Voltage Drop Calculator

Calculate AC voltage drop on single-phase or three-phase circuits using National Electrical Code (NEC) 2020 conductor resistance — the K-factor for conductors smaller than #2 AWG and Chapter 9 Table 9 AC resistance for #2 and larger. Results flag compliance against the 3% branch and 5% combined feeder + branch recommendations in NEC 210.19 and 215.2.

Circuit inputs

Assumes PVC conduit, unity power factor (resistive load), and NEC Chapter 9 Table 9 conductor values at 75°C.

How to use this calculator

  1. Enter source voltage and select phase configuration (single or three).
  2. Enter load current in amps and the one-way circuit distance in feet.
  3. Select conductor material and AWG size.
  4. Results update as you adjust inputs. The status badge indicates compliance with the NEC 3% branch and 5% combined recommendations.

NEC reference

Conductor resistance comes from NEC 2020 Chapter 9. For conductors smaller than #2 AWG the calculator uses the K-factor (K = 12.9 for copper, 21.2 for aluminum, derived from Table 8). For #2 AWG and larger it uses Table 9 AC resistance (PVC conduit), which accounts for the skin and proximity effects the K-factor misses on large conductors. Values are taken at the published 75°C basis at unity power factor. Compliance status references NEC 210.19(A)(1) Informational Note No. 4 and NEC 215.2(A)(1) Informational Note No. 2.

Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.

Voltage drop formula

Voltage drop is the conductor resistance times the current over the round-trip length:

Single-phase: VD = 2 × R × I × L / 1000

Three-phase:  VD = √3 × R × I × L / 1000

R is resistance in ohms per 1000 ft, I is current in amps, and L is the one-way distance in feet. The 2× factor on single-phase accounts for the round trip; three-phase uses √3 for the line-to-line relationship, which works out to about 86.6% of the single-phase drop. For conductors below #2 AWG, R comes from the K-factor (R = K × 1000 / circular mils); for #2 and larger it comes directly from NEC Chapter 9 Table 9. Circular-mil and DC-resistance data live in Table 8.

Worked example

A 20A load on #12 copper, 240V single-phase, 100 feet one way:

# 12 copper is smaller than #2 → K-factor, K = 12.9

R = 12.9 × 1000 / 6,530 cmil = 1.976 Ω/1000 ft

VD = 2 × (1.976 / 1000) × 20 × 100 = 7.90 V

VD% = 7.90 / 240 = 3.29%

At 3.29% this exceeds the NEC 210.19 branch-circuit recommendation of 3%. It is acceptable only as part of a combined feeder-plus-branch budget under NEC 215.2. Stepping up to #10 copper drops it to about 2.07%, back within the 3% branch recommendation.

Common mistakes

  • One-way vs round-trip distance. Enter the one-way run length. The calculator applies the 2× (single-phase) or √3 (three-phase) multiplier. Entering the there-and-back length double-counts the conductor.
  • K-factor on large conductors. The K-factor shortcut (VD = 2KIL/CM) tracks DC resistance and diverges from real AC resistance on large conductors as skin and proximity effects grow. This tool switches to Table 9 AC resistance at #2 AWG for that reason.
  • Confusing voltage drop with ampacity. These are separate checks. A conductor can satisfy its ampacity rating and still fail voltage drop on a long run — size for the worse of the two.
  • Using this tool for DC. This is an AC calculator. DC drop on solar, battery, and automotive circuits uses DC resistance — use a dedicated DC tool.

Frequently asked questions

What is the NEC limit for voltage drop?

The NEC does not set a hard limit. NEC 210.19 recommends no more than 3% drop on a branch circuit and NEC 215.2 recommends 5% combined on feeder plus branch. Both appear as informational notes, so they carry recommendation weight at the federal level. Some local jurisdictions adopt them as enforceable through amendments — check your adopted code.

Does the NEC require voltage drop calculations?

No. The 3% and 5% figures are informational notes in NEC 210.19 and 215.2, not mandatory rules. Voltage drop remains a functional design constraint regardless of code: motors, electronics, and lighting misbehave with excessive drop. Some local amendments make the calculation mandatory, and many engineering specs require it. Confirm your jurisdiction's amendments.

How do I calculate voltage drop on a three-phase circuit?

Use the √3 multiplier in place of the 2× used for single-phase: VD = √3 × R × I × L / 1000, where R is conductor resistance per 1000 ft and L is one-way distance. For the same conductor, current, and length, three-phase drop is about 86.6% (√3/2) of the single-phase value. Select "Three-phase" and the calculator applies the factor automatically.

Should I use one-way or total circuit length?

Enter the one-way length — source to load. The calculator applies the round-trip multiplier internally. Entering the total there-and-back length double-counts the conductor and roughly doubles the result.

What's an acceptable voltage drop for motor circuits?

NEC 430 doesn't mandate a figure, but motors are sensitive: low terminal voltage raises current and heat, and starting torque falls with the square of voltage. Many designers hold motor branches to 3% and total to 5%. This calculator assumes unity power factor; motors run at a lower power factor, which increases drop, so for precise motor work use a calculator that accepts a custom power factor.

When does voltage drop matter most?

Long runs, high current, and small conductors. Drop scales with current and length and inversely with conductor cross-section, so a long feeder or a small-AWG circuit pushed near its ampacity is where it bites. It also matters for sensitive loads — VFDs, control circuits, and LED drivers — where low voltage causes nuisance faults.

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