Average magnetic fields in homes in the USA appear to be about 2-3 times bigger than in the UK (see above).

There are several features of wiring practices that help explain this:

• Magnetic fields come from "net currents" which are produced when the neutral is multiply earthed or grounded.  In both countries the final distribution circuits ("secondaries" in the USA) are multiply earthed, allowing net currents.  But in addition, in the USA, the next voltage up - "primaries" - are multiply grounded too.  The equivalent voltage in the UK, 11 kV, is not.
• Further, transformers in the USA are smaller and serve fewer homes each, but there are a lot more of them.  So the USA primaries - which supply each transformer - tend to get closer to more homes than 11 kV circuits in the UK.
• Because the transformers are smaller (and usually single-phase as well), there is a greater tendency for loads to be unbalanced, so resulting in a bigger neutral current and in turn bigger net currents.
• The USA voltage for final distribution is 120 V, half (roughly) the UK's 230 V, so for the same power, currents (and magnetic fields) are doubled.
• Finally, electricity consumption is higher in the USA anyway - perhaps 2.5 times higher.

Against these factors:

• USA homes usually have two "hot legs", each at 120 V, sharing a neutral.  Smaller appliances are connected to one or other hot leg, larger appliances are connected at 240 V across the two.  So there is some cancellation of the neutral currents from loads on the two hot legs, which should reduce the neutral current and hence the magnetic field.

These factors were first listed in a paper published in 1994.