The capacity of the distribution system is determined in most cases by the thermal ratings of the equipment. In more rural areas with low load density it may be determined by voltage limits.
The distribution substation capacity depends on the size of transformers and the provision of an additional spare transformer. If a substation has two transformers, all load must be supplied by the remaining one if one fails.
In this case, the substation capacity will depend on the capability of the remaining transformer to carry the load for the time required to replace the failed transformer, with the capacity being lower if the replacement time is longer.
For substations with a single transformer, load is limited to what can be transferred to other substations at remote feeder tie points.
The allowable primary feeder loading can be limited by the size of conductors used and the characteristics of the load supplied. If the load varies, higher maximum loads can be carried by the feeder than steady loads, since the rating of the feeder depends on the heating effect of the current over time.
Feeder loading may also be limited by the voltage drop that occurs at the end of longer feeders.
The distribution transformer capacity is determined by the size of the transformer and the characteristics of the load. In some cases, the distribution transformers are single phase. When a three phase supply is needed, three single phase transformers can be used, each connected to a different phase of the three-phase primary supply.Alternatively, a three-phase transformer may be used in which the three phases are in a single tank. The capacity of the secondaries is determined by the size of wire used, their length, and the characteristics of the load they supply.