Installing Electric
Service-Entrance Systems

[lesson title]

Power-panel (Figure 1-24) and service-drop amperages must match. A smaller amperage power panel cannot be used as a main disconnect for a larger amperage service drop. A 210-ampere cable must be used to feed a 200-ampere power panel.

Figure 1-24. Power panel.

Conductors carrying current within a building are disconnected at the main power panel (Figure 1-25). When the main switch or circuit breaker is in the OFF position, all branch circuits will be disconnected.

Figure 1-25. Main switch inside a power panel.

A building can have no more than six switches or circuit breakers mounted in a single enclosure that disconnects all power from the building (Figure 1-26). In an emergency situation, all power from this service to the building can be disconnected by using no more than six operations of the hand. When a main switch (breaker) is provided inside the same panel, 42 branch circuit breakers can be installed.

Figure 1-26. Switches in a power panel.

Two- or three-pole breakers that can be joined together with a tie handle or a master handle can be counted as a single switch if all conductors are disconnected with a single pull or push of a set of switches. By adding tie handles, the power panel in Figure 1-27 follows the six-switch-maximum NEC rule. Label each breaker to indicate the load serviced.

Figure 1-27. Breakers joined with a tie handle.

The NEC contains important exceptions to the maximum-switch rule (Figure 1-28). The rule can be exceeded for a fire pump or an emergency service such as a hospital, but the switch should be located away from the other disconnects and the power panel should be labeled.

Figure 1-28. Labeled power panel.

The code also states that up to 42 overcurrent devices can be put into one panel; however, the power must be disconnected from all circuits with six hand operations. For the 42-device role, a two-pole circuit breaker is counted as two circuit breakers (Figure 1-29). One main disconnect removes the power from all circuits in 'this panel. Not counting the main, there are ten circuit breakers in this panel and four of them are tied together.

Figure 1-29. 42 overcurrent devices in a power panel.

No circuit should be loaded greater than 80 percent of the breakers amperage capacity. In Figure 1-30, the breaker is rated at 20 amperes. Therefore, the ampere load to be applied cannot be greater than 16 amperes. Each heater uses 8 amperes. This circuit has the maximum allowable load.

Figure 1-30. Maximum ampere load.

Power panels must be accessible. This is accomplished by mounting the panel so that the top of the panel is no higher than 6 1/2 feet and the middle no lower than 4 12 feet (see Figure 1-31).

Figure 1-31. Panel mounting location.

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