Contactor vs. Relay vs. Motor Starter: How to Size, Wire, and Test Them

A relay switches low-current control circuits, a contactor switches motor and power loads, and a motor starter is simply a contactor with an overload relay attached to protect the motor. That one distinction resolves most of the confusion in a control panel — but picking the right size, matching the coil voltage, and diagnosing a failure all take a bit more. This guide covers NEMA and IEC sizing, how to decode Allen-Bradley, Siemens, ABB, Schneider Electric, and Eaton catalog numbers, and how to bench-test a contactor or relay with a multimeter. Every specification here is taken from current manufacturer publications, cited at the end.

Relay vs. Contactor vs. Motor Starter vs. Soft Starter

Control Relay Contactor Motor Starter Soft Starter
What it switches Control circuits, pilot lights, PLC I/O interposing Motor and power loads, lighting, heaters, capacitors Motor loads, with overload protection Motor loads, with controlled voltage ramp
Typical current 3–16 A (up to 40 A on power relays) 9 A to 2,650 A Same as its contactor Sized to motor FLA
Poles SPDT, DPDT, 3PDT, 4PDT 3 or 4 power poles + auxiliaries 3 power poles + overload 3-phase
Overload protection None None Built in (the overload relay) Built in, electronic
Arc management Minimal — not for motor loads Arc chutes, double-break contacts Same as its contactor Solid-state, no arcing
Example families Allen-Bradley 700-HA, 700-HK, 700-HC; Omron MY, LY, G2R Allen-Bradley 100-C / 100-E / Bulletin 500; Siemens 3RT2; ABB AF; Schneider LC1D; Eaton XTCE Allen-Bradley Bulletin 509; Siemens 3RA; Schneider LC1D + LRD; Eaton XTAE Allen-Bradley 150-SMC; ABB PSR/PSE; Siemens 3RW
Use it when You need to isolate or multiply a control signal You need to switch power but protection lives elsewhere You need across-the-line motor control with thermal protection Inrush or mechanical shock is the problem

The practical rule: if the load is a motor, use a contactor or starter, not a relay. Relays are rated in NEMA pilot-duty terms (B300, A600, C300) for control circuits. Contactors are rated in IEC utilization categories (AC-1, AC-3, AC-4) or NEMA sizes for power circuits. They are not interchangeable even when the current numbers look similar.

Sizing a Contactor: NEMA Sizes

NEMA sizing is deliberately coarse. You pick by horsepower and voltage, and the size carries reserve capacity for inrush. These figures are from Rockwell Automation publication 500-SG008A-EN-P for the Bulletin 500, 500F, and 509 line, and are matched by Siemens SIRIUS NEMA-rated contactors and ABB AF NEMA-rated contactors.

NEMA Size Continuous Amps Service-Limit Amps Max HP 200V 3Ø Max HP 230V 3Ø Max HP 460V 3Ø Max HP 575V 3Ø
00 9 11 1-1/2 1-1/2 2 2
0 18 21 3 3 5 5
1 27 32 7-1/2 7-1/2 10 10
2 45 52 10 15 25 25
3 90 104 25 30 50 50
4 135 156 40 50 100 100
5 270 311 75 100 200 200
6 540 621 150 200 400 400
7 810 932 — 300 600 600
8 1215 1400 — 450 900 900
9 2250 2590 — 800 1600 1600

Two points that trip people up. First, the motor's full load current must not exceed the continuous ampere rating — the horsepower column alone is not the whole check. Second, the service-limit current is the maximum RMS current the controller may carry for extended periods in normal service, and the overload relay's trip current must never exceed it. Size up, never down. If the motor sits between two sizes, take the larger one.

Cross-brand equivalents at the same NEMA size: Allen-Bradley Bulletin 500/509 sizes 0–4 correspond to Siemens 3RT2018, 3RT2027, 3RT2036, 3RT2046, and 3RT1056; ABB covers sizes 00, 0, and 1 with AF09, AF16, and AF26.

Sizing a Contactor: IEC Utilization Categories

IEC sizing is by rated operational current for a specific duty. The category matters as much as the number.

  • AC-1 — non-inductive or slightly inductive loads, resistance furnaces. Highest current rating for a given frame.
  • AC-2 — slip-ring motors: starting and reversing.
  • AC-3 — squirrel-cage motors: starting, and switching off only after the motor is up to speed. This is the number most motor applications should use.
  • AC-4 — squirrel-cage motors with plugging (reversing while running) and inching or jogging. Far more demanding; contact life drops sharply.
  • AC-15 — switching electromagnets: contactor coils, solenoid valves, lifting magnets. This is the auxiliary-contact rating.

