[xenadu02]

We use a center-tap neutral except for commercial/industrial that receives three phase.

Most small-to-medium homes/businesses have two hot legs coming off each side of the transformer coil. The neutral is connected to the center of the coil and bonded to earth/ground so it becomes a 0v reference. Each hot leg to neutral is 120v. Between hot legs gives 240v. That neatly supports both voltages in a backwards-compatible way. Typically clothes dryers, hot water heaters, ovens/stoves, etc are 240v appliances. Lamps, USB chargers, and other small day-to-day stuff is 120v.

There are two failure modes that can happen but they are rare and usually only affect the customers attached to the affected transformer or a single customer.

1. Floating neutral. If the neutral becomes disconnected that causes floating voltages as the electricity backs up across the neutral and returns via the opposing hot leg. This presents as randomly fluctuating high/low voltages to 120v appliances but most 240v appliances don't use then neutral and don't care.

2. Damaged hot leg. One hot leg partially arcs to ground or is otherwise damaged. This causes half the 120v appliances to flicker/brown out. 240v appliances will see random low voltages.

Three phase is often delivered as wild leg/high leg delta so a neutral can be derived. It is usually setup so one phase (eg A/C) is center-tapped to make the neutral and two hot legs. This gives three phase power per normal and the same setup as a normal home would have: A/C forms two 120v legs wrt the center tap neutral. However you get 208v between the other phases and neutral so for high density housing you also need to balance the phases resulting in some apartments having 208v power rather than 240v. Thus most 240v appliances also support 208v here but unless you've lived in an apartment or worked on commercial/restaurant systems you'd never see that voltage.

Our breaker panels have 3-phase variants. You'd usually install both: a 240/120 panel for "normal" loads and a 3-phase panel for 3-phase and 240v split phase loads. Breaker design is the same: 3-pole takes up three slots and the bus bars alternate by 3 so every third point is on a different phase.