Voltage Drop Calculator

Voltage Drop Calculator
Electrical

Voltage Drop Calculator

Uses standard uncoated conductor DC resistance values (ohms per 1,000 ft) as a general reference, similar to NEC Chapter 9, Table 8.
Please enter valid, positive values for voltage, current, and wire length.
Voltage Drop
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Percent Drop
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Voltage at Load
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Wire Resistance
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NEC Guideline (3%/5%)
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Disclaimer: This calculator provides a general estimate using standard DC conductor resistance values and does not account for AC skin effect, conduit type, ambient temperature derating, or other job-specific factors. It is not a substitute for a licensed electrician's calculations or local electrical code review. Always verify wire sizing and voltage drop against the National Electrical Code (NEC) or your local code before installation.

Voltage Drop Calculator: How to Size Wire Runs Correctly the First Time

Quick answer: A voltage drop calculator uses your circuit’s current, one-way conductor length, wire size, and material (copper or aluminum) to estimate how many volts are lost across the run, then flags whether that drop stays within the NEC’s recommended 3% limit for branch circuits and 5% for feeders. Below you’ll find the exact single-phase and three-phase formulas, real forum-sourced worked examples, and the wire-gauge mistakes that most often cause circuits to fail voltage drop checks.

Anyone who has run a long conductor to a detached garage, a well pump, or an outbuilding has run into the same problem: the wire that’s rated for the amperage on paper still causes dim lights or an underperforming motor once you account for distance. That’s what a voltage drop calculator is built to catch before you buy wire and pull it through conduit. This guide covers the underlying formula, how the National Electrical Code (NEC) treats voltage drop, and several fully worked examples pulled from the kind of real-world questions electricians ask in the field.

The Voltage Drop Formula

The standard formula used by electricians and voltage drop calculators alike is:

Single-phase: VD = (2 × K × I × L) / CM

Three-phase: VD = (1.732 × K × I × L) / CM

Where:

  • VD = voltage drop, in volts
  • K = resistance constant (12.9 for copper, 21.2 for aluminum, at 75°C)
  • I = load current, in amps
  • L = one-way conductor length, in feet
  • CM = circular mils, the cross-sectional area of the conductor (from NEC Chapter 9, Table 8)
  • 2 = constant for single-phase circuits (accounts for the round-trip of current)
  • 1.732 = constant for three-phase circuits (√3)

Percentage voltage drop, which is the figure the NEC actually references, is then:

VD% = (VD / Source Voltage) × 100

According to NEC guidance, the voltage drop for feeders should not exceed 3%, and combined feeder-plus-branch-circuit voltage drop should not exceed 5%, for efficient equipment operation — figures that show up consistently across electrical code discussion forums and reference material.

Worked Example #1: Single-Phase Circuit (From a Real Field Question)

This example mirrors a commonly asked electrician-forum question: a 3-hp pump sits 220 feet from a 230-volt panel, wired with #12 AWG copper, drawing 17 amps.

  • K = 12.9 (copper)
  • I = 17 amps
  • L = 220 feet
  • CM = 6,530 (circular mils for #12 AWG)

VD = (2 × 12.9 × 17 × 220) / 6,530 VD = 96,492 / 6,530 VD ≈ 14.78 volts

VD% = (14.78 / 230) × 100 ≈ 6.4%

That’s more than double the NEC’s recommended 3% limit for a branch circuit, meaning #12 AWG is undersized for this run. Solving the formula backward for the minimum circular mils needed to hit 3% shows that roughly a #8 AWG conductor is required to bring the drop back into an acceptable range — a conclusion that matches what electricians reach when working this exact problem by hand.

Worked Example #2: Sizing Wire for a 200-Amp Feeder

Suppose you need to run a 200-amp, single-phase, 240-volt feeder, and you want to stay within the NEC’s 3% recommendation (7.2 volts on a 240V circuit).