Rockwell defines AC-3 test conditions as a making current of 6 × Ie, and AC-4 as both making and breaking at 6 × Ie — which is why the same frame carries a much lower AC-4 rating. ABB specifies AC-3 making capacity at 10 × Ie and breaking at 8 × Ie per IEC 60947-4-1.

Frame AC-3 (400V) AC-1 (40°C) HP 230V 3Ø HP 460V 3Ø HP 575V 3Ø
Allen-Bradley 100-C09 9 A 32 A 2 5 7-1/2
Allen-Bradley 100-C16 16 A 32 A 5 10 15
Allen-Bradley 100-C23 23 A 32 A 7-1/2 15 15
Allen-Bradley 100-C37 37 A 65 A 10 25 30
Allen-Bradley 100-C55 55 A 85 A 15 40 40
Allen-Bradley 100-C97 97 A 130 A 30 75 75
ABB AF09 9 A 25 A 2 5 7-1/2
ABB AF16 18 A 30 A 5 10 15
ABB AF26 26 A 45 A 7-1/2 15 20
ABB AF38 38 A 50 A 10 20 25
Schneider LC1D09 9 A 25 A 2 5 7-1/2
Eaton XTCE009B 9 A 20 A 3 5 7-1/2

Note how the same 9 A AC-3 frame carries 20–32 A AC-1 depending on brand. If you are switching a resistive load rather than a motor, the AC-1 column is the one that governs.

Decoding Coil Voltage Codes

Ordering the right contactor and getting the wrong coil is the single most common sourcing mistake. Every manufacturer appends the coil voltage as a suffix or infix, and the codes are not consistent between brands.

Allen-Bradley Bulletin 100-C (IEC)

The catalog number as listed is incomplete — insert the coil code. Example: 100-C0910 with a 120V 60 Hz coil becomes 100-C09D10.

  • KJ — 24V 50/60 Hz
  • D — 110V 50 Hz / 120V 60 Hz
  • L — 200…220V 50 Hz / 208…240V 60 Hz
  • KF — 230V 50/60 Hz
  • T — 240V 50 Hz / 277V 60 Hz
  • G — 400…415V 50 Hz
  • B — 440V 50 Hz / 480V 60 Hz
  • EQ / EJ — 12V / 24V DC electronic coil with integrated diode (100-C09…C55)
  • DJ / DD / DA — 24V / 110V / 220V DC with integrated diode (100-C60…C97)

All 100-C contactors ship with coil terminals on the line side. For load-side coil terminations, insert a U before the voltage code: 100-C09UD10.

Allen-Bradley Bulletin 500 / 509 (NEMA)

The NEMA size is a letter in the second position — A = size 0, B = 1, C = 2, D = 3, E = 4, T = 00, F = 5, G = 6, H = 7, J = 8, K = 9. Coil codes: J = 24V 50/60 Hz, D = 110V 50 Hz / 120V 60 Hz, H = 200–208V 60 Hz, A = 220V 50 Hz / 240V 60 Hz, F = 277V 60 Hz, N = 380V 50 Hz, I = 415V 50 Hz, B = 440V 50 Hz / 480V 60 Hz, C = 550V 50 Hz / 600V 60 Hz, VL = 24V DC, VG = 125…250V DC. So 500-BOD930 is a NEMA size 1 open contactor, 120V coil, three power poles with one N.O. auxiliary.

Siemens SIRIUS 3RT2

AC coils: C2 = 24 VAC, K6 = 120 VAC, P6 = 240 VAC, U6 = 277 VAC, V6 = 480 VAC, T6 = 600 VAC. DC: B4 = 24 VDC. Wide-range UC coils on 3RT203/204: B3 = 20…33V AC/DC, F3 = 83…155V AC/DC, P3 = 175…280V AC/DC.

Eaton XT

Frames A–M: A = 110V 50 Hz / 120V 60 Hz, B = 220V 50 Hz / 240V 60 Hz, C = 415V 50 Hz / 480V 60 Hz, F = 230V 50 Hz, L = 380V 50 Hz / 440V 60 Hz, T = 24V 50/60 Hz, TD = 24 VDC. So XTCE009B10A is a 9 A Frame B contactor, 1 N.O. auxiliary, 120V 60 Hz coil.

ABB AF

ABB takes a different approach entirely: the AF electronic coil interface accepts a wide control voltage range, so one coil covers many nominal voltages. Four coil ranges cover 24…500V 50/60 Hz and 20…500V DC — a single 100…250V 50/60 Hz–DC coil works worldwide without a change. AF..Z versions cover 24…250V 50/60 Hz or 12…250V DC and accept direct PLC control at ≥24V DC, 500 mA.

