4-Pole Contactor Symbol
Definition: The 4-Pole Contactor symbol represents an electromechanical switching device with four independently-switched main contacts (L1, L2, L3, L4 → T1, T2, T3, T4) operated by a single electromagnetic coil, used in schematics to denote a contactor that simultaneously switches all four conductors of a three-phase-plus-neutral or two-phase power circuit, as specified under IEC 60947-4-1 and ANSI/NEMA ICS 2.
Also known as: four-pole contactor, 4-pole motor contactor, 4-way contactor, neutral switching contactor, 4-pole AC contactor.
What the 4-Pole Contactor symbol means
The 4-pole contactor symbol denotes a contactor that switches four circuits simultaneously — typically the three phases and the neutral conductor of a three-phase four-wire system, or all four conductors of a two-phase supply. Adding a fourth pole to a standard 3-pole contactor allows complete isolation of the load from all supply conductors, which is required by some electrical safety codes and metering standards for generator changeover, transfer switch, and certain industrial applications.
The 4-pole contactor appears in transfer switch schematics (where it switches the load between mains and generator supply), multi-zone lighting control circuits, and industrial applications where neutral switching is mandated by local wiring regulations. The reference designator is KM (IEC 60617) or M (NEMA). A 4-pole contactor is physically larger and more expensive than a 3-pole of the same current rating, and the fourth pole is sometimes used as an auxiliary or signalling contact rather than as a fourth power contact.
How to identify the 4-Pole Contactor symbol
The 4-pole contactor symbol is drawn as four sets of normally-open main contacts arranged in parallel, each pair representing one phase or neutral switching element. Line-side terminals are labelled L1, L2, L3, L4 and load-side terminals T1, T2, T3, T4. A dashed enclosure or mechanical coupling line groups all four contact symbols together to indicate they operate simultaneously from the coil. The symbol is visually similar to the 3-pole contactor but has one additional contact pair, making it wider or taller depending on the drawing orientation.
Function in a circuit
The 4-pole contactor simultaneously connects or disconnects four conductors when the coil is energised or de-energised. In three-phase-plus-neutral applications, L1/L2/L3 carry the three phase voltages and L4 carries the neutral conductor, ensuring that when the contactor opens, the load is completely isolated from all live conductors. In transfer switch applications, two 4-pole contactors — one for mains and one for generator — are interlocked so that only one can be closed at a time, providing clean source-to-source transfer with no possibility of paralleling mains and generator supply.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-4-1 covers 4-pole contactors under the same framework as 3-pole devices, extending the utilisation categories (AC-1, AC-3, AC-4) to the fourth pole. IEC 60617-07 specifies four mechanically-linked contact symbols. Some regulations (e.g. IEC 60364-4-46) specifically require neutral switching in certain transfer applications. |
|---|---|
| ANSI/IEEE 315 | ANSI/NEMA ICS 2 and IEEE 315-1975 cover 4-pole contactor ratings and symbols in the same format as 3-pole devices with an additional contact column. UL 508A (industrial control panels) addresses 4-pole contactor use in transfer switch and changeover circuits. |
| Key difference | IEC and ANSI/NEMA symbols for 4-pole contactors are functionally identical — both show four normally-open contact pairs with a mechanical linkage — but IEC drawings use the IEC 60617 bridge-contact style while NEMA drawings may use a simplified line convention. The fourth pole's function (power neutral vs auxiliary) is noted in the drawing annotation. |
Terminals / pins
| Pin | Name |
|---|---|
| l1 | L1 |
| l2 | L2 |
| l3 | L3 |
| l4 | L4 |
| t1 | T1 |
| t2 | T2 |
| t3 | T3 |
| t4 | T4 |
Typical values
Rated current: 9 A to 630 A (4-pole versions, IEC). Coil voltage: 24 V AC/DC, 110 V AC, 230 V AC (standard options). Rated operational voltage: up to 690 V AC (IEC 60947). Fourth-pole current rating: may be rated the same as or lower than the three main poles. Mechanical endurance: typically 10–20 million operations. Response time: 20–50 ms pick-up; 20–100 ms drop-out.
Where the 4-Pole Contactor symbol is used
- Automatic transfer switches (ATS) — paired 4-pole contactors with electrical and mechanical interlocks transfer three-phase-plus-neutral loads from mains to generator supply without momentary parallel connection.
- Generator changeover panels — manual or automatic changeover units use 4-pole contactors to completely isolate the load from the grid before connecting to a standby generator, satisfying regulations requiring neutral switching.
- Three-phase four-wire distribution boards — main incomer contactors in 4-wire TN-S systems switch all four conductors simultaneously to ensure complete de-energisation for maintenance.
- Multi-zone lighting control — 4-pole contactors provide group switching of large lighting loads across all four conductors in 400 V + N circuits used in warehouse and commercial building lighting control systems.
- Industrial capacitor bank switching — AC-6b rated 4-pole contactors are used where neutral switching of capacitor banks is required by local utility regulations or harmonic filter designs.
