The two main advantages of unit substations are lower engineering and installation costs. Engineering time is saved because the manufacturer performs all internal wiring details. Installation is simplified because it is only necessary to prepare a flat foundation. Then the unit substation can simply be set in place and connected.
Unit substations can be ordered for any of the distribution systems previously discussed. The radial type unit substation is probably the single most common type in use today. The double-ended secondary type unit substation is also common. It has a normally open tie breaker connecting the buses so that either transformer can supply the load.
Unit substations are generally economical at 500 kVA and larger. While smaller sizes will price out well under some circumstances, these loads are usually supplied by conventional feeders and load centers. The cost of a 240 Volt substation is likely to be about 40% greater than a 480 Volt substation of the same kVA size.
