Secondary Circuit Analysis Calculations
Secondary Circuit Analysis relies on several calculations to perform 'what if' scenarios. These calculations are based on the following assumptions:

Single Source, Radial Circuit Topology: These analysis calculations require that a secondary circuit contain no loops and receive electricity from only one source. When confronted with a secondary circuit that contains loops, Secondary Circuit Analysis breaks each loop at an arbitrary point. When confronted with a secondary circuit fed by more than one transformer, Secondary Circuit Analysis ignores all but one of the transformers (selected at random).

Constant Voltage Bus at Transformer Primary: The transformer primary terminal is considered an "infinite bus." Its voltage is assumed to be constant regardless of the amount of power it supplies to the secondary circuit.

Single Line Equivalent Circuit Model: A singleline equivalent circuit model is used to perform the load flow calculation. The type of transformer (3phase or 1phase) feeding the circuit determines the way in which service loads are interpreted and added to the singleline equivalent.

If the transformer is 3phase, each service load is assumed to be a 3phase balanced load, and its total value is divided by three. In this case it is also assumed that load current flows in one direction only (i.e., it doesn't need to return via a neutral conductor).

If the transformer is 1phase, each service load is assumed to be on the same phase as every other service load, and the load current is assumed to flow in both a phase and a neutral (return) conductor.
Given these interpretations, one can generally expect voltage drops in a circuit headed by a 1phase transformer to be about six times greater than voltage drops in a similarly configured and loaded circuit headed by a 3phase transformer.
