Overload Relay

The correct overload relay will embrace the full load current of the motor within its range of available settings.

• If it's larger, it might not trip before the motor burns out, so it will not fulfil its intended purpose.
• If it's smaller, the motor will trip the overload before full load current is reached, which can be classed as "nuisance tripping".

PWSlack's answer is comprehensive.

To probe a bit, at this power level, are you using DOL starting? Unusual, i may say. If you are using a ΥΔ starter for example, it is normal for the relay to be within the Δ, and you should set it to the phase current, ie., 123/√3 A.

"If you are using a ΥΔ starter for example, it is normal for the relay to be within the Δ, and you should set it to the phase current, ie., 123/√3 A."

Oooohh, I'll bet that confused him...

If you are in NEC (or CEC) world, the overload setting of the DOL starter motor is generally no more than 1.15% of the motor nameplate full load current.

Here is the maximum overload setting for continuous duty motors from NEC 430.32 (A)(1):

Motors with a marked service factor 1.15 or greater: 125% of nameplate full load current

Motors with a marked temperature rise 40°C or less: 125% nameplate full load current

All other motors: 115% nameplate full load current

I think PWSlack's comment is per IEC standard.

- MS

1.15%? Really?

Thanks for pointing it out. I think, it is understood that 1.15% is a typo. It should be 115%.

- MS