USA distribution wiring

flag of USA

How grounding practices on USA distribution circuits produce magnetic fields

We explain on our main page on low-voltage distribution that magnetic fields arise when the neutral is earthed in more than one place.  This allows the current to divert out of the neutral conductor and creates a "net current".  We explain here how this happens, specifically, in USA distribution systems.

Distrubution wiring in the USA typically has small transformers, fed from a primary circuit, and supplying just a few homes.  Each transformer produces two "hot wires" at 120 V.  These two wires plus the neutral go into each home so that appliances can be connected either at 120 V from one hot wire to neutral, or at 240 V between the two hot wires.

digram of usa wiring

The neutrals of the primary and secondary are connected together and  grounded at the transformer.  Each home has a ground bus.  This is connected to the neutral conductor and also usually to two other grounds - a driven ground rod, water pipes, or structural steel.

This means the neutral is multiply grounded and net currents are created, which are a source of magnetic field.