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Residential Service Entrance Panel Figure 4 shows an example of a residential application. Loadcenters are typically rated 225 amps or less, and a maximum of 240 volts. Most homes have 200 amp service and a 120/240-volt, single-phase, three-wire system. Three conductors run from the Distribution Transformer to the service entrance panel. This transformer converts the utility voltage into a voltage and current supply suitable for use in most residential applications. Two of the Main Service Conductors (or Mains) are ungrounded (“hot”), and the third is the neutral. The “hots” are connected to the Main Circuit Breaker in the loadcenter. The neutral is attached to the neutral bar. If a Voltmeter reading is taken between the two hot conductors (“A” and “B”), it will measure 240 volts. If a reading is taken between a hot conductor and the neutral (“N”), it will measure 120 volts.
Figure 4. Typical Single-Phase, Three-Wire Loadcenter This is convenient because 240 volts are needed to power central air conditioners, electric clothes dryers, and electric range tops. At the same time, 120 volts are required to service lighting, small appliances, and receptacle loads. Residential Distribution Panel Now, let’s consider the case of a residential distribution panel. Remember that a distribution panel is a panel used at a point beyond the service entrance and is sometimes called a subpanel. This means that the distribution panel is typically fed by a service entrance panel. You can add on to a loadcenter in two ways. The first way is to use a branch circuit breaker. The second way is to use Feed-Through Lugs.
Figure 5. A Service Entrance Panel Feeding a Distribution Panel In the example shown here, the service conductors (the two “hots” and the neutral) run from the feed-through lugs of the service entrance panel to the Main Lugs of the distribution panel. In this way, the distribution panel itself is protected by the main circuit breaker in the service entrance panel . The neutral conductor is isolated. The importance of this will be discussed later. Usually, a residential distribution panel is used to supply an air-conditioner, hot tub, garage, out-building, and/or home addition. Commercial/Industrial Service Entrance Panel Many commercial/industrial service entrance panels are supplied with a different power system than the residential one. A distribution transformer provides a 120/208 volt, three-phase, four-wire system to the panel. Four conductors go to the service entrance. The three mains are ungrounded (“hot”), and the fourth is the neutral. The “hots” are connected to the main breaker in the loadcenter. The neutral is attached to the neutral bar and is connected the same way as in the residential application. If a voltmeter reading is taken between any two hot conductors (“A,” “B” and “C”), it will measure 208 volts. If a reading between any hot conductor and the neutral (“N”) is taken, it will measure 120 volts.
Figure 6. Typical Three-Phase, Four-Wire Loadcenter This system can handle single-phase or three-phase applications. The 120/208 volt single-phase power is used the same way as in the residential application. The three-phase, 208-volt power is rarely used in loadcenters. For this reason, we will not discuss three-phase any further in this module. Commercial/Industrial Distribution Panel The distribution panel is treated the same as the residential distribution panel. It is typically fed by the service entrance panel and it can be single phase or three phase. Again, it is important to remember that the neutral is isolated.
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