How a power line’s magnetic field fades with distance

People sometimes worry about the magnetic field from a power line near their home. They wonder how far away they need to be. The answer is not simple, but the basic physics is. Any wire carrying 60 Hz alternating current makes a low-frequency magnetic field around it. That field gets weaker as you move away. It does not fall off in a straight line. It drops fast at first, then slower. Right beside a large transmission line at average load, the field might be in the tens of milligauss. Walk fifty feet away, and it can drop below one milligauss. Walk two hundred feet, and it is often too small to measure above the general background.

The field strength depends on the load on the line, not just the voltage. A line carrying a lot of power makes a stronger field than the same line carrying little power. That is why one measurement taken at one moment does not tell the whole story. The load changes all day and all year. A reading at noon in summer might be higher than one at midnight in winter. Voltage matters too, but in a different way. Higher voltage lines tend to carry more current, but the field comes from the current, not the voltage itself. A low-voltage line carrying heavy current can have a stronger field than a high-voltage line carrying light current.

Another reason a single reading can mislead is that the magnetic field inside a house often comes from the wiring, not the line outside. In a survey of 992 U.S. homes, the average reading room by room, away from appliances, was under one milligauss. But some rooms had higher fields because of wiring mistakes. Current flowing on a water pipe, or a circuit where the outgoing and returning currents do not cancel, can raise the field across a whole room. A licensed electrician can track that down. The field from a transmission line outside may be lower than the field from a bad wire inside.

On health, the International Agency for Research on Cancer classified extremely low frequency magnetic fields as possibly carcinogenic in 2002. That was mostly because of studies that linked higher average exposure to childhood leukemia. Very few children live with fields that high, and those studies cannot show cause and effect. For comparison, the international reference level for the public is 2,000 milligauss, far above what homes measure. If you have health concerns, talk to a doctor. If you want to know about the line near your house, call your utility. The Federal Communications Commission handles radio frequency radiation, not the power line frequencies. And no sticker, shield or device blocks these magnetic fields. They pass through walls and bodies like light through glass. The only reliable way to lower exposure is to increase distance from the source.

A closer look is at power line magnetic fields by distance.