Voltage Drop Calculator: Size Wire Right Every Time

Getting wire size wrong costs money — or worse, causes flickering lights, tripped breakers, and equipment damage. A voltage drop calculator is the single most practical tool you can use before pulling a single foot of wire, whether you’re running a 20-amp branch circuit in a kitchen remodel or feeding a 200-amp subpanel in a detached shop 150 feet from the main panel. This guide breaks down exactly how voltage drop works, what the NEC says about it, and how to use real numbers to make confident decisions on every project.

What Is Voltage Drop and Why Does It Matter?

Voltage drop is the reduction in voltage that occurs as electrical current travels through a conductor. Every wire has resistance, and that resistance converts a small portion of the electrical energy into heat instead of delivering it to the load. The longer the wire run and the smaller the wire gauge, the more voltage you lose.

Here’s why that matters in practical terms:

• Motors run hot and inefficiently. A 240V well pump receiving only 222V draws more amperage to compensate, shortening its lifespan significantly.
• LED drivers and electronics malfunction. Sensitive controls, VFDs, and smart-home devices can behave unpredictably when supply voltage sags below their rated input range.
• You waste energy. Every volt lost in the wire is energy you’re paying for but never using. On a 100-amp feeder with excessive drop, that waste adds up to hundreds of dollars per year.
• Lighting dims noticeably. Even a 5% drop on a 120V lighting circuit means luminaires receive only 114V — visible to the naked eye and unacceptable in commercial work.

NEC Recommendations for Voltage Drop

The National Electrical Code doesn’t mandate a maximum voltage drop as a hard rule, but NEC 210.19(A) Informational Note No. 4 and 215.2(A) Informational Note No. 2 recommend the following:

• 3% maximum for the longest branch circuit
• 5% maximum total from the service entrance to the farthest outlet (combining feeder and branch circuit drop)

On a 120V circuit, 3% equals just 3.6 volts. On a 240V circuit, 3% is 7.2 volts. These are tight margins, and exceeding them is the number-one reason inspectors flag wire sizing on long runs.

The Voltage Drop Formula Explained with Real Numbers

The standard single-phase voltage drop formula is:

VD = (2 × L × I × R) ÷ 1000

• VD = voltage drop in volts
• L = one-way length of the conductor in feet
• I = load current in amps
• R = resistance of the conductor per 1,000 feet (from NEC Chapter 9, Table 8)

For three-phase circuits, replace the “2” with 1.732 (√3).

Example 1: 120V, 20A Circuit at 100 Feet

Suppose you’re wiring a garage workshop with a dedicated 20-amp, 120V circuit. The panel is 100 feet away. You’re considering 12 AWG copper (THHN), which has a resistance of 1.93 ohms per 1,000 feet.

VD = (2 × 100 × 20 × 1.93) ÷ 1000 = 7.72V

Percentage drop: 7.72 ÷ 120 = 6.43% — well over the 3% recommendation.

The fix: upsize to 10 AWG copper (1.21 ohms/1,000 ft).

VD = (2 × 100 × 20 × 1.21) ÷ 1000 = 4.84V → 4.03%

Still above 3%. For strict compliance, you’d go to 8 AWG (0.764 ohms/1,000 ft):

VD = (2 × 100 × 20 × 0.764) ÷ 1000 = 3.06V �� 2.55%

That’s within spec. Yes, 8 AWG on a 20-amp circuit sounds oversized — until you realize 100 feet is a genuinely long residential run and the NEC recommendation exists for good reason.

Example 2: 240V, 50A Feeder to a Detached Workshop at 200 Feet

You’re running a subpanel feeder: 240V, 50 amps continuous, 200 feet one-way. Start with 6 AWG copper (0.491 ohms/1,000 ft):

VD = (2 × 200 × 50 × 0.491) ÷ 1000 = 9.82V → 4.09%

That feeder alone eats 4.09%, leaving almost no room for branch circuit drop before you hit 5% total. Upsizing to 4 AWG copper (0.308 ohms/1,000 ft):

VD = (2 × 200 × 50 × 0.308) ÷ 1000 = 6.16V → 2.57%

Now you have roughly 2.4% of headroom for branch circuits in the shop — a much safer design.

Common Mistakes When Calculating Voltage Drop

• Forgetting to double the distance. Current flows out on the hot conductor and returns on the neutral (or second hot in 240V). The “2” in the formula accounts for this round-trip distance. Omitting it cuts your calculated drop in half and gives you a dangerously optimistic number.
• Using ampacity tables instead of voltage drop calculations. NEC ampacity tables (Table 310.16) tell you the maximum safe current for a wire gauge based on heat. They say nothing about voltage drop. A 12 AWG wire is rated for 20 amps at any distance per the ampacity table, but as Example 1 showed, it fails on voltage drop at 100 feet.
• Ignoring conduit fill and temperature derating. When you upsize wire for voltage drop, verify that the larger conductors still fit your conduit per NEC Chapter 9 tables, and recheck ampacity after applying any temperature correction factors from Table 310.15(B)(1).
• Assuming aluminum performs the same as copper. Aluminum conductors have roughly 1.6 times the resistance of copper at the same gauge. If you’re using aluminum (common for large feeders to save cost), you typically need to go up two wire sizes compared to copper to achieve the same voltage drop.

When to Use a Voltage Drop Calculator Instead of Manual Math

The formula is straightforward for simple single-phase circuits, but real-world jobs get complicated fast. Consider these scenarios:

• Three-phase feeders with mixed resistive and inductive loads where power factor affects actual drop
• Parallel conductor runs where you must calculate per-conductor current sharing
• Multiple tap points along a single run where each segment carries a different current
• Comparing copper vs. aluminum costs at multiple gauges simultaneously

In each of these cases, a dedicated calculator eliminates arithmetic errors and lets you compare options in seconds rather than minutes. Contractors who bid competitively know that accurate wire sizing prevents costly change orders — oversizing wastes material budget, and undersizing fails inspection.

Quick Reference: Maximum One-Way Distance Before 3% Drop (120V, Copper)

• 14 AWG at 15A: ~37 feet
• 12 AWG at 20A: ~47 feet
• 10 AWG at 30A: ~39 feet
• 8 AWG at 40A: ~37 feet
• 6 AWG at 55A: ~33 feet

These distances are shorter than most people expect. If your run exceeds these figures, you need to upsize — and a voltage drop calculator will tell you exactly how far to go.

Size Your Wire Correctly — Use Our Free Voltage Drop Calculator

Stop guessing and stop risking failed inspections. The free voltage drop calculator at electricalcalcpro.com lets you plug in your voltage, phase, load current, wire gauge, conductor material, and run length to get instant results with percentage drop and a pass/fail indicator against the NEC 3% and 5% recommendations. Try it now before your next pull — it takes ten seconds and could save you a trip back to the supply house.