Wire size for a 300 amp service

A 300 amp service for a house needs 250 kcmil copper or 350 kcmil aluminum service entrance wire, sized for 249 A (83% of 300 A). The grounding electrode conductor for a 300 amp service is 2 AWG copper, or 6 AWG copper when it runs only to ground rods.

300 amp service wire size

The full rating row is for a service that does not carry the whole load of one home. Conduit sizes are for three THHN wires of the service size: two hots and a full-size neutral. A smaller neutral, allowed in many cases, can reduce the conduit.

How the 300 amp service wire is sized

300 A × 0.83 = 249 A. The smallest wire rated for at least 249 A at 75 °C is 250 kcmil copper (255 A) or 350 kcmil aluminum (250 A). Without the 83% rule the same service would need 350 kcmil copper or 500 kcmil aluminum.

The 83% sizing applies only to the 120/240 V single-phase wires that carry the whole load of one home. Where the wire also needs a correction for heat or bundling, such as a run through a hot attic or a conduit crowded with other circuits, it is sized for 83% of the rating divided by the correction factor, which can mean one size larger.

Ground wire size for a 300 amp service

Aluminum grounding electrode conductor may not be terminated within 18 in. of the earth outside a building or listed enclosure, and a ground rod clamp sits at the earth, so the wire to ground rods is copper. Aluminum is used only where it runs to a connection above that height, such as a water pipe or building steel.

Who sizes the service wire

The utility decides the size of its own drop or lateral and the meter equipment. The service entrance wire from the meter to the main panel is the owner's, and the local inspector checks it. Many utilities publish their own requirements; check them before buying wire.

Typical 300 amp service uses

• Large house with electric heat