Wire Size at 400 Feet (400 ft)

Voltage Drop at 400 Feet

At a distance of 400 feet, the voltage drop across any conductor is proportional to the current draw, the conductor resistance, and the type of circuit (single-phase or three-phase). On a 120-volt single-phase circuit, the 3% limit allows a maximum of 3.6 volts of drop. On a 240-volt circuit, the 3% limit allows 7.2 volts. The tables below show the recommended wire gauge for every common voltage and amperage combination at exactly 400 feet, making it easy to find the right conductor without performing manual calculations.

The recommendations consider both ampacity (from NEC Table 310.16 at 75 degrees C) and voltage drop (using NEC Chapter 9, Table 8 resistance values). The recommended gauge is the smallest standard size that satisfies both requirements simultaneously. When the 3% column and 5% column show different gauges, voltage drop is the controlling factor, and you may be able to use the smaller gauge if your circuit qualifies for the 5% combined feeder-plus-branch limit.

120V Wire Sizes at 400 ft

Recommended wire gauges for 120-volt circuits at a one-way distance of 400 feet. The copper columns show the recommended gauge at both 3% and 5% voltage drop limits. The aluminum column shows the 3% recommendation, which is typically one to two sizes larger than copper.

240V Wire Sizes at 400 ft

Recommended wire gauges for 240-volt circuits at a one-way distance of 400 feet. The copper columns show the recommended gauge at both 3% and 5% voltage drop limits. The aluminum column shows the 3% recommendation, which is typically one to two sizes larger than copper.

208V Wire Sizes at 400 ft

Recommended wire gauges for 208-volt circuits at a one-way distance of 400 feet. The copper columns show the recommended gauge at both 3% and 5% voltage drop limits. The aluminum column shows the 3% recommendation, which is typically one to two sizes larger than copper.

277V Wire Sizes at 400 ft

Recommended wire gauges for 277-volt circuits at a one-way distance of 400 feet. The copper columns show the recommended gauge at both 3% and 5% voltage drop limits. The aluminum column shows the 3% recommendation, which is typically one to two sizes larger than copper.

480V Wire Sizes at 400 ft

Recommended wire gauges for 480-volt circuits at a one-way distance of 400 feet. The copper columns show the recommended gauge at both 3% and 5% voltage drop limits. The aluminum column shows the 3% recommendation, which is typically one to two sizes larger than copper.

Tips for 400-Foot Runs

At 400 feet, voltage drop dominates wire sizing decisions for all but the lowest current circuits. The conductor cost for maintaining acceptable voltage drop can be substantial. Consider alternative strategies: install a subpanel closer to the load and run a higher-amperage feeder (voltage drop is more efficient at higher amperages relative to the conductor size), use a higher distribution voltage with a step-down transformer, or use aluminum conductors to reduce material costs. For agricultural and rural installations, a separate utility service may be more economical than very long conductor runs.

When burying conductors for outdoor runs of 400 feet, use UF cable for direct burial or install individual conductors in PVC conduit. Direct burial cable must be installed at a minimum depth of 24 inches for 120V circuits (18 inches under a 2-inch concrete slab or under a residential driveway) per NEC Table 300.5. PVC conduit reduces the burial depth requirement to 18 inches. For long runs, the cost of trenching and backfill is often more significant than the conductor cost, so there is little reason not to upsize the conductor while the trench is open.

For above-ground runs to detached structures, the conductor must be supported and protected per NEC requirements. Overhead spans require a minimum clearance of 10 feet above finished grade, 12 feet above driveways, and 18 feet above public streets. Underground installation is generally preferred for permanent structures. Always install a grounding electrode system at the detached structure per NEC 250.32.

Other Distances

Browse wire sizing recommendations for other common run distances. As distance increases, voltage drop grows linearly, and the required conductor size increases to compensate.