Short answer: Set the dial to the motor’s full-load current (FLC) from the nameplate, then make three corrections. Choose a setting range that brackets that current near its mid-point. Allow for ambient temperature when the relay is not temperature-compensated. Set the trip class to match how hard the motor starts. The nameplate current is the destination; the range, the class and the ambient are the route.
HAZAM is an independent trading company – not a manufacturer, and not an appointed distributor of any brand. What follows is how an NR2 thermal overload relay is set on the starters we quote, and the corrections that decide whether the setting holds in service.
What a thermal overload relay does
A thermal overload relay protects a motor against sustained overcurrent. In IEC/EN 60947-4-1 terms it is a separate protective device, mounted with a contactor, that carries the motor current through three bimetal elements. When the current stays above the setting long enough, the bimetal bends, the trip mechanism releases, and a normally-closed auxiliary contact opens the contactor coil, stopping the motor before the winding is damaged.
Separate the two roles: the contactor switches, the relay protects, and short-circuit protection comes from the breaker or fuse ahead of the circuit. The NR2 adds phase-loss protection, so a single lost phase trips the starter instead of cooking the remaining windings.
The range and the set point
Two numbers are often confused, and swapping them is how a starter is mis-set in the field. The setting range is what a given catalog number can cover – a band such as one of the NR2 bands that together span 0.1 to 150 A. The set point is where you turn the dial inside that band. The range is a property of the part you ordered; the set point is a decision you make on site.
| Item | What it is | Decided when |
|---|---|---|
| Setting range | The band the ordered relay covers (for example a band inside 0.1-150 A) | At quotation, from the motor nameplate current |
| Set point | The dial value inside that band | At commissioning, on site |
| Trip class | How long a stated multiple of current is allowed to pass (class 10A on the NR2 family) | At selection, from the motor’s starting behaviour |
| Reset mode | Manual or automatic after a trip | At selection, from the panel’s operating rules |
Choose the range so the motor’s FLC falls near the middle of the band, not at an end. Sitting at the very bottom of a band makes the dial sensitive and leaves no headroom; sitting at the very top leaves no margin for a runaway load. A mid-band set point is the calm choice.
Setting an NR2 in four steps
- Read the nameplate. Take the full-load current at the connection actually used – delta or star, and the voltage actually supplied. A nameplate that lists two currents is telling you two different connection cases, not two options.
- Pick the band. Confirm the ordered NR2 band brackets that current comfortably, near its middle.
- Turn the dial. Set it to the nameplate FLC first, with no fudge factor. Then apply the corrections below – ambient first, duty second – and record what you actually set.
- Test and log. Prove the trip with a secondary-injection set or a start under load, and write the set point, the class and the date on the panel schedule.
Trip class and the starting curve
The trip class says how the relay behaves at roughly six times its set current. It is the difference between a relay that tolerates a heavy start and one that trips on every heavy start. Class 10 is the general-purpose choice on the NR2 family; when motors start slowly, or under a long loaded start, a higher class may be the honest answer rather than turning the dial up.
| Class | Behaviour at about 6x setting | Typical use |
|---|---|---|
| Class 10 | Trips in a short, standard window after a sustained overload | General squirrel-cage starters – pumps, fans, conveyors |
| Class 20 | Allows a longer window before tripping | Motors with a longer or heavier run-up |
| Class 30 | Longest common window | Very hard starts, high-inertia loads |
The trap is using the class to solve a problem the class cannot solve. Turning the dial up past the nameplate current to stop nuisance trips removes the protection the relay exists to give. If a correctly set class 10 relay trips on every start, the starting duty is asking for a higher class, a larger motor, or a soft start – not a higher setting.
Ambient, enclosure and derating
A bimetal relay senses current as heat. If the panel runs hot, or the relay shares an enclosure with other heat sources, the relationship between current and temperature shifts. A temperature-compensated relay corrects for this across a stated ambient band; a non-compensated one does not, and the set point should be adjusted for the ambient at installation. State the ambient when you specify, and separate the relay from other hot components in the panel.
The same logic applies to frequency. The bimetal is broadly frequency-independent, but the contactor coil it pairs with is not – a coil wound for the wrong grid frequency will chatter or overheat. That is why the coil is quoted per frequency even when the relay is not; see contactor coil voltage and 50 Hz versus 60 Hz.
Star-delta and phase loss
On a star-delta starter the relay sits in the delta leg, so the current flowing through it is not the line current the nameplate states at the motor terminals. Set the relay to the delta current the manufacturer’s scheme specifies, not to the line current, or the starter will be over- or under-protected. This is the single most common field error on star-delta starting. Phase-loss protection is the other reason to fit a proper relay rather than rely on the thermal setting alone: an open phase raises the current in the remaining windings faster than the overload element alone would capture, and a relay with phase-loss detection trips first.
Where NR2 fits the ranges
| Contactor range | Rated band | Overload relay |
|---|---|---|
| CJX2 / NC1 | AC-3 motor duty from 9 A to 95 A at 400 V | NR2 plugs directly onto the contactor |
| NXC | Ie 6-630 A, AC-3 motor power to 335 kW at 400 V | NXR relay family for the larger frames |
| NC2 | Ie AC-3 at 400 V 115-800 A | NXR relays 80-800 A |
NR2 covers a 0.1-150 A setting band, plugs onto CJX2 and NC1 contactors, and can be mounted separately on MB-1 to MB-4 bases when the layout needs it. It carries overload and phase-loss protection, manual or automatic reset, and is declared to IEC/EN 60947-4-1 up to 690 V. Certificates for the models we quote are listed per exact ordering code. The full starter family is on the contactors and overload relays page, and the duty categories that decide the contactor are explained in AC-1 versus AC-3 versus AC-4.
Relay or MPCB?
A relay-plus-contactor is a two-part solution: the contactor switches, the relay protects, short-circuit protection sits upstream. A combined manual motor starter – a motor protection circuit breaker – does isolation, short-circuit and overload protection in one device. Where panel space matters the combined device wins; where a large frame or a specific coil frequency matters the pair still wins. The comparison is set out in motor protection circuit breaker selection, and the short-circuit device ahead of a motor branch is covered under miniature circuit breakers.
FAQ
Q: Do I set the relay to the motor’s full-load current or slightly higher?
A: To the full-load current stated on the nameplate for the connection in use, with no fudge factor, then apply the documented ambient and class corrections. Setting it higher to stop nuisance trips removes the overload protection and moves the failure into the winding.
Q: What is the difference between the setting range and the set point?
A: The range is the band a given catalog number covers; the set point is where you turn the dial inside that band. Order so the motor current falls near the middle of the range, and set the dial on site to the nameplate current.
Q: Why does my correctly set relay trip on every start?
A: The starting duty, not the setting, is the likely cause. A long or heavy run-up may need a higher trip class, a larger motor, or a soft start. Turning the dial past the nameplate current hides the symptom and removes the protection.
Q: Where is the relay set on a star-delta starter?
A: In the delta leg, to the delta current the scheme specifies – not to the line current on the nameplate. The two are different numbers, and using the wrong one is the classic star-delta setting error.
Q: Does grid frequency change the relay setting?
A: The bimetal element is broadly frequency-independent, but the contactor coil it pairs with is not. Quote the coil for the destination grid; the relay setting itself is unchanged.
Related: Contactors, overload relays and starters · AC-1 vs AC-3 vs AC-4 contactor selection · Contactor coil voltage 50 Hz vs 60 Hz · Motor protection circuit breaker selection · How to size a distribution board · Request a quotation