Rearranging the formula to solve for circular mils:

CM = (2 × K × I × L) / VD

With K = 12.9, I = 200 amps, an assumed length of L feet, and VD = 7.2 volts, solving for a feeder run requires roughly 200,000+ circular mils, which corresponds to a 4/0 AWG copper conductor — a sizing outcome consistent with load calculations electricians commonly reach for feeder circuits carrying a full 200-amp load.

Worked Example #3: A 250-Foot RV/Trailer Cord Run

A real-world example that comes up often in electrical forums: a 50-amp, 120/240V RV trailer cord run 250 feet using #6 AWG wire, which has a resistance of about 0.491 ohms per 1,000 feet.

VD = 0.491 × 2 × 250 × 50 / 1,000 VD ≈ 12.3 volts

VD% = (12.3 / 240) × 100 ≈ 5.1%

That’s above the 3% branch-circuit target and even above the combined 5% guideline, illustrating why long outdoor runs to RVs, sheds, or barns are one of the most common places voltage drop becomes a real, noticeable problem rather than just a code technicality.

Step-by-Step: How to Use a Voltage Drop Calculator

  1. Enter the load current (I) in amps — use the actual expected running current, not just the breaker size.
  2. Enter the one-way conductor length (L) in feet, measuring from the source panel to the load.
  3. Select the conductor material — copper or aluminum — since aluminum has a higher resistance constant and will show a larger voltage drop for the same wire gauge.
  4. Select single-phase or three-phase circuit type.
  5. Enter the source voltage (commonly 120V, 208V, 240V, 277V, or 480V).
  6. Review the output: voltage drop in volts, percentage drop, and whether it falls within NEC-recommended limits — many calculators will also suggest the minimum wire gauge needed to stay compliant.

Common Mistakes People Make

  • Using the breaker size instead of the actual load current, which overstates the voltage drop and can lead to oversized (and needlessly expensive) wire.
  • Forgetting the single-phase multiplier of 2, which accounts for current traveling out and back through the circuit.
  • Mixing up one-way and round-trip length when entering the run distance into the formula.
  • Ignoring temperature correction, since the K constant of 12.9 assumes 75°C conductor temperature and can shift at different operating temperatures.
  • Treating voltage drop as a code violation rather than a design recommendation — the NEC lists 3%/5% as informational guidance for efficient operation, not a mandatory rule in most jurisdictions, though some local codes do enforce it.

Frequently Asked Questions

What is an acceptable voltage drop percentage?

NEC guidance recommends no more than 3% voltage drop on a branch circuit and no more than 5% combined for a feeder plus branch circuit, though these are design recommendations rather than strict violations in most cases.

Does wire gauge or wire length matter more for voltage drop?

Both matter directly — voltage drop increases proportionally with length and current, while thicker wire (more circular mils) reduces resistance and therefore reduces voltage drop, so long runs typically need a larger gauge than the ampacity table alone would suggest.

Is voltage drop worse with copper or aluminum wire?

Aluminum has a higher resistance constant (about 21.2 versus copper’s 12.9), so an aluminum conductor of the same gauge and length will have a larger voltage drop than copper.

Do I need to calculate voltage drop for every circuit?

Short, low-current circuits close to the panel rarely have a meaningful voltage drop, but long runs — like those to a detached garage, well pump, or outbuilding — should always be checked, since that’s where drop most often exceeds recommended limits.

How does three-phase voltage drop differ from single-phase?

Three-phase circuits use a multiplier of 1.732 (√3) instead of 2, which generally results in a lower voltage drop than an equivalent single-phase circuit carrying the same current over the same distance.

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Conclusion

A voltage drop calculator is really just a shortcut for the same formula electricians have used for decades — current, length, and wire size run through a resistance constant — but having it automated removes the risk of a rounding error costing you an undersized feeder or a dim motor at the end of a long run. Whether you’re wiring a subpanel, an RV pad, or a well pump, plug in your actual load current and one-way distance, check the result against the NEC’s 3%/5% guidance, and size up your conductor if the percentage comes back too high.