Many homes contain a sub panel, which is a second electrical panel, smaller than the main panel, that is connected to (and fed by) the main panel downstream to provide power to branch circuits in additional areas of the property. Often times, additions, garages and other isolated structures or areas will contain a sub panel.
Sub panels are generally easier to wire than main panels for the simple fact that they are smaller and contain fewer breakers and wires, which means less work. Because they are easier to wire, though, some homeowners with a bit of electrical knowledge and experience attempt to wire sub panels on their own without realizing that they should not be wired in exactly the same way as a main panel.
One of the most common mistakes I come across is bonding of the neutral and ground bars in sub panels. This is necessary and standard practice in the main panel, which is the one and only place it is allowed, but it is dangerous and prohibited in a sub panel. Let’s examine why bonding the neutrals and grounds in a sub panel is dangerous…
Electricity always seeks to return to its source. In the case of a home, the main electrical panel is where the electricity enters and exits the home, and can thus be thought of as both the starting point and the finish line of the electrical current’s “lap.” The goal is to prevent the current from traveling anywhere other than along one intended path. That path is from the breakers in the panel, through the “hot” wires that supply power throughout the house, and back to the main panel along the neutral wires. If a ground-fault occurs, which essentially creates a “detour” for the current, we want the electricity to travel only along that predetermined ground path. So, the goal is to establish one primary path for the current as well as one alternate path in case something goes wrong along the normal route.
With that in mind, what happens when the neutrals and grounds are bonded in a sub panel? Rather than having only one path for the neutral current to return to the main panel on, it now has two – the neutral and ground wires. And unfortunately, electricity does not magically choose to travel along one path and not the other; if both are available to conduct electricity, both will conduct it. This creates additional energized lines between the sub and main panels, which creates more possibilities for someone to be shocked. This can also cause some problems with GFCI receptacles and interference with other systems.
So, by bonding the ground and neutral bars in a sub panel, you create multiple paths for returning neutral current when we only want one. Bonding is fine in the main panel because it is the beginning and end point of the current and is concealed and safe, unlike bonding in a sub panel which creates potentially dangerous points that exist in the middle of the path.
If you’re thinking of adding a sub panel, be sure to hire a qualified electrician to do the job and make sure he or she does it properly by keeping the grounds and neutrals separate. If you already have a sub panel, make sure the ground and neutral bars aren’t bonded (connected) or that the neutral and ground wires aren’t all on the same bar. You’ll be much safer if your sub panel is installed properly by someone who knows and follows this essential rule.
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