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Wire Gauge Voltage Drop Calculator

Check how much voltage a copper wire run loses over distance, and whether it's within the recommended 3% limit.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax
49.21 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

Voltage drop

1.94 V

High confidence
Voltage drop
1.62 %
Voltage at the load
118.06 V

At the values currently entered, the voltage drop works out to 1.94. The largest intermediate quantity is voltage at the load, at 118 V — check that step first if the total looks off. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 1.944 V — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard single-phase voltage drop formula: Vdrop = (2 x K x one-way length in ft x amps) / circular mils, K=12.9 for copper
  • NEC recommends keeping voltage drop to 3% or less for branch circuits (and 5% total including feeders) for reasonable efficiency, though it is a recommendation, not a hard code violation

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Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Why does voltage drop matter?
Excessive voltage drop wastes energy as heat in the wire and can cause motors to run hotter and less efficiently, dim lights, or trip sensitive electronics — the NEC recommends staying under 3% for branch circuits as a best practice.
Is exceeding 3% a code violation?
No — the NEC's voltage drop guidance is an informational recommendation (in an appendix), not an enforceable code rule like breaker sizing is. Still, exceeding it is a real efficiency and equipment-longevity concern worth fixing.
What's the easiest fix for high voltage drop?
Use the next larger (lower-numbered) wire gauge, or shorten the run if possible — both directly reduce the resistance the current has to push through.
Does this apply to aluminum wire?
No — this uses the resistivity constant for copper. Aluminum has roughly 1.6x the resistance of copper at the same gauge, so it needs a thicker gauge for the same voltage drop.