Electrical – How are the individual homes connected in a 3 phase power distribution system (power grid) so that they have correct voltage

acsingle-phasethree phasevoltage

For a single phase distribution system it's simple, each home has one wire coming from the neutral and one from the live, in turn each outlet has a live and a neutral wire. The live is 120V relative to earth, which is the neutral wire's voltage (ignoring ground because that is just there as a safety feature).

For 3 phase (I am talking about standard North America three phase where two phases are given to each house, not industrial 3 phase motors) there is a neutral wire, but it is thinner than the live and meant to simply carry away current from unbalanced loads, not used under ideal circumstances, so since it is not a main source being used to carry the electricity and I will assume this is an ideal circumstance so we can ignore the neutral wire. I would then think that this would mean that two live wires would need to feed to one outlet, with one of the live wires in place of the neutral, creating the circuit. But the standard voltage of a wall outlet is 120V, and two live wires makes 208V because they are only 120 degrees out of phase. From my research I know that the 208V is used in some appliances, but most use 120V.

So my question is this: How do you get 120V from a wall outlet without using a neutral wire, if each wire individually is 120V relative to ground, and two lives connected makes 208V? This diagram may help you understand my question.

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Best Answer

> residential

> 3-phase, 2 phases to each house

> 208V between phases

You are either in New York City or certain countries in Central America which do that. Brazil does something similar but at 127/220V, which is close enough to work with most North American appliances.

If you have two poles / phases L1 and L2 neither near earth, and you want 120V, you will need an isolation transformer. An autotransformer won't do because 120V requires a neutral wire that is near earth potential for safety. It might be OK if the 120V machine is double insulated.

Easier is to just pull a cable to that location which has a neutral wire.

You cannot use equipment safety ground as a current return. If anything goes wrong it will light up your entire grounding system. Things which are supposed to be safe will be lethal.

Everywhere else in North America, 3-phase is only used for transmission. Near your house, one phase is tapped by a single phase transformer. The transformer gives 240V center tap which goes to the houses.