Wire Sizing for Long Cable Runs

How to select the right wire gauge for long-distance circuits based on voltage drop, current, and conductor material

Why Cable Runs Need Careful Sizing

Voltage drop increases with distance, current, and wire resistance. For short runs, the minimum wire size for ampacity is usually sufficient. But for longer cable runs, voltage drop often requires upsizing the wire beyond the ampacity minimum.

Wire gauge needed for a 20A circuit (NEC, copper, THHN)

Distance	20A @ 120V	20A @ 240V
50 ft	#12 AWG	#12 AWG
100 ft	#10 AWG	#12 AWG
150 ft	#8 AWG	#10 AWG
200 ft	#6 AWG	#10 AWG

Circuits at 120V need larger wire for the same distance because the percentage drop is calculated against a lower base voltage. A 3.6V drop is 3% at 120V but only 1.5% at 240V.

Sizing Cables for Common Scenarios

Click any scenario below to open the calculator with the values pre-filled. Each link sets the standard, voltage, current, distance, and conductor type.

Residential

Commercial / Industrial

European (IEC)

Copper vs Aluminum for Long Runs

Aluminum conductors cost less per foot than copper but have only 61% of copper's conductivity. This means you need a larger gauge aluminum wire to carry the same current with the same voltage drop.

Aluminum is commonly used for feeders and service entrance cables where the cost savings justify the larger conduit. For branch circuits, copper is preferred due to its smaller size and easier termination.

When to Use Parallel Conductors

For very high-current or very long runs, running multiple sets of smaller conductors in parallel can be more practical than a single oversized wire. Parallel sets share the total current equally.

Try parallel conductors →

Parallel conductors must be the same length, size, material, and insulation type per NEC 310.10(G). Unequal lengths or sizes will cause uneven current distribution.

Sources

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