Schneider TeSys D

The coil code is the trailing suffix: LC1D09G7 is the 120 VAC version, LC1D09M7 is 220 VAC. The numeric prefix is the AC-3 current rating — LC1D18 is 18 A AC-3.

What "Coil Voltage" Actually Tolerates

A coil is not a simple on/off device. Every manufacturer publishes a pick-up band and a drop-out band, and understanding them explains most field failures.

Device Pick-up range Drop-out Inrush vs. sealed
Allen-Bradley 100-C (AC coil) 0.85…1.1 × Us 0.3…0.6 × Us 75–400 VA inrush / 9.5–24 VA sealed by size
Allen-Bradley Bulletin 500 size 1 (60 Hz) 85…110% — 192 VA inrush / 29 VA sealed
Allen-Bradley Bulletin 500 size 3 (2–3 pole) 85…110% — 660 VA inrush / 45 VA sealed
ABB AF09–AF38 0.85 × Uc min…1.1 × Uc max ≤60% of Uc min 50 VA pull-in / 2.2 VA hold (AF)
Schneider LC1D09 0.8…1.1 Uc (50 Hz); 0.85…1.1 Uc (60 Hz) 0.3…0.6 Uc 70 VA inrush / 7.5 VA hold-in
Allen-Bradley 700-HA relay 80…110% nominal 20% AC, 10% DC 3.3 VA inrush / 2.2 VA sealed (50 Hz)
Omron MY relay Must operate at ≤80% of rated Must release at ≥30% AC, ≥10% DC ≈0.9–1.3 VA at 60 Hz

The inrush-to-sealed ratio is why an undersized control transformer causes intermittent pull-in problems: a NEMA size 3 contactor draws 660 VA for the few cycles it takes to close, then settles to 45 VA. If several contactors pick up simultaneously, the transformer sags below 85% and nothing latches.

Signs a Contactor or Relay Is Failing

  • Contactor chattering or buzzing. The coil is receiving voltage in the gap between pick-up and drop-out — typically low control voltage, a sagging transformer, a long control run with excessive voltage drop, or a partially open contact in the control circuit. On AC coils, also suspect a damaged shading ring in the magnet assembly.
  • Contactor not pulling in. Verify coil voltage at the A1/A2 terminals while the circuit is calling. Below 85% of nominal on most IEC and NEMA coils, the armature will not seat. If voltage is correct, the coil is open — measure it.
  • Coil burnt or running hot. Usually a coil ordered for the wrong voltage, a 50 Hz coil on a 60 Hz supply, or an armature mechanically blocked so the coil never transitions from inrush to sealed current. A blocked armature holds the full inrush VA continuously and cooks the coil in minutes.
  • Contacts welded closed. The motor keeps running after the stop command. Caused by repeated AC-4 duty (plugging or jogging) on a contactor sized only for AC-3, by short-circuit current beyond the coordination rating, or by simple end-of-life erosion. Replace the contactor — do not file the contacts.
  • Overload tripping repeatedly. Check the FLA dial against the motor nameplate first. An E1 Plus set to Class 10 will trip on a long-accelerating load that needs Class 20 or 30. Also check for single-phasing: Allen-Bradley E1 Plus detects phase loss in about 3 seconds, and Schneider LRD relays trip at 130% of Ir when one phase drops out.
  • Relay clicks but the load does not switch. The coil is fine; the contacts are not. Measure contact resistance — Omron specifies 50 mΩ maximum for MY-series relays, and a reading in the ohms means eroded or oxidized contacts.
  • Relay will not drop out. Leakage current from a PLC or triac output can hold a sensitive coil energized. Allen-Bradley 700-HL terminal-block relays are available with a leakage current suppression circuit rated to 2.2 mA off-state current specifically for this.
  • Coil failure after a few months on DC. Missing surge suppression. A DC coil's collapsing field produces a transient many times system voltage.

How to Test a Contactor or Relay with a Multimeter

Safety first: de-energize the panel, apply lock-out/tag-out, and verify absence of voltage with a meter you have just proven on a known live source. Bench-test removed components whenever possible.

