Why is it unsafe to bond neutral and ground wiring at subpanels?
Monday, June 18, 2018
To understand why it is problem, we have to start with the basic principle that a 120-volt circuit begins and ends at the transformer on a pole, or the ground, outside the home. The neutral wiring completes the loop of the circuit back to transformer from one of the two 120-volt hot wires that serve the panel. Ground wiring is intended as an alternate route when a “ground fault” occurs, which is essentially when the electric current strays from its intended path and may be dangerous. The ground wiring allows a fault to complete a circuit with an unimpeded surge of current that trips a breaker, stopping a potentially dangerous event.
When the ground wires are connected (bonded) to the neutral at the main service panel, the current flows readily through the neutral from there to the transformer. But when the neutral and ground wires are connected further back at a subpanel, they both carry current back to the main panel. This means that you effectively have two neutral wires running in parallel. The ground wire should not carry current continuously, and there is the potential to electrify the metal components connected to it that the ground wiring is intended to protect from becoming electrically charged.
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