Three-Phase Power Explained: Why Factories Get 400 V

Part 14 of 23 in Cobler's Electricity Fundamentals series. New here? Start with the course map.

Look at the incoming supply to a Malaysian terrace house and you see two wires doing the work: one live, one neutral. Walk into a factory next door and the incomer has four: three lives and a neutral. The house runs on single-phase power; the factory runs on three-phase power, and its nameplate says 400 V where the house says 230 V. That difference is not an upgrade you pay extra for. It is the reason big motors turn at all, and the reason the grid can move serious power without burying the street in copper.

This is the piece of the puzzle that ties the whole series together: why the grid alternates, why it settles at 50 Hz, and why reactive power matters all lead here, to the moment three overlapping sine waves start doing something a single one never could.

Why does a house get two wires and a factory get four?

Because their loads want different things. A house runs lights, sockets, a fridge and an aircon or two. None of that needs constant torque, so a single live-and-neutral pair is enough. A factory runs induction motors: compressors, pumps, fans, chillers, conveyors. Those want steady, smooth torque and they want to start themselves, and that is exactly what three phases deliver and a single phase cannot.

Three-phase supply is three separate live conductors, each carrying an AC voltage of the same size and frequency, but staggered in time. Phase two peaks a third of a cycle after phase one; phase three a third of a cycle after that. In degrees, the three are spaced 120 apart around the 360-degree cycle. That even 120-degree spacing is the whole trick.

Why does single-phase power pulse but three-phase power stay smooth?

Because instantaneous power in an AC circuit rises and falls with the voltage, and a single phase spends part of every cycle at zero. On 50 Hz mains the voltage passes through zero 100 times a second, and a single-phase supply's power pulses down toward zero in step, 100 times a second. The power arrives in lumps, dropping toward nothing in between.

Picture one person pushing a playground merry-go-round. They shove, let go to reset their hands, shove again. The ride surges and coasts, surges and coasts. Now put three people around it at staggered positions: as one finishes their push, the next is already starting. Someone is always pushing, so the ride turns at a constant, even speed. Three-phase power works the same way. When you add the three staggered phases together, the dips of one are filled by the peaks of the others, and a balanced three-phase load draws constant total power every instant. Constant power means constant torque, which means a big motor runs smoothly without a heavy flywheel to iron out the bumps.

How do three phases spin a motor with no moving contacts?

They create a rotating magnetic field, for free, just by existing. Feed those three currents, each 120 degrees apart in time, into three sets of stator windings (the fixed coils lining the inside of the motor), spaced 120 degrees apart in physical position, and their magnetic fields add up to a single field of constant strength that sweeps around the inside of the motor at a speed set by the supply frequency and the number of magnetic poles the motor is wound for (its synchronous speed). It is like the chasing lights on a signboard: no bulb moves, but the bright spot travels around the loop.

Drop a metal rotor into that rotating field and the field drags it along. There are no brushes, no commutator, no sliding contacts to wear out or spark. This is the induction motor, and it is why three-phase won. Nikola Tesla filed seven polyphase patents in 1887 and was granted his electromagnetic motor patent on 1 May 1888; Westinghouse bought the portfolio. Mikhail Dolivo-Dobrovolsky at AEG then engineered the practical three-phase system, and in 1891 it went public in the most convincing way possible: the Lauffen-to-Frankfurt demonstration sent power 175 km, stepped up to about 15 kV at roughly 75% efficiency, running a 100 hp motor and around a thousand lamps at the far end. Long-distance transmission and the self-starting motor arrived in the same package. That package still runs your factory.

Three phases also save copper. Because the return currents in a balanced three-phase system partly cancel, three conductors carry more power per kilogram of metal than three separate single-phase circuits would, and a balanced system barely loads its neutral. More phases would help a little more, but with sharply diminishing returns for extra conductors. Three is the minimum that gives you both constant power and a rotating field, so three is where the world stopped.

Why is it 400 V, not 460 V? The square-root-of-three answer

Because the two phases you measure between do not peak at the same moment, so their voltages do not simply add. In the Malaysian low-voltage system each phase sits at 230 V measured from that phase to neutral. Measure between any two phases instead and you get 400 V, the number on the factory nameplate.

If the phases peaked together you would expect 230 plus 230, or 460 V. They do not. They peak 120 degrees apart, so at the instant one phase is at its maximum the other is already on its way down. The geometry of two waves that size, offset by 120 degrees, works out to a factor of the square root of 3, about 1.732. And 230 multiplied by 1.732 is roughly 398, which we round to 400. No trigonometry needed to use it: just remember that phase-to-neutral times 1.732 gives phase-to-phase. That is why LV three-phase is quoted as "400/230 V" (the older Malaysian figure was "415/240 V", now legacy nomenclature). It is also why a factory gets a bigger voltage without any new generation: the extra volts are geometry, not a bigger supply.

Why does your electrician spread the circuits across all three phases?

To keep the load balanced, so no single phase carries more than its share. In a balanced system the three phase currents are roughly equal and their returns cancel in the neutral, which is why the neutral can be thin and why total power stays smooth. Pile all your single-phase loads onto one phase and you get the opposite: that phase overheats, its cable and the transformer winding run hot, and the voltage on it sags while the other two sit lightly loaded. The neutral, which should be quiet, ends up carrying the leftover unbalanced current.

So a competent electrician wiring a shoplot or factory deliberately shares the lighting, sockets and small single-phase loads across all three phases, aiming for roughly equal current on each. It is the same logic TNB uses across a whole street, spreading houses across phases so the network stays balanced. Every serious motor, meanwhile, is wired to all three at once, which is the neatest balanced load there is.

When does a building actually need three-phase (and the wire-colour trap)?

The moment its load outgrows a single phase or includes a real induction motor. A home's few kilowatts of lighting and appliances fit comfortably on a single phase, and nothing in a house needs a constant-torque three-phase motor. A shoplot with a couple of larger aircon units or a small compressor might take a three-phase supply. Anything with real induction motors, a factory, a chiller plant, a workshop, takes three-phase as a matter of course, because that is the only economical way to run those motors and to balance heavy load across the incoming supply.

One practical warning for the Malaysian transition. The old British-derived colour code used Red, Yellow and Blue for the three phases, Black for neutral, Green for earth. The new IEC code being adopted here uses Brown, Black and Grey for the phases, Blue for neutral, and green-and-yellow for earth, with full compliance required by 1 January 2029. Notice the trap: Blue used to be a live phase, and now it is the neutral. During the changeover, never assume a wire's job from its colour alone on unfamiliar work. Test it.

Three phases, staggered 120 degrees apart, buy you three things at once: constant power, a rotating field that turns motors without contacts, and a 400 V line supply from a 230 V phase. That is why the factory next door gets four wires, and why every heavy motor in it is a three-phase machine.

This is Part 14 of 23 in Cobler's Electricity Fundamentals series. Previous: Why 50 Hz? The Accident Behind the Grid Frequency. Next: How Electric Motors Work (and Run Everything).

Those three-phase motors are also where most of a facility's reactive power and power-factor cost comes from. If you want to see how your motors load each phase and what that does to your TNB demand and power-factor charges, see what CobiNeural monitors.