How to Set a Motor Overload Relay from Nameplate FLA
The overload relay is the device that lets a motor survive its own load: it models winding heat from measured current and opens the starter before the insulation cooks. Setting it is a two-minute job that regularly gets done wrong, because the dial interacts with the nameplate, the wiring configuration, the service factor, and the relay's own trip curve. This page walks through where the number on the dial comes from, how trip class is chosen, and how to verify the setting against the motor's real running current.
Set an Overload Relay from Nameplate FLA in one line: To set a motor overload relay, read the full-load amps from the motor nameplate for the voltage and connection actually wired, and set the relay dial to that FLA value on most relays - the margin for normal service is built into the trip curve, which per IEC 60947-4-1 must carry the motor continuously at just above set current and trip within a defined time at overload multiples. Choose the trip class to match the start: Class 10 for loads that accelerate quickly, Class 20 as the common general-purpose choice, Class 30 for long, high-inertia accelerations. The NEC caps how high the protection may be set, at 125 percent of FLA for motors with a service factor of 1.15 or more and 115 percent for others, and how a given relay's dial relates to that cap is defined by its manufacturer - selection and setting decisions beyond the dial belong to qualified personnel.
Read the Nameplate Before Touching the Dial
The number you need is full-load amps, and the trap is that many motors carry more than one. A dual-voltage motor lists two FLA values, one per voltage, and the correct one is the value for the voltage and winding connection the motor is actually wired for - setting a relay from the low-voltage column on a motor wired for high voltage roughly doubles the intended protection level. Multi-speed motors, and motors rewired between wye and delta, carry the same trap in different clothes. Confirm the connection in the peckerhead or on the connection diagram, then take the matching amps.
Two more nameplate items shape the setting. The service factor tells you how much thermal margin the motor was built with, and it drives the NEC's cap on protection level: motors with a service factor of 1.15 or more, or a marked temperature rise of 40 C or less, may be protected up to 125 percent of FLA, while all others are capped at 115 percent, per NEC 430.32. And the ambient matters: a relay mounted in a hot enclosure sees heat the motor does not, which is why ambient-compensated relays exist and why an uncompensated relay in a hot panel can nuisance-trip in summer at settings that were fine in winter.
Set the Dial and Pick the Trip Class
On most modern bimetallic and electronic overload relays, the dial is set directly to nameplate FLA, because the relay's trip curve already contains the running margin: IEC 60947-4-1 requires a thermal overload relay to carry a small margin above its set current for a sustained period without tripping, and to trip within a defined window at higher multiples. In other words, the dial is calibrated in motor FLA, not in trip threshold. The exception is relays whose manufacturer specifies a different setting basis, which is why the relay's own instruction sheet outranks habit; if the basis is unclear, that is a question for the electrical engineer of record, not a guess.
Trip class is the second decision, and it describes how long the relay tolerates the locked-rotor current of a start. The class number is the maximum trip time at 7.2 times the set current under IEC 60947-4-1: a Class 10 relay trips within 10 seconds at that multiple, Class 20 within 20 seconds, Class 30 within 30 seconds. Loads that reach speed quickly, such as most centrifugal pumps, suit Class 10; Class 20 is the common general-purpose choice; Class 30 exists for high-inertia loads such as large fans and some crushers whose normal acceleration would trip a faster class. The class must be long enough for the real start and no longer, because every extra second of tolerated stall current is thermal abuse the motor absorbs during an actual jam.
Electronic relays add features worth enabling deliberately rather than by default: phase-loss detection that trips quickly when one supply phase disappears, unbalance sensitivity, and in some models thermal memory that models cooling between starts. Each is a protection decision with consequences for both the motor and process availability, so the enabling and configuration of these functions should follow the site's electrical engineering review rather than the commissioning technician's preference.
Verify Under Load, and the Mistakes That Undo the Setting
A setting is verified with a clamp meter, not by the absence of trips. With the motor at its normal working load, measure the actual running current in each phase and compare it against the dial. A healthy motor at normal load typically draws noticeably less than nameplate FLA, and roughly equal current in all three phases. Running current at or above FLA at what should be normal load is a finding about the load or the supply, not a reason to raise the dial. Where the starter's current is monitored by the control system, trending it in a platform such as Merobix turns this one-time check into a continuous one, and a slow upward drift in running current becomes visible long before it reaches the trip curve.
The classic mistakes are all versions of the same move: silencing the relay instead of listening to it. Cranking the dial up because the relay keeps tripping treats the symptom while the cause - low voltage, unbalance, a failing bearing, a jammed load, an undersized motor - keeps cooking the windings. Setting the dial from the breaker or fuse size instead of the nameplate confuses short-circuit protection with overload protection, which are different functions sized by different rules. And picking Class 30 to survive a long start on a motor never designed for that acceleration converts a nuisance trip into eventual rewinds. When a correctly set relay trips repeatedly, the escalation path is an investigation, and any decision to operate with modified protection belongs to qualified personnel under the site's electrical program.
Frequently Asked Questions
Do I set the overload dial to FLA or to 125 percent of FLA?
On most modern relays the dial is set to nameplate FLA, because the trip curve already includes the running margin defined by IEC 60947-4-1; the NEC's 125 percent figure is the cap on the resulting protection level for high-service-factor motors, not an instruction to multiply the dial. Some relays are labeled on a different basis, so the relay's instruction sheet is the deciding document, and doubtful cases go to the electrical engineer rather than to arithmetic on the panel door.
What trip class should I choose for my motor?
Match the class to the acceleration time of the real load. Class 10 suits loads that come up to speed quickly, including most centrifugal pumps; Class 20 is the common general-purpose default; Class 30 is reserved for genuinely long, high-inertia starts. The class only needs to outlast the normal start, because everything beyond that is stall current the relay will tolerate during a real fault, heating the motor the whole time.
Why does the relay trip even though it is set exactly to nameplate FLA?
Because the motor is genuinely drawing more current than its rating, and the relay is doing its job. Common causes are low supply voltage or phase unbalance, a mechanical problem such as a dragging bearing or tightened packing, a process load that has grown, or a wrong FLA taken from the other voltage column of a dual-voltage nameplate. Measuring the actual running current with a clamp meter tells you which story you are in; raising the dial without that measurement just hides the evidence.
Automation services
Need help turning this into a working system?
Merobix integrates SCADA, programs Allen-Bradley and Siemens PLCs, and designs and fabricates industrial control panels.
Meeting requests are reviewed before confirmation.