What is Inside the Service Panel?
This article will center around the anatomy of a “Main Breaker” style service panel because that is the type of circuit breaker panels found in most private homes. There is another type of panel known as a “Main Lug” panel which does not contain a “Main Disconnect” because the panel disconnect is in another power distribution panel located somewhere ahead of it. Main lug panels may be found in some apartments, but, since renters cannot work on the electrical systems, we will not concern ourselves with Main Lug panels in this article. The National Electric Code only permits homeowners to do their own electrical when they not only own the home but also occupy the home as the principle resident.
The Anatomy of a Main Breaker Panel.
Once you have removed the panel's outer cover and its protective inner cover you will begin to see its real construction and lay out. Depending on how old the panel is and on how many “electricians” that have been in it, the wiring may be neat as the wiring appearing in my photos, or it may be a scrambled mess. Neatness really is next to Godliness when dealing with electrical work. There is one catch all rule in the NEC that can get your project Red Tagged quicker than any other, NEC Article 110.12Mechanical Execution Of Work "Electrical equipment shall be installed in a neat and workmanlike manner." When an inspector sees sloppy wiring in a panel or anywhere else, he or she will start to look really hard at everything else you did or did not do.
The Main Breaker (Main Disconnect).
The Main Breaker/Service Disconnect is located at the very top of this panel, but, depending on the panel that you are working with, it might also be at the very bottom of the panel, or located on the left or right side of the panel, at the top or bottom. The two, thick, black wires attached to the main breaker are the two phase wires bringing the electricity into the panel. It is these two wires and the terminals that they are attached to that remain “Hot” even when the Main Breaker is in the “OFF” position. There are 240-Volts between these two terminals, so do not touch them. Good practice is to keep a small piece of rubber in your tool kit and secure that rubber over theses terminals when you are working in the panel.
The “A-Phase” and “B-Phase” Bus Bars.
In this photo, I have removed most of the branch circuit breakers to expose the Bus Bars that the circuit breaker receive their power from. As you progress in your DIY Electrical studies and work, you will hear the phrase. “A Balanced Panel,” over and over again. A balanced panel is one in which the electrical loads on both phases are roughly equal. The easiest way to do that is to dive the sum of the circuit breaker ampere ratings equally between the two phase bus bars.
The neutral bus bar.
The Service Panel's Neutral Bar may be located along the right side of the panel as shown here, or it may be located at the to left side of the panel, or at the bottom of the panel, depending on the style of panel you are working with. No matter where it is located on your panel, it is easily recognized by the fact that third service drop wire is connected to it and all the branch circuit, white, neutral wires are connected to it. This is important, never secure more than one neutral wire under any one screw. You may see two or more wires under one screw and you should correct that because it is a direct violation of the electrical code.
The Panel's Grounding Bar.
The panel's Grounding Bar is easily identifiable by the thick, bare system grounding wire attached to it, as well as by its green colored screws and all the bare copper branch circuit ground wires attached to it. The panel box itself is bonded to this bar by a Bonding screw that passes through the bar and screws into the back of the panel box. The same rule of only one wire per screw applies.
The Branch Circuit Breakers.
With the branch circuit breakers back in place, the branch circuit hot wires are secured under the circuit breaker's terminal screws. Actually the person who made these connections did some sloppy work because there is too much bare copper showing on all of the breaker except for the breaker on the bottom left. It would pass inspection, but it is still sloppy workmanship.