How to Calculate Voltage Drop: Step-by-Step Examples

What Is Voltage Drop and Why Does It Matter?

Voltage drop is the reduction in voltage as electrical current flows through a conductor. Every wire has resistance, and the longer the wire and the more current it carries, the more voltage is lost between the source and the load. The result: equipment at the end of the run doesn't get the voltage it needs to operate properly.

Diagram of a single-phase branch circuit showing source voltage, one-way run length, conductor resistance and the resulting voltage at the load — Voltage drop is a function of four things: current, distance, conductor material and cross-sectional area.

The NEC addresses voltage drop in informational notes rather than mandatory requirements, but inspectors and engineers treat the recommendations seriously. Poor voltage drop means flickering lights, overheating motors, tripped breakers, and unhappy customers.

NEC 210.19(A) Informational Note No. 4 — Recommends that branch circuit voltage drop not exceed 3%, and that the total voltage drop for both feeder and branch circuit not exceed 5%.

The Voltage Drop Formula

For single-phase circuits, the standard voltage drop formula is:

VD = (2 × K × I × D) / CM

Single-Phase Voltage Drop Formula

Where:

VD = Voltage drop (in volts)
K = Resistivity constant (12.9 for copper, 21.2 for aluminum)
I = Current in amperes (load current)
D = One-way distance in feet (source to load)
CM = Circular mil area of the conductor

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The "2" in the formula accounts for the round-trip distance — current flows out on the hot conductor and returns on the neutral.

For three-phase circuits, replace the 2 with 1.732 (the square root of 3):

VD = (1.732 × K × I × D) / CM

Three-Phase Voltage Drop Formula

Step-by-Step: How to Calculate

Step 1: Gather Your Values

You need four pieces of information: the conductor material (copper or aluminum), the load current in amps, the one-way distance from the panel to the load in feet, and the wire size you're planning to use (which gives you the circular mil area).

Step 2: Look Up Circular Mils

Common circular mil values for copper conductors:

#14 AWG — 4,110 CM
#12 AWG — 6,530 CM
#10 AWG — 10,380 CM
#8 AWG — 16,510 CM
#6 AWG — 26,240 CM
#4 AWG — 41,740 CM
#2 AWG — 66,360 CM
#1/0 AWG — 105,600 CM
#2/0 AWG — 133,100 CM

Step 3: Plug In and Calculate

Example 1 — Residential Branch Circuit

Scenario: 20A load on #12 AWG copper, 150-foot run, 120V single-phase.

VD = (2 × 12.9 × 20 × 150) / 6,530

VD = 77,400 / 6,530 = 11.85 volts

Percentage: 11.85 / 120 = 9.9% — This exceeds the 3% recommendation significantly. You need to upsize the conductor.

With #8 AWG: (2 × 12.9 × 20 × 150) / 16,510 = 4.69V (3.9%) — Still over 3%. Consider #6 AWG or reducing the circuit length.

Example 2 — Commercial Three-Phase Feeder

Scenario: 100A load on #1/0 AWG copper, 200-foot run, 208V three-phase.

VD = (1.732 × 12.9 × 100 × 200) / 105,600

VD = 446,976 / 105,600 = 4.23 volts

Percentage: 4.23 / 208 = 2.03% — Under the 3% recommendation. This wire size works.

Step 4: Check Against NEC Recommendations

The NEC recommends no more than 3% voltage drop on the branch circuit and no more than 5% total (feeder + branch circuit combined). These aren't hard code requirements — they're informational notes — but most inspectors and engineers treat them as the standard.

Common Mistakes

Forgetting the round-trip: The "2" in the formula is there for a reason. Current travels out and back.
Using the breaker size instead of actual load: If you have a 20A breaker but the actual load is 16A, use 16A for a more accurate calculation.
Ignoring temperature: The K factor changes at higher temperatures. 12.9 for copper is at 75°C. At higher temperatures, resistance increases.
Not accounting for the feeder: Your branch circuit might be fine at 2.5%, but if the feeder already drops 3%, you're over 5% total.

When to Upsize Conductors

If your calculation shows excessive voltage drop, you have two options: upsize the conductor (use a larger wire gauge with more circular mils) or shorten the run (relocate the panel or sub-panel closer to the load). In practice, upsizing the conductor is usually the more practical solution.

On long commercial runs, it's common to go up one or two wire sizes specifically for voltage drop — even when the ampacity of the smaller conductor would be sufficient for the load.

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Frequently asked questions

Is voltage drop actually required by the NEC?

The commonly cited 3 percent and 5 percent figures live in informational notes, which are advisory rather than enforceable. Several specific applications do carry mandatory voltage drop language, notably sensitive equipment and certain fire pump and EV circuits, and job specifications routinely make the percentages contractual. Treat them as a design requirement even where the code text is advisory.

What K value should I use?

About 12.9 for copper and about 21.2 for aluminum. These are approximations at typical conductor operating temperature. If you need precision, use the actual ohms-per-1000-feet values from Chapter 9, Table 8 for your conductor and raceway type.

Do I use one-way or round-trip distance?

Use the one-way distance. The factor of 2 in the single-phase formula, and the 1.732 in the three-phase formula, already account for the current's full path.

Does voltage drop affect required conductor ampacity?

No. Ampacity and voltage drop are separate checks. A conductor can be perfectly legal for ampacity and still drop too much voltage over a long run. Size for ampacity first, then verify the drop, then upsize if needed.

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// DisclaimerThe National Electrical Code® (NEC®) and NFPA 70® are registered trademarks and copyrighted publications of the National Fire Protection Association (NFPA). This article is an independent educational resource and is not affiliated with, endorsed by, or sponsored by the NFPA. All code references in this article are paraphrased for educational purposes only and do not reproduce the official text of any NEC edition. This content is not a substitute for the official NFPA 70 publication. Electricians, contractors, and students are encouraged to purchase the official National Electrical Code from the NFPA at nfpa.org for complete and authoritative code text. Always verify requirements with the code edition adopted by your jurisdiction and your Authority Having Jurisdiction (AHJ).

How to Calculate Voltage Drop — Step by Step