leg
A branch circuit; one phase of a polyphase system.
leg: one conductor in a multi-phase power circuit
In electrical distribution, a leg is a single conductor or phase of a multi-phase power system. Three-phase systems, standard in industrial settings, use three legs to transmit power; single-phase systems use two. Each leg carries voltage relative to neutral (in grounded systems) or to the other legs (in delta configurations). The term reflects the literal structure: multiple conductors radiating from a transformer or panel like the legs of a table.
Three-phase systems typically operate at 208V, 277V, 480V, or 600V between any two legs (line-to-line), with each phase offset 120 degrees in time. This staggered delivery of power produces smoother torque in motors and allows smaller conductors than single-phase for the same load. Industrial facilities almost always use three-phase because motors and equipment are designed around it. Residential service is usually single-phase, with two legs delivered from a center-tapped transformer.
A leg differs from a branch circuit, though the terms are sometimes conflated. A leg is the main feeder phase itself; a branch circuit is a smaller wire that draws from one or more legs to serve a specific load or group of outlets. The distinction matters for code compliance and load balancing. In a three-phase panel, uneven loading across the three legs creates neutral current, heat, and voltage imbalance that stresses transformers and motors.
Voltage imbalance, the most common problem with unequal leg loading, occurs when one phase carries significantly more current than the others. A 5 percent imbalance can reduce motor efficiency by 1 to 2 percent and raise winding temperature by up to 10 degrees Celsius. Balancing loads across all three legs is a core task in facility design and maintenance, particularly as equipment is added or reconfigured.
Modern load monitoring equipment can track current and voltage on each leg in real time. Power factor correction devices and variable frequency drives operate on individual legs or all three phases depending on design. Understanding the characteristics of each leg, including harmonic content, is essential when troubleshooting power quality issues or planning upgrades to aging infrastructure.