How is voltage drop calculated?

For a two-conductor path, voltage drop is 2 times current times resistance times the one-way run length divided by 1,000, using the candidate-conductor resistance in ohms per 1,000 feet. Percentage drop divides that result by system voltage.

Does the calculator choose the wire size for me?

No. This worksheet does not choose wire size, determine ampacity, or establish electrical-code compliance. Resistance should come from current manufacturer data, and electrical work should be designed and checked for the applicable installation.

What inputs does the worksheet need?

Enter voltage, current, one-way run length, and candidate-conductor resistance in ohms per 1,000 feet. Empty or invalid inputs, prohibited negatives, zero denominators, impossible relationships, and unsupported options do not produce a misleading numeric result.

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Latest update: Aug 14, 2026

Electrical planning starts with knowing what the wiring will carry. The cable size calculator estimates conductor size (AWG) from amps, voltage, run length, and material, applying voltage-drop and derating checks in a preliminary, jurisdiction-specific worksheet. Reach for it when adding a circuit, running a long feeder, or wiring a new room: enter the planned load and measured one-way run. The worksheet shows every limiting check but does not choose wire size, determine ampacity, or establish code compliance — use manufacturer resistance data and have the design checked for the applicable installation. Pair it with the stud spacing calculator for cable runs through framed walls, and the ladder safety calculator for elevated work. Explore the Construction & Home collection.

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Electrical Cable Size Calculator

Estimate electrical cable size (AWG) from amps, voltage, run length, conductor material, derating factors, voltage drop, and simplified ampacity limits.

Updated Aug 14, 2026

Verified Aug 14, 2026

Frequently asked questions

How is voltage drop calculated?: For a two-conductor path, voltage drop is 2 times current times resistance times the one-way run length divided by 1,000, using the candidate-conductor resistance in ohms per 1,000 feet. Percentage drop divides that result by system voltage.
Does the calculator choose the wire size for me?: No. This worksheet does not choose wire size, determine ampacity, or establish electrical-code compliance. Resistance should come from current manufacturer data, and electrical work should be designed and checked for the applicable installation.
What inputs does the worksheet need?: Enter voltage, current, one-way run length, and candidate-conductor resistance in ohms per 1,000 feet. Empty or invalid inputs, prohibited negatives, zero denominators, impossible relationships, and unsupported options do not produce a misleading numeric result.