum

Pickup Setting Guidelines

  • Phase OC: 1.25 - 1.5 × FLA
  • Ground OC: 0.1 - 0.5 × FLA
  • Consider cold load pickup
  • Motor starting: 1.5 - 2.0 × FLA