6.9 Primary Distribution

The selection of a primary distribution system, i.e., radial, loop, primary selective, secondary selective, network, etc., should be evaluated on a case by case basis, with consideration given first to safety, then to cost and reliability. Generally, radial or loop systems are preferred.

The primary distribution system design should be based on the estimated demand load plus 25 percent spare capacity.

See Chapter 7: Fire Protection Engineering for additional requirements.

Medium Voltage Switchgear. When required, medium voltage service switchgear may be provided with either air, vacuum or SF6 circuit breakers or fused air interrupter switches. Provide voltmeter, ammeter and watt-hour meter with demand register. Meters should be pulse-type for connection to the BAS. Providing a power monitoring and management system is an acceptable option.

Conductors. Conductors should be insulated with cross linked polyethylene (XLP) or ethylene propylene rubber (EPR). 133 percent insulation should be provided. Conductor size should not exceed 240 mm2 (500 Kcmil).

Spot Network Transformers. In cases where reliability is an absolutely critical concern - the IRS office that processes refund checks, for example - network transformers should be considered. In large cities, where load densities are very high, utility companies may choose to supply power through network transformers. If so, these systems should be utility owned and maintained.

Double-ended Substations. If reliability is critical and spot networks cannot be provided by the utility, double ended substations should be used. Transformers may be equipped with fans to increase the rated capacity. The sum of the estimated demand load of both ends of the substation must not exceed the rating of either transformer, and it must not exceed the fan cooling rating. All double-ended substations should be equipped with two secondary main breakers and one tie breaker set up for open transition automatic transfer.

Substation transformers must be dry-type with epoxy resin cast coils or silica oil filled type. Liquid filled transformers may be used outdoors. Substations should be located at least 30 m (100 feet) from communications frame equipment to avoid radio frequency interference. Provide lightning arrestors on the primary side of all transformers. Consider surge suppression on the secondary and/or downstream busses.

Transformers located in underground vaults must not be positioned directly adjacent to or beneath an exit way.

Where silica oil filled transformers are used, the design must comply with all spillage containment and electrical code requirements.

Last Reviewed 2016-08-02