Testing the coil

  1. Disconnect the coil leads (A1/A2, or pins 13/14 on an 8-pin relay socket).
  2. Set the meter to resistance and measure across the coil. An open circuit (OL) means the coil is burnt. A near-zero reading means a shorted coil.
  3. Compare against the published value. Omron MY-series coils, for example, measure approximately 180 Ω at 24 VAC, 4,430 Ω at 120 VAC, 650 Ω at 24 VDC, and 11,000 Ω at 110 VDC, with a tolerance of +15%/−20% on AC and ±15% on DC at 23 °C.
  4. If no published figure is available, a coil that reads within roughly 20% of an identical known-good unit is acceptable.

Testing the contacts, de-energized

  1. Check continuity across each N.O. power pole: L1–T1, L2–T2, L3–T3. All should read open.
  2. Check each N.C. auxiliary: it should read closed, near zero ohms.
  3. Any N.O. pole showing continuity at rest indicates welded contacts. Replace the unit.

Testing the contacts, energized

  1. Apply rated coil voltage from an isolated, appropriately fused source. The armature should pull in cleanly and stay seated without buzzing.
  2. Re-check each N.O. pole — all should now read closed, and each N.C. auxiliary should read open.
  3. Measure resistance across each closed pole. Readings should be in the low milliohms and consistent pole to pole. One pole reading significantly higher than the others means uneven erosion and eventual single-phasing.
  4. Remove coil voltage. The contactor should drop out instantly. Allen-Bradley 700-HA relays specify 12 ms pick-up and drop-out; Schneider LC1D09 specifies 12–22 ms closing and 4–19 ms opening; ABB AF09–AF38 specify 40–95 ms closing. A sluggish or sticking drop-out indicates mechanical wear.

What a multimeter cannot tell you

Contact erosion under load, insulation degradation, and coil-to-contact dielectric breakdown do not show up on a handheld meter. Manufacturers rate electrical life separately from mechanical life — the Allen-Bradley 100-C is rated 13 million mechanical operations but 1.3 million electrical operations at AC-3 and 400V, and Allen-Bradley 700-HA relays are rated over 20 million mechanical but 100,000 electrical minimum. A contactor that passes a bench test can still be near the end of its contact life.

Overload Relays: Trip Class Is the Setting That Matters

The overload relay is what turns a contactor into a starter. Trip class defines how long the relay tolerates locked-rotor current before tripping. IEC 60947-4-1 specifies that a Class 10A device must trip between 2 and 10 seconds at 7.2 times the setting current.

  • Class 10 — standard. Most pumps, fans, and general loads that accelerate quickly.
  • Class 15 / 20 — longer-accelerating loads: loaded conveyors, centrifuges, high-inertia fans.
  • Class 30 — very long acceleration. Use only when the motor can actually tolerate it.

Manufacturer specifics, from current publications:

  • Allen-Bradley E1 Plus: the 193-ED is fixed Class 10, manual reset only. The 193-EE and 592-EE are DIP-switch selectable for Class 10, 15, 20, or 30 and for manual or automatic reset, with a 5:1 adjustment range, a trip rating of 120% FLA, phase-loss reaction typically 3 seconds, and typical automatic reset time of 120 seconds.
  • Allen-Bradley E100: the basic 193-1EE offers Class 10 and 20, manual reset only. The advanced 193/592-1EF offers Class 10, 15, 20, and 30 with manual or automatic reset, and automatic reset times that vary by class — approximately 90 seconds for Class 10, 135 for Class 15, 180 for Class 20, and 270 for Class 30.
  • Schneider LRD: Class 10A per IEC 60947-4-1, with a tripping threshold of 1.14 ± 0.06 Ir, phase-failure sensitivity that trips at 130% of Ir when one phase is lost, and temperature compensation from −20 to 60 °C. The LRD16 covers 9–13 A and mounts under LC1D12 through LC1D38 contactors.
  • Siemens: 3RU21 thermal overload relays to 100 A; 3RB20/21 and 3RB30/31 solid-state relays to 630 A, dimensionally coordinated with SIRIUS NEMA-rated contactors.
  • Eaton: XTOB bimetallic relays mount directly to the load-side terminals of the matching XTCE frame; XTOE electronic relays cover the same frames with wider adjustment.

Whatever the brand, set the dial to the motor nameplate FLA, not to the breaker size and not to the contactor rating. Rockwell also cautions that the trip current of the overload relay must never exceed the service-limit current rating of the controller.

