IEC vs NEMA Starter Chart
The two rating systems for motor starters, decoded. IEC 60947-4-1 rates contactors by utilization category — what the contacts must make and break, defined per duty — while NEMA ICS 2 publishes standardized sizes with fixed HP tables. The first table below is the IEC category chart every catalog references; the second is the honest comparison for choosing between the two on a job.
IEC utilization categories — motors
| Category | Defined load | Make / break duty | Typical applications |
|---|---|---|---|
| AC-1 | Non-inductive or slightly inductive loads; resistance furnaces (PF ≥ 0.95) | Makes and breaks ≈ rated current — minimal arcing | Resistive heating, furnaces, distribution — not motors |
| AC-2 | Slip-ring (wound-rotor) motors: starting, switching off | Makes and breaks starting current | Wound-rotor motor starters, high-torque applications |
| AC-3 | Squirrel-cage motors: starting, switching off while running | Makes motor inrush (≈ 5–7 × Ie per manufacturer guides); breaks rated current | The default motor category — pumps, fans, compressors, elevators |
| AC-4 | Squirrel-cage motors: starting, plugging, inching | Makes AND breaks ≈ 6–8 × Ie, repeatedly | Cranes, jogging, rapid reversing — expect a deep derate vs AC-3 |
Other utilization categories
| Category | Defined load |
|---|---|
| AC-5a | Electric discharge lamp controls |
| AC-5b | Incandescent lamps |
| AC-6a | Transformers |
| AC-6b | Capacitor banks |
| AC-8a | Hermetic refrigerant compressors, manual-reset overloads |
| AC-8b | Hermetic refrigerant compressors, auto-reset overloads |
NEMA vs IEC — the trade-offs
| Attribute | NEMA (ICS 2) | IEC (60947-4-1) |
|---|---|---|
| Selection inputs | Horsepower and voltage — the size ladder does the rest | Motor load, duty cycle, and full-load current, matched to a utilization category |
| Sizing philosophy | Standardized general-purpose steps with built-in reserve | Application-rated — sized tightly to the defined duty |
| Physical size | Larger frames; 30.5 mm pilot devices | 30–80% smaller below ~100 A (difference fades above); 22.5 mm pilot devices |
| Cost | Higher first cost | Generally less expensive, especially in small sizes |
| When it fails | Repairable — contacts and coils replace in place | Below ~100 A treated as disposable: off the DIN rail, new one on |
| Short-circuit coordination | Traditionally by UL listing with the specified OCPD | Type 1 (safe but repair/replace after fault) or Type 2 (fit for further use) per IEC/UL 60947-4-1 |
How to choose
Start from duty, not brand allegiance. Steady pump-and-fan duty with clean starts is AC-3 territory and either system handles it — the IEC device will be smaller and cheaper, the NEMA one more forgiving if the duty was estimated optimistically. Jogging, plugging, and reversing push toward NEMA (whose sizes carry published jog tables — see the NEMA starter size chart) or an honest AC-4 selection with the derate taken seriously. For panels an OEM ships in volume, IEC with Type 2 coordination is the industry default; for a plant floor where electricians rebuild starters at 2 a.m., NEMA's repair-in-place still earns its footprint.
Common questions
What does AC-3 mean on a contactor?
AC-3 is the IEC 60947-4-1 utilization category for squirrel-cage motors that start and switch off while running — the normal pump/fan/compressor duty. An AC-3-rated contactor must make the motor inrush (about five to seven times rated current per manufacturer guides) but only break rated running current. It is the rating to read when comparing an IEC contactor to a motor application.
When do I need AC-4 instead of AC-3?
When the application jogs, inches, plugs, or rapidly reverses — anything that makes the contactor interrupt locked-rotor-level current instead of running current. The same physical contactor carries a much lower AC-4 rating than its AC-3 number (manufacturer examples run to a quarter or less of the AC-3 kW), and electrical contact life in AC-4 duty is typically 10–20% of AC-3 life. Cranes and positioning drives are the classic AC-4 loads.
Which is better, an IEC or a NEMA starter?
Neither — they price different philosophies. NEMA sizes are generously standardized: pick by HP and voltage, get built-in reserve, repair contacts in place, pay more and use more panel space. IEC devices are sized tightly to a defined duty: smaller (30–80% below 100 A) and cheaper, but the selection must honestly match the duty category, and small frames are replaced rather than repaired. OEM panels lean IEC; heavy industry with jogging duty and in-house maintenance often stays NEMA.
What is Type 1 vs Type 2 coordination?
Both mean the starter fails safely on a short circuit; they differ in what is left afterward. Type 1: no hazard to people or the installation, but the contactor and overload may need repair or replacement before returning to service. Type 2: the starter must be fit for further use — at most light contact welding that separates easily. Type 2 requires the specific fuse or breaker the manufacturer tested with, so honor the coordination tables.
Is there an IEC equivalent of a NEMA size?
Not directly. A NEMA size is one number that implies a standardized HP table; an IEC contactor is specified by rated operational current (Ie) in a category at a voltage, so the "same" contactor carries several ratings. Crosswalk by duty: match the motor FLC and the true duty cycle to the AC-3 (or AC-4) rating — never by comparing frame sizes on a shelf.
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