EVSE showing open ground

CCIE said:

Sub-panels can also be tricky if they're not properly setup with neutrals & grounds separated. The panels usually come with a screw in place that bonds the neutrals & grounds. That bonding should only occur in the main panel (technically inside the main disconnect, but that's usually the main panel). So, in a sub-panel the screw needs to be removed and separate bus-bars must be used for neutrals & grounds.


If your garage sub-panel has neutrals & grounds mixed together on the same bus-bars, you could wind up with stray current on the ground that the EVSE doesn't like.

Click to expand...

Actually, incorrect bonding between the neutral and ground in a sub-panel could make the Volt's EVSE less likely to fault. (though I wouldn't suggest that as a fix)

The EVSE checks for proper grounding by measuring between neutral and ground. Ideally there will be a circuit reading close to zero ohms. Problems can arise with long wire runs: a load on the hot and neutral will cause voltage drop, but the ground wire has no load and thus no drop.

Assume 100 feet of 12ga wire. If it's fed with 120.0V and loaded to 15 amps the voltage at the far end of the wire would be 115.2V between hot and neutral, 117.6V between hot and ground, and 2.4V between neutral and ground.

This isn't usually an issue for the EVSE because it's typically the only load on the circuit, but with a subpanel all of the connected loads affect the voltage drop on the cable feeding the subpanel.