Motor Overload Relay Setting Calculator

Motor Overload Relay Setting Calculator supports engineering calculations with transparent assumptions, practical result interpretation, and links to next-step technical resources.

Trip Class & Startup Envelope

Formula

IFLC = Pout / (sqrt(3) x V x eta x PF)

Iset range = IFLC x [100% .. 115/125%] x AmbientFactor

Startup stress = (Istart / IFLC)^2 x tstart

Trip envelope: tlimit = Class x (7.2 / Iratio)^2

This tool is a front-end relay setting estimator. Final settings should be validated against relay manufacturer curves, motor thermal limits, and project protection coordination requirements.

Protection Loop

Trip Class Curves (Dynamic)

Enter motor data and startup assumptions to render trip-class startup envelope.

Inputs & Outputs

Phase System

Starter Type

Motor Rated Power

Supply Voltage

Efficiency (%)

Power Factor (0-1 or 0-100)

Service Factor

Load Factor (%)

Ambient Temperature (C)

Selected Trip Class

Start Current Multiplier (xFLC)

Start Time (s)

Relay Decision

Awaiting valid inputs

Recommended class: --

Motor rated output--

Full-load current (FLC)--

Operating current--

Recommended OLR setting--

Setting range--

Startup current estimate--

Selected class limit at startup ratio--

Selected class startup pass--

Selected class startup margin--

Overload Relay Setting Fundamentals

Reliable overload settings require balancing thermal protection intent with real startup behavior. Both running current and acceleration profile must be evaluated to avoid false trips or under-protection.

Thermal Protection Window

Set too low: nuisance trips during normal load transitions.

Set too high: delayed overload response and potential motor stress.

Trip Class Fit

Class behavior must align with startup current and acceleration duration.

Startup envelope mismatch is a frequent root cause of commissioning trips.

Equation Matrix

Core equations used for overload setpoint and trip-class screening.

Topic	Equation	Meaning
Motor full-load current	IFLC = Pout / (sqrt(3) x V x eta x PF)	Base relay setting reference for three-phase motors from electrical and mechanical assumptions.
Overload setting range	Iset in [IFLC x 100%, IFLC x 115/125%] x AmbientFactor	Practical setting window based on service factor and ambient influence assumptions.
Startup stress index	Stress = (Istart / IFLC)^2 x tstart	Fast screening metric for selecting trip class robustness against motor acceleration events.
Trip-class envelope	tlimit = Class x (7.2 / Iratio)^2	Approximate time-current envelope for comparing startup profile against selected class behavior.

Commissioning Workflow Matrix

Connect overload setting outputs to practical field commissioning and maintenance decisions.

Scenario	Objective	Recommendation	Critical Checks
Motor branch protection commissioning	Set overload relay for reliable startup without nuisance tripping	Calculate FLC first, then tune setpoint inside allowable range based on service factor, ambient, and measured load profile.	Actual running current, acceleration time, process duty cycle
Starter-type migration project	Re-check overload class after switching DOL to soft starter or VFD	Update startup multiplier and start time assumptions, then compare selected trip class margin before field rollout.	Ramp profile, torque demand, bypass mode transitions
Maintenance troubleshooting	Diagnose frequent overload trips under known process conditions	Compare measured startup events with class envelope and setting range to identify if trips are thermal-true or setting-driven.	Mechanical drag, voltage sag, relay health and calibration