Panelboards and Voltage Requirements
A panelboards is defined as a part of an electrical supply arrangement that separates electrical power input into subsidiary circuits. It also has a protective circuit breaker or fuse for each circuit, in an enclosure. This is a fairly simplified description of panelboards, which are a very common and vital part of any industrial facility. They can be open (not included within an enclosure) or enclosed in a suitable cabinet.
NEMA Recognized Panelboards
There are two NEMA recognized types of panelboards: lighting and appliance or power and feeder. In this article the power and feeder (or distribution) type is discussed. Distribution panelboards can be used for any of several voltage systems and current protection arrangements. For switch and fuse branch circuits with pull out disconnects the ratings range from 120/240V single phase to 480/277V Wye-connected three-phase, four-wire. For dead front operated and molded-case circuit breaker switch and fuse panels the voltage rating are from 120/240V single phase to 600/347V Wye-connected three-phase, four-wire. These boards must meet the UL 67 standard and enclosures must meet the UL 50 and 50E standards. Some conditions that may require special consideration are ambient temperatures below -30ºC or above 40ºC; explosive or corrosive atmospheres; high dust or humidity; altitude above 2000 meters and some other extremes. If extreme conditions exist the engineer should consult the panel manufacturer for recommendations.
Service entrance equipment must conform to UL67 and UL869A standards. Ground-fault protection is required on 1000A or more service equipment where the voltage exceeds 150 volts to-ground (and less than 600 volts phase-to-phase). This protection must be adjusted to 1200A maximum and one second or less time delay for 3000A or more ground faults. The ground-fault protection must disconnect all ungrounded wires.
To meet NEMA rating standards a panel must be rated at the smaller of the current capacity of the main buss bars or the trip rating of the main circuit breaker. Frequency ratings must be 50, 60 or 50/60Hz. Short-circuit rating is in RMS amps and must not exceed the interrupting rating of any installed component.
NEMA Standards
The NEMA standard for panelboards includes several pages of additional specifications. To the average user these standards are completely meaningless. They only become important when a component fails and must be replaced; when a branch circuit is added; or when designing a new installation. There are standards for wire sizes, wiring connections, labeling, grounding (or bonding) and even the enclosure in which the panel is mounted. All these standards exist to protect the safety of technicians working on the equipment, people in the areas near the panels and the equipment connected to the panel. There are no less than 6 different types of enclosures specified in the NEMA standard and an entire section of the standard is dedicated to the design tests for temperature rise, dielectric, short-circuit, environmental and mechanical.
Panel manufacturers have documentation available online for their respective products and most offer custom design assistance and ordering help for new installations. When replacing failed components of panelboards, it is important that the replacement part meet the same requirements as the part being replaced.
