How to Read an Overload Relay Coordination Table
What is an overload relay and contactor coordination table? A coordination table is a manufacturer-published test matrix, run to IEC 60947-4-1, that pairs a specific short-circuit protective device (SCPD) — fuse or MPCB — with a specific contactor and overload relay for a stated motor rating and prospective short-circuit current, then assigns the combination a Type 1 or Type 2 rating. Step outside a listed combination and the starter's behavior under fault current is unverified: nobody has tested whether the contactor welds shut, the overload relay survives, or the panel stays safe to open. This article covers why the table exists, how Type 1 differs from Type 2, the columns worth reading closely, how the overload relay's trip class fits into the picture, and where the published tables live.
Why the Coordination Table Exists
A motor starter is three devices working in sequence: the SCPD clears a short circuit, the contactor switches load current, and the overload relay trips on sustained overload. Each device is rated on its own, but a short circuit stresses all three at once — the SCPD has to open before the let-through energy destroys the contactor or overload relay downstream. IEC 60947-4-1 does not let a manufacturer claim a rating by adding up three independent datasheets. The SCPD, contactor, and overload relay have to be tested together, at a declared prospective short-circuit current, and the result published as a table.
That is the point of the table: it replaces a calculation nobody can run reliably with a test result somebody already ran. Cross-reference the motor's full-load current (FLC) and the site's prospective fault current against the table, and the combination it returns is pre-verified for that fault level.
Type 1 vs Type 2 Coordination
Type 1: after a short circuit, the SCPD has cleared the fault, but the contactor or overload relay may be damaged beyond reuse. No danger to personnel or the surrounding installation — the starter itself may need replacing.
Type 2: after the same fault, the SCPD has cleared it and the contactor and overload relay show no damage except light contact welding that separates with a screwdriver or is cleared by the next normal operation. The starter stays in service.
Type 2 costs more up front — it usually forces a specific fuse or MPCB size paired with a specific contactor frame, sometimes a size larger than the load alone would need. What we see in the field: panel builders default to Type 1 on low-value, easily-replaced motors and reserve Type 2 for starters that are expensive to access or that cannot tolerate downtime, like a submerged pump or a rooftop unit.
| Criteria | Type 1 | Type 2 |
|---|---|---|
| Contactor/overload after fault | May be damaged, may need replacement | No damage; light, separable contact welding only |
| Starter serviceable after fault | Not guaranteed | Yes, without part replacement |
| SCPD sizing | Matched to load; less restrictive | Often one size up from Type 1, per the table |
| Typical use | Low-cost, easily-swapped motors | Critical, hard-to-access, or downtime-sensitive motors |
Anatomy of a Coordination Table: Reading the Columns
Manufacturer coordination tables (Schneider, ABB, Siemens all publish them, usually as a PDF appendix to the contactors catalogue) share a common column set:
- Motor rating — kW or HP at a stated voltage (400V is the common European reference).
- FLC — the full-load current the table assumes for that motor rating.
- SCPD reference and rating — the exact fuse (gG/aM, amp rating) or MPCB (model, trip range) tested.
- Contactor reference — the exact catalog number, e.g. LC1D or 3RT2.
- Overload relay reference and setting range — the exact catalog number and its amp dial range, e.g. LRD or 3RU21.
- Prospective short-circuit current (Icc) — the fault current level the combination was tested at, usually stepped: 50 kA, 65 kA, 100 kA.
- Coordination type achieved — Type 1 or Type 2 for that Icc.
The Icc column is the one most often skipped, and it is the one that matters most. A Type 2 rating published at 50 kA does not apply at a site with 65 kA of prospective fault current — read the table at the fault level your installation actually has, not the first row on the page.
Matching the Overload Relay's Trip Class to a Listed Combination
The overload relay row in the table specifies a catalog number, which fixes its trip class — the table was tested with that class, not a substitute. Swap a Class 10A relay for a Class 20 relay of the same amp range and the coordination result is no longer the one published, because the electronic or bimetal trip curve changed the time the SCPD has to clear the fault before the overload relay is stressed further.
