Voltage Drop in Electrical Wiring: What It Is and When It Matters

Our voltage drop calculator gives you the drop from conductor DC resistance for a wire gauge, length, and load. Voltage drop is the reduction in voltage that occurs as electrical current flows through a wire. Every wire has resistance, and that resistance converts some of the electrical energy into heat instead of delivering it to your load. On short runs with large wire, the drop is negligible. On long runs with undersized wire, it can cause lights to dim, motors to overheat, and equipment to malfunction.

This guide explains the underlying physics and helps you determine when voltage drop is a problem worth solving.

Why Voltage Drop Matters

Voltage drop does not just waste energy. It affects equipment performance. A motor designed for 120V that receives only 108V (10% drop) draws more current to produce the same torque. That extra current generates heat in the motor windings, shortening the motor's life. A National Electrical Code informational note recommends keeping total voltage drop (feeder plus branch circuit) under 5% for efficiency and proper equipment operation.

The problem is worse on low-voltage systems. A 12V solar system running 20 amps over 30 feet of 12 AWG copper drops 2.37V, 19.75% of the supply voltage. The same 2.37V drop on a 240V circuit is 0.99%. This is why wire sizing for 12V DC systems is far more critical than for household AC. Our wire distance calculator takes the current, the one-way distance, the system voltage and your maximum drop percentage, and returns the recommended copper gauge with its voltage drop.

The Voltage Drop Formula

The calculation is straightforward. Here is how to do it by hand.

1. Identify your variables. You need: current in amps (I), one-way wire length in feet (L), and the wire resistance in ohms per 1,000 feet (R). The resistance depends on the wire gauge (AWG) and material (copper or aluminum).
2. Apply the formula. For DC and single-phase circuits: Voltage Drop = 2 x I x L x R / 1,000. The factor of 2 accounts for the round-trip distance: current flows out on one conductor and returns on the other. For three-phase circuits, replace 2 with the square root of 3, which Adams Electric Cooperative and Encore Wire print rounded to 1.732; the calculator uses the square root of 3 itself.
3. Calculate the percentage. Divide the voltage drop by the source voltage and multiply by 100. A 6V drop on a 120V circuit is 5.0%. A 6V drop on a 12V circuit is 50%, a critical failure.
4. Compare against limits. The NEC recommends no more than 3% drop on a branch circuit and 5% total (feeder plus branch). These are informational recommendations, not mandatory code requirements, but exceeding them leads to real operational problems. For sensitive electronics and LED lighting, a lower target on the branch circuit leaves more margin.

Wire Resistance by AWG Gauge

This table shows the DC resistance of Class B stranded copper and aluminum conductors per 1,000 feet at 75°C from Anixter's Tables 7.2 and 7.4, along with the copper ampacity at a 60°C insulation rating. One cell is not Anixter's print: for 6 AWG aluminum, Anixter's 75°C DC column prints 0.080 beside its AC cells of 0.808, a slip in the print; the table keeps 0.808, which is the calculator's own figure for that wire to three places. The calculator itself takes R as K x 1,000 / circular mils, with K = 12.9 for copper and 21.2 for aluminum as Adams Electric Cooperative prints them (12 AWG copper: 12.9 x 1,000 / 6,530 = 1.9755), so its figures can differ from Anixter's in the third digit; the examples below use the calculator's figures.

The key pattern, by the calculator's circular mils: each step of two gauge numbers in this table (14 to 12, 12 to 10, 10 to 8, 8 to 6, 6 to 4, 2 to 1/0) cuts resistance by 37%, and each step of one gauge number (4 to 3, 3 to 2, 1/0 to 2/0) cuts it by 21%. Going from 12 AWG to 10 AWG cuts the calculator's resistance from 1.9755 to 1.2428 ohms per 1,000 ft, a 37% reduction that cuts the voltage drop by the same share. If you are borderline on voltage drop, jumping one wire size is often the simplest fix. For the relationship between wire gauge, current, and wattage, the HP-to-watts guide covers how motor power ratings translate to the amps that drive voltage drop calculations.

Worked Example: Wiring a Detached Shed

You are running a 20A, 120V circuit to a workshop shed that is 150 feet from your main panel. You plan to use 12 AWG copper wire.

Voltage drop: 2 x 20A x 150ft x 1.9755 / 1,000 = 11.85V. That is 9.88% of 120V, nearly double the 5% total the NEC note recommends. Your tools will underperform and motors may overheat.

