Single-phase vs three-phase power: a guide for business

Most small business premises run on the same single-phase supply as a house next door. Once equipment gets heavier, businesses typically move to a three-phase supply, which carries far more power through a connection of the same physical size. Here is what the difference actually means, how much each can provide, and how to tell which one your premises needs.

What is the difference between single-phase and three-phase power?

A single-phase supply carries electricity on one live conductor and a neutral. A three-phase supply carries it on three live conductors, each carrying the same voltage but offset from one another in timing, plus a neutral. The Electricity Safety, Quality and Continuity Regulations 2002 declare the standard low voltage supply at 230 volts between a phase (live) conductor and neutral, with a permitted variation of 10% above or 6% below that figure.

On a three-phase supply, each of the three live conductors is still 230 volts to neutral. Because the three phases are offset from each other, the voltage measured between any two of them works out at roughly 400 volts, not 460 volts as simple addition might suggest. This 400V figure between phases is why you will sometimes see three-phase described as a “400V supply” and single-phase as a “230V supply”.

Why does three-phase carry more power through the same size cable?

Each of the three live conductors in a three-phase supply carries its own 230V current, offset from the other two so that the three waveforms are evenly spaced across each electrical cycle. Because the conductors are staggered rather than all peaking at the same instant, three conductors of a given size can jointly carry roughly three times the power of one conductor of the same size, without each individual conductor having to carry any more current than a single-phase conductor would. That is the basic reason a three-phase connection at a given amp rating provides so much more capacity than the single-phase equivalent, as the National Grid Electricity Distribution figures below show.

How much power can each type of supply provide?

For the same size of connection, a three-phase supply provides roughly three times as much capacity as single-phase, because it is effectively three 230V supplies running in parallel through separate conductors. National Grid Electricity Distribution’s own connection application guidance puts it simply: an 80 amp single-phase supply can provide up to around 18kVA, while an 80 amp three-phase supply can provide up to around 69kVA.

Connection	Typical maximum capacity
Single-phase, 80A	Up to around 18kVA
Single-phase, 100A	Up to around 23kVA
Three-phase, 80A	Up to around 69kVA
Three-phase, 100A	Up to around 69 to 100kVA, subject to DNO approval

National Grid Electricity Distribution also confirms that the normal maximum capacity for an individual domestic-style property is 18.4kVA, and that its typical small business connection, covering supplies up to 69kVA, is usually processed within 15 working days of a full application. Larger loads move into high voltage (HV) connection territory, which our guide to maximum demand meters covers in more detail.

Does my business need a three-phase supply?

If your total electrical demand is likely to exceed around 18kVA to 20kVA, or you are running equipment specifically designed for a three-phase input, you need a three-phase connection. A few signs point towards needing three-phase:

• Heavy motors. Larger motors, compressors, pumps and lifts are commonly built for three-phase input because it runs them more efficiently and with a smoother start-up than single-phase.
• Commercial kitchens. Combination ovens, large extraction systems and multiple pieces of catering equipment running at once can push total demand past what a single-phase supply comfortably provides.
• Workshops and light industrial units. CNC machinery, welding equipment and industrial compressors often specify three-phase on their nameplate.
• EV charging at scale. Multiple simultaneous EV charge points, or any rapid charger, typically needs three-phase capacity to deliver enough power without exceeding a single-phase connection’s limits.

If you are not sure, National Grid Electricity Distribution’s own guidance for calculating your power requirement suggests your electrician works this out for you, adding up the connected load of your equipment and applying an appropriate diversity factor, since not everything runs at full power simultaneously.

What if I only need a small amount of extra power?

Not every capacity problem needs a three-phase upgrade. If your existing single-phase supply is close to its limit but you do not run equipment that specifically requires three-phase, increasing your single-phase capacity, up to the roughly 23kVA available on a 100A single-phase connection, may be enough. Your DNO’s connection team can confirm what your local network can support.

Why do motors run better on three-phase?

