Contactor Size Calculator
Find the motor contactor size two ways: the IEC AC-3 rated current (IEC 60947-4-1) or the NEC/UL horsepower-rated NEMA starter size (NEC 430.83) using Table 430.250 motor current.
Contactor Size Calculator
How to Use Contactor Size Calculator
- 1Choose the standard. IEC sizes by AC-3 rated current (IEC 60947-4-1). NEC sizes the controller by horsepower (NEC 430.83) and gives a NEMA starter size.
- 2Enter the motor. Power (kW or HP), supply voltage and phase. For NEC, enter HP at a nameplate voltage (115, 200, 208, 230, 460 or 575 V) so the calculator can use the code table current. If you know the current, tick the manual option.
- 3Set efficiency, power factor and margin. Use catalogue values for efficiency and PF. Leave the margin at 100% to get the smallest compliant size, or raise it for hot enclosures or frequent starts.
How to Calculate Contactor Size
1. Motor current
Single-phase: I = P × 1000 / (V × PF × η)
P in kW (1 HP = 0.746 kW), V in volts, PF = cos φ, η = efficiency as a decimal. Under the NEC, use the full-load current from Table 430.250 (three-phase) or Table 430.248 (single-phase) instead of the nameplate or a calculated value when sizing the controller rating, conductors and short-circuit protection (NEC 430.6(A)(1)). The table values apply to the voltage ranges 110–120, 220–240, 440–480 and 550–600 V.
2. Selection rule by standard
| Item | IEC (IEC 60947-4-1) | NEC / UL (NFPA 70 Art. 430) |
|---|---|---|
| Rating that must cover the motor | Rated operational current Ie for AC-3 at the supply voltage ≥ motor rated current | Controller horsepower rating ≥ motor HP at the application voltage (430.83(A)(1)) |
| Motor current used | Nameplate or catalogue current (or the formula above) | Table 430.250 / 430.248 FLC (430.6(A)(1)); nameplate FLA for overloads (430.6(A)(2)) |
| Typical device | AC-3 rated contactor, frames such as 9, 12, 18, 25, 32, 40, 50, 65, 80, 95 A | NEMA Size 00–9 starter (NEMA ICS 2), or a listed IEC-type contactor with UL HP ratings |
| Short-circuit coordination | Type 1 or Type 2 with the stated SCPD (manufacturer tables) | Listed combination and SCCR marking for industrial control panels (UL 508A) |
3. Design margin
Neither IEC 60947-4-1 nor the NEC requires a 125% factor on the contactor. The 125% in NEC 430.22 applies to branch-circuit conductors, not the controller. A margin is a design choice: it buys longer electrical life, headroom for a hot enclosure, or room for a larger replacement motor. The calculator shows both the smallest compliant size (100%) and the size with your margin.
IEC example: 15 kW, 415 V, three-phase
Without margin: smallest AC-3 frame ≥ 27.28 A = 32 A
With 125% margin: 27.28 × 1.25 = 34.10 A → 40 A (AC-3)
Check the chosen contactor's AC-3 rating at 415 V in the catalogue. For a motor that is inched or plugged, use the manufacturer's AC-4 rating instead, which is lower for the same frame.
NEC example: 10 HP, 460 V, three-phase
Controller HP rating ≥ 10 HP at 460 V → NEMA Size 1 (27 A, 10 HP at 460 V)
IEC-type alternative: 14 × 1.25 = 17.5 A → 18 A AC-3 frame, provided its UL HP rating at 480 V is at least 10 HP
The same 10 HP motor on 230 V draws 28 A from the table and needs a NEMA Size 2, because Size 1 is rated only 7.5 HP at 230 V.
