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Voltage drop calculator (you enter the resistance)

Type the conductor’s resistance from your code book or the maker’s data, the one-way length, the load current and the supply voltage. You get the drop in volts and percent, and what is left at the load. Nothing here is a code table — that is what keeps it right on any code cycle, CEC or NEC.

Circuit

Voltage drop—
Drop as %—
Volts at the load—
Against your limit—

Resistance-only estimate using the resistance you entered. Check your code book for the resistance to use and the drop allowed.

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How it works

  • VD = 2 × I × R × Lsingle-phase
  • VD = √3 × I × R × Lthree-phase, balanced
  • % = VD ÷ V × 100against the supply

A conductor is a long, thin resistor. Push current through it and Ohm’s law takes its cut: V = I × R, where R is the resistance per unit length times the length. On a single-phase circuit the current makes the trip twice — out and back — so the length counts twice. R here is per single conductor per unit length, and L is the one-way distance from the source to the load. The result ignores the conductor’s reactance, which is small on branch-circuit sizes and matters more on big feeders; your code book or the maker’s data covers the fuller method.

Worked examples

Single-phase, 100 ft, 16 A on 120 V

  1. Resistance: say your book gives 2.0 Ω per 1000 ft for the conductor you are using (a round practice number — use your own). Per foot that is 0.002 Ω.
  2. VD = 2 × 16 × 0.002 × 100 = 6.40 V.
  3. As a percent: 6.40 ÷ 120 × 100 = 5.33 %, leaving 113.6 V at the load. Compare that with the limit your book sets.
  4. The same run in metric: 2.0 Ω/1000 ft = 6.562 Ω/km, and 100 ft = 30.48 m. 2 × 16 × 0.006562 × 30.48 = 6.40 V — the same answer, as it must be.

Three-phase, 200 m, 30 A on 600 V

  1. Practice resistance: 0.5 Ω/km, so 0.0005 Ω per metre.
  2. VD = 1.732 × 30 × 0.0005 × 200 = 5.20 V.
  3. 5.196 ÷ 600 × 100 = 0.87 %. On a 208 V system the same drop would be 2.5 % — voltage drop hurts more on lower voltages.

Questions people ask

Where do I get the resistance number?

From the conductor-properties table in your code book (CSA C22.1 in Canada, the NEC in the US) or the cable maker’s data sheet, for the size, the metal (copper or aluminium) and the temperature you are working at. This site deliberately does not carry that table — it changes by edition, and you should be reading it in your own book.

What percent drop is allowed?

Your code book says, and the limit depends on the kind of circuit and the edition. Type it into the “your limit” box and the calculator tells you whether this run is within it. Treat the number here as arithmetic, not as approval.

Why ×2 for single-phase and ×√3 for three-phase?

On a single-phase circuit the current goes out on one conductor and back on the other, so it crosses the length twice. On a balanced three-phase circuit the return is shared among the other phases, and the line-to-line drop works out to √3 (1.732) times the drop in one conductor.

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