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Electrical power comes through the service and gets distributed to feeders and then to branch circuits. But when doing a design, installation, or shutdown, you go in the opposite direction, starting with the branch circuits. Why is this?
Design. Before you can properly size your electrical service, you need to know what loads it must support. You start by determining branch circuit loads and working toward the feeders.
Installation. From a project perspective, it seldom makes sense to install a service that has no loads to connect to it. Typically, loads are installed by crews using temporary power (Article 590) for lights and tools,
Shutdown. Flick off a household light switch, and you aren’t interrupting much current. But operate a 1200A breaker under load, and it’s a different story. You want to operate that breaker with little or no load to reduce risk of arc blast and arc flash.
To shut down a facility for maintenance, begin by opening the individual branch breakers. Work toward the supply through the various branch panels, then the feeder panels that supply those branch panels. Then open the main breakers.
In all cases, you’re going from lower energy levels to higher energy levels. Following this practice in all electrical work provides a safety advantage. And that’s why the Chapter 2 load side power distribution sequence begins with branch circuits in Article 210 and ends with services in Article 230
Source: Mark Lamendola | Mindconnection