Work out relay contact life instantly with clear inputs, formula shown and shareable results.
Contact life falls steeply with load current — roughly as the inverse 1.5 power — and depends heavily on load type. Inductive loads cut life to around a third of resistive because the arc on break is far more energetic, and lamp inrush is worse still.
Contact life
life = rated operations x (rated current / load current)^1.5 x load type factor
Interrupting current in an inductance produces an arc that transfers metal from one contact to the other. Freewheel diodes on DC and RC snubbers on AC greatly extend life.
For frequent switching of inductive loads, a solid-state relay has no contacts to erode. The trade is on-state voltage drop, heat and no galvanic isolation of the load.