North American "Low Voltage" Electric Services

This is a chart depicting all three of the North American “low voltage” electric services typically available. Most 120V 15A appliances use plugs and cords, and most that use more than that are hard wired and stationary, however, there is almost always a weird looking NEMA socket for any wiring configuration up to 60A, and even above that we can use connectors that aren’t NEMA sockets (like Stage-Pin that goes up to 100A 240V, or other specialized connectors that go even higher).

Many different types of services are derived from standard pole transformers these transformers have two 120V windings in series to make 240V, though if you open the transformers up you can connect them in parallel to get only 120V at higher current.

Single Phase:
This is the most common, and most residential buildings have this service. All convenience receptacles in houses have one of the hot wires and the neutral wire, to get 120V. However, larger appliances (air conditionings, ovens/stoves, electric dryers, electric water heaters, welders, etc) use the two hot wires to get 240V. 240V circuits have 2-pole breakers that break both phases at the same time, even if only one of them has a fault. Very very few homes have anything other than this, though some small or temporary construction site services are 30A 120V service, with a neutral and only one phase.

120/208 Wye:
This is the most common 3-phase service today. You are able to use one of the phases at a time to get 120V for all convenience outlets, while using the 3 phases together to power large motor appliances. Some non-motor appliances will only use two of the three phases for what is essentially single-phase 208V, and sometimes only two of these three phases are given to residents of large condominiums, so they don’t have as much freedom with appliance selection. Sometimes, 208V 3-phase is given to 240V 3-phase motors (see below) (instead of using the proper 208V motor), but this lowers the motor’s power, and isn’t done as much any more. The transformers used for this service are internally put in a parallel configuration, so that only 120V is output by each transformer leaving one of the terminals unused, making this easy to identify in the wild.

120/240 Delta:
This service is not used very commonly anymore, though in my opinion it really should be. This is one of the more complicated services, with the neutral conductor in the center of the bottom two phases of the triangle. This makes for the standard 120/240 single-phase service for use with convenience outlets and large appliances (single-phase service), but also a third phase providing 3-phase 240V for use with large motor appliances. The two “hot” wires are identical to the respective wires in single-phase service (120V to ground), while the orange “High” or “Wild” phase is 208V to ground, used alone with the neutral wire for some larger 208V non-motor appliances. Sometimes in smaller services the transformer labeled T#1 is omitted, using only two transformers. This is called Open-Delta service, and is recognizable easily by only two out of three transformers being present on the pole bracket.

More services exist than the ones shown here, but they are classified as “high voltage” services and I might make a drawing of them later.