- Solar and renewable energy systems — grid-tie inverters and microgrid systems use 4-pole contactors as grid isolation devices to disconnect all conductors including neutral when the grid supply fails or islanding is detected.
Example
In an automatic transfer switch (ATS) panel wiring diagram, two 4-pole contactors — KM1 (mains) and KM2 (generator) — are shown with their coils and contact symbols. The L1, L2, L3, L4 (neutral) line-side terminals of KM1 are connected to the mains incomer busbar; its T1–T4 load-side terminals connect to the distribution board. KM2's line-side connects to the generator output. An interlock (KM1 NC auxiliary contact in series with KM2 coil, and vice versa) prevents both contactors from closing simultaneously, ensuring the load is never connected to both sources at once.
Key facts
- The 4-pole contactor symbol shows four mechanically-linked normally-open contacts labelled L1/T1, L2/T2, L3/T3, L4/T4; the fourth pole switches the neutral conductor in three-phase-plus-neutral applications or an additional phase in two-phase circuits.
- The IEC reference designator for a 4-pole contactor is KM; in NEMA drawings it may appear as M4P (motor, 4-pole). All four contacts operate simultaneously from one coil (A1–A2).
- IEC 60947-4-1 classifies 4-pole contactors under the same utilisation categories as 3-pole devices; the fourth pole must be rated for the conductor it switches, which may differ from the three main poles in some multi-voltage designs.
- Neutral switching (fourth pole) is required by IEC 60364-4-46 and local wiring regulations in certain countries for safe isolation of loads and for transfer switch applications where neutral potential must be controlled.
- In transfer switch applications two 4-pole contactors are always used with electrical and mechanical interlocks to prevent both contacts from closing simultaneously and paralleling two separate sources (mains and generator).
- Pins on this symbol: L1 (x=0 y=10), L2 (x=0 y=22), L3 (x=0 y=34), L4 (x=0 y=46) on line side; T1 (x=60 y=10), T2 (x=60 y=22), T3 (x=60 y=34), T4 (x=60 y=46) on load side.
- A 4-pole contactor is physically larger and costlier than a 3-pole of the same current rating due to the additional contact assembly; where neutral switching is not required, a 3-pole contactor is the preferred choice.
- IEC 60947-4-1 and ANSI/NEMA ICS 2 both specify that a contactor is not a protective device; it must always be paired with a circuit breaker or fuse for short-circuit protection.
Frequently asked questions
What does the 4-pole contactor symbol look like?
The 4-pole contactor symbol shows four sets of normally-open main contacts arranged in parallel, with line-side terminals L1, L2, L3, L4 at the input and load-side terminals T1, T2, T3, T4 at the output. A dashed box or mechanical linkage line groups all four contacts to indicate simultaneous operation. It is visually the same as the 3-pole contactor symbol with one additional contact pair.
What is the difference between a 3-pole and 4-pole contactor?
A 3-pole contactor switches three conductors (the three phases) and is used for standard three-phase loads. A 4-pole contactor switches four conductors (typically three phases plus neutral) to provide complete isolation of the load from all live conductors. The 4-pole version is required in transfer switch applications and where local electrical codes mandate neutral switching.
When is a 4-pole contactor required instead of a 3-pole?
A 4-pole contactor is required when the neutral conductor must also be switched — for example, in automatic transfer switches (ATS) between mains and generator supply, in countries where IEC 60364-4-46 mandates neutral switching for certain circuits, or in applications where complete isolation of all live conductors is necessary for safety during maintenance.
What standard defines 4-pole contactor ratings and symbols?
IEC 60947-4-1 (Low-voltage switchgear — contactors and motor-starters) defines ratings, utilisation categories, and test requirements for 4-pole contactors. IEC 60617-07 specifies the schematic symbol. The North American equivalent is ANSI/NEMA ICS 2. IEEE 315-1975 covers drawing symbols.
What is the reference designator for a 4-pole contactor?
The IEC reference designator for any contactor (3-pole or 4-pole) is KM. In NEMA North American drawings the designator M or M4P is sometimes used. Multiple contactors are numbered KM1, KM2, etc., with all associated auxiliary contacts and the coil sharing the same number throughout the schematic.
Can the fourth pole of a 4-pole contactor be used as an auxiliary contact?
Yes. In some installations the fourth pole is used as a high-current auxiliary contact for control or signalling circuits rather than as a power neutral contact. This is acceptable provided the pole is rated for the applied voltage and current. When used this way the fourth pole effectively acts as a normally-open relay contact that operates simultaneously with the main three contacts.
What is an ATS and why does it use 4-pole contactors?
An ATS (Automatic Transfer Switch) automatically transfers electrical loads from the main utility supply to a standby generator when the utility fails. It uses two 4-pole contactors — one for mains and one for generator — that are electrically and mechanically interlocked. The fourth pole switches the neutral conductor, ensuring the load neutral is always connected to the correct source neutral and preventing neutral-to-neutral voltage from appearing across the load during transfer.
Place the 4-Pole Contactor symbol on a wiring diagram or schematic in the free online circuit diagram maker — no download required.