How to Size Motor Overload Relays: NEC 430.32 Guide

An overload relay is a protective device that opens the motor starter circuit when motor current remains too high for too long, and you size it from the motor nameplate Full Load Amps (FLA), not from the NEC tables. 

NEC 430.32 sets the maximum ceiling at 125% of nameplate FLA for motors with a 1.15 service factor or higher, and 115% for most others. Set the dial at nameplate FLA to start, pick the right trip class, then test the overload relay before you close the panel door.

How Overload Relays Work and Which Type to Use

An overload relay monitor motor current and opens the small control contact when heat builds up in the motor faster than it can leave. That contact drops out the contactor coil, and the motor stops. It does not clear short circuits. The fuses or breaker upstream do that, and NEC 430.40 says those devices must protect the relay too.

Most panels use one of four types.

• Thermal (bimetal). Current heats a metal strip that bends and trips the relay. Cheap and simple, but panel heat moves the trip point.
• Eutectic. A solder-type alloy melts under steady overcurrent and releases a ratchet. It can’t be reset until the alloy cools, which is normal. Common on older NEMA starters.
• Electronic. Sensors read each phase; the trip class is adjustable, and most models trip on phase loss and imbalance. Siemens 3RB and the Rockwell 193 series are common examples.
• Motor protection relay (MPR). Adds RTD or thermistor inputs, ground fault, jam, and underload trips for larger or critical motors.

Pick electronics when the ambient swings or the motor matters to production. Thermal suits a small load in a stable panel.

NEC Rules for Overload Relay Sizing

NEC 430.32(A)(1) covers continuous-duty motors above 1 hp that use a separate overload device. The ceiling, as a share of the nameplate FLA, depends on what the plate says.

Service factor is the extra load a motor can carry continuously without harm, so 1.15 means 115% of rated load. A motor without a service factor marked falls inside the remaining row.

You need a measured start to prove that the lower setting fails and you must select the overload relay under (A)(1) first, and the higher 430.32(C) percentages come only after that. Get a measured start to prove the lower setting fails. Motors of 1 hp or less have their own rules in the same section.

Check 430.32 in the edition your inspector enforces.The 2026 NEC introduced Design BE and Design CE motor types to Article 430, along with updated conductor and short-circuit protection rules. The percentage ceilings remain the same as the 2023 text -125% and 115% – and most 2026 training materials still reference these figures.

Nameplate FLA vs Table FLC for Overload Relay

NEC Tables 430.248 and 430.250 list full-load current (FLC) by horsepower and voltage. Those table values are for wire size and short-circuit protection under 430.6(A)(1). For the overload relay, 430.6(A)(2) sends you to the nameplate current instead.

The gap matters. The table gives 14 A for a 10 hp, 460 V motor, but a modern high-efficiency motor often draws less- say, a plate that reads 12.8 A. At 125%, that is 16.0 A from the plate and 17.5 A from the table. The extra 1.5 A is heat going into the winding before the relay reacts.

If the plate is painted over or gone, get the FLA from the maker’s data sheet using the serial number before you fall back on a table value. On dual-voltage motors, use the FLA for your wired voltage.

Step-by-Step Overload Relay Sizing

1. Read the plate. Write down FLA at your voltage, service factor, temperature rise, and duty.
2. Work out the ceiling. Maximum setting (A) = nameplate FLA × 1.25 or × 1.15, using the table above.
3. Choose an overload relay whose dial range puts your FLA near the middle, so you can adjust up or down.
4. Match the overload relay to the contactor. The maker must list it for that starter size, and mixing brands usually breaks that.
5. Pick the trip class from the load.
6. Set the dial at nameplate FLA. Go up only after you have measured running current and ruled out mechanical causes.
7. Test the overload relay and record the setting.

Trip Class Selection

Trip class is the time an overload relay takes to trip when current reaches 600% of its setting from a cold start. Class 10 trips within 10 seconds, Class 20 within 20 seconds, and Class 30 within 30 seconds. A lower class protects the motor faster, so use the lowest class that lets the motor accelerate without tripping.

Look at the motor’s locked-rotor or safe stall time on its data sheet. The relay must trip before that time runs out, and Class 30 on a motor with a short stall time can quietly overheat the winding. IEC relays are tested at 7.2 times the setting instead of 6, so curves from different makers don’t match one to one. Motor maker data beats any generic chart.

Sizing Chart and Decision Table

These are typical values only. Always use the actual nameplate.

Voltage doesn’t change the math; the plate FLA does. A motor on a 208 V system draws different current than on 460 V, so read the FLA for your voltage.