A three-phase motor receives power from three offset supplies rather than one, which lets it produce a steady rotating magnetic field directly. A single-phase motor has to create that rotating effect artificially, usually with a starting capacitor or winding, which makes it less efficient and harder to build in larger sizes. This is why most motors above a few kilowatts, whether in a walk-in fridge compressor, an extraction fan or a lift, are specified by their manufacturer to run on three-phase.

What is involved in upgrading to a three-phase supply?

Upgrading an existing single-phase premises to three-phase is a job for your DNO and a qualified electrician working together, not something a supplier arranges. Broadly, it involves:

1. Working out your required capacity. Your electrician adds up the connected load of your equipment, applying diversity where not everything runs simultaneously, to reach a kVA figure to apply for.
2. Applying to your DNO. You submit a connection application (sometimes called an augmented connection, since you already have a supply) specifying the new capacity and whether you need single or three-phase.
3. DNO assessment and quote. The DNO checks whether the local network, including the substation and cabling to your premises, can support the new load, or whether reinforcement work is needed first. This is reflected in the quote you receive.
4. Internal wiring work. Your electrician fits three-phase distribution equipment inside the building and rewires or replaces circuits and equipment connections as needed.
5. Meter exchange. Your supplier arranges a three-phase meter once the new supply is live.

Timescales and costs vary considerably depending on whether the local network already has three-phase capacity nearby, or whether new cabling or a larger transformer is needed. Because of this, it is worth starting the DNO application early if a fit-out or expansion depends on the new supply being ready by a particular date.

How do I get a three-phase supply connected?

You apply to your local distribution network operator (DNO), not your energy supplier, for a new connection or to upgrade an existing single-phase supply to three-phase. Our guide to what a DNO is explains who runs your local network and how to find them.

The DNO will assess the load you need, check what the local network can support without reinforcement work, and issue a formal connection offer. For straightforward small business connections up to 69kVA, National Grid Electricity Distribution’s guidance points to a typical processing time of around 15 working days for the quotation, though the physical connection work itself can take longer. If you are connecting a site that has never had a supply before, our guide to new business energy connections covers the wider process, including gas.

Worked example: comparing single-phase and three-phase capacity

This example shows how the single-phase figure in the table above is derived, using the declared voltage set out in the Electricity Safety, Quality and Continuity Regulations 2002.

Single-phase, 80A supply:

• Voltage: 230V
• Capacity = 230V x 80A = 18,400VA = 18.4kVA, matching National Grid Electricity Distribution’s own domestic maximum of 18.4kVA

For three-phase, each of the three conductors can carry its own current, so the total capacity scales up with the number of phases rather than simply with the amp rating of one conductor. National Grid Electricity Distribution states this directly for its own network: an 80A three-phase supply can provide up to around 69kVA. The exact figure your DNO quotes will depend on its own connection standards and the headroom on the local network, so always treat your formal connection offer as the definitive number rather than a calculation you do yourself.

Can I run single-phase equipment from a three-phase supply?

Yes. A three-phase supply still provides 230V on each individual phase, so ordinary single-phase equipment, sockets and lighting circuits can be wired to draw from just one of the three phases. A good electrical installation spreads single-phase loads evenly across all three phases so that none of them is left carrying most of the building’s demand, which is why rebalancing circuits is a common part of any three-phase upgrade.

How does three-phase affect my meter and bill?

Moving to three-phase does not change your unit rate. Gas and electricity unit prices are set per kWh regardless of how many phases deliver it. What does change is:

• Your meter. Three-phase supplies need a three-phase meter, wired to measure all three live conductors, rather than a single-phase meter.
• Your agreed capacity. Larger three-phase connections usually carry a higher Maximum Import Capacity (MIC), agreed with your DNO in kVA, and you pay a capacity charge on that whether you use all of it or not. Our guides to capacity charges and kVA and half-hourly meters set out how capacity charges and excess capacity charges work.
• Possible half-hourly metering. Larger three-phase sites are more likely to have demand over the 100kW threshold that makes half-hourly metering compulsory.

If you want to check exactly what is on your bill once a three-phase supply is in place, our guide to reading a business energy bill explains where these charges typically appear, and you can compare business energy quotes with us once you know your connection type and capacity.

Frequently asked questions