What is the difference between 240 volts three phase to 120 volts three phase compared to 400 volts to 240 volts?

Answer for USA, Canada and countries running a 60 Hz supply service.To try and simplify the answer (and question), let's consider a basic house service. Most houses today have a 240 volt single phase (a so-called "split-phase" service) coming into the service box, along with a neutral (zero) voltage wire.

An appliance that takes 120V, like a toaster, is connected to one half of the split 240 volt single phase and to the neutral. The neutral should always run at zero voltage and is required to return the current which comes back from the load to the power station.

A large appliance, like an electric stove or a clother dryer, is usually of the type that needs a lot more power, so - to keep the current draw as low as possible - it requires more voltage. This is supplied by connecting it to the full 240 volts single phase service by using both "split phases": the 120V "hot" with red insulation and the 120V "hot" with black insulation, along with the neutral. The neutral is used to provide power at 120 volts to a time clock and/or an appliance programmer (e.g. for the drying cycle time on a dryer) which don't usually run on 240 volts.

Some 240 volt appliances, like water heaters and some air conditioners, take the full 240 volts from both red and black hots but don't need a neutral.

Equipment having much larger motors, like elevator motors or motors driving machine tools in factories, usually use a 3 phase service, either with no neutral - called a "delta" configuration - or with a neutral, which is called a "star", "Y" or "wye" configuration.

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