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A three-phase board almost always has a mix of three-phase loads and single-phase loads, and that is exactly where the 3P versus 4P decision gets made badly. The two devices look similar, take four modules between them, and are often specified interchangeably — but only one of them switches the neutral, and the consequences of getting it wrong range from a failed inspection to an isolation procedure that does not actually isolate.
The short answer: use a 3P breaker when the circuit is three-phase with no neutral, such as a motor, a pump, or a three-phase heater. Use a 4P breaker when the circuit includes a neutral that must be switched and isolated — a mixed three-phase and single-phase distribution board, a TT system, or a circuit feeding equipment with a neutral connection.
Three device types are sold under two names, and the catalogue entry does not always make the difference obvious:
| Dispositivo | Modules (18 mm) | Poles switched | Poles protected | Uso habitual |
|---|---|---|---|---|
| 3P | 3 | L1, L2, L3 | L1, L2, L3 | Three-phase load without neutral: motors, heaters, pumps |
| 3P+N | 4 | L1, L2, L3, N | L1, L2, L3 only | Mixed load where the neutral is switched but not protected |
| 4P | 4 | L1, L2, L3, N | Depends on the device — check the datasheet | Circuits where the neutral carries current that must be protected |
That last row is the trap. A device sold as “4P” is frequently a three-pole breaker with a switched neutral pole — internally a 3P+N. It switches the neutral, which is what most installations need, but it will not detect a neutral overload. If neutral protection is the reason you are buying a 4P device, look for a datasheet that states four protected poles, not four poles.
If the load has no neutral connection, a 3P device is the right answer and adding a fourth pole buys nothing. Motors, three-phase heaters, most pumps, and three-phase transformers fall into this group. The neutral, if one runs to the equipment at all, is there for control or instrumentation and is not part of the load circuit.
There is a second case: a three-phase submain feeding a downstream board that has its own neutral bar and its own neutral disconnection. In that arrangement the neutral is solid at the upstream device and is broken downstream, which is a legitimate design in TN systems. It does mean the upstream device alone does not isolate the submain completely, so the isolation procedure has to name both points.
Mixed boards. Once a distribution board carries both three-phase and single-phase circuits, the incoming device has to break the neutral, because otherwise opening the main switch leaves a live neutral connected to every single-phase circuit in the board. A 3P incoming device with a separate neutral bar is common in older installations and is a frequent finding in inspections.
TT systems. The same reasoning as for single-phase circuits applies. In a TT installation the neutral is not tied to the installation earth in the same way, and equipment may have to be isolated on all poles.
Neutral as a current-carrying conductor. Non-linear loads — switch-mode power supplies, LED drivers, variable frequency drives, IT equipment — produce triplen harmonics whose currents add in the neutral instead of cancelling. In a heavily loaded four-wire circuit the neutral current can exceed the phase current. A three-pole device cannot see that current at all, and a 4P device with only three protected poles cannot either. This is the one case where a genuinely four-pole device earns its price.
All-pole isolation requirements. Some codes require every live conductor, including the neutral, to be isolated for particular equipment or locations. That is a project-specific requirement and needs to be checked against the applicable standard rather than assumed.
In a TN-C system the neutral and protective functions share one conductor, the PEN. That conductor must not be switched or disconnected: breaking it removes the protective earth connection for everything downstream, which turns a routine isolation into a shock hazard. A four-pole device must not be inserted in a PEN conductor.
In TN-C-S the combined conductor is split into separate neutral and protective conductors at the origin of the installation. Downstream of that split the neutral is an ordinary N conductor and may be switched. Where the split happens determines which device is appropriate, and it is worth confirming on the drawing rather than assuming.
One extra module per device, on every three-phase way in the board. On a 12-way three-phase board that is 12 extra modules — most of an 18-module row — which usually comes out of either the spare capacity or the physical size of the enclosure.
The cost is not only rail space. Four-pole devices are longer to wire, need the neutral brought to the device and taken away again, and add a terminal pair that has to be torqued and checked. Boards that were sized for 3P devices and then changed to 4P late in the project frequently run out of wiring space before they run out of modules.
A balanced three-phase load has no neutral current, which is why the neutral is often omitted. But balance is an assumption, not a guarantee. Single-phase loads distributed across the three phases are never perfectly balanced, and the neutral carries the difference. Add harmonic content and the neutral current can be higher than any single phase.
If you are specifying a 4P device for a mixed board, the neutral rating matters as much as the phase rating. A device with a reduced neutral rating, sometimes marked as a 3P+N device with a lower N current, is not suitable for a harmonic-rich load.
For the single-phase version of the same decision, including the 1P+N middle option, see 1P vs 2P MCB for single-phase circuits. The board-level consequences — how many ways the fourth pole consumes and how much spare capacity to keep — are covered in distribution box layout and spare module space.
No. A motor has no neutral connection, so the fourth pole has nothing to switch. A 3P device with the correct utilisation category and setting is the normal choice.
Not necessarily. Both take four modules and both switch the neutral. A true 4P device has a protective element in every pole; a 3P+N device has three protected poles and one switched neutral. The datasheet is the only reliable way to tell them apart.
Only if the neutral is solid and does not need to be isolated at that point, for example where a downstream board breaks it. Otherwise the circuit is not fully isolated when the breaker is open.
Triplen harmonics — the third, ninth, fifteenth and so on — add in the neutral rather than cancelling. With a high proportion of switch-mode loads this can push neutral current above the phase current, which is why some installations specify a full-size or oversized neutral.
If the circuit includes a neutral and the RCD is protecting it, the device has to sense and switch that neutral. A four-pole RCCB or a 3P+N device is the usual answer, and mixing a 3P RCD with a separately routed neutral defeats the protection.
When you are ready to specify, the AC MCB range lists single-pole to four-pole versions with module widths, and the circuit breaker category covers MCCB and RCCB options for larger frames.
