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27kW water heater three phase supply

Updated: 21 October 2026

You want hot water at a tap without a storage cylinder, and a 27 kW instantaneous heater looks like the way to get it. The supply has to come first. A 27kW water heater three phase connection is how UK manufacturers rate this output, at about 400 V, while a normal flat or house is still a single-phase service at 230 V. Buy the heater only after someone competent has confirmed that 400 V is already there, or after the distribution network operator has agreed to provide it. The [27kW instantaneous water heater comparison](/hub/hot-water/27kw-instantaneous-water-heater) lines the appliances up. Peter F. prepared this from manufacturer sheets and public UK guidance on 20 October 2026. It is not a laboratory test.

You want hot water at a tap without a storage cylinder, and a 27 kW instantaneous heater looks like the way to get it. The supply has to come first.

Our top picks

WINTEMP WINTEMP YQ-WN27K

Best overall

WINTEMP WINTEMP YQ-WN27K
1.8/10about £408.53
View offer
GRS GRS 27 kW

Rank 2

GRS GRS 27 kW
1.6/10about £388.71
View offer
Stiebel Eltron Stiebel Eltron DHE 27 Set

Rank 3

Stiebel Eltron Stiebel Eltron DHE 27 Set
6.9/10about £821.57
View offer
On this page
  1. 1.230 V in the flat and 400 V between the phases
  2. 2.What 27 kW draws on 400 V
  3. 3.Flats, risers and the network operator
  4. 4.What an electrician still has to design

230 V in the flat and 400 V between the phases

In most UK homes the cut-out describes one live and a neutral, at a nominal 230 V. Three-phase adds two further lives, and the voltage between any two is about 400 V. Stiebel Eltron's DHE 27 Set is specified at 380 to 400 V and wired 3/PE. Photograph the cut-out label and leave the cover, the meter and any sealed fuse carrier closed. One fuse carrier and a 230 V marking usually mean the ordinary domestic supply. A block of flats catches people out: new-build risers often carry 400 V, then one phase into each flat. Shower and basin totals belong on the buying guide once the supply is settled.

What 27 kW draws on 400 V

On the DHE 27 Set, Stiebel states 39 A at 400 V, protected by 40 A fuses. Heathfield's data for the same class matches: 3/PE, 380 to 400 V, about 39 A, and 40 A fuses. Dividing 27,000 watts by 400 volts and by the square root of three, at a power factor of one, lands on about 39 A. Dividing by 230 volts gives about 117 A, which only shows the mismatch with a normal UK service. Cable selection stays with the electrician. A spare 32 A way is already too small. If a listing is marked 240 V, read the 240 V contrast before you treat the title wattage as this circuit.
WINTEMP WINTEMP YQ-WN27K

Testsieger

WINTEMP WINTEMP YQ-WN27K
1.8/10about £408.53
View offer

Flats, risers and the network operator

A three-phase riser still leaves many flats on a single phase. Designing 400 V into the dwelling belongs in a new block and is a substantial change in an existing one. The distribution network operator owns the equipment up to the service head. An application for three-phase goes to that operator. An electrician often sends it, because the form needs a proper description of the load. Elec-Mate describes these connections as quoted work, timed in weeks, priced only inside that guide's own bands. Those bands are not a tariff for your postcode. Until the operator has agreed the supply in writing, leave the plumber's date.

What an electrician still has to design

Confirmation of 400 V is the start of the design. In England, Approved Document P makes a new circuit notifiable, and Approved Document G expects the electrical work to comply with BS 7671. Scotland and Northern Ireland have their own building standards. The cable to this one heater stays at full load. A diversity sum from the IET On-Site Guide estimates the maximum demand of a whole installation and leaves the single 27 kW unit at the full 39 A. An IET note says not to apply diversity to reduce the cable to one instantaneous heater. Give the electrician the data sheet and a photograph of the unopened cut-out.

Common questions

Is my home already three-phase?
Look at the cut-out label and count the service fuses, without removing any cover. A single 230 V supply, which is what most UK flats and houses have, is one phase to neutral. Three-phase shows three live conductors and about 400 V between them, often with three fuse carriers. A three-phase riser in the block can still leave your own meter on one phase. If the label is unclear, ask an electrician to confirm it, and leave the cut-out closed while you wait.
What does 400 V mean if each phase to neutral is 230 V?
Both numbers describe the same kind of supply, measured in different places. From any one phase to neutral the nominal voltage in the UK is 230 V. Between two phases it is about 400 V. A 27 kW instantaneous heater in this class is built for that phase-to-phase voltage, at 380 to 400 V on the Stiebel and Heathfield sheets. A socket only shows one phase, so 230 V at the socket tells you little about the service head.
How many amps is 27 kW?
On a 400 V three-phase supply, 27 kW is about 39 A per phase when the power factor is taken as one. Stiebel's DHE 27 Set states 39 A and 40 A fuses, and Heathfield's 27 kW class uses the same order of protection. Dividing 27,000 watts by 230 volts gives about 117 A, and that current does not fit a normal domestic service. Quote 39 A to an electrician, and leave cable selection off a home calculation.
Who applies to the network operator?
You can apply as the customer for the address, and an electrician often does it because the load has to be described properly. The operator owns the cables in your area. The heater manufacturer does not issue the connection, and the company that sends the energy bill is a supplier, which is a different role. Published cost bands on a trade website are that site's ranges. Wait for the written offer before you buy the heater.
Does diversity let me put 27 kW on a spare 32 A way?
A 27 kW heater at 400 V draws about 39 A, so a spare 32 A way is already too small for it. The diversity figures for instantaneous water heaters, 100 per cent of the first two and 25 per cent of the rest, estimate the maximum demand of a whole installation. They leave this one circuit at full load. An electrician sizes the cable and the protective device under BS 7671. A sum copied from the On-Site Guide is the wrong tool for that breaker.

Related pages

Best 27kW instantaneous water heater

27kW instantaneous water heater

27kW instantaneous water heater UK: the buying checks

How to use an instantaneous water heater at 27 kW output

240V tankless water heater UK, not the 400 V heater

Sources

Public UK sources named below. amazon.co.uk prices, stars and review counts on these three listings were read on 19 October 2026. Page updated 21 October 2026.

  1. Approved Document G, with 2024 amendments (retrieved 19 October 2026)
  2. Approved Document P, 2013 (retrieved 19 October 2026)
  3. Water Supply (Water Fittings) Regulations 1999 (retrieved 19 October 2026)
  4. HSE, landlords and legionella (retrieved 19 October 2026)
  5. Stiebel Eltron UK, DHE 27 Set (retrieved 19 October 2026)
  6. Stiebel Eltron UK comparison sheet (retrieved 19 October 2026)
  7. Stiebel Eltron UK, three-phase water heating (retrieved 19 October 2026)
  8. Flexiheat UK, electric instantaneous water heaters (retrieved 19 October 2026)
  9. amazon.co.uk, Stiebel Eltron DHE 27 Set B08L5B34YH (retrieved 19 October 2026)