Voltage Drop Calculator

Voltage drop formula

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

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:

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.

Guides

• How to Calculate Voltage Drop Percentage: the 3% rule and the K-factor method, worked step by step.
• Wire Size for a 200-Foot, 30-Amp Circuit: why the ampacity-minimum conductor fails on a long run.

Related calculators

• Wire Size Calculator: size a conductor for a target ampacity and voltage drop.
• DC Voltage Drop Calculator: for solar, battery, and automotive DC circuits.
• Wire Ampacity Calculator: conductor ampacity with temperature and conduit-fill derating.