Leg: circuit branch or body part

To most people, a leg is a limb: the part of an animal or person that runs from hip to foot. Furniture has legs too, the supporting pillars holding up a table or chair. The word derives from Old Norse, and both senses are anatomical or architectural, something that stands, something that supports.

In energy and utilities, a leg means something quite different. It is a branch circuit; one conductor or phase of a polyphase electrical system. A standard three-phase power system has three legs, each carrying current at a different phase angle, usually 120 degrees apart. In single-phase service to a home, there are typically two legs, each at opposite potential to neutral ground. Electricians speak of "a 120-volt leg" or "the hot legs" of a panel when they mean specific conductors or phases within a larger distribution network.

The metaphor bridges the two meanings through the idea of division and support. Just as a table's four legs each support the top, the legs of a polyphase circuit each carry a portion of the total load and contribute to the stability and balance of the system. Each leg is a separate path for current, yet together they form one complete electrical supply.

Other trades do not commonly repurpose the term in the way utilities do. Mechanical engineers speak of "legs" on machinery, but they mean literal support structures. The electrical sense is particular to power systems and emerged as AC polyphase technology developed in the late 19th and early 20th centuries. As three-phase power became the backbone of industrial and grid distribution, the language needed a short, intuitive term for each phase conductor, and "leg" was natural and widely adopted.