Upgrade to 10 AWG: 2 x 20A x 150ft x 1.2428 / 1,000 = 7.46V = 6.21%. Better, but still above 5%.

Upgrade to 8 AWG: 2 x 20A x 150ft x 0.7813 / 1,000 = 4.69V = 3.91%. This is under the 5% total the NEC note recommends, though above its 3% for a branch circuit; 6 AWG drops 2.95V = 2.46%, under 3%. Yes, 8 AWG on a 20A circuit is larger than required by ampacity tables, but voltage drop on long runs demands it. The extra wire cost is the price of a proper installation. Use the amps draw calculator to quickly check the current for any combination of load and voltage.

This is a common real-world scenario. Many electricians have seen sheds, barns, and outbuildings wired with the minimum code-required wire gauge that meets ampacity but fails the voltage drop recommendation because no one checked the distance. Motor loads make the problem worse because full-load amps draw 3-10x more current than lighting or receptacle loads, amplifying the drop.

Copper vs. Aluminum: When Each Makes Sense

Aluminum has 61% of the conductivity of copper by the calculator's constants (K = 12.9 ohm-cmil/ft for copper and 21.2 for aluminum, as Adams Electric Cooperative prints them). For the same voltage drop, aluminum needs 1.64 times the circular mils of copper; two gauge numbers up give 1.59 times (6 AWG at 26,240 cmil to 4 AWG at 41,740 cmil), so on the shed run above 6 AWG copper drops 2.95V and 4 AWG aluminum drops 3.05V.

Aluminum is used for service entrance conductors and large feeder runs, where its lower weight and cost count. For the 150-foot shed run above, 6 AWG aluminum drops 4.85V (4.04%) against 4.69V (3.91%) for 8 AWG copper, but requires AL-rated connectors at every termination point. Many electricians prefer copper for branch circuits and residential work because the connections are simpler and more forgiving.

For 12V and 24V DC systems (solar, battery, automotive), copper is nearly universal. The wire sizes needed for low-voltage high-current runs are already large, and aluminum's additional size increase makes routing and termination impractical in tight spaces. If you are sizing cable between a battery and inverter, our inverter cable size calculator handles the low-voltage sizing math where voltage drop is most critical.

Voltage Drop in Solar and Battery Installations

Low-voltage DC wiring is where voltage drop causes the most trouble. A 48V battery bank feeding a 3,000W inverter draws about 69 amps at full load once inverter losses are counted (3,000 ÷ 48 ÷ 0.90). The 0.90 is an inverter efficiency entered for this example, and the 1.25 used below is the continuous-duty factor. Over a 10-foot cable run in 3 AWG copper (the calculator's 0.2452 ohms/1000ft), the drop is 2 x 69 x 10 x 0.2452 / 1,000 = 0.34V, 0.70% of 48V. Our inverter cable size calculator returns 2 AWG for a 3,000W inverter rating, a 48V battery, a 10-foot one-way cable length, a 3% maximum voltage drop, an inverter efficiency of 0.90, continuous duty on, which sizes cable ampacity at 125% of the current (3,000 ÷ 48 ÷ 0.90 x 1.25 = 86.8A), and its default setting of inverter DC terminals not marked for 75 °C. The 60 °C column of the table above rates 3 AWG at 85A, below that 86.8A; at that rating the smallest row that carries it is 2 AWG (95A). With the terminals marked for 75 °C, it returns 3 AWG, the size the drop above is worked for. Adjust the inputs in the calculator for your unit.

But the same system at 12V draws about 278 amps once inverter losses are counted (3,000 ÷ 12 ÷ 0.90), and continuous-duty sizing at that 1.25 factor asks for 347A of ampacity (3,000 ÷ 12 ÷ 0.90 x 1.25), more than a single 4/0 run carries. Two parallel runs of 2/0 copper over 10 feet, 139 amps in each, drop 2 x 139 x 10 x 0.0969 / 1,000 = 0.27V, 2.25% of 12V. A single 500 kcmil run drops less again, but it is beyond this calculator's list of gauges, which stops at 4/0. The Copper Development Association (page no longer available at time of checking) publishes detailed resistance and ampacity data that engineers use for these calculations.

This is one reason to choose a 24V or 48V battery bank for off-grid solar: higher voltage means lower current, thinner wire, and less voltage drop for the same power delivery. If you are planning cable runs between batteries, charge controllers, and inverters, our inverter cable sizing guide covers the full process including safety derating and temperature correction.