# How do utility lines wire transformers to go from 3-phase power to residential split phase?

I'm trying to understand how power distribution lines go from 3-phase to single-phase and haven't been able to find a concrete explanation.

I understand that power companies transmit with 3-phase power (delta usually) at 60 kV and above, eventually, get to a substation to step down, and go from anywhere from 7-13 kV to another transformer to a single phase.

I guess my real question is how the transformer converts 3 phases at 120º apart to a single (split phase) that is now 180º apart (which allows for 240V loads common in residential).

• (As a rule, it does not, for cost reasons more than anything else before even considering something beyond 60/50 Hz. The single "split phase transformer" connects to two phases, or one and neutral.) Jan 23 at 16:06
• The pole-mounted transformers in my neighborhood each connect to only one phase of the high-voltage distribution network. Presumably, the utility company distributes them in a way that keeps the loads on the three phases approximately in balance. The secondary windings are center-tapped. The two residential "phases" simply are the two ends of the secondary winding, and the neutral is the center tap. Jan 23 at 16:20
• The transmission of power over three wires can come from a delta or wye secondary hence, when you say "delta usually", you might mean 3-wire and no neutral? There isn't a 3-phase to single phase transformer; there are three transformers boxed together that can be used to produce 3 single phase outputs. Jan 23 at 17:20

I guess my real question is how the transformer converts 3 phases at 120º apart to a single(split phase) that is now 180º apart (which allows for 240V loads common in residential).

It's not necessary. The single-phase transformers are fed from one or two phases only.

To convert 3 phase to two single-phase sources that distribute their load equally on all 3 phases, a Scott-T circuit can be used. That's a very specific application, though - say if you had a machine that used two single-phase servo drives to run two motors at similar power, but had to be fed from a 3-phase supply and also there was a reason to distribute the load evenly on all 3 phases. In most cases using a 3-phase input servo drive would be a better choice - less stress on the rectifier, and much less iron in the cabinet - unless you need an isolated supply for some reason.

On the other hand, residential customer loads all average out to "about the same", so the phase loads are balanced in practice, even though the individual loads are not balanced across phases at all.

Assuming you mean typical North American distribution, the transformer does not convert three phase voltage.

The transformer connects to one phase and neutral (or two phases) so the input is single phase.

The output is also single phase, but center tapped neutral means you have single phase split into two phases.