If you want to know what the magnetic field is around a power line, the best thing to do is walk away from it. Stand near the line and then move back ten feet, then twenty feet. You will see the reading on a meter drop faster than you might expect. That is because magnetic fields from power lines follow a simple rule. They are strong close to the source and weaken quickly as distance increases. The field from a single wire drops by the square of the distance. Double your distance and the field falls to a quarter of what it was. For a three-phase line, the drop is even steeper because the fields from the different wires partly cancel each other.

The load on the line matters a lot. A transmission line carrying a light load produces a smaller field than the same line carrying a heavy load. The voltage on the line does not change the magnetic field much. What matters is the current, the flow of electrons. A line with higher voltage but low current can have a weaker field than a lower voltage line with high current. The field changes throughout the day as the load changes. It is usually lower at night and higher during the day when people use more power.

A single reading taken at one spot at one time does not tell the full story. The field at your property line might be different at noon than at midnight. It might be different on a hot summer day when air conditioners run than on a cool spring morning. The field also depends on what else is around. Water pipes, metal fences, and wiring in the house can change how the field spreads. A reading taken right next to the line might look alarming, but the field drops to low levels just a short distance away.

In the United States, typical fields in homes are low. A survey of hundreds of homes found that the average reading in rooms away from appliances was under one milligauss. Right beside a large transmission line the field can be in the tens of milligauss. That sounds high, but the field falls off quickly. The international reference level for the public is 2,000 milligauss. That is much higher than anything you would find near a power line.

If you have questions about the field near your home, you can ask your utility to measure it. Some utilities do this for free. If you are worried about health effects, talk to your doctor. The International Agency for Research on Cancer classified low-frequency magnetic fields as possibly carcinogenic in 2002. That classification was based on studies that linked higher average exposure to childhood leukemia. Very few children live with fields that high, and the studies do not show that the fields cause the disease.

A meter reading at one spot is just a snapshot. The real picture is more complicated. The field changes with distance, with load, and with time. A few measurements in different places at different times give a better sense of what is going on.

For more on this, see power line magnetic fields by distance.