Transformers constructed to reduce the primary voltage down to the customer's utilization voltage are commonly referred to as distribution transformers. The distribution transformers may be single-phase, normally used for residential customers, or three-phase, normally used for commercial or industrial customers. The transformers are constructed for mounting on poles or racks, in vaults, or on pads constructed with concrete, plastic, or fiberglass and for direct burial in the earth. The construction, theory of operation, and the method of making connections to distribution transformers are described in Chap. 15, Distribution Transformers.

Distribution Transformer Size. The transformer capacity required to supply any given load may be estimated as follows:

Residential Loads. Residential loads may be estimated in accordance with Table 36.1, which gives the demand per house. As the table shows, a small house is taken as having a demand of 3500 watts, a medium-sized house 6000 watts, and a large house 15,000 watts.

A large home utilizing electricity for heating may have a demand of 20,000 to 30,000 watts. When several houses are served from the same transformer, there will be diversity between the peak use of energy, as shown in Table 36.2.

The size of distribution transformer required to serve serveral houses is determined by totaling the estimated watts for each house obtained from Table 36.1, dividing the total by 1000 to obtain kilowatts, and multiplying by the diversity factor obtained from Table 36.2.