electric main

The electric main is the heavy-gauge conductor that enters a building from the utility pole or underground service, carrying the full amperage supplied by the grid to that structure. It terminates at the service entrance, usually connecting directly to the main breaker or switch panel before splitting into branch circuits. For residential service in North America, this is typically a two-phase or three-phase feed; voltage is usually 120/240 volts single-phase or 277/480 volts three-phase in industrial settings.

Wire gauge on an electric main depends on the service rating. A residential main supplying 100 amps might use 1 AWG copper or 1/0 AWG aluminum; larger commercial services of 400 to 1000 amps use multiple conductors in parallel or larger single conductors, sometimes 4/0 AWG or beyond. The main is always protected by a main breaker or disconnect rated to handle the full service amperage without nuisance tripping during normal peak demand.

Physical routing and voltage drop

The electric main runs from the service head (where the utility drops off power) through the building's exterior and into the meter socket and main panel, typically no more than 10 meters in residential work to keep voltage drop under 3 percent. In large industrial plants, feeder mains run from the substation switchgear to distribution centers; at these scales, voltage drop calculation becomes critical because even 1 or 2 volts lost across a long run affects motor performance and equipment efficiency. The main is almost always run in metallic conduit, cable tray, or buried in rigid conduit for mechanical protection and grounding continuity.

A common failure mode is corrosion where the main enters the building, especially in damp climates or coastal areas where salt spray accelerates oxidation of aluminum conductors. Loose connections at the meter or main breaker lugs generate heat and can fail suddenly under load; these are found during thermal imaging surveys of electrical enclosures. Undersizing occurs when a building is retrofitted for higher demand (adding heavy machinery, heat pumps, or large loads) but the utility is never asked to upgrade the main, forcing load shedding or frequent breaker trips.

The term main comes from its role as the trunk of the electrical tree: it is the single path through which all power entering the building must flow, making it the obvious place to mount the main disconnect switch required by electrical code. This allows safe isolation of the entire building from the grid during maintenance or emergency.