ElectroHub

Overload Relay & Contactor Calculator

The relay is set to the current it carries, and on a star-delta starter that is not the current on the nameplate. The relay sits in the delta leg, so it sees phase current — FLC/√3, about 58 % of the line current. Setting it to the nameplate figure means it needs 73 % more current than the motor can survive before it trips, which is to say it never trips.

Overload relay & contactor

The relay is set to the current it actually carries, which on a direct-on-line starter is the nameplate full-load current — and on a star-delta starter is FLC / √3, because the relay sits in the delta leg and sees phase current, not line current.

Full-load current
Relay carries
Set the dial to
Relay band to buy
Contactors
ContactorCarries (A)AC-3 rating (A)
Line13.9518
  • On a direct-on-line starter the relay is in the line, so the dial goes to the nameplate full-load current.
  • A thermal overload relay does not protect against short circuits — it is far too slow. It is always backed up by a fuse or an MPCB, and the pair has to be a combination the maker has type-tested.
Common trap: Setting a star-delta relay to the nameplate current is the commonest motor-protection mistake there is. The relay then needs 73 % more current than the motor can survive before it trips — which means it never trips, and the winding burns out with the protection fitted and working.
Relay bands and AC-3 contactor ratings are the standard series the major makers sell (25 bands, 0.1 A to 120 A). A relay and its backup fuse or MPCB must be a combination the manufacturer has type-tested — the pair is certified together, not separately.

The formula

DOL: setting = I_fl ; star-delta: setting = I_fl / √3 ; contactor ≥ the current its contacts carry

I_fl
motor full-load line current from the nameplate
√3
1.732 — present because the relay is inside the delta
AC-3
the contactor duty rating for squirrel-cage motors
SF
service factor — only if the nameplate declares one

Worked example

A 7.5 kW, 415 V three-phase motor at 88 % efficiency and 0.85 power factor, on a star-delta starter.

  1. I_fl = 7500 / (√3 × 415 × 0.88 × 0.85) = 13.95 A
  2. The relay is in the delta leg: 13.95 / √3 = 8.05 A
  3. Standard band that brackets 8.05 A: 7–10 A
  4. Main and delta contactors carry 8.05 A; the star contactor carries 13.95/3 = 4.65 A

Set the dial to 8.05 A on a 7–10 A relay, with 9 A AC-3 contactors. Setting it to 13.95 A would leave the motor unprotected.

Where you'll use it

Commissioning any motor starter, and diagnosing the motor that burnt out with its overload relay fitted and apparently healthy. A thermal relay is not short-circuit protection — it is far too slow — so it always sits behind a fuse or an MPCB, and the pair has to be a combination the maker has type-tested.

The laws behind it

Parts this applies to

Resistor (fixed)Varistor / MOVTVS / Transient SuppressorIGBTSCR (Thyristor)Linear Regulator (78xx / LDO)Switches (toggle/push/slide/rotary/DIP)Solid-State Relay (SSR)

Related calculators

Motor Full Load Current CalculatorI = kW × 1000 / (√3 × V × η × cos φ) [three phase]MCB & RCCB Selection CalculatorI_b ≤ I_n ≤ I_zSynchronous Speed and Slip CalculatorN_s = 120f / PShort Circuit Fault Current CalculatorI_sc = I_fl × 100 / %Z