How to Select a Contactor for an Inductive Motor Load (AC-3 Sizing Guide)

When buyers ask us at Electrical Center B2C for a contactor, the #1 mistake we see is picking by the big amp number on the front cover. That number is usually the AC-1 resistive rating — heaters, ovens, pure resistive loads. An induction motor is inductive, switches at 5–8× inrush every start, and breaks current with a lagging arc. Size a motor contactor wrong and you get welded contacts, burned coils, and a panel that dies in 6 months.

This guide walks through the correct order: nameplate → AC-3 current → utilization category → coil voltage → overload relay → short-circuit coordination. All examples use the CHINT NXC series we stock (NXC-09 / 12 / 18 / 25 / 32, 3-pole, AC-3).

1. Start from the motor nameplate, not the kW guess

For a 3-phase squirrel-cage motor you need three numbers:

• Rated voltage (230V Δ / 400V Y, or 380V, 460V, etc.)

• Full-Load Current (FLC / Iₑ) — amps at rated load

• Starting method — direct-on-line (DOL), star-delta, soft starter, VFD

If you only have kW, estimate FLC:

Iₑ ≈ P(kW) × 1000 ÷ (√3 × U(V) × η × cosφ)

(η≈0.85–0.92, cosφ≈0.80–0.88)

Example: 2.2 kW @ 400V → Iₑ ≈ 2200 ÷ (1.732 × 400 × 0.85 × 0.85) ≈ 4.4 A.

2. Pick the utilization category — AC-3 is the default for motors

IEC 60947-4-1 defines the duty that actually matters:

Rule: for 80% of industrial motor jobs, AC-3 is correct. A contactor rated 32A AC-1 may only be ~12–14A AC-3 — never cross-use the columns.

The CHINT NXC series is built for AC-3 motor duty, IEC/EN 60947-4-1 compliant, 690V max, 3-pole.

3. Size the contactor: AC-3 Iₑ ≥ motor FLC, with margin

Practical rule of thumb engineers use:

• Motors ≤ 30 kW: AC-3 Iₑ ≥ 1.25 × FLC

• Larger motors: ≥ 1.15 × FLC

• Frequent starts (>30/hr) or hot panel (>40°C): go one frame size up

Worked example — 2.2 kW @ 400V, FLC ≈ 4.4A → 1.25× = 5.5A → NXC-09 (9A AC-3) is comfortable.

7.5 kW @ 400V, FLC ≈ 15A → 1.25× = 18.7A → NXC-25 (25A AC-3).

15 kW @ 400V, FLC ≈ 28A → 1.25× = 35A → NXC-32 (32A AC-3) is tight, step to NXC-38 (38A) if cycling is high.

CHINT NXC AC-3 reference (3-pole, 380–400V)

Coil options on these frames: AC 24V / 36V / 110V / 220V / 380V (DC coil variants exist in wider NXC family).

4. Coil voltage = control circuit, NOT motor voltage

A 400V motor can have a 24V DC, 110V AC, or 220V AC coil — whatever your PLC or control transformer supplies. Mismatched coil = buzzing, chattering, burnt coil.

• PLC panel → 24V DC coil

• Legacy industrial → 110V AC or 220V AC

• Simple manual starter → 380V AC coil off one phase

All NXC-09…32 frames accept the above AC coil voltages.

5. A contactor is NOT protection — add an overload relay

The contactor only switches. You must add:

• Thermal overload relay (e.g. CHINT NXR series matched to NXC) set to 0.95–1.05 × motor FLC

• Short-circuit protection upstream: MCB/MCCB or fuse, coordinated to IEC Type 1 or Type 2 (Type 2 = no contactor damage after fault)

Example: 2.2 kW motor FLC 4.4A → NXR overload set ~4.2–4.6A, upstream 6A C-curve MCB (or 10A if cable allows and overload still protects).

👉 Our CHINT Kunlun NXR thermal overload relays (0.4–100A, multiple frame sizes):

https://www.electric-b2c.com/products/chnt-kunlun-nxr-25-nxr-38-nxr-100-thermal-overload-relay-0-4-100a

These NXR relays directly couple to NXC contactors via the side bracket — no extra wiring needed for the trip contact. Available in NXR-25 (up to 22A), NXR-38 (up to 34A), and NXR-100 (up to 90A), covering motors from fractional HP to ~45 kW.

6. Don't ignore the environment

• Panel ambient >40°C → derate ~10–15% (a 25A AC-3 contactor behaves like ~21A at 60°C)

• Altitude >2000 m → extra derate

• Enclosure IP rating, vibration, duty cycles/hour → all eat electrical life (NXC rated ~1.2M electrical ops AC-3, 10M mechanical)

Quick 4-step checklist

1. Read motor FLC from nameplate (don't trust kW alone)

2. AC-3 Iₑ ≥ 1.25 × FLC, pick NXC frame

3. Coil voltage = control supply (24V DC / 110V AC / 220V AC / 380V AC)

4. Pair with NXR overload + MCB/fuse, check Type 2 coordination

Shop the CHINT NXC contactor series

We stock original CHINT NXC-09, NXC-12, NXC-18, NXC-25, NXC-32 — 3-pole AC magnetic contactors, AC-3 rated, coil voltages 24V–380V AC, DIN-rail mount, IEC 60947-4-1.

👉 Product link: https://www.electric-b2c.com/products/original-chint-nxc-09-nxc-12-nxc-18-nxc-25-nxc-32-lc1-nxc-contactor-magneti

Free Standard Express on orders > US$30 (15–25 days), or Expedited DHL/UPS/FedEx 7–10 days at checkout. Bulk OEM inquiries: info@electric-b2c.com

Bottom line: inductive motor load = AC-3, size to FLC × 1.25, match coil to control not motor, always add overload + short-circuit protection. A CHINT NXC-25 on an 11 kW pump is a textbook-correct pick; the same NXC-25 used "because it says 40A" on a 15 kW motor (AC-1 column) is a fire waiting to happen.