subfeed

A subfeed is a secondary power distribution line that originates from a main feeder or service entrance and delivers electricity to a specific area, building section, or load center. Unlike a primary feeder, which carries power from a utility source or transformer to a building, a subfeed operates downstream, splitting the main supply into smaller, manageable circuits. Subfeed cables typically run through conduit or cable trays from a panelboard, switchboard, or distribution point to serve an isolated section of a facility.

In large commercial and industrial installations, subfeeds allow flexibility in site layout and load management. A building might have a main 480V three-phase service with multiple subfeeds branching to different floors, production areas, or tenant spaces. Each subfeed can be sized independently based on its connected load, allowing designers to optimize copper usage and reduce voltage drop across long distances. For example, a 200-ampere subfeed with three-phase wire might run 150 feet to serve a machine shop, while a separate 100-ampere subfeed reaches an office wing.

Subfeed sizing depends on anticipated demand and code requirements. The National Electrical Code (NEC) requires subfeeds to be protected at their origin with overcurrent devices matched to their conductor ampacity. A 4/0 copper subfeed cable might be protected by a 150-ampere breaker, limiting the actual available current. Voltage drop becomes critical over longer runs; a subfeed extending 200 feet at rated current might see a 3 to 4 percent drop, enough to affect motor starting or precision equipment operation.

Variants and Installation

Subfeeds may be routed as individual conductors in conduit, bundled cables (such as 4-conductor service entrance cable), or cable assemblies. Some installations use parallel conductors to reduce impedance and cost: four runs of 1/0 copper instead of a single 4/0, for instance. Temporary subfeeds are common in construction, using flexible cords or portable cable from a distribution box to temporary load centers; permanent subfeeds require rigid protection, proper grounding, and integration into the site electrical design.

The term subfeed also appears in antenna and RF system design, where it denotes a cable or waveguide segment that carries signal from a main feed network to a radiating element or secondary feed point. In this context, subfeed losses and matching become critical for antenna efficiency. In electrical power systems, however, subfeed refers almost exclusively to the secondary distribution function described here, distinguishing it from feeder (main), branch circuit (final), and service line (utility).