AC-1 vs AC-3 vs AC-6a: Contactor Utilization Categories Explained
A utilization category is the IEC 60947-4-1 code stating what kind of load a contactor is proven to switch. The same contactor carries a different amp rating in each category — typically Ith ≥ AC-1 ≥ AC-3 ≥ AC-4 — because switching resistive load is far easier than switching a motor. An amp figure without a category attached is not a rating.
A contactor labelled "400 A" may be proven to switch 400 amps, or 132 amps, depending on which category that number refers to. Here is how the system works and how to avoid the four mistakes that cause wrong selections.
What a utilization category is
Switching different loads is not equally difficult. Turning off a bank of heaters and turning off a running motor place completely different demands on the same contacts.
So IEC 60947-4-1 defines separate categories, each with its own test conditions, and gives a contactor a separate current rating in each. One physical contactor therefore carries several different amp ratings at once. They are all correct. They apply to different jobs.
The contactor utilization categories table
| Category | Load type | What the test demands |
|---|---|---|
| AC-1 | Non-inductive or slightly inductive, power factor ≥ 0,95 — heating, resistive loads, mixed distribution feeders | Make and break at about 1 × rated current |
| AC-2 | Slip-ring motors — starting and switching off | Moderate multiples |
| AC-3 | Squirrel-cage motors — start, and switch off while running | Make at ~6 × rated current (locked-rotor inrush), break at 1 × |
| AC-4 | Motors — starting, plugging, inching, reversing | Make and break at ~6 × rated current |
| AC-6a | Switching of transformers | Must withstand magnetising inrush, typically 8–12 ×, at low power factor with DC offset |
| AC-6b | Switching of capacitor banks | Capacitive inrush — very high, very brief |
| AC-15 / DC-13 | Auxiliary contacts switching electromagnet coils | Control circuits only, fractions of an amp |
Read the right-hand column and the system makes sense. AC-1 asks the contactor to break roughly what it carries. AC-3 asks it to close onto six times that. AC-4 asks it to break six times that.
Same device, same contacts — completely different difficulty.
Difference between AC-1 and AC-3 — and why the order never changes
For any given contactor the ratings almost always fall in this sequence:
Ith ≥ AC-1 ≥ AC-3 ≥ AC-4
Easier duty, bigger number. Worth committing to memory, because getting the direction backwards leads straight to specifying the wrong device.
A real example from a widely-sold industrial contactor:
| Rating | Value |
|---|---|
| Ith (thermal current) | 32 A |
| AC-1 | ~32 A |
| AC-3 | 18 A |
| AC-4 | 7,7 A |
Same copper, same springs, same coil. 32 A down to 7,7 A — a factor of four — purely because the job got harder.
On large feeder-type contactors the spread is wider still. A frame rated 400 A in AC-1 may be rated only 132 A in AC-3. Fit it to a motor circuit on the strength of the AC-1 figure and it will fail.
Ith vs Ie: the other number people confuse
Two current ratings appear on every contactor datasheet, limited by two completely different failure modes.
Ith — conventional thermal current. Contacts closed, nothing moving. Current flows, metal warms. Ith is a heat limit: how much can flow continuously without overheating. No arcing involved.
Ie — rated operational current. What the contactor can actually make and break. As contacts separate under current, an arc forms across the widening gap at several thousand degrees, burning metal off the contact faces. Ie is an arc erosion limit — and it is always quoted per utilization category.
The analogy: a person can hold a 50 kg weight at chest height for a long time. Ask them to lift and lower it a hundred thousand times and they fail long before. Holding capacity and lifting capacity are different numbers. Ith is holding. Ie is lifting.
Ith is normally the largest honest number on a datasheet, which is why it is often the one quoted. It is rarely the rating that decides suitability.
AC-6a: switching transformers, and why it needs its own category
Energising a transformer produces magnetising inrush — a brief but severe surge as the core flux establishes.
Three things make it harder than motor inrush:
- Magnitude — typically 8–12 × rated current for oil-filled distribution transformers, higher for some dry-type units
- Power factor — below 0,3, much worse than the 0,35–0,45 typical of motor starting
- Waveform — strongly asymmetrical with a DC offset, so the first peak is far higher than a symmetrical current of the same RMS value
That combination is why a contactor proven under AC-3 is not automatically proven under AC-6a. A symmetrical 6 × making test does not demonstrate an asymmetrical 12 × one.
