Voltage Drop Calculator

How to Use This Voltage Drop Calculator

Enter the system voltage, the circuit type and the load current. Then pick the conductor material and wire size, and measure the one way distance from the panel to the load. The calculator returns the drop in volts, the same number as a percentage, the voltage the load actually sees, and the smallest wire size that would meet your target.

The default example is a 240 volt single phase circuit carrying 40 amps through 100 feet of 8 AWG copper. That conductor has 0.778 ohms per 1,000 feet, so 100 feet is 0.0778 ohms. The round trip doubles it to 0.1556 ohms. At 40 amps the loss is 6.22 volts, or 2.59% of 240 volts. The load sees 233.8 volts, and the wire gives off about 249 watts of heat while the circuit runs.

Two more outputs are worth a look. The maximum run tells you how far the same wire and current could go before crossing your target, which is useful when planning a new circuit. The ampacity card gives the 75°C rating from NEC Table 310.16, so you can see at a glance whether the conductor is legally large enough for the current, separate from the voltage drop question.

If the drop percentage is over target, raise the wire size one step and recalculate. If the ampacity is below the load, the wire is illegal regardless of how small the drop looks. Both conditions have to pass.

The Voltage Drop Formula, in One Line

Voltage drop follows Ohm's law applied to the conductor. The wire is a resistor in series with the load, and the voltage across that resistor is the drop.

Single phase: Vdrop = 2 × I × R × L / 1000

Three phase: Vdrop = 1.732 × I × R × L / 1000

I is the current in amps. R is the conductor resistance in ohms per 1,000 feet, taken from NEC Chapter 9 Table 8 at 75°C. L is the one way length in feet. The factor of 2 in the single phase formula accounts for the two current carrying conductors, one out and one back. In a balanced three phase circuit the factor becomes the square root of 3, because the three currents add as phasors rather than as simple sums.

Divide the result by the system voltage and multiply by 100 to get the percentage. That percentage is the number the 3% rule is written against, so the calculator shows both forms.

The resistance in Table 8 is a direct current value. Alternating current adds a small skin effect and reactance term, which matters most on large feeders and at high current. For the branch circuits and small feeders a homeowner is likely to run, the difference is small enough that the direct current table is the standard field method. An engineer sizing a 400 amp service would use the alternating current impedance values instead.

What the 3% and 5% Numbers Actually Mean

NEC 210.19(A) and 215.2(A) carry informational notes that recommend a maximum voltage drop of 3% on a branch circuit and 5% from the service point to the farthest outlet. Those notes are guidance, not a mandatory rule in a typical dwelling. Local amendments and some energy codes turn them into requirements, and many inspectors still expect a design to respect them.

The split matters. A 3% branch circuit fed from a feeder that already loses 4% puts the total at 7%, well past the 5% target. That is why a subpanel feeder deserves its own calculation, and why the total is what the equipment actually experiences.

Three percent of 120 volts is 3.6 volts. Three percent of 240 volts is 7.2 volts. The same percentage is a different number of volts, which is why a circuit that passes at 240 volts can fail at 120 volts on the same wire and current. The formula does not change; only the voltage in the denominator does.

Voltage drop is also not the only limit. A conductor has to carry the current without overheating, which is the ampacity question. It has to be protected by the right breaker, which is the overcurrent question. And it has to be installed with the correct insulation and support. Meeting 3% is one check among several, not a substitute for the others.

Wire Size, Resistance and Drop at a Glance

The table below uses copper conductors at 75°C, a 240 volt single phase circuit and a 20 amp load. Drop is shown for a 100 foot one way run. The last two columns give the ampacity from Table 310.16 and the longest run that stays inside 3% at that current.

The 14, 12 and 10 AWG ampacities in the table are the Table 310.16 values, but NEC 240.4(D) limits the breaker on small conductors to 15, 20 and 30 amps respectively for copper. A 14 AWG conductor is not protected above 15 amps in a typical branch circuit even though its table rating is 20 amps. The voltage drop math is unaffected; only the overcurrent protection changes.

The maximum run column assumes the current stays at 20 amps. At a higher current the same wire reaches 3% sooner, in direct proportion. Double the current and the allowable run is cut in half.

Copper and Aluminum: The Same Job at Different Sizes

Aluminum has roughly 1.6 times the resistance of copper for a given size. An 8 AWG aluminum conductor is 1.26 ohms per 1,000 feet against 0.778 for copper. Run the same current over the same distance and the aluminum wire loses about 60% more volts and heat.

That does not make aluminum a bad choice. It costs less and weighs much less, which matters on long service drops and large feeders where the copper equivalent would be heavy and expensive. The usual practice is to go two sizes larger than the copper selection. An 8 AWG copper feeder becomes 6 AWG aluminum, and the resistances end up close enough that the drop is similar.

Small aluminum branch circuits are uncommon, and most electricians avoid them below 10 AWG. Aluminum also needs connectors rated for the metal and a torque wrench on the terminations. Loose aluminum connections are the classic cause of a warm receptacle, and a warm connection adds resistance exactly where the calculation assumes none.

