Do I actually need a three phase supply?

What is a three phase supply, in plain terms?

A normal house has a single phase supply. One live conductor comes in, and the declared voltage between that live and neutral is 230 volts. Everything in the building runs off that one live.

A three phase supply brings three lives in instead of one. Each still gives 230 volts to neutral, so ordinary sockets and lights work exactly as before. Between any two of those lives there is roughly 400 volts, and the building can draw close to three times as much current.

It is not cleaner power or a better grade of electricity, and on its own it makes nothing run more smoothly. It is more capacity, plus the 400 volts that certain motors and machines are built to use.

How do I know which one I already have?

Look at the cut out, the sealed box where the supply cable enters before the meter. Your network operator puts it plainly: a single phase supply typically has one fuse in it, and a three phase supply has three.

The board gives you a second clue. A three phase board has a main switch that breaks three poles at once rather than one, and the circuits are usually labelled L1, L2 and L3. Do not open anything to check: the cut out is sealed, it belongs to the network operator, and only they may break that seal.

It is worth checking rather than assuming. Plenty of older commercial and industrial units already have a three phase supply capped off when the machinery left, which turns a long application into an afternoon’s work.

When do you genuinely need it?

There are three honest reasons, and a building usually has one of them clearly rather than a vague sense that more power would help.

• A machine that only exists in a 400 volt version. Larger motors, lathes, bigger compressors, lift gear, and much commercial catering and refrigeration plant. Above roughly three kilowatts a motor is usually three phase because that is how it is built.
• A total demand a single phase supply cannot carry. A commercial kitchen with several large appliances running at once, or a workshop where three or four machines are used together.
• More than one meter in the building. Separate tenancies, flats above a shop, or units inside a larger building are normally fed from a three phase intake, split across the phases.

Charging is the newer one. A 22kW charge point needs three phases by definition, and a bank of chargers for a fleet or a staff car park is a three phase job from the start. A single home charge point is not.

When is the honest answer no?

More often than the internet suggests. These are the four we are asked about most, and none of them is fixed by a new supply.

• The lights dim when a machine starts. That is almost always volt drop on an undersized or overlong submain. A bigger supply feeding the same undersized cable changes nothing.
• It keeps tripping. A breaker that trips has found a fault or is protecting an overloaded circuit. Running out of supply capacity looks different, and it is rare. This is a job for fault finding, not for the network operator.
• We want to future proof. A supply you are not using still carries a standing charge and still had to be paid for, and the demand you eventually have is rarely the one you guessed at.
• It is a house. Domestic three phase is almost never the right call. A heat pump, an induction hob and a charge point together usually still fit inside a 100 amp single phase supply, with the load management modern equipment already has built in.

What fixes most of these is unglamorous. A maximum demand and diversity assessment is a designer’s job under BS 7671, and it tells you what a building really draws rather than what the nameplates add up to. The answer is then a larger main fuse, a properly sized submain, or load management so the big items take turns. We would rather do that assessment and tell you the supply is fine.

How do you actually get one?

The application goes to your distribution network operator, not to the company that bills you. They are different organisations and people mix them up constantly. You can find yours by postcode through the Energy Networks Association, or by ringing 105.

They quote for and carry out everything up to and including the new cut out. Everything after it is the electrician’s work: the tails, a three phase distribution board, the submains, the containment, and rebalancing the existing circuits so the phases carry roughly equal loads. Both halves need designing together, which is why we would rather be involved before the application goes in than after.

Lead time is the part that catches people out. If three phase already runs past the building the work can be modest. If it does not, the job may involve a longer cable route, a road opening and a permit, and that is months rather than weeks. For a business there is a duty behind all of it as well: the Electricity at Work Regulations require the system to be constructed and maintained so as to prevent danger, so far as is reasonably practicable. Our three phase and distribution page covers how we handle the design and the application together.

What changes the price of the job?

Two separate bills, and they move for different reasons. The network operator’s side is driven almost entirely by civils.

• Whether three phase already runs down your street, or has to come from further away.
• The distance from the network to your intake position, and whether the intake must move.
• Whether the route crosses a road, a pavement or somebody else’s land, which brings permits, reinstatement with it.
• Whether the supply can be brought in live, or the building must be shut down for it.

The electrical side is more predictable: the size of the new board, the length and rating of the submains, the containment they run in, and how much existing wiring has to be reconnected and rebalanced across the phases. A unit with one tidy board in a sensible position is a different job from one with four ageing boards and no schedules for any of them.

What we will not do is put a number on either half before somebody has looked. The part that usually makes a phone quote wrong is the ground outside, which nobody can price from a description.

Questions we get asked about this