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Electrical supply for commercial air conditioning: single or three phase?

Updated 23 October 2026 · SEO Dons Editorial

Which units need single-phase and which need three-phase power?

The model decides. Mitsubishi Electric's September 2012 service manual for its PUHZ-SHW range lists the 80V and 112V outdoor units as single-phase at 230 V and the 112Y and 140Y units as three-phase at 400 V. Its wiring table gives a 40 A main switch for the single-phase SHW112V and 16 A for the three-phase SHW112Y and 140Y, so the same capacity class asks for a much smaller protective device per phase when the load is spread across three.

Daikin's VRV IV manual for the RXYLQ10 to 14 outdoor units gives 3N~ 50 Hz at 380 to 415 V, while its wiring figure shows the indoor units on a separate 230 V single-phase supply. The supply has to match the unit: Daikin's commissioning checklist says the voltage at the local supply panel must correspond to the nameplate. In Great Britain a low voltage supply is declared as 230 volts between phase and neutral.

What does the unit's circuit need?

  • A dedicated circuit. Mitsubishi Electric's indoor unit manual says to use dedicated power supplies and that special circuits should also be used, because a circuit lacking capacity may cause a risk of electric shock or fire.
  • Isolation. Regulation 12 of the Electricity at Work Regulations 1989 requires suitable means for cutting off the supply and for isolating electrical equipment where necessary to prevent danger. The manuals add detail: Mitsubishi Electric asks for a switch with at least 3 mm contact separation in each pole, and Daikin lists a main switch.
  • Protective devices. Daikin says the supply must have a main switch, a slow blow fuse on each phase and an earth leakage protector, and an instantaneous-action RCD. Mitsubishi Electric says to use a leakage breaker compatible with higher harmonics because the unit is equipped with an inverter.
  • Cable sizing. Daikin says wiring is sized in accordance with the applicable legislation using its minimum circuit ampacity and recommended fuse table. For combinations not in the table, it says to add the minimum circuit amps of each unit, multiply by 1.1 and select the next higher recommended fuse. Mitsubishi Electric says wiring size must comply with local and national codes.

Daikin's own worked example shows the method. Combining an RXMLQ8 (minimum circuit ampacity 16.1 A) with an RXYLQ10 (22.0 A) gives 38.1 A, which multiplied by 1.1 and rounded up to the next recommended fuse size gives 45 A. It is a maker's illustration for those models, not a rule for others.

What do inverter units change?

Daikin says not to install a phase advancing capacitor because the unit has an inverter, and Mitsubishi Electric says not to build a system whose power supply is switched on and off frequently. Daikin also puts a supply-quality duty on the installer or user: to ensure, by consultation with the distribution network operator if necessary, that the unit is connected only to a supply with a system impedance not above the stated maximum. Its error list includes reversed supply phase order and phase loss, so phase order is part of getting a three-phase unit running.

The two manuals checked do not list a starting current. They size the supply through circuit ampacity and fuse tables instead, so the figures to design from are the published ones for the exact model.

What must the electrician certify?

The standard is BS 7671, the IET Wiring Regulations. The IET says the transition period for Amendment 2 ended on 15 October 2026, that Amendment 2 is now obsolete for new installation work, specifications and certification, and that BS 7671:2018+A4:2026 is the current edition. Daikin's manual says all wiring must be performed by an authorised electrician and comply with the national wiring regulation, and regulation 16 of the Electricity at Work Regulations 1989 bars anyone from work needing technical knowledge unless they have it or are suitably supervised.

The IET's model forms include the Electrical Installation Certificate (EIC) and the Minor Electrical Installation Works Certificate. The minor works form is only for work that does not include a new circuit, and a dedicated supply to a new outdoor unit is normally a new circuit, so the EIC with its schedules of inspection and test results is the usual record for a fresh installation. The electrician makes that call. Useful papers to ask for alongside it are the circuit schedule, the protective device ratings used and where each isolator sits. The certificate belongs in the handover pack described in the commissioning and handover guide.

Does Part P apply to a commercial building?

Approved Document P is titled for dwellings, and its limits on application cover installations in or attached to a dwelling, the common parts of a building serving dwellings, and a building that receives its electricity from a source located within or shared with a dwelling. For business premises, the document says Part P applies where they are connected to the same meter as the electrical installation in a dwelling, such as shops and public houses below flats, and does not apply to business premises in the same building as a dwelling with separate metering.

An office, warehouse or standalone shop is therefore outside Part P, and BS 7671 and the Electricity at Work Regulations are the route. A shop below a flat on a shared meter is inside it, and regulation 12(6A) makes the installation of a new circuit notifiable work. Which case applies is a metering fact to confirm before work starts.

What about the building's incoming supply?

Everything above sits on the customer's side of the meter. Whether the incoming supply can carry the new load is a separate question for the network operator, covered in the guide to electrical capacity for new HVAC plant. Operators publish their own thresholds: Northern Powergrid, for example, describes a small connection as typically up to 69kVA for a new build or small commercial premises. Readers planning a full installation can see the commercial air conditioning installation page and send an enquiry through the site's form. The retrofit guide shows where this fits in an existing building.

Sources

Common questions

Can a three-phase unit run from a single-phase supply?

Not by changing a connection. Daikin's checklist says the supply voltage must correspond to the voltage on the nameplate, and makers publish single-phase and three-phase versions as separate models. A building with only single-phase supply needs a single-phase model or a supply upgrade.

Does a split system need a local isolator?

Regulation 12 of the Electricity at Work Regulations 1989 requires suitable means of cutting off the supply and isolating equipment where necessary to prevent danger, and Mitsubishi Electric's manuals ask for a switch with at least 3 mm contact separation in each pole. The exact device and position are the designer's decision.

Is the electrician's certificate the same as the F-gas certificate?

No. Wiring is electrician's work, certified under BS 7671. Refrigerant work needs a company certified to install, repair, maintain or decommission F gas equipment, and the F-gas certificate check guide shows how to verify it. A complete installation needs both.

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