1P, 1P+N, 2P, 3P and 4P Circuit Breakers: How Many Poles You Need
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A pole is one set of contacts inside a circuit-breaker, switching one conductor. A single-pole breaker interrupts one conductor, a double-pole breaker two, and so on up to four. The number of poles you need is not decided by the number of wires in the cable but by which conductors have to be protected against overcurrent, which ones have to be isolated, and what the earthing system allows you to leave connected.
The types you will meet
| Marking | Switches | Protects | Typical use |
|---|---|---|---|
| 1P | Line | Line | Final circuits in TN systems, where the neutral may stay connected |
| 1P+N | Line and neutral | Line only | Final circuits where the neutral must be switched, in a compact width |
| 2P | Two conductors | Both | Single-phase circuits in TT systems, 240 V split-phase loads in North America, DC circuits |
| 3P | Three lines | Three lines | Three-phase loads without neutral: motors, three-phase heaters |
| 3P+N | Three lines and neutral | Three lines | Three-phase loads with neutral where the neutral must be switched |
| 4P | Three lines and neutral | All four | Changeover between sources, high harmonic loads, TT and IT systems |
The difference between 1P+N and 2P, and between 3P+N and 4P, catches people out. In a 1P+N device the neutral pole opens and closes with the line pole but has no tripping element of its own. In a 2P device both poles can trip. For a single-phase circuit fed from line and neutral the two usually behave the same in practice, because the line and neutral currents are equal. The distinction becomes real in three-phase systems, which we come back to below.
The one rule with no exceptions
BS 7671 regulation 132.14.1 says it plainly: a single-pole fuse, switch or circuit-breaker shall be inserted in the line conductor only. A single-pole device in the neutral leaves the circuit live with the breaker off. Anyone who then works on the circuit believing it is dead is exposed to full line voltage on every terminal that should be safe.
This turns up in two situations: an older installation where line and neutral were swapped at the board, and a replacement where the new breaker was simply wired the way the old cables happened to arrive. A polarity test at every point catches both, which is why it is on the schedule of tests.
When the neutral has to be switched
Whether the neutral needs a pole depends on the earthing system.
In TN-S and TN-C-S systems the neutral is connected to earth at the source, and at the origin of the installation in the case of TN-C-S. BS 7671 regulation 461.2 lets you leave the neutral unswitched and unisolated where it can be regarded as reliably at earth potential. That is why the outgoing ways of a domestic board on a TN supply are normally single-pole.
In a TT system the neutral is earthed only at the transformer, far away, and its potential at your installation can float by a few volts under normal conditions and much more under fault. Means of isolation in a TT installation have to disconnect the neutral as well. The main switch must be double-pole, and in practice the RCDs that every TT installation relies on are double-pole devices too.
In an IT system nothing is solidly earthed, so every live conductor, neutral included, is treated as potentially dangerous and is switched.
When four poles are really needed
Changeover between two sources. A building with a standby generator that has its own earthed neutral needs a four-pole changeover. If the neutral were left permanently connected, the neutral currents would split between two earthed star points and part of the load current would return through the protective conductors. RCDs downstream would trip at random, and the earthing arrangement would be quietly compromised. Switching the neutral with the lines keeps each source a separate system.
High harmonic content. In a three-phase four-wire circuit feeding computers, LED lighting or other non-linear loads, the third harmonic currents of the three phases add up in the neutral instead of cancelling. The neutral current can exceed the phase current. IEC 60364-4-43 requires overcurrent detection in the neutral where this can happen, and a 4P breaker with a protected neutral is the usual way to provide it.
Reduced neutral cross-section. Where the neutral of a large supply cable is smaller than the line conductors, it needs its own overcurrent protection unless the load is balanced and harmonic-free.
Width and labelling
On DIN-rail equipment one module is 17.5 or 18 mm. A standard MCB takes one module per pole, so a 1P is one module wide, a 2P two and a 4P four. Many manufacturers make a compact 1P+N in a single module for domestic boards. RCBOs vary: some single-module types carry line and neutral in one module, others take two.
On the front you will see the pole markings at the terminals: 1–2 for the first pole, 3–4 for the second, and N–N for a neutral pole. On a 4P device the position of the neutral pole differs between manufacturers, left on some and right on others, and getting it wrong on a busbar-fed board puts the neutral on a phase bar. Check the drawing on the device before you connect.
North American practice
In a 120/240 V split-phase panel a single-pole breaker feeds a 120 V circuit and a double-pole breaker feeds a 240 V load such as a range, a dryer or an air-conditioning condenser. A double-pole breaker has a common internal trip, so a fault on either leg opens both. Two single-pole breakers joined by a handle tie switch together by hand, but only open together on fault if they are listed for common trip. The NEC rules on this are in section 240.15(B).
Mistakes we see most often
- A single-pole breaker in the neutral after a careless replacement.
- A single-pole main switch in a TT installation.
- A 3P breaker on a three-phase supply with neutral in a TT system, leaving the neutral connected when the circuit is isolated.
- A 3P+N device on a heavily non-linear load, where the neutral needed a protected pole.
- Generator changeover switches with three poles and a solid neutral link between two separately earthed sources.
Related: how to read circuit-breaker markings, TT earthing system, neutral conductor.
References
- BS 7671:2018+A2:2022, Regulations 132.14.1 and 461.2, and Chapter 53.
- IEC 60364-4-43, protection against overcurrent, protection of the neutral conductor.
- IEC 60898-1 and IEC 60947-2, product standards for circuit-breakers.
- NFPA 70 (NEC), section 240.15(B).