Three-Phase Supply
What is a three-phase electricity supply?
If you’ve ever walked into a small office and seen the same kind of fuse board you have at home, you’ve seen single-phase. If you’ve ever walked into a workshop, restaurant kitchen, or industrial unit and seen a much chunkier panel with multiple separate breakers, you’ve probably been looking at three-phase. The difference matters because what you can plug in, what it costs to install, and how your electricity bill is calculated all depend on which one you’ve got.
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Three-phase supply is one of those things UK businesses inherit rather than choose. The previous occupier put it in, or didn’t, and the cost of changing it later is significant. Worth understanding what you actually have before you commit to new equipment that won’t run on it.
What three-phase actually is
Mains electricity in the UK is supplied as alternating current, which means the voltage rises and falls 50 times a second. A single-phase supply uses one live conductor doing that rise-and-fall, plus a neutral. A three-phase supply uses three live conductors, each doing the rise-and-fall but offset from each other by 120 degrees.
The practical upshot is that three-phase delivers power more smoothly and at higher total capacity than single-phase. The voltage between any one phase and neutral is 230 volts (the same as single-phase). The voltage between any two phases is 400 volts. That’s why heavy equipment uses three-phase. Motors, pumps, ovens, kilns, and large air-con units run better on it.
Three-phase vs single-phase
Most homes and small offices in the UK run on single-phase. The supply comes in on one live wire plus neutral, and everything in the building runs off that. It’s simple, cheap to install, and fine for everyday loads.
Three-phase comes in on three live wires plus neutral. Loads inside the building can be split across the three phases to balance them out, which means a three-phase site can draw more total current without overloading any single conductor.
The rough rule of thumb. Single-phase tops out at around 23 to 25 kW of demand. Three-phase can comfortably handle 50 kW, 100 kW, or much more, depending on the connection’s Authorised Supply Capacity.
Capacity and load handling
The capacity advantage isn’t just about total power. It’s about the way three-phase handles load. Each motor or appliance running on three phases gets a smoother power delivery, which makes the equipment run cooler, last longer, and use slightly less energy than the single-phase equivalent.
For motor-driven loads in particular. Anything with a compressor, pump, or significant rotation. Three-phase motors are simpler, cheaper to manufacture, and more reliable than single-phase ones. Industrial equipment is almost universally specified for three-phase as a result.
The other side of the picture is reactive power. Three-phase sites running large motors can develop a power factor issue, where reactive power starts costing money. It’s worth knowing this exists before you find out via a surcharge on the bill.
Who needs three-phase
You’ll typically find it where there’s:
- Heavy machinery or motors. Workshops, manufacturing, packing lines.
- Commercial kitchens. Large ovens, fryers, refrigeration banks.
- Significant HVAC. Climate control in offices, hotels, retail spaces over a few hundred square metres.
- Electric vehicle charging. Most rapid chargers (50 kW+) need three-phase.
- Larger retail or hospitality sites. Restaurants, pubs, hotels, gyms typically all draw enough load to warrant three-phase.
- Data centres or server rooms. Load density and resilience both push toward three-phase.
Smaller offices, retail units under about 200 m², and most service businesses do fine on single-phase. The line between the two is roughly a 25 kW peak demand, but it’s not a sharp cutoff.
How to tell what you have
The easiest visual check is your main fuse and meter. Single-phase has one fuse and one meter terminal pair coming in. Three-phase has three fuses (often in a row) and three pairs of terminals.
Inside the building, three-phase distribution boards are taller and have three rows of breakers, each row covering one phase. You’ll often see labels like L1, L2, L3 next to each row.
Your bill won’t tell you directly, but a half-hourly meter on a higher ASC is a strong hint you’ve got three-phase. The first two digits of your MPAN tell you the distributor; the topline doesn’t directly state the phase but your supplier can confirm.
Cost of installing three-phase
Three-phase isn’t expensive to specify when you’re putting in a new connection. The cost gap from single-phase is modest. The expensive part is retrofitting it after a single-phase connection is already in.
For a new build or fit-out where the meter installation hasn’t happened yet, asking for three-phase typically adds a few hundred to a couple of thousand pounds on top of the single-phase quote. The DNO has to lay the right cable and fit the right meter, but the marginal cost is manageable.
Upgrading from single to three-phase later is a different story. The DNO has to bring a new cable from the street, may need to upgrade transformer capacity, and the work itself usually involves excavation. Costs commonly run from £3,000 to £15,000+, depending on distance to the supply and how much network reinforcement is needed.