Compatibility Caveats Before You Order

  • Coil voltage and frequency must match exactly. A 50 Hz coil on a 60 Hz supply, or a 230V coil on a 240V system at the top of its tolerance, runs hot and fails early. Check the frequency column in the coil code table, not just the voltage.
  • Surge suppression differs by brand. ABB AF contactors have built-in surge protection and require no external suppressor. Schneider LC1D contactors are supplied without a built-in suppressor module. Allen-Bradley 100-C requires a plug-in RC, varistor, or diode module selected by coil voltage — for instance 100-FSC280 for 110…280V AC, 100-FSV136 for 56…136V AC / 78…180V DC, 100-FSD250 for 12…250V DC. Fitting the wrong type does nothing useful, and on DC coils a diode module extends drop-out time.
  • Auxiliary contact limits are real. On the Allen-Bradley 100-C, up to 8 auxiliary contacts are possible — front-mounted a maximum of 4 N.C. on AC or DC, side-mounted a maximum of 2 N.O. Exceeding the published combinations is not supported.
  • Safety circuits need the right contact type. Mechanically linked contacts per IEC 60947-5-1 Annex L and mirror contacts per IEC 60947-4-1 Annex F are not marketing terms — they guarantee the N.C. auxiliary will not change state when a power pole welds. Allen-Bradley 100S-C safety contactors, ABB AF built-in N.C. auxiliaries, Schneider LC1D auxiliaries, and Allen-Bradley 700-HPS safety control relays (certified to EN 50205) carry these ratings. A standard contactor does not.
  • Relay socket rating can be lower than the relay. An Allen-Bradley 700-HK SPDT relay is rated 16 A, but the 700-HN121 socket is rated 10 A at 70 °C and the 700-HN221 is 16 A at 50 °C, derating to 12 A at 70 °C. Omron similarly warns not to exceed the carry current of the socket in use.
  • Bifurcated and gold-plated contacts exist for a reason. Standard silver contacts need a minimum load to stay reliable — Allen-Bradley 700-HA specifies 10V at 5 mA minimum, while the gold-plated 700-HAX drops to 5V at 2 mA. For low-energy PLC signals, specify the gold version.
  • Size up, never down, on contactors, breakers, fuses, and overload ranges. And when replacing, match the series and revision, not just the catalog number.

Frequently Asked Questions

What is the difference between a contactor and a relay?
A relay switches control circuits at low current — typically 3 to 16 amps — and is rated for pilot duty. A contactor switches power loads, is built with arc-quenching chambers and double-break contacts, and is rated in IEC utilization categories or NEMA sizes. Contactors also carry auxiliary contacts for interlocking and feedback. Use a relay to interpose a PLC output; use a contactor to switch the motor.

How do I test a contactor with a multimeter?
With power off and locked out, measure the coil across A1 and A2 — an open reading means a burnt coil. Then check each N.O. power pole for continuity; all should be open at rest. Apply rated coil voltage from an isolated source and re-check: all N.O. poles should now read closed in the low milliohms, consistent pole to pole, and the unit should drop out instantly when the coil is de-energized.

Why is my contactor chattering?
The coil is sitting between its pick-up and drop-out thresholds. Most AC contactor coils pick up at 85% of nominal and drop out somewhere between 30% and 60%. Check for a sagging control transformer, voltage drop on a long control run, a loose terminal, or a failing shading ring in the magnet assembly.

What NEMA size starter do I need for a 10 HP motor?
At 460V three-phase, a NEMA Size 1 is rated to 10 HP with 27 continuous amps. Verify that the motor's nameplate full load current does not exceed 27 A, and confirm the overload relay range covers the nameplate FLA. If the motor runs at 230V, 10 HP requires Size 2.

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Browse in-stock inventory: Contactors & Motor Starters · Industrial Relays · Relay Sockets · Overload Relays · Safety Relays & Machine Safety · Auxiliary Contacts & Contact Blocks

Sources

Specifications in this guide are taken from: Rockwell Automation publication 100-TD013Q-EN-P, IEC Contactor Specifications (May 2026); Rockwell Automation publication 500-SG008A-EN-P, Bulletin 500 Line Open Type Modular Kits and Components Selection Guide; Rockwell Automation publication 193-TD011D-EN-P, E1 Plus Overload Relay Specifications (April 2019); Rockwell Automation publication 193-TD013D-EN-P, E100 Electronic Overload Relay Specifications; Rockwell Automation publication 700-TD552E-EN-P, Relay and Timer Specifications (June 2020); Siemens SIRIUS NEMA Rated Contactors Selection & Application Guide; ABB AF09…AF38 3-pole Contactors Technical Data; Schneider Electric product data sheets for LC1D09 and LRD16; Eaton XT IEC Power Control Contactor & Starter Selection Guide; and Omron MY Miniature Power Relays specifications. Always confirm against the manufacturer documentation for your specific catalog number and series before installing.

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