Most homes and small shops run happily on single-phase. You need three-phase if you run equipment built for it, such as larger workshop machines, some commercial kitchen equipment or a 22kW EV charger, or if your total load is more than a single-phase supply can carry. Before applying for three-phase, check the alternatives: load management, a bigger single-phase main fuse, or a phase converter for a single machine.
What is three-phase power, in plain English?
Three-phase is a supply with three live conductors instead of one. It delivers more power through the same building and suits large motors better.
A single-phase supply has one live and one neutral, giving 230V. A three-phase supply has three lives, usually with a neutral. Between any two of the lives you get 400V, and between any one live and neutral you still get the normal 230V.
That means a three-phase building can run 400V machines and ordinary 230V sockets and lights at the same time. The lighting and sockets are simply shared out between the three phases. The three phases peak one after another, so a three-phase motor gets a smooth, continuous push. That's why larger motors are often built for three-phase: they tend to start more easily and run more smoothly.
Who actually needs three-phase?
Mainly businesses with heavy equipment, and a small number of large homes. If you're not sure, the rating plate on your equipment is the best guide.
- Workshop machinery
- Larger lathes, mills, saws, spray booths and welders are often three-phase only.
- Commercial kitchens
- Combination ovens, large fryers and pass-through dishwashers in cafés and takeaways.
- Larger heat pumps
- Some bigger units, mostly in large homes or commercial buildings.
- 22kW EV charging
- Single-phase home chargers top out around 7kW. 22kW needs three-phase.
- Big homes with several large loads
- A heat pump, EV charger, electric heating and more, all on one supply.
How can I tell if I already have three-phase?
Look at your cut-out and meter, but treat what you see as a clue rather than proof. Don't open or touch the cut-out: it belongs to the network operator and is live.
The cut-out is the sealed unit where the supply cable enters the building, usually next to the meter. If you see three separate fuse carriers side by side, that usually means a three-phase supply. A single fuse carrier usually means single-phase. A three-phase meter is often larger and may show three sets of readings or markings such as L1, L2 and L3.
There are exceptions. Some buildings have a three-phase supply but only one phase is in use, and some cut-outs and meters are hard to read. Your energy supplier or the network operator can confirm, and an electrician can check safely at a survey. Clear photos of the cut-out, meter and main board are often enough to tell.
What can I do instead of getting three-phase?
Often more than people think. If the problem is total load rather than a three-phase machine, one of these may be cheaper and quicker:
- Load management
- A sensor reduces big loads, such as an EV charger, when the house is busy.
- Smart charging
- Charging the car overnight, when the rest of the house uses little power.
- A bigger main fuse
- The network operator may upgrade, for example from 60A to 80A or 100A.
- A phase converter
- Runs some three-phase machines from single-phase, within limits.
- Spreading the load
- Not running every heavy item at once, by habit or by timer.
Is a main fuse upgrade enough?
It often is for homes. Many houses still have a 60A main fuse, and upgrading to 80A or 100A gives noticeably more headroom without changing to three-phase.
The main fuse belongs to the network operator, so they do this work, not your electrician. In Leicestershire that's National Grid Electricity Distribution. They'll check whether the cable into your home can take the higher rating. If it can't, or if your supply is shared with a neighbour (a looped supply), they may need to do more work first. Whether there's a charge, and how long it takes, is up to them.
Your meter tails and main switch also need to be suitable for the higher rating, which is something your electrician can check. A main fuse upgrade has a ceiling, though. Around 100A is usually the most a single-phase domestic supply offers, so if your planned load is well beyond that, three-phase may be the only real answer.
When does a phase converter make sense?
When you have one or two three-phase machines and no other reason to upgrade the supply. It's common for a lathe or mill in a home or small workshop.
There are a few types. An inverter drive (a variable speed drive) can run a single three-phase motor from single-phase and often adds speed control. Static and rotary converters can run some machines, with different trade-offs in starting power and running performance.
The limits matter. Converters suit smaller motors better than large ones, and machines with several motors, complex controls or heavy starting loads can be hard to run this way. A converter also doesn't add capacity: if your total load is too big for the supply, it won't fix that. Check the machine's rating plate and the maker's advice, and get an electrician to confirm the converter, cable and protection are right for it.
How do the costs and timescales compare?
The cheaper, quicker options sit at the single-phase end. A new three-phase supply is usually the slowest and most expensive route, but sometimes the only one that works.
- Load management or smart charging
- Usually the quickest. Often part of a charger installation anyway.
- Main fuse upgrade
- Done by the network operator. Often fairly quick, but depends on your supply.
- Phase converter
- Equipment plus installation. Quick to fit, but only for suitable machines.
- New three-phase supply
- Quoted by the DNO, plus the electrician's work. Weeks to months.
- Existing unused three-phase
- If you already have it, bringing it into use is often the best value.
So which should I choose?
Start from what you need to run, not from the supply. Then work through it in this order:
- If you already have three-phase
- Use it. Get the board and circuits checked and extended as needed.
- If your machine is three-phase only and large
- Choose a three-phase supply. A converter is unlikely to cope well.
- If you have one small three-phase machine
- Consider a phase converter or inverter drive first.
- If you want a 7kW charger with a heat pump
- Try load management and a main fuse check before three-phase.
- If you need 22kW charging
- You'll need three-phase, and a car that can actually take 22kW.
- If several big loads exceed 100A
- Three-phase is likely the long-term answer. Plan for future loads too.
What should I have ready before asking for advice?
A little information saves a lot of guesswork. Take photos of your cut-out, meter and main board, and list the equipment you run or plan to add, with the power rating from each rating plate.
Note where the equipment will go and how far that is from the board. If you've had any letters from the network operator, keep them handy. With that, an electrician can usually tell you which route is realistic and what the next step is.
At NT Electric we look at your photos or visit, explain the options honestly, and give one fixed price for our part of the work.
Questions
Is three-phase more expensive to run?
Not in itself. You pay for the energy you use, so the same machine costs roughly the same to run. Standing charges and tariffs can differ for three-phase supplies, so check with your energy supplier before you switch.
Can I have three-phase at home?
Yes. It's less common in homes, but the network operator can provide it where the local network allows. They decide whether it's possible and what it will cost. Your electrician then handles the board and circuits after the meter.
Will three-phase stop my fuse box tripping?
Not usually. Tripping is normally caused by a fault, damp or an overloaded circuit, not by the supply type. A main fuse blowing because of overload is different, and more capacity can help there. Get the fault found first.
Can I plug a three-phase machine into a normal socket?
No. A three-phase machine needs a three-phase supply or a suitable converter, and a proper connection with its own isolator. Never try to adapt the plug. Large single-phase machines may also need their own circuit rather than a 13A plug.
Does a three-phase supply need a different meter?
Yes, usually. Changing from single-phase to three-phase normally means a new meter, which your energy supplier arranges. It's worth contacting them as soon as the network operator confirms a connection date.
Do I need three-phase for a heat pump?
Most home heat pumps run on single-phase. Larger units for big homes or commercial buildings may need three-phase. Check the model's specification and ask the installer to work out the total load with everything else in the building.