Overload Examples for Calculation

These use example nameplates, not real manufacturer data.

Pump motor. Example nameplate: 10 hp, 460 V, three-phase, FLA 12.8 A, service factor 1.15. Ceiling = 12.8 × 1.25 = 16.0 A. Pick a 10 to 16 A overload relay, Class 10, dial at 12.8 A.

Conveyor. Example nameplate: 5 hp, 460 V, FLA 7.0 A, service factor 1.15, and it trips when started under load. Ceiling = 7.0 × 1.25 = 8.75 A, start limit = 7.0 × 1.40 = 9.8 A. Try Class 20 first. If it still trips, raise the dial in small steps and clamp the running current. If it stays above 7.0 A, the belt is dragging, and a higher setting only hides that.

Crusher. Example nameplate: 50 hp, 460 V, FLA 60 A, service factor 1.0. Ceiling = 60 × 1.15 = 69 A, start limit = 60 × 1.30 = 78 A. Use a Class 30 overload relay with the dial at 60 A.

Single-phase air compressor. Example nameplate: 3 hp, 230 V, FLA 15.0 A, service factor 1.15. Ceiling = 15.0 × 1.25 = 18.75 A, start limit = 15.0 × 1.40 = 21.0 A. Set 15.0 A, and check the overload relay data sheet for single-phase pole connections.

Special Cases

VFDs. A drive with built-in electronic overload can replace a separate overload relay if it has thermal memory (it keeps the heat count when power is cycled). Enter nameplate FLA in the motor current parameter, not the drive rating. A 15 hp motor with 19.0 A FLA on a 34 A drive should be set to 19.0 A.

 One drive feeding several motors requires an overload relay for every motor. Self-cooled motors run warmer at low speed, so check that the drive overload is speed-sensitive or add a motor thermistor.

Protectors inside the motor. A thermally protected motor’s trip current is capped at 170% of full-load current up to 9 A, 156% from 9.1 to 20 A, and 140% above 20 A. These use table values.

Hot panels. Except for ambient-compensated relays, ambient heat above 40°C lowers a relay’s trip current, and a cooler panel raises it. Turning the dial up is the wrong fix. Read the derating curve, or move to an ambient-compensated or electronic relay.

Large motors. These often use an overload relay fed by current transformers. The dial then reads secondary amps, so divide nameplate FLA by the CT ratio.

Automatic reset. NEC 430.43 says a relay that can restart a motor by itself must be approved for that motor, and can’t be used where a surprise restart could hurt someone. Use manual reset on conveyors, mixers, and anything with guards.

Refrigerant compressors. Hermetic compressors fall under Article 440 and use rated-load current, so the rules above don’t apply to them.

Common Mistakes

Testing and Documenting the Setting

Lock out and tag out the motor circuit before you open the starter, and wear the PPE your arc-flash label calls for. With control power off, press the test button and set a meter on continuity. On most IEC-style relays, the normally closed contact 95-96 opens and the normally open contact 97-98 closes. The contactor drop-out check needs control power, so do it only if your site procedure allows.

Then start the motor and read each phase with a clamp meter under normal load. A big gap between phases points to voltage imbalance or a loose connection. Some plants also need injection testing on electronic relays; follow your site standard.

Write the motor tag, FLA, dial setting, trip class, reset mode, date, and your initials on a label inside the panel door and in the maintenance record. The local inspector has the final say on code.

FAQ

What percentage of FLA should an overload relay be set to?

Usually at nameplate FLA. The NEC ceiling is 125% for service factor 1.15 or higher, or temperature rise of 40°C or less, and 115% for other motors. Go to 140% or 130% only if the motor can’t start on the lower setting.

Can I use NEC table FLC to set an overload relay?

No. NEC 430.6(A)(2) says to use nameplate current for overload protection. Table values are for conductors and short-circuit protection.

What trip class should I use for a pump motor?

Class 10 for most centrifugal pumps. Move up only if the motor trips on a normal start and the measured current is fine.

Can a VFD replace an overload relay?

Yes, if the drive has an approved electronic overload with thermal memory, the FLA is entered correctly, and the drive feeds one motor.

What is the distinction between an overload relay and a circuit breaker?

An overload relay guards the motor towards long, moderate overcurrent and can not open a short circuit.

What happens if an overload relay is set too high?

The motor runs hot for longer before the relay trips, so winding insulation ages faster. On a stall or lost phase, it may trip too late, and a setting above the NEC ceiling is a code problem.

Before you leave the panel, write the dial setting on the door label so the next person doesn’t have to guess.