Do I Need Three-Phase Power? London Electrician's Sizing Guide

A standard London single-phase supply is rated at 80A or 100A — capable of delivering 18.4kW or 23kW continuously. That covers a 3-bed family home running induction hob, electric oven, electric shower, washer-dryer and a 7kW EV charger simultaneously, with comfortable headroom.

For most homeowners the practical ceiling is one EV charger (7kW) plus normal domestic load. A modern dual-RCD or all-RCBO consumer unit on a 100A main fuse handles this cleanly. There is no benefit to upgrading to three-phase if your usage shape fits inside the single-phase envelope.

The single-phase main fuse is your real constraint, not the consumer unit. Many older London properties still have an 80A or even 60A main fuse. UKPN can uprate to 100A free of charge in most cases if your service cable supports it — typically a 30-minute visit.

Multiple EV chargers. Two 7kW chargers running simultaneously is 14kW — within single-phase headroom but leaves nothing for the rest of the house. Add a heat pump and you are oversubscribed. Single 11kW or 22kW chargers are explicitly three-phase devices and require three-phase supply.

Air-source heat pump (ASHP) + induction + EV charger + battery inverter. A 12kW ASHP draws up to 5kW electrically, an induction hob can pull 7kW briefly, a 7kW EV charger is constant, and a 5kW battery inverter exports at full rate. Four loads, each capable of 5–7kW, on an 80A main fuse will trip during peak coincident demand.

Workshop equipment. Anything with a three-phase motor — lathes, large compressors, kilns above 6kW, professional kitchen extract systems — is inherently three-phase. Single-phase emulation via VFDs works for some applications but adds cost and inefficiency.

Large home batteries. Battery systems above 10kWh increasingly use three-phase inverters (Sigenergy, Sungrow SH series, Tesla Powerwall 3) to deliver higher continuous output and faster charging. Single-phase batteries cap at around 5kW continuous; three-phase units routinely deliver 10–15kW continuous.

In London, the Distribution Network Operator (DNO) is UK Power Networks (UKPN) for almost the whole city, and SSEN (Scottish and Southern Electricity Networks) for the small western fringe around Slough and parts of Hillingdon.

A three-phase upgrade requires a connection application to the DNO. The DNO assesses whether the local network can support the extra capacity, surveys the property to confirm the cable route, and quotes for the works. Domestic three-phase quotes in London typically come back in 6–10 weeks.

The application is usually made by your electrician on your behalf, with you signing the connection agreement. You will need title plans, intended max demand calculation, the desired ampere rating per phase, and a willingness to allow excavation for a new service cable if the existing one is single-phase only.

Engineering Recommendation G99 governs the connection of any embedded generation — solar PV, battery storage, EV-bidirectional chargers — above 16A per phase. Below that threshold, the simpler G98 fast-track applies.

A three-phase property with solar above 16A per phase (roughly 11kW PV) needs a G99 application before commissioning. The application is free, takes 28 days to assess, and is now standard for any decent solar contractor to handle for you.

Where you are planning solar + battery + bidirectional EV charging in a single project, three-phase makes the G99 conversation considerably simpler. Each phase can be loaded to 16A independently, multiplying your G98 fast-track headroom.

UKPN three-phase service connection: £3,200–£6,500 for a domestic property where the existing single-phase cable can be replaced in-situ. Where new ducting is required across pavement or driveway, add £800–£1,800.

Three-phase consumer unit and tails: £950–£1,800 fitted, including isolator switch, main protective device, and certification. All-RCBO Type A specification is the modern default.

Internal rewiring to spread load across phases: rarely needed in a domestic context. Most three-phase upgrades retain single-phase final circuits (each circuit is single-phase, just drawn from a different phase at the board) and only convert specific high-load circuits like EV chargers or workshop sockets.

Total realistic spend for a London family home moving to three-phase: £4,500–£8,000 all-in including all electrical works and the DNO connection. Where the existing cable is on a difficult route or the local network needs reinforcement, costs can run higher. Always get the DNO quote in writing before committing to internal works.

Stay single-phase if: you have one EV charger or none, no heat pump, and your generation is under 11kW PV. The 100A main fuse covers it.

Move to three-phase if: you are planning 2+ EV chargers, a heat pump and an EV together with battery storage, a home workshop with three-phase tooling, or a solar/battery system above 16A per phase per the G99 threshold.

Decide before you commission. Adding three-phase later means re-doing the DNO application (a second connection fee), re-doing the consumer unit (you cannot retrofit three-phase to a single-phase board), and disrupting the property for the cable upgrade. Planning the upgrade once is materially cheaper than splitting it across two events.

A 4-bedroom Victorian terrace in Wandsworth planning a full electrification project: ASHP (12kW) replacing the gas boiler, induction kitchen, 2 x EV chargers (one home, one rental to neighbour), 6kW solar PV, 10kWh battery, retained electric shower.

Coincident maximum demand calculation: ASHP 4kW + induction 4kW (diversified) + 2x EV charger 14kW + shower 9kW + base load 1.5kW = roughly 32kW. Well above the 23kW headroom on a 100A single-phase supply with diversity factors applied.

UKPN three-phase quote came in at £4,800 with no road excavation required (existing duct re-used). Internal works: new three-phase consumer unit £1,250 fitted, distribution upgrade to EV bay £450, generation circuit for solar £350. Total project electrical: £6,850 plus the cost of the ASHP, EVSEs, panels and battery.

Net result: no demand-side compromise, full simultaneous use possible during the worst winter evening, future headroom for a third EV charger or workshop upgrade without revisiting the DNO. The three-phase decision paid off because the property had four high-load systems running concurrently.

Before committing to three-phase, modern load-management hardware can stretch a single-phase 100A supply considerably. Dynamic EV chargers (Zappi, Wallbox Quasar, Ohme) modulate charging current based on the rest of the house load — pausing the car during peak coincident demand to avoid tripping the main fuse.

ASHP soft-start kits and battery inverter scheduling (e.g. via Home Assistant or the inverter's native API) let you orchestrate when high-load devices run. Heat the hot-water tank at 02:00 when the EV is not charging, run the dishwasher at 14:00 from solar surplus, and the coincident demand graph flattens dramatically.

For most London 3-bed semis adding one heat pump + one EV, smart load management plus a 100A single-phase supply is the cheaper and faster route than three-phase. The break-even tips toward three-phase when you genuinely cannot schedule loads off-peak — typical of commercial use, ground-floor mixed-use property, or households with shift workers.

Always test the load-management approach against your worst-case winter evening before locking in. The week between Christmas and New Year, with three EVs being charged for return-to-work, ASHP on continuous, and oven + induction at dinner-time, is the stress test that exposes single-phase limits if they exist.

A pragmatic test: if you can identify two times of day when at least one of your major loads can be deferred, single-phase will probably do. If everything you own needs to run together (because a household member is on a shift pattern that does not match cheap-rate windows), three-phase becomes the right answer regardless of headline maximum demand.

One more variable: the smart-tariff economics. Single-phase with a load-managed Octopus Go or Flux tariff captures most of the optimisation upside that three-phase otherwise unlocks. The case for three-phase strengthens sharply when you are running multiple high-load systems simultaneously without ability to defer; it weakens when your usage shape already aligns with cheap-rate windows.