b. Individual protection is not required where such existing panelboards are used as service equipment in supplying an individual residential occupancy. For example, take a split-bus panelboard in which the line section contains three to six circuit breakers or fuses, none of which are rated 20 amperes or less. In such an arrangement, one of the main overcurrent devices supplies the second part of the panel that contains l5- or 20-ampere branch-circuit devices. The other main overcurrent devices (over 20 amperes) supply feeders or major appliances, such as cooking equipment, clothes dryers, water heaters, or air-conditioning equipment (fig. 5-4). Thisarrangement is permitted only for existing panelboards in existing individual residential occupancies.
2. A lighting and appliance branch-circuit panelboard is limited to not over 42 overcurrent devices (excluding the main overcurrent devices) in any one cabinet or cutout box (fig. 5-5). When such devices are numbered, a single-pole circuit breaker is counted as one overcurrent device; a two-pole circuit breaker, as two overcurrent devices; and a three-pole circuit breaker, as three overcurrent devices.
In addition, the panelboards will be provided with physical means to prevent the installation of more overcurrent devices than the panelboard was designed, rated, and approved to handle. Figure 5-6 shows a suitable arrangement for overcurrent devices.
Figure 5-4. - Typical arrangement of a split-bus lighting panelboard.
Figure 5-5. - Typical arrangement that shows NEC® rules for lighting panelboards.
Figure 5-6. - Suitable arrangement for an existing 200-ampere lighting panelboard used as service equipment for individual residential occupancy.
A typical lighting panelboard is a circuit-breaker type with a main 200-ampere circuit breaker and thirty-two 20-ampere single-pole breakers. This type of panel is used for a four-wire, three-phase, grounded neutral system. The main breaker is three-pole.
Other NEC® provisions that apply to all types of panelboards are as follows:
1. Panelboards, equipped with snap switches rated 30 amperes or less, will have overcurrent protection not in excess of 200 amperes. Circuit breakers are not considered snap switches.
2. Panelboards that have switches on the load side of any type of fuse will not be installed except for use as service equipment. Panelboard equipment with the snap switch is on the line side of the plug fuses and satisfies the Code.Continue Reading