AC Utilisation Categories (IEC 60947-4-1)
The same contactor frame has different current ratings in each category. Always compare the motor current with the rating for the duty you actually have.
| Category | Typical load | What the contacts must handle |
|---|---|---|
| AC-1 | Non-inductive or slightly inductive loads, resistance heaters | Make and break about rated current; highest current rating of the frame |
| AC-2 | Slip-ring motors: starting, switching off | Make and break limited starting current |
| AC-3 | Squirrel-cage motors: starting, switching off while running | Make locked-rotor current, break about running current |
| AC-3e | As AC-3, for high-efficiency motors with higher locked-rotor current (added in the 2018 edition) | Higher inrush than AC-3; relevant for IE3/IE4 motors (IEC 60034-30-1) |
| AC-4 | Squirrel-cage motors with plugging, inching or reversing from running | Make and break locked-rotor current; much shorter electrical life |
| AC-6b | Capacitor banks | High inrush; use capacitor-duty contactors |
Contactor Size Calculator Chart
IEC: three-phase 400 V motors
Calculated with PF 0.85 and efficiency 90%. Small motors often have a lower PF and efficiency, so their real current is higher; use the nameplate value when you have it.
| Motor (kW) | Current (A) | AC-3 frame, no margin | Current × 1.25 (A) | AC-3 frame, 125% margin |
|---|---|---|---|---|
| 0.75 | 1.42 | 9 A | 1.77 | 9 A |
| 2.2 | 4.15 | 9 A | 5.19 | 9 A |
| 4 | 7.55 | 9 A | 9.43 | 12 A |
| 5.5 | 10.38 | 12 A | 12.97 | 18 A |
| 7.5 | 14.15 | 18 A | 17.69 | 18 A |
| 11 | 20.75 | 25 A | 25.94 | 32 A |
| 15 | 28.30 | 32 A | 35.38 | 40 A |
| 18.5 | 34.91 | 40 A | 43.63 | 50 A |
| 22 | 41.51 | 50 A | 51.89 | 65 A |
| 30 | 56.60 | 65 A | 70.75 | 80 A |
| 37 | 69.81 | 80 A | 87.26 | 95 A |
| 45 | 84.90 | 95 A | 106.13 | 115 A |
| 55 | 103.77 | 115 A | 129.72 | 150 A |
| 75 | 141.51 | 150 A | 176.88 | 185 A |
Frame current steps follow a common catalogue series; some ranges use 38 A, 115 A or 330 A frames and others use different steps. Use the AC-3 kW or amp rating printed for your voltage.
NEC: Table 430.250 current and NEMA starter size
| Motor (HP) | FLC 230 V (A) | NEMA size 230 V | FLC 460 V (A) | NEMA size 460 V |
|---|---|---|---|---|
| 1 | 4.2 | 00 | 2.1 | 00 |
| 3 | 9.6 | 0 | 4.8 | 0 |
| 5 | 15.2 | 1 | 7.6 | 0 |
| 7.5 | 22 | 1 | 11 | 1 |
| 10 | 28 | 2 | 14 | 1 |
| 15 | 42 | 2 | 21 | 2 |
| 20 | 54 | 3 | 27 | 2 |
| 25 | 68 | 3 | 34 | 2 |
| 30 | 80 | 3 | 40 | 3 |
| 50 | 130 | 4 | 65 | 3 |
| 75 | 192 | 5 | 96 | 4 |
| 100 | 248 | 5 | 124 | 4 |
| 150 | 360 | 6 | 180 | 5 |
| 200 | 480 | 6 | 240 | 5 |
NEMA starter sizes (NEMA ICS 2, three-phase)
| NEMA size | Continuous amps | HP at 200 V | HP at 230 V | HP at 460 / 575 V |
|---|---|---|---|---|
| 00 | 9 | 1.5 | 1.5 | 2 |
| 0 | 18 | 3 | 3 | 5 |
| 1 | 27 | 7.5 | 7.5 | 10 |
| 2 | 45 | 10 | 15 | 25 |
| 3 | 90 | 25 | 30 | 50 |
| 4 | 135 | 40 | 50 | 100 |
| 5 | 270 | 75 | 100 | 200 |
| 6 | 540 | 150 | 200 | 400 |
| 7 | 810 | – | 300 | 600 |
The calculator uses the 200 V column for 208 V systems. Plugging or jogging duty needs a larger size; see the manufacturer's derating table.