The lesson, if you’re choosing right now between specifying single or three-phase for a new fit-out. Even if you don’t immediately need three-phase, fitting it during the initial connection is usually cheaper than discovering you need it later.
How three-phase shows on your bill
The supply phase isn’t usually printed on your bill, but its consequences are. Three-phase sites tend to:
- Be on half-hourly metering rather than NHH, given the higher load.
- Have a separately broken-out DUoS charge, not bundled into the unit rate.
- Carry an ASC figure measured in kVA, with a daily availability charge attached.
- Be exposed to excess capacity charges if peak demand creeps above the agreed ASC.
Most three-phase sites also benefit from being on a pass-through contract rather than a fully fixed one, because the non-energy costs are large enough that fixing them locks in a premium.
Upgrading and capacity changes
If your usage has grown. New equipment, more staff, longer hours, or an EV fleet. You may need more capacity than your current three-phase connection provides. That’s a separate process from upgrading single to three-phase. It’s negotiated with your DNO and involves uplifting your ASC.
The opposite case also comes up. Sites that have downsized. Equipment removed, shifts shortened, electric loads moved off-site. May be paying for ASC capacity they no longer need. Reducing ASC has to go through the DNO too, and there’s typically a wait while they confirm the change.
Either way, the work doesn’t usually require new cable, just a meter or settings change. Costs are much lower than the original install. The harder part is knowing whether to make the change. That’s where bill validation and an honest look at your peak demand pattern matter.
Frequently asked questions
What is a three-phase electricity supply?
A three-phase supply delivers power through three live conductors offset by 120 degrees, plus a neutral. It provides higher total capacity and smoother power delivery than single-phase, and is used at sites with heavy equipment or significant load.
How is three-phase different from single-phase?
Single-phase uses one live conductor and is suitable for everyday loads up to about 25 kW. Three-phase uses three live conductors offset by 120 degrees, can handle 50 kW or much more, and runs heavy equipment more smoothly.
What voltage is three-phase supply in the UK?
Each phase to neutral is 230 volts, the same as single-phase. The voltage between any two phases is 400 volts. Heavy equipment uses the 400V three-phase configuration directly.
How do I know if I have three-phase electricity?
Look at your main supply. Single-phase has one fuse and one pair of meter terminals. Three-phase has three fuses, often in a row, and three pairs of terminals. Distribution boards inside the building also have three rows of breakers labelled L1, L2, L3.
How much does it cost to upgrade from single-phase to three-phase?
Typically £3,000 to £15,000 or more, depending on distance to the supply and what network reinforcement is needed. The cost is much lower if you specify three-phase at the time of new connection rather than retrofitting later.
What businesses typically need three-phase?
Workshops, manufacturing, commercial kitchens, larger retail and hospitality sites, hotels, gyms, sites with significant HVAC, sites with EV charging at rapid charger speeds, and data centres or server rooms.
Can I use single-phase equipment on a three-phase supply?
Yes. Single-phase loads can be connected to any one of the three phases. Distribution boards typically balance these across the phases to keep the load even.
Does three-phase electricity cost more per kWh?
Not the energy itself, but three-phase sites usually pay more in non-energy charges. They typically have a higher Authorised Supply Capacity, broken-out DUoS charges, and a daily availability charge tied to ASC.
Will I see three-phase listed on my bill?
Not usually as a single line. The consequences show up. Half-hourly metering, kVA-denominated ASC, broken-out DUoS, and potentially excess capacity charges if peak demand exceeds the agreed ASC.
Who do I contact to install three-phase?
Your Distribution Network Operator (DNO). They assess what cable is needed, what infrastructure is available locally, and quote for the work. The Energy Networks Association postcode tool helps you find your DNO.
How long does a three-phase upgrade take?
Lead times vary by region but typically 4 to 12 weeks from confirmed quote to live connection. Sites needing significant network reinforcement can take 6 months or more.
Can a three-phase site downsize back to single-phase?
Technically yes, but it is rarely worth it. If your load has dropped, reducing the Authorised Supply Capacity within three-phase is usually a faster and cheaper route than physically downgrading the connection.
Your DNO’s connections guidance · Elexon Balancing and Settlement Code guidance (elexon.co.uk) · Ofgem (ofgem.gov.uk)