Energy Academy
Level 1/Electrical Science Refresher
Three-Phase Systems5 / 10

Three-Phase Basics

What three-phase power is, why UK sites use it, and the voltages decoded: 400 V vs 415 V, 230 V vs 240 V, and the √3 linking line to phase.

10 min read · Jacob Willis, Net Zero Lead · Last reviewed July 2026

Walk into any plant room and the supply arriving at the main panel is not the single live wire of a domestic socket but three, working together. In the UK, a three-phase supply is nominally 400 V measured between any two lines and 230 V from a line to neutral; the 415 V and 240 V still printed on older nameplates describe the same supply using the pre-1995 nominal figures. Three-phase power runs almost every commercial building and every industrial site, and the reasons are practical rather than historical: it delivers power more smoothly, uses less copper, and starts motors without help. This lesson explains what three-phase actually is, in plain terms, and decodes those numbers.

Three waves, evenly spaced

A single-phase AC supply is one voltage wave, rising and falling 50 times a second. At two moments in every cycle that wave passes through zero, so the power delivered pulses rather than flows steadily.

A three-phase supply is three such waves on three separate conductors, deliberately staggered by a third of a cycle each. When one phase is at zero, the other two are mid-swing. The result is that the combined power delivered never dips: something is always pushing. For anything converting electricity into rotation, that is the difference between a smooth pull and a series of jerks.

The three conductors are the lines (L1, L2, L3), and most supplies add a neutral, the return path that also lets single-phase loads hang off any one line.

The progression from one wave to three is best seen, not read. Step through it below and watch the pulsing power of a single phase turn into the flat line that three phases deliver together:

From one wave to three1 / 4
-3250325010203040Time (milliseconds)Voltage (V)zero crossings: nothing pushingL1
One wave: single phase

A single-phase supply is one voltage wave, rising and falling 50 times a second. Twice in every cycle it passes through zero: at those instants this wire is pushing nothing at all.

Why sites use it

Three benefits explain its universality:

  • Smooth, efficient power delivery. Constant combined power means less vibration and stress in machines, and for the same power delivered, three-phase needs substantially less conductor material than the equivalent single-phase supply. Networks are cheaper to build in three-phase.
  • Self-starting motors. Three staggered phases create a naturally rotating magnetic field, so a three-phase motor starts and runs without any extra components. Single-phase motors need capacitors and starting mechanisms to fake this, which is why serious motors, effectively everything above a few kW, are three-phase. The motors course picks this up in depth.
  • Flexibility. Big loads connect across the three lines; small loads (sockets, lighting) connect between one line and neutral. One supply serves both.

Decoding 230 V and 400 V

UK low-voltage supplies quote two numbers, and they describe the same system measured two ways. Between any line and neutral you measure 230 V: that is what a socket sees. Between any two lines you measure 400 V. The two differ by a fixed factor, the square root of 3 (about 1.732), which comes from the geometry of three waves spaced a third of a cycle apart:

400 V ÷ √3 ≈ 230.9 V

So "a 400 V three-phase supply" and "230 V single-phase sockets" are the same installation. Nameplates on larger equipment quote 400 V because those machines connect across the lines; small appliances quote 230 V because they connect line to neutral.

400 V or 415 V?

Older engineers and older nameplates say 415 V where newer ones say 400 V, and both are right. Until January 1995 the UK nominal voltages were 240 V single-phase and 415 V three-phase (240 × √3 ≈ 415.7 V). European harmonisation then relabelled the nominals as 230 V and 400 V, with a tolerance of +10% to −6% (so a single-phase supply may legitimately sit anywhere between 216 V and 253 V). The physical network barely changed: measure a UK supply today and you will typically read something closer to the old 240 V and 415 V. Treat 400 V and 415 V as the same supply described by different eras, not two different systems, and check a machine's rated voltage range rather than the single number on the front.

Balance matters

A three-phase supply works best when the load is spread evenly across the three lines. If one phase carries much more than the others, its cable and any shared neutral run hotter, voltage drops unevenly, and capacity is wasted: the supply is only as available as its most loaded phase. Checking phase balance, three clamp meter readings at the main panel, is one of the quickest health checks an energy manager can do, and unbalanced readings usually trace to how single-phase circuits were allocated over years of alterations.

Look, measure, but do not touch

Everything in this course supports reading nameplates, meters and clamp readings taken by competent persons. Work inside live panels is for qualified electricians operating under the site's safety rules. An energy manager's job is to know what the numbers mean, not to hold the screwdriver.

The next lesson adds the arithmetic: how to compute power in a three-phase system, and how to estimate what a motor really draws from a single current reading.

Sources and further reading

Frequently asked questions

What is the three-phase voltage in the UK?
UK three-phase supplies are nominally 400 V between any two lines and 230 V between a line and neutral, with a permitted tolerance of +10% to −6%. In practice, measured voltages often sit closer to the older nominal figures of 415 V and 240 V, because the physical network changed very little when the nominal values were harmonised in 1995.
Is UK three-phase 400 V or 415 V?
Both labels describe the same supply. 415 V was the UK nominal line-to-line voltage until January 1995 (240 V × √3); European harmonisation renamed the nominals 400 V and 230 V without materially changing the network. Equipment rated for either will normally be compatible, but always check the rated voltage range on the nameplate rather than the single headline figure.
What is the difference between single-phase and three-phase power?
A single-phase supply is one alternating voltage wave on one live conductor, so the power it delivers pulses rather than flows steadily. A three-phase supply is three waves staggered by a third of a cycle on three conductors, so their combined power delivery is constant. That smoothness, and the rotating magnetic field three phases create, is why motors above a few kilowatts are almost always three-phase.
When does a building need a three-phase supply?
Broadly, when total demand approaches the limit of a single-phase supply (typically 100 A in the UK, about 23 kW) or when any single machine above a few kilowatts, such as a lift, compressor or large motor, must be powered. Most commercial buildings and effectively all industrial sites take three-phase by default.
How can I tell if a supply is three-phase?
Look at the incoming supply position: a three-phase supply has three fuses (or a three- or four-pole main switch) where single-phase has one, and the meter will be marked as three-phase. On equipment, a 400 V or 415 V rating indicates a three-phase connection; 230 V or 240 V indicates single-phase.