What distinguishes a 3 wire distribution circuit from a 4 wire distribution circuit?

The distinguishing feature between a 3 wire distribution circuit and a 4 wire distribution circuit is indeed the presence of the neutral wire. In a 3 wire system, typically used for single-phase electrical systems, there are three conductors: two phase wires and one neutral. This setup allows the distribution of electricity and provides a return path for current.

However, in a 4 wire distribution circuit, there are two phase wires, one neutral wire, and one grounding wire. The inclusion of the additional wire enhances safety and allows for additional phase configurations, including the use of balanced loads.

The neutral wire serves several essential functions, such as maintaining the voltage balance in multi-phase circuits and providing a return path for unbalanced loads. This distinction in the number and function of the wires significantly impacts how electrical systems are designed and used in various applications.

The other factors listed, including power output, type of transformer, and length of the span, may affect the operation and configuration of power distribution systems but do not define the basic structure and purpose of 3 wire versus 4 wire circuits. Therefore, the correct answer hinges on recognizing the critical role of the neutral wire in differentiating these two types of circuits.