EET 323 – Electrical System Design Lecture 11: Panel-boards and Switch-boards Instructor: Radian Belu, PhD

Introduction The distribution of power for lighting, receptacles, and other loads within a building is accomplished by the use of panel-boards and switch-boards. Panel-boards are used as the point of supply for the branch circuits in a certain area of a building, and often they contain the last over-current protection device for the circuits it supplies. The supply for the panel-board is typically the service entrance or a feeder from a main distribution panel or switch-board. A main over-current device is commonly installed in the panel-board to protect it. The rating of the panel-board and over-current device must be sufficient to supply the load to be served.

Lecture Objectives

Panel-Board Bus-bar Arrangements The function of a panel-board is to distribute the power to the branch circuits and feeders through suitable over-current protection. The internal bus structure of the panel-board must allow for the application of proper voltage to the branch circuits and feeders. A single-phase panel-board applied to a 120/240 V, single-phase, 3-wire system must properly distribute power to individual branch circuits such that the voltage is either 120 V phase-to-neutral or 240 V phase-to-phase. A single-pole breaker would be installed in the panel-board to protect the phase conductor of 120 V branch circuit, and a two-pole breaker would be installed to protect both ungrounded conductors of the 240 V branch circuit. Figure 11-1 shows how the panel-board accomplishes the distribution of voltage. It is possible for a main circuit breaker to be used instead of the main lugs, where the incoming supply is directed.

Over-current Protection of Panel-Boards Panel-boards must be protected by an over-current device having a rating equal or less than the rating of the panel main bus-bars. Section 408.16(A) of the NEC specifically requires over-current protection for all lighting and appliance branch circuit panel-boards. Section 408.16(B) of the NEC requires over-current protection for power panel-boards that have the system neutral brought to panel-board and have more than 10% of the branch circuit over-current rated 30 A or less. Panel-boards may be provided with the necessary over-current protection by use of a main circuit breaker, mounted inside the panel-board enclosure and has one set of terminal lugs for connection of the supply conductor (Figure 11-7(a)). The rating of the main circuit breaker must not exceed the rating of the panel-board bus-bars.

Over-current Protection of Panel-Boards A panel-board may be provided with over-current protection by the feeder over-current device, which provides the protection for both the panel-board and the feeder conductors (Figure 11-7(B)).

Panel Schedules A panel schedule is designed to keep the record of the branch circuits, listing items such as: circuit number, circuit load description, number of poles, circuit loading, an any special notes regarding the branch circuits, allowing easy tabulation of the loads on the individual branch circuit. The panel schedules aid in the development of the required circuit directory of the panel. The poles or spaces, as well as circuit numbers are numbered from left to right, top to bottom, starting with space number 1 at the top left-hand side (Figures 11-9 and 11-10). The branch circuit loading is expressed in amperes.

Panel Balancing Proper electrical design requires an even loading of the service and feeder conductors, so the branch circuit loads must be evenly distributed among the phase conductors of a panel-board, which is determined by calculating the percentage unbalance of the loads in the panel. For proper design, the total percentage unbalance must not exceed 10%. The percentage unbalance is determined by computing the total loading on each phase, using the panel schedule. The average loading on the panel is determined by calculating the average value among the phases. The deviation for each phase is calculated by subtracting the average loading from the total loading on that particular phase, and taking the absolute value.

Main Distribution Panel Schedules Installation requiring the distribution of power to several panel-boards located throughout the building typically use main distribution panel-boards (MDPs), serving service entrance equipment of the larger services and supply feeders to other panels. MDPs ranging from 800 A to 5000 A are available. The schedule for the MDP consists of a pictorial view of the front panel (Figure 11-11(A)). The MDP consists of several vertical sections bolted together to form the complete switchboard assembly. Full-height vertical sections, designated A,B, C, and so on are required for auxiliary sections to allow for top feed of cables, main switch, or main circuit breaker.

Medium-Voltage Switchgear and Unit Substations

Clearances Around Panel-boards and Switchgear To ensure a safe electrical installation and adequate working space, proper clearances must be maintained around electrical panels and switchgears (Figure 11-16(A)). Section 110.26(A)(2) of the NEC specifies a clear working space of at least 30”, unless the equipment itself is wider than 30”, in which case the required working space in front of the equipment must not be less than the width of the equipment. Section 110.26(A)(3) of the NEC requires that the height of the working space must be at least 6 ½” or the height of the equipment, whichever is larger.

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