Whatever the material, the calculator uses the 75°C resistance column. That matches the temperature rating of most modern THHN, XHHW and SER cable. Older conductors with 60°C insulation have slightly higher resistance and a lower ampacity, so a circuit built with them may measure worse than the calculator predicts.

Where Voltage Drop Matters Most

Short lighting circuits tolerate drop. The loads that care are the ones with motors, compressors or a continuous draw.

• EV chargers. A Level 2 charger pulls 32 to 48 amps for hours. A 60 amp circuit run to a detached garage 120 feet away is a textbook case for a wire size jump. The EV charging cost calculator prices the energy, and this page sizes the circuit that delivers it.
• Well pumps and sump pumps. A pump motor that sees low voltage draws more current, runs hotter and may fail to start. Long runs to a well head are common, and the starting current is several times the running current.
• Air conditioners and heat pumps. The compressor is the biggest motor in most homes. Undersized wire on the condenser circuit shortens compressor life, and the HVAC BTU calculator sets the load this circuit has to carry.
• Subpanel feeders. A garage or shop subpanel carries every load in the building. The feeder has to meet the 3% target on its own, because the branch circuits downstream add their own drop.
• Workshops and welders. A welder draws a large intermittent current, and a long run to a detached shop is where a 10 AWG extension cord becomes a fire risk.

A 120 volt circuit is the most sensitive case. Losing 7 volts is 5.8% at 120 volts and only 2.9% at 240 volts. If a circuit is marginal at 240, it is probably failing at 120.

How to Fix a Circuit That Fails the Check

The order of fixes runs from cheapest to most expensive.

1. Check the terminations first. A loose lug or a backstabbed receptacle adds resistance that no wire size table accounts for. Turn the power off, inspect the connections, and torque them to the manufacturer's spec. A warm cover plate is a sign.
2. Shorten the run. Moving the load closer, or moving the panel, can cut more drop than a wire size jump. A 100 foot run cut to 60 feet loses 40% less voltage on the same conductor.
3. Reduce the load. A smaller charger or a lower current setting on an EV unit may bring the circuit inside the target without new cable. Many chargers have a dial or an app setting for exactly this reason.
4. Increase the wire size. This is the reliable fix, and the calculator's recommended size tells you where to land. Going from 10 AWG to 8 AWG cuts resistance by about 37%. Going to 6 AWG cuts it by 60%.
5. Parallel conductors or a transformer. These are engineering solutions for large feeders and long services, not for a garage circuit. A licensed electrician or engineer should design them.

One fix that does not work is a larger breaker. The breaker protects the wire, and raising it to match a load the wire cannot carry is how fires start. Fix the conductor, not the protection.

Common Mistakes When Sizing for Voltage Drop

• Using the round trip distance. The formula's factor of 2 already counts the return conductor. Enter the one way length, or the calculated drop doubles.
• Comparing percentages across different voltages. A 5% drop at 240 volts is 12 volts; at 120 volts it is 6 volts. The percentage is what the rule uses, and the volts are what the equipment feels.
• Ignoring the feeder. A 3% branch on top of a 4% feeder is a 7% total. Calculate the feeder first, then the branch, and keep the sum under 5%.
• Forgetting the starting current. A motor that draws 10 amps running may draw 60 amps at startup. The drop during that instant is six times larger, which is why long pump circuits sometimes fail to start even though the running numbers look fine.
• Assuming the table value is the installed value. Terminations, splices, conduit fill and ambient temperature all change the real resistance and ampacity. The table is a design starting point, not a measurement.
• Choosing wire by ampacity alone. A conductor can be legal for the current and still lose too much voltage over a long run. Both checks are needed, which is why this page shows them side by side.

Worked Example: A Detached Garage Charger

A homeowner wants a 48 amp Level 2 charger in a detached garage 110 feet from the main panel. The charger needs a 60 amp circuit. The plan is 6 AWG copper, which has 0.491 ohms per 1,000 feet.

The one way resistance is 0.491 times 0.110, or 0.054 ohms. The round trip is 0.108 ohms. At 48 amps the drop is 48 times 0.108, or 5.19 volts. That is 2.16% of 240 volts, comfortably inside the 3% target. The load sees 234.8 volts, the wire sheds about 249 watts during a charge, and 6 AWG copper at 65 amps of 75°C ampacity covers the 48 amp continuous load with the 125% factor the code requires for EV charging.

Now change one variable. Suppose the run is 200 feet instead. The same 6 AWG wire now drops 9.4 volts, or 3.93%, and fails the 3% target. The calculator recommends 4 AWG, which brings it back to about 2.5%. That is the kind of decision the tool is built for: the wire that works at 110 feet is not the wire that works at 200 feet, even with the same charger and the same panel.

Before buying cable, confirm the panel has capacity for the new 60 amp circuit. The electrical load calculator runs the NEC 220 service calculation, and the generator size calculator covers what happens to that charger during an outage.