Formula: Trip Class Test Point — Source: IEC 60947-4-1, trip class table
ttrip at I = 7.2 × Iset, from cold
| Symbol | Description | Unit |
|---|---|---|
| ttrip | Time to trip, bounded by class (10A: 2-10s, 10: 4-10s, 20: 6-20s, 30: 9-30s) | seconds |
| Iset | Overload relay dial setting (motor FLC) | amps |
Coordination tables almost always list Class 10 or 10A relays, because that is the standard pairing for general-purpose motors. If a high-inertia load forces a Class 20 or 30 relay — see our note on high-inertia and long-start loads — check whether the manufacturer publishes a separate coordination table for that class before assuming the Type 2 rating still holds.
Fuse-Based vs MPCB-Based Coordination
A gG or aM fuse and an MPCB (motor protection circuit breaker) clear a short circuit differently, and the table reflects it. A fuse limits let-through energy by current-limiting action above its rated current — the higher the fault current, the faster and more current-limiting it becomes, which tends to protect the downstream contactor well at high Icc. An MPCB opens on its magnetic instantaneous element, which is fast but not current-limiting in the same way a fuse is; its let-through energy at very high Icc can be higher, so MPCB-based Type 2 coordination is often only published up to a lower Icc ceiling than the fuse-based version of the same contactor and overload relay.
This is one reason a panel spec sometimes calls out "fuse-protected starter" specifically — not preference, but coordination margin at the site's fault level. Pair the SCPD choice with the motor protection circuit breakers or fuse gear the table was actually tested with, and read our Type 1 vs Type 2 coordination breakdown for how the two SCPD types compare in more detail.
What Happens When You Deviate From a Listed Combination
Substitute any one of the three devices — a different fuse amp rating, a different contactor frame, an overload relay from a different family — and the coordination rating on the datasheet no longer applies. This is not a formality. A cheaper contactor swapped into a Type 2-rated starter, with the SCPD and overload relay left alone, can turn a serviceable-after-fault starter into one that needs a new contactor and possibly a new overload relay every time it clears a fault.
Some engineers assume any same-brand, same-frame swap is safe because the parts look interchangeable. That assumption fails often enough to be worth checking every time: a contactor's auxiliary block, coil voltage variant, or even a manufacturing revision can shift the tested coordination without changing the outward part number pattern. When in doubt, match the exact catalog references printed in the table, not the nearest equivalent.
Building the Full Starter From the Table
Once the table gives a verified SCPD + contactor + overload relay combination for the motor's FLC and the site's Icc, the rest of the selection follows in order: confirm the overload relay's setting range covers the FLC (see our guide on selecting and setting an overload relay), confirm the contactor's coil voltage matches the control circuit, and confirm the SCPD's interrupting rating meets or exceeds the site's prospective fault current — not just the Icc the coordination table was tested at.
Browse verified thermal overload relays against the exact catalog references your coordination table calls for, and cross-check contactor coil and auxiliary options in the same family before ordering. For the broader picture of how overload relays fit into motor protection, our thermal overload relay engineering guide covers bimetallic and electronic types, trip classes, and reset modes in full.
Frequently Asked Questions
What is a coordination table in motor starter design?
It is a manufacturer test result, published as a table, that verifies a specific SCPD, contactor, and overload relay combination survives a stated short-circuit current at a stated Type 1 or Type 2 level, per IEC 60947-4-1.
What's the practical difference between Type 1 and Type 2 coordination?
Type 1 allows the contactor or overload relay to be damaged after a fault as long as no one is endangered. Type 2 requires the starter to stay serviceable after the same fault, with at most light, easily-separated contact welding.
Can I substitute a different overload relay than the one listed in the table?
Not without invalidating the published coordination rating. The table was tested with that exact catalog reference and trip class; a different relay changes the trip time and the stress the SCPD has to clear before it acts.
Does the SCPD type — fuse or MPCB — change the coordination result?
Yes. Fuses are current-limiting and often support Type 2 coordination to a higher prospective short-circuit current than an MPCB paired with the same contactor and overload relay, because the MPCB's instantaneous trip does not limit let-through energy the same way.
Where do I find the coordination table for a specific contactor and overload relay?
Manufacturers publish them as an appendix or standalone document alongside the contactor and overload relay catalogs — search the brand's technical documentation by contactor family (e.g. TeSys D, SIRIUS 3RT2, A/AF) for "coordination table" or "short-circuit coordination."
Conclusion
A coordination table is the only place a Type 1 or Type 2 claim is actually backed by a test, not an assumption. Read the Icc column against the site's real fault current, match the SCPD, contactor, and overload relay to the exact catalog references listed — not the nearest equivalent — and treat any substitution as a reason to re-check the rating before it goes in the panel.
