What is a three-phase supply and do I need one?

Household and many small-business supplies are single-phase. Three-phase is the heavier-duty option. The differences here are standard electrical practice, so for your own site confirm with a qualified electrician or your network operator.

Do you need it?

Typical business	Likely supply
Small office, shop, salon	Single phase
Café with a small kitchen	Often single phase; check large ovens
Commercial kitchen, bakery, workshop, farm with machinery	Often three-phase
Several EV chargers, lifts, big motors	Usually three-phase

How to tell what you have

• Look at the incoming cable or main fuse board: a three-phase board has three main fuses or a three-pole switch.
• Check your bill or supply details for a stated capacity in kVA.
• Ask your electrician.

Cost and bills

A three-phase connection is not automatically more expensive per unit of electricity. Charges depend on your supply capacity (measured in kVA), your standing charge and the unit rate. A larger capacity can mean higher fixed network charges. See supply capacity and kVA charges.

Rough capacity example

Three-phase capacity is roughly the voltage times current times 1.73 (the square root of 3) divided by 1,000. At 400 volts and 100 amps: 400 x 100 x 1.73 / 1,000 = about 69 kVA. Treat this as a guide only; your DNO sets the formal figure.

Getting a three-phase supply

Upgrading from single to three-phase normally means applying to your distribution network operator (DNO), which quotes for the work; the cost depends on the work involved at your site. Timelines vary; see new commercial connections.

Before you buy equipment, ask the supplier of the machine what supply it needs. Retrofitting later is costlier than planning up front.

See also the wider three-phase glossary entry.

Questions to ask your electrician

1. What is my current supply: single or three-phase, and what is the main fuse rating?
2. What would my planned equipment draw at peak?
3. Do I have headroom, or do I need to ask for more capacity?
4. Is the existing cable sized for an upgrade?

Why it matters for your energy contract

A change to your connection can change your agreed capacity and your charges. Tell your supplier when work happens so the account details stay right. If you extend a building, check whether you need a new connection; see new business energy connections.

Signs you may be outgrowing single phase

• The main fuse or breaker trips when several large appliances start together.
• An equipment supplier says a new oven, compressor or charger needs a three-phase connection.
• Your electrician says the existing supply has little spare capacity.

Load management, which staggers when heavy equipment runs, can sometimes delay the need to upgrade. Ask your electrician whether it suits your site before paying for new network work.

EV charging example

A single 7 kW charger on a single-phase supply draws about 30 amps at 230V (7,000 / 230 = about 30). Four at once is about 120 amps, which usually exceeds a single-phase supply and is a reason to consider three-phase or load management.