We recently brought power in to our cabin for the first time. The service panel and internal wiring had previously been in place. We hired an electriacian to make the connection between the meter board and the service panel. I am planning to add a couple new circuits and, when I removed the panel cover for the first time, I noticed that some of the existing circuits have both the ground wire and the neutral wire secured to the ground bus bar. Other circuits have the ground wire to the ground bus bar and the neutral wire to the neutral bus bar. The electrian assured me this was fine and everthing currently in place works fine. What is the reason for the difference? And if I'm adding new circuits, which of the two options should I use? Thanks for shedding some light on this for me.

 

The question is whether this is a MAIN panel or a SUB panel. The MAIN panel is fed *directly* from the meter. From what you stated, it sounds like this is a MAIN panel, thus your situation is OK.

In a MAIN panel, the neutral and ground is bonded. Usually, there is only one bar, and both neutral and ground wires are connected to it. When there are multiple bars, the bars themselves are electrically connected together, thus, the neutral and ground wires can be connected to either bar.

In a SUB panel, the neutral and ground are separate. Neutral wires connect to a bar that is isolated from the panel inclosure (mounted to plastic insulators or in some way to prevent an electrical connection between the neutral bar and the panel box). Ground wires connect to a bar that is connected directly to the panel box (thus grounding the panel box itself). Neutral and ground wires MUST be connected to the appropriate bars.

 

In the panel housing the main disconnect, and only in this panel, the neutral wires and the grounding wires may (and must) be interconnected. The reason they must is that otherwise the fault current would have no place to go. The reason that they must not be connected elsewhere is that the grounding wires should never carry any current other than fault current.

The above is the nutshell version. The full explanation can and does fill many books.

So in the main panel, the neutral and grounding bars are really interchangeable. But you may not put a neutral wire and a grounding wire under the same screw, nor may you put more than one neutral wire under the same screw.