How to select a contactor for transformer switching
Rather than reading a single amp figure:
- Calculate the transformer's full-load current
- Estimate the expected inrush peak using an appropriate multiplier for the transformer type
- Verify the contactor's making capacity exceeds that peak
- Confirm thermal withstand — the I²t energy the contactor absorbs against what the inrush delivers
Some specifications permit an AC-6a rating to be demonstrated either by direct transformer-switching test or by derivation from the AC-3 rating in accordance with IEC 60947-4-1. Where derivation is allowed it becomes a documentation route rather than a new test — but the derivation method comes from the standard, not from a rule of thumb.
The 4 mistakes that cause wrong contactor selections
1. Reading the model number as the rating
Manufacturers number frames inconsistently. In some series the number is the AC-3 rating; in others it is the thermal current, with a substantially lower AC-3 figure. An "F800" frame may be an 800 A thermal frame with an AC-3 rating below 500 A. Read the table, never the model name.
2. Quoting Ith as if it were a switching rating
Ith describes carrying, not switching. Ask explicitly: is that Ith or Ie? And if Ie, in which category?
3. Assuming AC-3 covers everything
Manufacturers lead with AC-3 because most customers buy for motor applications. It is habit, not deception — but AC-1 and AC-6a figures often will not appear unless you specifically request them.
4. Assuming AC-3 is the conservative choice
For resistive feeder duty the AC-1 rating is the relevant, larger figure — so sizing on AC-3 means over-buying. For transformer switching, AC-6a is harsher — so assuming AC-3 covers it means under-specifying. The direction of the error depends entirely on the application.
How to specify a contactor correctly
A usable datasheet line names the category:
Ie = 400 A in AC-1 at 400 V; Ith 500 A; electrical endurance 100 000 operations at AC-1; derating table to 55 °C.
An unusable one reads:
400 A contactor.
A bare amp figure is a model name, not a rating.
When requesting quotations, state your category, operating voltage and ambient temperature, then ask for:
- Rated operational current Ie in your category, at your voltage
- Conventional thermal current Ith
- Derating data if ambient exceeds the datasheet condition — most contactors are rated at 40 °C and ratings fall above it
- Mechanical and electrical endurance in operations, at your category
- Type-2 coordination data with the specific protective device upstream
Frequently asked questions
What is the difference between AC-1 and AC-3 on a contactor?
AC-1 is for resistive or near-resistive loads and requires the contactor to make and break at roughly its rated current. AC-3 is for squirrel-cage motors and requires making at around six times rated current. Because AC-3 is the harder test, the AC-3 rating of any given contactor is lower than its AC-1 rating — often by half or more.
Which utilization category applies to a distribution feeder?
Generally AC-1, since a mixed distribution load is largely resistive. If the feeder carries substantial motor content, check the AC-3 rating too. If the contactor energises a transformer, AC-6a applies.
Can I use an AC-3 rated contactor on an AC-1 load?
Yes, and it will be generously sized, because its AC-1 rating is higher. The reverse is not safe: an AC-1 rating does not demonstrate AC-3 capability.
Why does my contactor have four different current ratings?
Because it was tested under four different sets of conditions. All are valid; each applies to its own duty.
Does the utilization category affect contactor life?
Substantially. Electrical endurance falls sharply as the category gets harsher, because arc energy at make and break is what erodes contacts. The same contactor may achieve several times more operations in AC-1 than in AC-4.
What is Iq on a contactor?
Iq is the rated conditional short-circuit current — the largest prospective fault current the contactor can be subjected to, behind a specified protective device, and remain acceptable afterwards. A contactor cannot interrupt a fault, so it has no Icu. Iq is meaningful only when quoted with the protective device it was tested with and the coordination type.
What is the difference between Type 1 and Type 2 coordination?
After a short circuit, Type 1 requires no danger to people or the installation, but the contactor may be destroyed and need replacing. Type 2 requires the contactor to remain serviceable, with at most light contact welding that separates easily.
If you take one thing from this: never accept a contactor rating without its category. The number alone cannot tell you whether the device suits your application.
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