Short answer: A contactor is rated by the duty it will switch, not by a single current number. AC-1 is non-inductive or lightly inductive load, switched rarely: the contactor is rated near its thermal current. AC-3 is a squirrel-cage motor switched while running — the normal duty of a pump, fan or conveyor starter. AC-4 is the same motor switched while inching, plugging or braking — starting current is broken repeatedly, and the contactor’s usable current drops sharply. The same frame that carries a full load in AC-3 may carry only a fraction of it in AC-4, which is why two quotes for the “same” 32 A contactor can be honest and still differ by a frame size.
HAZAM is an independent trading company: not a manufacturer, and not an appointed distributor of any brand. What follows is how the utilisation categories work on the ranges we quote, and how to specify one without over- or under-buying.
What a utilisation category is
IEC 60947-4-1 does not rate a contactor with one number, because the wear on the contacts depends on what is being switched and how often. Instead it defines utilisation categories, and each category carries its own rated operational current (Ie) and its own making and breaking duty at a stated power factor. The device is then declared for several categories at once, and the installer picks the category that describes the real load.
In practice this means a contactor has several simultaneous ratings. Reading only the largest one — or only the frame label — is how starters get undersized.
The categories, side by side
| Category | Typical load | Rated duty at the contactor | Where it shows up |
|---|---|---|---|
| AC-1 | Non-inductive or slightly inductive: resistive heaters, lighting, distribution | Making and breaking at the thermal current; contacts carry close to the frame’s rating | Heater banks, resistive loads, feeder switching |
| AC-2 | Slip-ring motors: starting, inching, plugging | Heavier making and breaking than AC-1, lighter than AC-4 | Legacy slip-ring installations |
| AC-3 | Squirrel-cage motors: start, then switch off while running | Making at about six times rated current; breaking at rated current | Pumps, fans, conveyors, compressors — the standard starter duty |
| AC-4 | Squirrel-cage motors: inching, plugging, reversing, braking | Making and breaking at several times rated current | Crane and hoist control, machine inching, frequent reversing |
The direction of travel is the point: from AC-1 to AC-4 the current the contacts must break rises while the current the contactor may carry falls. A starter selected on its AC-1 figure and used on a plugging duty will be destroyed by a duty cycle the catalogue never promised.
How the ranges sit against the categories
| Range | Role | Rated current band | Notes for selection |
|---|---|---|---|
| CJX2 | Standard AC-3 contactor | Ie AC-3 at 400 V from 9 A to 95 A; motor power roughly 2.2 kW to 55 kW at 380 V | Coil options across AC 24–400 V; the last two digits of the code state the auxiliary contact arrangement |
| NC1 | Economical AC-3 contactor, same family logic | Up to 95 A | Declared for AC-3 and AC-4; same coil steps as CJX2 |
| NXC | Wider industrial range | Ie 6–630 A; AC-3 motor power to 335 kW at 400 V | AC coils across a wide band plus DC and AC/DC coil options on the larger frames |
| NC2 | Large-current contactor | Ie AC-3 at 400 V 115–800 A; AC-1 up to 1500 A; motor power 55–450 kW at 400 V | Double-break contacts; pairs with the larger thermal overload relays |
Notice that NC2 states both an AC-3 figure and a much larger AC-1 figure. That gap is not marketing; it is the category system working as designed. When you specify, quote the category first and then the current.
Specifying in five steps
- Name the duty. Random start and stop while running is AC-3. Inching, plugging, reversing or braking is AC-4 — even if the motor is small and the load looks light.
- Fix the motor’s rated current and the supply voltage. The category rating is declared at a stated voltage; read the figure for your grid, not the highest number in the table.
- Choose the frame from the category rating. Go up a frame when the duty is AC-4 or the starts per hour are high. Contact life falls with breaking current, so a lightly used AC-4 duty may still need a larger frame than the AC-3 rating suggests.
- Set the coil for the grid. The coil carries the control voltage and the frequency; a coil wound for one frequency used on another can lose pull-in or overheat. The detail is in contactor coil voltage at 50 Hz versus 60 Hz, and the grid itself in voltage and frequency by country.
