# Single-phase or three-phase: why balance the phases?

> Single-phase 230 V, three-phase 400 V: why spread loads over L1, L2 and L3, and how the most loaded phase sets the size of the source.

Source: https://www.darkside-energy.com/en/did-you-know/balancing-phases/ · Dark Side Energy · Updated: 2026-10-12

12 October 2026 · 2 min read

- On a 230/400 V network, you get 230 V between a phase and the neutral, and 400 V between two phases.
- A three-phase source is sized on its most loaded phase.
- Spreading single-phase equipment evenly over L1, L2 and L3 avoids oversizing the installation.

At an event, a lot of equipment plugs in as [single-phase](https://www.darkside-energy.com/en/glossary/#monophase-triphase), on an ordinary 16 A socket. Yet the distribution cabinet feeding it is usually three-phase. How do you get from one to the other, and why does the way the equipment is connected change the size of the source?

## Back to basics

We work on a 230/400 V network. A three-phase installation has three phases, labelled L1, L2 and L3. In a [four-pole (3P+N)](https://www.darkside-energy.com/en/glossary/#tetra) connection, a neutral and an earth are added. On this network:

- between a phase and the neutral, you get 230 V: this is **single-phase**;
- between two phases, you get 400 V: this is **three-phase**, which carries more power for the same current.

Three-phase equipment loads all three phases. Single-phase equipment loads only one. Single-phase loads are therefore spread over L1, L2 and L3, and that is where it all happens.

## The most loaded phase decides

A source is sized on its most loaded phase. That is the method used in our power assessment: source required = 3 × 230 V × current on the most loaded phase.

For example, take three single-phase appliances each drawing 10 A:

- spread over L1, L2 and L3, each phase carries 10 A, and the source required is 3 × 230 V × 10 A = 6,900 VA, or 6.9 kVA;
- all three plugged into L1, that phase carries 30 A, and you need 3 × 230 V × 30 A = 20,700 VA, or 20.7 kVA.

Same equipment, same consumption, but a source three times larger and a higher connection rating. An imbalance therefore forces you to oversize the installation, for nothing.

That is why our [power assessment](https://www.darkside-energy.com/en/power-assessment/) tool allocates single-phase lines automatically: it places them one by one, from the most to the least powerful, on the least loaded phase.

To sum up, single-phase powers the equipment, three-phase powers the installation, and the balance between the phases sets the size of the source.

> It is not the total power that sizes the source, it is the most loaded phase.

List your equipment: the tool allocates single-phase lines across the three phases and calculates the current on each one.

[Run a power assessment](https://www.darkside-energy.com/en/power-assessment/)

[How much power can you draw from a P17 socket?](https://www.darkside-energy.com/en/did-you-know/p17-socket-power/)
[What is event power distribution?](https://www.darkside-energy.com/en/did-you-know/event-power-distribution/)

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