# How does three phase add up?

I understand that three phase allows you to always have a voltage, when another phase is in neutral, other phases will be at maximum peak.

What I don't understand is how three phase's add up? If all the phases are separated 120 degrees apart, however they all reach maximum peak in one cycle, and lets say each phase is 5VAC Would it be 15VAC per cycle since they're all reaching max peak in one cycle? Just overlapped 120 degrees apart.

• Basically and fundamentally, they don't "add up". – Andy aka May 6 '17 at 15:33
• What does it mean when equipment or even houses use either a variation of 115VAC or 230VAC, etc? How does three phase manipulate this? It seems to me they're adding up the separate peaks, just don't know how it's done. – ApolloMission4 May 6 '17 at 15:38
• It's unclear what you're asking here. What do you want to know? – Hearth May 6 '17 at 15:39
• 115VAC and 230VAC refer to the RMS voltage. In ordinary appliances this is single-phase, not three-phase. Exactly how this corresponds to the three-phase power depends on the wiring configuration of the various transformers in the distribution system--I don't know what the standard is. – Hearth May 6 '17 at 15:40
• You use a transformer? I don't understand your question. – Hearth May 6 '17 at 15:42

## 1 Answer

The easiest way to think of three phase is as three "hot" wires and one neutral wire. Between any hot wire and neutral you will measure (say) 230 volts AC and usually a single household will receive one hot wire and neutral. Several houses further down the street a house might receive one of the other hot wires and neutral.

There are three circuits sharing a common wire called neutral and, as far as any single phase household is concerned they "see" a hot (live) and a neutral. Different names in different countries sometimes of course.

You could mistakingly supply two hot wires to a house and the voltage wouldn't be 230 V AC anymore - it would be about 400 volts because "line-to-line" voltage is $\sqrt3$ times bigger than line to neutral (often called "phase voltage"): -

So if Va is 230 volts and Vb is 230 volts, Vab is about 400 volts. It's just geometry.