Contactor Selection Checklist and Common Mistakes
- Use the AC-3 (or HP) rating, not AC-1. The AC-1 current on the same frame is much higher than the AC-3 value and does not apply to motors.
- Check the rating at your voltage. A frame rated 15 kW at 400 V carries a different kW rating at 230 V or 690 V; a NEMA Size 1 is 10 HP at 460 V but 7.5 HP at 230 V.
- Match the duty. Inching, plugging and reversing are AC-4 duty. Many starts per hour or a long run-up time also shorten contact life.
- Coordinate with the short-circuit device. Pick the contactor, overload relay and breaker or fuse from the same Type 2 table (IEC 60947-4-1) or a listed combination with a known SCCR (UL 508A).
- Coil and auxiliaries. Specify the coil voltage and AC or DC supply, and the auxiliary contacts needed for the control circuit.
- Enclosure temperature. Ratings apply up to a stated ambient. Check the derating if the panel runs hotter.
- A contactor is not an isolator. Provide a separate disconnecting means (NEC 430.102 and 430.109; isolation requirements of IEC 60364-5-53).
Contactor Size Calculator Frequently Asked Questions
Find the motor full-load current (nameplate, catalogue or I = P / (√3 × V × PF × η)). Under IEC, choose a contactor whose AC-3 rated current at the supply voltage is at least that current. Under the NEC, use the Table 430.250 current and choose a controller whose horsepower rating at the supply voltage is at least the motor HP (430.83(A)(1)).
A 7.5 kW, 400 V motor draws about 14–15 A, so an 18 A AC-3 contactor is the usual choice. A 10 HP, 460 V motor has a table current of 14 A and needs a NEMA Size 1; at 230 V the table current is 28 A and it needs a NEMA Size 2.
AC-1 covers resistive loads such as heaters, where the contacts only make and break about rated current. AC-3 covers squirrel-cage motors, where the contacts must make the locked-rotor current. The AC-3 current for a given frame is therefore lower than its AC-1 current.
No standard requires it. IEC 60947-4-1 only needs the AC-3 rating to cover the motor current, and the NEC rates controllers by horsepower. The 125% rule in NEC 430.22 is for conductors. Many designers still add a margin for hot panels or frequent starting.
They are defined in IEC 60947-4-1 for the starter and its short-circuit device. After a short circuit, Type 1 allows damage to the contactor and overload relay as long as there is no danger to people. Type 2 requires the starter to remain fit for use, with only light contact welding that can be easily separated. Type 2 needs the exact breaker or fuse listed by the manufacturer.
Yes, if it is listed for the North American market and has a UL horsepower rating at your voltage that is at least the motor HP. Use the UL rating, not the IEC kW rating, and include the device in the panel's short-circuit current rating (SCCR) under UL 508A.
Standards and References
The formulas and reference values on this page follow these standards. For design work, check results against the current edition that applies in your country and the equipment manufacturer's data.
- IEC 60947-4-1 – Low-voltage switchgear and controlgear: contactors and motor-starters (utilisation categories AC-1 to AC-4, Type 1 and Type 2 coordination). Published by IEC.
- IEC 60034-30-1 – Efficiency classes (IE code) of line-operated AC motors. Published by IEC.
- NFPA 70 (NEC) Article 430 – Motors and controllers: 430.6(A), 430.22, 430.83, 430.102, 430.109, Tables 430.248 and 430.250. Published by NFPA.
- NEMA ICS 2 – Industrial control and systems: controllers, contactors and overload relays (NEMA starter sizes and horsepower ratings). Published by NEMA.
- UL 60947-4-1 and UL 508A – Contactors and motor starters for North America, and industrial control panels including SCCR. Published by UL.
Related Calculators
Motor Contactor Size Calculator
Calculate the correct motor contactor size using motor power, voltage, efficiency and power factor.
Open calculator Panel DesignStar Delta Contactor Size Calculator
Calculate main contactor, delta contactor and star contactor ratings for star-delta starters using motor power and voltage.
Open calculator Panel DesignStar Delta Starter Component Calculator
Calculate star delta starter components including main contactor, delta contactor, star contactor, overload relay, MCCB…
Open calculator