At an event, a lot of equipment plugs in as single-phase, 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) 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 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.