Three Phase Installation: The Complete BS 7671 Guide

A three phase electrical supply uses three live conductors — designated L1, L2, and L3 — each carrying an alternating current at 50Hz but offset from each other by 120 degrees. This arrangement delivers significantly more power than a single phase supply for the same conductor size and current rating.

The voltage between any two phases (line-to-line voltage) is 400V. The voltage between any single phase and the neutral conductor (line-to-neutral voltage) is 230V. The relationship between these voltages is governed by the square root of 3 (approximately 1.732): 400V / 1.732 = 230V.

In the UK, the Distribution Network Operator (DNO) provides three phase supplies as standard to commercial and industrial premises. Domestic properties typically receive a single phase supply, but three phase can be requested where the maximum demand justifies it — for example, properties with multiple EV chargers, heat pumps, or high-power electric cooking appliances.

Three phase power is calculated as P = sqrt(3) x V_line x I_line x power factor. For a balanced load drawing 100A per phase at unity power factor, the total power is 1.732 x 400 x 100 = 69.3kW — compared with 23kW from a single phase 100A supply.

The decision to install a three phase supply depends on the total electrical demand of the premises. A single phase supply in the UK is typically rated at 60A or 100A (depending on the service fuse), providing a maximum of approximately 14kW to 23kW. When the calculated maximum demand exceeds this, three phase becomes necessary.

To determine whether three phase is needed, calculate the maximum demand using diversity factors from BS 7671 and the IET On-Site Guide. If the total demand after diversity exceeds the single phase supply capacity, three phase is required. Elec-Mate's max demand calculator applies the correct diversity factors automatically and tells you whether single or three phase is appropriate.

Load balancing is one of the most important aspects of three phase installation design. In a perfectly balanced system, each phase carries the same current and the neutral conductor carries zero current. In practice, perfect balance is rarely achieved, but the goal is to get as close as possible.

An unbalanced three phase system causes current to flow in the neutral conductor. The greater the imbalance, the higher the neutral current. In extreme cases, the neutral current can exceed the phase current, which risks overheating the neutral conductor — particularly dangerous in older installations where the neutral may be undersized relative to the phase conductors.

Elec-Mate's AI circuit designer can assist with phase allocation by analysing the connected load on each phase and suggesting redistribution to minimise imbalance. This is particularly useful on complex commercial installations with dozens of circuits.

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A TPN (Triple Pole and Neutral) distribution board is the heart of a three phase installation. Unlike a single phase consumer unit with one busbar, a TPN board has three phase busbars (L1, L2, L3) and a neutral bar. Each outgoing way can be connected to any one phase (for single phase circuits) or across all three phases (for three phase loads).

When selecting a TPN distribution board, consider the following requirements under BS 7671:

Labelling is essential on TPN boards. Every circuit must be clearly labelled with its function, phase allocation, and protective device rating. The circuit chart must show which phase each circuit is connected to. Under BS 7671 Regulation 514.9, the circuit chart must be fixed inside or adjacent to the distribution board and kept up to date.

Cable sizing for three phase circuits follows the same principles as single phase — you must ensure the cable can carry the design current (Ib), withstand the prospective fault current, and keep the voltage drop within BS 7671 limits. The key difference is that the current per phase is lower for a given total power, because the power is shared across three conductors.

For a balanced three phase load, the current per phase is calculated as: I = P / (sqrt(3) x V_line x pf), where P is the total power in watts, V_line is 400V, and pf is the power factor. For example, a 30kW balanced load at unity power factor draws 30,000 / (1.732 x 400 x 1.0) = 43.3A per phase.

The cable sizing process then applies the standard correction factors from BS 7671 Appendix 4:

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Testing a three phase installation requires the same sequence of tests as single phase, but with additional measurements between phases. The testing sequence under BS 7671 Chapter 64 and GN3 is:

Phase rotation is particularly important for three phase motor installations. Incorrect phase rotation causes motors to run in reverse, which can damage driven equipment. Always verify phase rotation at the distribution board and at the motor terminals before energising.

Record all test results on the Schedule of Test Results form. Elec-Mate's EIC certificate app includes the full schedule of test results and supports three phase circuit entries with per-phase readings.

Earthing and bonding requirements for three phase installations follow the same BS 7671 regulations as single phase, but the conductor sizes are often larger due to the higher fault currents associated with three phase supplies.

The main earthing conductor size is determined by BS 7671 Table 54.7 based on the largest phase conductor size. For a 100A three phase supply with 25mm2 phase conductors, the main earthing conductor must be at least 16mm2 (copper). The main protective bonding conductors must be at least half the size of the main earthing conductor, with a minimum of 6mm2 for supplies up to 35mm2 phase conductors and 10mm2 for larger supplies.

For detailed guidance on earthing arrangements including TN-S, TN-C-S, and TT systems, see our dedicated guide.

Every new three phase installation requires an Electrical Installation Certificate (EIC) under BS 7671 Part 6. The EIC must include the full schedule of test results for every circuit on every phase, the schedule of items inspected, and the designer, installer, and inspector declarations.

For three phase installations, the EIC must record:

If the three phase installation is notifiable under Part P of the Building Regulations (for example, a new installation in a dwelling or the installation of a new consumer unit), the work must be either carried out by a registered competent person who can self-certify, or notified to Building Control before work starts.