Electric Power Distribution System

Overview

Secondary Distribution Systems
The secondary distribution system is that portion of the network between the primary feeders and utilization equipment. The secondary system consists of step-down transformers and secondary circuits at utilization voltage levels.
Residential secondary systems are predominantly single-phase, but commercial and industrial systems generally use three-phase power.

While the various ways transformers are wired are interesting .. in the end what I am really interested in is how to model these various wiring patterns. I need to know how to wire my 3d printed transformers depending on whether they are feeding power to a residential or commercial/industrial system. To that is added the number of primary wires – if there is any rhyme or reason we can apply to our model.

Secondary Voltage Levels

The voltage levels for a particular secondary system are determined by the loads to be served. The utilization voltages are generally in the range of 120 to 600 V. Standard nominal system voltages are as shown to the left.

In residential and rural areas the nominal supply is a 120/240 V, single-phase, three-wire grounded system.

If three-phase power is required in these areas, the systems are normally 208Y/120 V or less commonly 240/120 V.

In commercial or industrial areas, where motor loads are predominant, the common three-phase system voltages are 208Y/120 V and 480Y/277 V.

The preferred utilization voltage for industrial plants, however, is 480Y/277 V. Three-phase power and other 480 V loads are connected directly to the system at 480 V and fluorescent lighting is connected phase to neutral at 277 V. Small dry-type transformers, rated 480-208Y/120 or 480-120/240 V, are used to provide 120 V single-phase for convenience outlets and to provide 208 V single- and three-phase for small tools and other machinery

This tells us (as modelers) quite a bit. If we are running power to a residential system it is single-phase 2 or 3 wire. My understanding is that 2 wire 120V systems were used early last century before residential power requirements were enough to require 240V.

Notice that in these residential and rural areas if three-phase is required the system is normally the 208Y/120V or (less commonly) 240/120V.

Commercial/Industrial systems (motor loads) it is 208Y/120V and 480Y/277V with the latter preferred for Industrial.