Main Panel vs Sub-Panel

What the main panel is

The main panel is where the utility service lands and where your main disconnect lives. It is the origin point for the whole building's power and the one place the grounded neutral and the grounding system are tied together, through the main bonding jumper.

Everything about the main panel says this is the source. There is exactly one main bonding point in a system, and it is here.

What a sub-panel is for

A subpanel is fed from the main and distributes power to a specific area: a garage, an addition, a workshop, a separate building. It lets you group circuits close to where they are used instead of running everything back to the main.

A subpanel is fed by a feeder, and that feeder must include four conductors: two hots, a neutral, and a separate equipment grounding conductor. That fourth wire is the crux of the whole topic.

The four-wire rule and floating neutral

In a subpanel, the neutrals and grounds must be kept separate. The neutral bar must be isolated from the enclosure, and the bonding screw removed. The ground bar bonds to the metal cabinet. Neutral and ground are bonded only once, back at the main.

• Feeder: two hots, one neutral, one ground.
• Subpanel neutral bar: isolated, floating.
• Subpanel ground bar: bonded to the enclosure.
• Bonding screw: removed in the subpanel.

Why the mistake is dangerous

If you bond neutral and ground together at a subpanel, normal neutral current starts flowing back on the ground wire and on any metal bonded to it: conduit, enclosures, even water pipe. Now those surfaces can carry current they never should, and a broken neutral could energize them.

Float that subpanel neutral, run the separate ground, and you have avoided the single most common and most dangerous panel error in the trade. Size the feeder and its overcurrent protection to the load and you are compliant.