Three-Phase vs Single-Phase: When London Properties Need an Upgrade

A standard UK single-phase supply delivers 230V between live and neutral. A three-phase supply delivers three lives, each 120° out of phase, plus a shared neutral. The voltage between any two phases is 400V (sometimes referenced as 415V — same thing, slightly different nominal). The voltage between any phase and neutral remains 230V.

Practical consequence: a three-phase property can run 230V loads on any one phase like a normal house, and can also run 400V three-phase motors directly — which is what large heat pumps, commercial kitchen ovens and 22kW EV chargers need.

Three-phase also lets you draw far more total power. A standard 60A single-phase supply can deliver 13.8kVA continuous. A 60A three-phase supply can deliver 41.4kVA — three times the headroom before the main fuse blows.

Older mansion blocks and ex-council blocks often have three-phase distribution at the riser, with each flat fed off a different single phase via a shared main switch board in the basement. The flat itself is single-phase but the building is three-phase.

Commercial conversions are the common surprise. A former shop or workshop converted to flats often retains the three-phase incoming supply even if currently fed single-phase to the new flat. UKPN often leaves the spare phases capped at the meter location — usable later for an upgrade.

Modern detached and large semi-detached properties built post-2018 with documented EV/heat-pump provision will sometimes have been specified as three-phase from new. Anything explicitly marketed as "EV-ready" in 2023+ is worth checking.

Single-phase meters have a single brown (live) tail entering and a single blue (neutral) tail leaving toward the consumer unit. The main fuse carrier (cutout) below the meter is a single unit, typically 60A, 80A or 100A.

Three-phase meters are physically wider — typically 250-300mm versus 150mm for single-phase. Three brown tails enter (or three labelled L1/L2/L3), one blue neutral. The main fuse cutout is a triple-pole unit with three separate 60A or 100A fuses.

A "Henley block" with three or four tails branching off can look like three-phase but might just be a single-phase splitter for a sub-board. The give-away is the meter itself: count the input tails.

A single-phase to three-phase upgrade for a London property means UKPN coming on site to: install a new triple-pole cutout, replace the meter, run a new service cable from the LV mains (if the existing cable cannot carry three phases), and recommission. The new tails into the consumer unit need to be three-phase rated.

Cost depends on whether the new supply needs trenching from the street. If the service entry point is within ~5m of the LV mains and the existing duct can accept a triple-core cable, cost is typically £3,000-£4,500. If trenching is needed across a driveway or garden, expect £5,500-£8,000.

Timeline: 12-16 weeks from application to commissioning is normal for UKPN London. Apply via the UKPN connections portal. A budget estimate is free; a formal quote is around £400, deducted from final cost if you proceed.

Run the maximum demand calculation. Standard appliance loads with diversity: electric oven 8kW × 0.4 = 3.2kW, induction hob 7.4kW × 0.6 = 4.4kW, shower 9kW × 1.0 = 9kW, EV charger 7kW × 0.75 = 5.25kW, ASHP heat pump 9kW × 1.0 = 9kW. Other lights and sockets ~4kW.

On a 60A single-phase supply (13.8kVA), the stack above is already over capacity. On 80A (18.4kVA) it is still tight. An 100A single-phase (23kVA) — the maximum standard domestic supply — handles most of the above with the EV throttled at peak heating times, especially with a load-balancing charger.

The genuine three-phase trigger is when the household stack exceeds 100A single-phase: multiple EVs, a 22kW commercial-grade EV charger, a 16kW+ ASHP, or a workshop with significant 230V motor load.

Load-balancing EV chargers (Zappi, Ohme, Wallbox Pulsar Plus with CT clamp) measure real-time household import and throttle the EV down dynamically. A 7kW charger can run at any current from 1.4kW (6A) to full 7kW depending on whether the shower, oven and heat pump are running. This single technology delays many three-phase upgrades by 5+ years.

Time-shifted heat-pump scheduling: most modern ASHPs can be set to prioritise hot-water and high-output heating outside peak appliance hours, smoothing the household load profile.

Battery storage (Tesla Powerwall, GivEnergy, Sigenergy) lets you import slowly from the grid into the battery during off-peak and discharge fast during peak — effectively decoupling instantaneous demand from grid supply.

Combined, these can keep a heavily-electrified household on an 80A or 100A single-phase service that would otherwise need three-phase. Cost comparison: load balancer £0 add-on (just choose a smart charger), battery £4-8k, versus £5-8k for three-phase upgrade plus board change.