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The Basics of Panelboards

Dec 1, 1998 12:00 PM, By Scott Falke

It's easy to make mistakes when installing and wiring a lighting and appliance panelboard. Let's look at some basics that apply to its installation. You can use this article as an abbreviated checklist to keep the electrical inspector from citing your installation.

• Short-circuit rating of bus and overcurrent devices. Apply interrupting ratings to branch-circuit and main circuit breakers and fuses. This minimizes excessive movement (deflection) and overheating (hot gas/shock wave formation) of conductive parts and prevents damage to adjacent insulating materials. (NEC Sec. 110-10).

• "Slash" voltage ratings. Watch for multipole, molded-case circuit breakers rated "120/240V" or "480Y/277V" compared to "240V" or "480V." Usually, the Code does not permit applying "slash" ratings to ungrounded, resistance-grounded, or grounded-delta systems. (Secs. 100, 110-9, 240-85).

• Bus overcurrent protection/main lugs only (MLO) versus integral molded-case circuit breaker or fusible switch. Do you have an upstream overcurrent device or disconnecting means? If so, then determine whether you need a panelboard with these components integral to the board. If you are feeding the panelboard from "tap" conductors, then you usually must install an external-fused switch or circuit breaker upstream of the MLO panelboard. (Secs. 215-3, 240-21, 384-14, 384-16)

• Attached insulated-conductor temperature rating. When determining conductor ampacity, use the temperature column that matches the temperature rating of the weakest (lowest temperature) connected termination, conductor, or device in the circuit. That is, if you have 60C lugs, don't use the 75C ampacity tables for your conductors. You can use conductors whose temperature ratings exceed those of other components for ampacity adjustment. And, there are limits to ampacity/temperature ratings when you go beyond 100A. Be sure to review the Code's extensive instructions on this. (Sec. 110-14(c), Table 310-16).

• Main breaker feed direction. Does your panel main circuit breaker come with an interchangeable trip mechanism? If so, then terminate incoming cable only to the line side of the breaker. This limits a degree of shock/flash hazard during service or replacement of the trip unit.

• Bolted versus "stab" bus-to-MCCB pole connections. When you have a bolted connection, each ungrounded (phase) overcurrent-device pole secures to the panel bus by a machine screw; as opposed to a spring-clip-type conductor. Make sure you have the right breaker before getting started!

• Main bonding jumper (install or not?) The main bonding jumper gives you "system grounding." If a transformer is immediately upstream of the panelboard, you must bond the neutral bus or neutral conductor to the panel enclosu re and to a (bare) grounding-electrode conductor. Generally, exceptions are for ungrounded service, like 480V, 3-phase, 3-wire service, and some 2-wire control circuits under special conditions. For most feeders serving this installation, like 120/240V, 3-wire, or 4-wire 208Y/120V and 480Y/277V circuits, you must keep the two electrically separate. You can place the bonding jumper in the transformer enclosure or panelboard. (Sec. 250).

• Concentric knockouts versus field-cut openings/51/2-in.-deep enclosures. Don't depend on concentric knockout rings (or reducing washers) to give you adequate grounding continuity. The Code requires bonding jumpers around them over 150V. If you have several enclosure openings, all those jumpers can be a headache. Specifying panelboards furnished "without knockouts" or "with blank-end walls" can eliminate this concern; while cutting conduit openings at the installation site eliminates these problems. Generally, panelboard enclosures 51/2 in. deep or more are available without factory knockouts. (Sec. 250).

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