Single-phase, two-phase, and three-phase feeders

A capacity number almost always describes the feeder as a whole, but a feeder consists of three wires that don't carry the same load.

Most distribution feeders are three-phase, but residential service is commonly single-phase, run off just one of the three legs, and rural or lightly-loaded stretches sometimes use two-phase or open-delta configuration rather than a full three-phase build.

Industrial motors, HVAC compressors, and data center UPS systems are typically made to run on three-phase power specifically, because three-phase delivers smoother, more continuous power than an oscillating leg can. A site fed only by a single-phase lateral can show up on a capacity map with a number attached to its parent feeder even if the pole wasn’t made to carry a three-phase load.

Feeder imbalance

Even where three-phase service exists end to end, the three legs don't necessarily share the load evenly. Single-phase customers get assigned to one of the three phases more or less as they're added over time, and nothing forces that process to stay balanced as a neighborhood or an industrial corridor grows. The result is phase imbalance, with one leg running hotter than the other two. This imbalanced current increases losses, drives unwanted current through the neutral conductor, and can overheat three-phase motors connected downstream, since a motor sees the imbalance as an effective loss of efficiency even when its own local wiring is fine.

The commonly cited engineering guideline, drawn from NEMA and IEEE standards for motor operation, caps voltage unbalance at around one percent before a motor's insulation life and thermal margins start eroding. For a distribution operating limit, utilities tolerate closer to 10 to 15% on a loaded feeder.

Utilities actively rebalance feeders by moving individual customers between phases during planned work because imbalance left unmanaged shows up later as avoidable equipment stress and additional asset reinforcement that wouldn't otherwise have been needed.

A feeder can have capacity in aggregate and still have one phase sitting much closer to its limit than the published figure implies, because the published figure is usually a feeder-level total. A new three-phase load draws from whichever phase or combination of phases it's tied into, and if that happens to be the already-loaded one, the available room is smaller than the site's aggregate number would suggest.

Before a site gets chosen, planning engineers should clarify what phase configuration serves this specific site and whether that feeder's phases are anywhere close to balanced.