Previously: wiring
It used to be that subpanels would run a ground wire back to the main panel, and that was good enough. That's 4 conductors: hot/hot/neutral/ground. The neutral and the ground are bonded in the main panel, meaning they connect to the same bus bar. In a subpanel, though, neutral and ground are kept separate, to stop the ground conductor from carrying the neutral current.
It used to be that subpanels would run a ground wire back to the main panel, and that was good enough. That's 4 conductors: hot/hot/neutral/ground. The neutral and the ground are bonded in the main panel, meaning they connect to the same bus bar. In a subpanel, though, neutral and ground are kept separate, to stop the ground conductor from carrying the neutral current.
Today subpanels get their own ground rods. These are 8' long, 5/8" diameter galvanized steel rods. You drive them in to the ground 6' apart, and then attach a ground wire to them with special ground rod clamps.
Grounding has two important jobs. First, if lightening strikes the electrical system, the ground wires will carry the current away safely. Second, the metal housing on many machines & appliances is connected to the ground. Normally the ground conductor carries no current, but if a hot wire ever touches that metal housing, it will short & trip the breaker. If it's not grounded, then the housing gets energized, and can shock you if you touch it.
Around here, ground rods are trouble. The subsoil gets extremely hard around 4' below grade. When they put the water main in the street, they had to use a big jackhammer to break it up. They said it was harder than breaking up concrete.
There's no reasonable way to put ground rods in to an 8' depth. Luckily I already knew this from watching the pro install the main panel. You drive the rods in as far as they'll go, then bend them over in the trench. I rented a rotary hammer to put in the ground rods. I planned ahead so they'd still be 6' apart when bent over. When I was done, I discovered they were too close by about 2". I don't know if an inspector would get out a tape measure, or whether they'd reject the work for 2". It was easy to fix, though - I just turned one of them around, to face the other way in the trench. Unfortunately the ground wire ended up much longer, to be able to go from subpanel to one ground rod and then the other. That wire alone cost $25.
(In some places they require the ground wire to have both ends in the subpanel, so it makes a big loop. Guess we're lucky.)
Next: torque.
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