6.10 Secondary Distribution

Main Switchboards. 208V and/or 480V service switchboards as well as substation secondary switchboards should be provided with a single main service disconnect device. This main device should be molded case, insulated case, power air circuit breaker or fusible switch (where appropriate) individually mounted, draw-out type (as applicable). Insulated case and power air circuit breakers should be electrically operated.

The meter section should contain a 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.

Feeder devices of switchboards 2,000 AMPS and larger should be molded case, insulated case, power air circuit breakers or fusible switches where appropriate, individually mounted, draw-out type as applicable and electrically operated. Feeder devices of switchboards below 2,000 AMPS may be group-mounted, molded case circuit breakers or fusible switches.

Switchboards should be front and rear accessible. In smaller switchboards, front access only is acceptable if space is limited.

Grounding. All grounding systems must be carefully coordinated, especially in regard to: NEC grounding electrode systems; lightning protection; communications grounding; and computer room signal reference guide. Power distribution system grounding must be in accordance with the National Electrical Code. Also reference general design criteria (this chapter) for equipment grounding conductor. Grounding for communications systems must follow the requirements in EIA/TIA Standard 607: Commercial Building Grounding (Earthing) And Bonding Requirements For Telecommunications (and related bulletins).

Ground Sources. The ground source for the electrical power system must have a maximum resistance to ground of 5 ohms, except in small buildings i.e. less than 5000 m2 (50,000 square feet) that have only minimal communications systems. Grounding systems for these buildings may have a resistance up to 10 ohms. The grounding design must be based on a soils resistivity test and ground resistivity calculations. Below-grade connections should be exothermically welded.

A wall-mounted, 6 mm by 50 mm (0.25-inch by 2-inch) copper ground bus should be provided in each electrical room. The ground bus should be located in the rear access aisle of the room and should extend at least 1 m (3 feet). It should be interconnected with the ground electrode and ground bus in the switchgear or switchboard.

Isolated Grounding Panels. Provide separate panels for computer loads to separate from general electrical loads in lieu of an IG system which is more complex and prone to miswiring.

Submetering. Electric power meters must be provided on the services to all spaces planned to be outleased, to all computer rooms and to the parking garage, if any.

Power Factor Correction. If the utility rate structure has a power factor penalty, non-PCB centralized automatic power factor capacitors should be connected at the main electrical service on the load side of the utility metering. Power factor capacitors should be designed to automatically correct a lagging power factor to a value that will avoid penalty charges. Switching circuits should be specifically designed to prevent electrical noise from entering the electrical power distribution system.

Motor Control Centers. Grouped motor controls should be used where more than six starters are required in an equipment room. Motor control center construction should be NEMA Class I, Type B with magnetic (or solid state if appropriate) starters and either circuit breakers or fuses. Minimum starter size should be size 1 in motor control centers. Each starter should have three overload relays. Control circuit voltage should be 120V connected ahead of each starter via control transformer as required.

Reduced voltage starters may be used for larger motors to reduce starting KVA.

In the design of motor control centers on emergency power, time delay relays should be considered to reduce starting KVA on the generator.

Elevator Power. Elevators should be powered from a shunt trip circuit breaker located in the elevator machine rooms. Electrical design standards in elevator standard ANSI/ASME A17.1 must be followed.

Secondary Distribution Systems
Secondary electrical power distribution systems in federal buildings are classified as normal, emergency and uninterruptible. Normal power serves the general power and lighting loads in the building. Emergency power is distributed to life safety and critical loads. Uninterruptible power is required for critical loads, which cannot be interrupted.

In typical GSA office buildings it is recommended that 208/120V normal power be subdivided to isolate the office electronic equipment load. Figure 6-2 shows a typical power distribution scheme.

Figure 6.2 Typical Power Distribution Scheme

Figure 6.2 Typical Power Distribution Scheme

Bus Duct. Where plug-in bus duct is used, it should have an integral ground bus, sized at 50 percent of the phase bus to serve as the equipment grounding conductor.

Conductors. Aluminum or copper conductors are acceptable for motor windings, transformer windings, switchgear bussing, switchboard bussing and bus duct, where the conductor is purchased as part of the equipment. Aluminum conductors shall not be used for primary feeders, branch feeding or branch circuits.

Power Distribution Panels. In general, circuit breaker type panels will be the standard of construction for federal buildings. With the exception of lighting and receptacle panel boards, fusible switches may be considered if specific design considerations warrant their application, such as in electrical coordination of electrical over-current devices.

Lighting and Receptacle Panelboards. Lighting and receptacle panelboards shall be circuit breaker type. Provide minimum 30 poles for 100 amps panelboards and minimum 42 poles for 225 amp panelboards.

Lighting panelboards shall have minimum of three 20-amp 1-pole spare circuit breakers.

Receptacle panelboards should have minimum of six 20-amp 1-pole spare circuit breakers. For initial planning purposes, the number of receptacle circuits may be estimated by assuming 19 m2 (200 square feet) per circuit.

All panelboards must be located in closets. In circumstances where horizontal runs would become excessive and another riser is not warranted, shallow closets, at least 600 mm (24inches) deep, may be used for additional panelboards.

Panelboards Serving Electronic Equipment. Electronic equipment panelboards serving personal computers, computer terminals or dedicated work stations should have an isolated ground bus. The service to the electronic panelboard should be supplied from an isolation transformer. Consideration shall be given to providing equipment with 200 percent neutrals. For initial planning purposes, the number of receptacle circuits may be estimated by assuming 19 m2 (200 square feet) per circuit.

Feeders and branch circuits serving electronic load panels should be provided with isolated ground conductors.

Last Reviewed: 2019-02-26