- Add overload protection and remember it is separate. A contactor switches; it does not protect against sustained overload. A thermal overload relay sized to the motor’s full-load current — or a motor protection device — does that job.
The code tells you the auxiliary contacts
On the CJX2 family the last two digits of the ordering code state the auxiliary contact arrangement, and it changes what you can wire: 10 for one normally-open contact, 01 for one normally-closed, and 11 for one of each. The main poles are three-pole, with the auxiliary block carrying the seal-in and interlock circuits. Get it wrong and the starter will not hold itself in, which is a commissioning fault, not a design one.
On the larger frames the arrangement shifts, so the code is read against the catalogue rather than assumed from the small-frame rule. For the full range and the auxiliary blocks that fit, see contactors, overload relays and starters.
Where overload and motor protection fit
Three-part starters are the normal build: contactor, thermal overload relay, and a short-circuit device. The thermal relay is set to the motor’s full-load current and provides sustained-overload and phase-loss protection; it does not clear a short circuit. Thermal overload relays in the ranges we quote cover a setting range from a fraction of an amp up to the large frames, mounted directly onto the contactor or on a separate base.
Where the installation wants one device instead of a small starter, the job is done by a motor protection device: a combined isolating, overload and short-circuit unit that also protects against phase loss. The choice between a three-part starter and a single combined device is a space and maintenance question as much as a technical one — see how to size a distribution board for the space side, and moulded case breakers where the short-circuit device is a larger frame.
Why AC-4 is the expensive category
Contact erosion is proportional to the energy broken, and AC-4 breaks several times the current that AC-3 breaks. Two consequences follow for a buyer. First, a small motor with a reversing duty can need a contactor rated well above its nameplate current. Second, expected contact life in AC-4 is expressed in a far smaller number of operations than in AC-3, so a high-starts-per-hour application may consume a larger frame faster than the price difference suggests. Buying the AC-3 rating for an AC-4 job is the single most common way a starter fails early.
For control circuits and changeover duties the same category logic reappears; ATS versus manual changeover covers the switching side, and miniature circuit breakers cover the final circuits the motor control centre feeds.
FAQ
Q: Can I use an AC-3 contactor for an AC-4 duty if the current is low?
A: Only after checking the AC-4 rating of that frame, which is always lower than the AC-3 rating. If the AC-4 figure is not declared or is below the load, step up a frame. Inching and plugging break several times the running current, and that is what wears the contacts.
Q: What does AC-1 mean in practice?
A: AC-1 is non-inductive or lightly inductive load switched at or near the frame’s thermal current — heaters, lighting, resistive loads. The contactor is used close to the current the frame can carry continuously, because the current broken is close to the current carried.
Q: Why do two catalogues give different currents for the same frame?
A: Because the current is declared per utilisation category and per voltage. The same device has an AC-1 rating, an AC-3 rating and an AC-4 rating, and they are different numbers by design. Compare like with like: same category, same voltage.
Q: Does the contactor protect the motor?
A: No. A contactor switches; it does not protect against overload or short circuit. Overload protection comes from a thermal overload relay set to the motor’s full-load current, or from a combined motor protection device; short-circuit protection comes from the breaker or fuse ahead of it.
Q: How do I read the last two digits of a CJX2 code?
A: They state the auxiliary contact arrangement: 10 is one normally-open, 01 is one normally-closed, and 11 is one of each. The three main poles are the same across those variants, so the code decides the control wiring, not the power circuit.
Q: What else changes the selection besides the category?
A: The grid frequency, because it sets the coil winding; the starts per hour, because it sets contact wear; and the ambient temperature and enclosure, because they set how much current the frame can carry in that installation. State all of them and the frame becomes a calculation rather than a guess.
Related: Contactors, overload relays and starters · Contactor coil voltage 50 Hz vs 60 Hz · Voltage and frequency by country · How to size a distribution board · ATS vs manual changeover switch · Request a quotation