Calculators · Voltage Drop Calculator

Electrical

Voltage Drop Calculator

Enter the voltage, current, length and copper wire size. Get the voltage drop in volts and percent.

Type of circuit

DC and single-phase use two wires. Three-phase uses the line-to-line voltage.

Supply voltage (V)

For example 12, 120, 240 or 480.

Load current (A)

Amps the circuit carries.

One-way length

From the source to the load, not the round trip.

Copper wire size

AWG or kcmil.

Conductor type

Stranded adds 2 percent to the resistance.

Conductor temperature

Resistance rises with heat. 75 °C is a cautious choice.

Target voltage drop (%)

Used to find the smallest size that meets it.

How to use the voltage drop calculator

Choose the type of circuit and type the supply voltage, the load current and the one-way length. Pick the copper wire size and the conductor temperature. The calculator gives the drop in volts and in percent, the voltage left at the load, and the resistance it used. It also shows the smallest listed size that stays within your target drop, by voltage drop alone. Use the length from the source to the load, because the formula already counts the return path.

The voltage drop formula

Voltage drop is the current times the resistance of the wire. For a DC or single-phase circuit the current flows out and back, so the length counts twice. For a balanced three-phase circuit the factor is the square root of three. This is the form used in Southwire's voltage drop guide (checked October 2026), where the resistance comes from a material constant and the circular mils of the wire.

Circuit	Voltage drop in volts	Percent drop
DC or single-phase	VD = 2 × I × R × L ÷ 1,000	VD ÷ V × 100
Three-phase (balanced)	VD = √3 × I × R × L ÷ 1,000	VD ÷ V × 100

I is the current in amps, R is the resistance in ohms per 1,000 feet, and L is the one-way length in feet. V is the supply voltage, which for three-phase is the line-to-line voltage.

Where the resistance comes from

The calculator does not use a pasted table. It works the resistance out from the properties of copper published in the NBS Handbook 100, Copper Wire Tables (a U.S. government publication, checked October 2026). The Handbook gives annealed copper a resistivity of 10.371 ohm-circular mils per foot at 20 °C and a temperature coefficient of 0.00393 per °C. It lists the area of each AWG size in circular mils, and it says stranded conductors have a resistance 2 to 5 percent above the solid equivalent, with 2 percent adopted up to 2,000,000 circular mils. So:

Solid resistance at 20 °C = 10.371 × 1,000 ÷ circular mils, in ohms per 1,000 ft
Stranded = solid × 1.02
At temperature t = resistance at 20 °C × [1 + 0.00393 × (t − 20)]

Our results agree with the stranded-copper values in the Handbook's own table 17 (8 AWG and larger) to within about 0.5 percent, which is the rounding of the Handbook's three-digit values. Smaller stranded sizes use the same 2 percent rule. Here is what the calculator uses for stranded copper at 75 °C and at 25 °C.

Size	Circular mils	Ω per 1,000 ft at 75 °C	Ω per 1,000 ft at 25 °C
18 AWG	1,620	7.9413	6.6582
16 AWG	2,580	4.9864	4.1807
14 AWG	4,110	3.1302	2.6244
12 AWG	6,530	1.9701	1.6518
10 AWG	10,380	1.2394	1.0391
8 AWG	16,510	0.7792	0.6533
6 AWG	26,240	0.4903	0.4111
4 AWG	41,740	0.3082	0.2584
2 AWG	66,360	0.1939	0.1625
1/0 AWG	105,600	0.1218	0.1021
4/0 AWG	211,600	0.0608	0.0510
500 kcmil	500,000	0.0257	0.0216
1000 kcmil	1,000,000	0.0129	0.0108

Example: a 20 amp load 100 feet away on 120 volts

A 20 amp single-phase load sits 100 feet from the panel, on a 120 volt circuit. The wire is stranded copper at 75 °C. The table shows how the size changes the drop. The target is 3 percent, which is 3.6 volts.

Size	Operation	Drop	Percent	At the load
14 AWG	2 × 20 × 3.1302 × 100 ÷ 1,000	12.52 V	10.43 %	107.48 V
12 AWG	2 × 20 × 1.9701 × 100 ÷ 1,000	7.88 V	6.57 %	112.12 V
10 AWG	2 × 20 × 1.2394 × 100 ÷ 1,000	4.96 V	4.13 %	115.04 V
8 AWG	2 × 20 × 0.7792 × 100 ÷ 1,000	3.12 V	2.60 %	116.88 V
6 AWG	2 × 20 × 0.4903 × 100 ÷ 1,000	1.96 V	1.63 %	118.04 V

Only 8 AWG and larger stay under 3 percent in this run, so the calculator names 8 AWG as the smallest size by voltage drop. That says nothing about whether the wire may be used for a 20 amp circuit. Ampacity and overcurrent protection are a separate check done under the electrical code.

Example: a three-phase feeder on 208 volts

A balanced three-phase load draws 100 amps at 208 volts line to line, 150 feet from the source. The wire is stranded copper at 75 °C. With 1/0 AWG the drop is √3 × 100 × 0.1218 × 150 ÷ 1,000 = 3.17 volts, or 1.52 percent. With 3 AWG it is 6.35 volts (3.05 percent), and with 3/0 AWG it is 1.99 volts (0.96 percent).

What target drop should I use?

There is no single number. A 3 percent target is common and is the figure the U.S. Department of Energy cites in a motor systems tip sheet (checked October 2026). Southwire's guide shows 3 and 5 percent targets in the context of the Canadian code. Sensitive equipment, motors and low-voltage lighting often need less, because a few volts are a large share of a small supply voltage. Check your local code, the equipment data and the job specification.

What this calculator does not do

Ampacity. It does not tell you whether a wire can carry the current safely.
Reactance and skin effect. It uses DC resistance only. Southwire's guide notes that long runs, large conductors and motor loads call for an impedance method.
Aluminum. No open source we could check was available, so it is not included.
Flexible cords and fine-stranded cable. Their resistance is higher than the 2 percent used here. Use the maker's data.
Motor starting and voltage at the panel. It works from the supply voltage you type.

To turn watts into amps first, use the watts to amps calculator. For the relation between volts, amps and ohms, see the Ohm's law calculator.

Frequently asked questions

How do you calculate voltage drop?

Multiply the current by the resistance of the wire, and count the wire both ways. For a DC or single-phase circuit, the drop in volts is 2 × amps × ohms per 1,000 ft × length in feet ÷ 1,000, where the length is the one-way distance. For a balanced three-phase circuit, replace the 2 with √3 (1.732). Divide the drop by the supply voltage to get the percentage.

How much voltage drop is there in 100 feet?

It depends on the wire size and the current. For 20 amps over 100 feet one way on 12 AWG stranded copper at 75 °C, the drop is 7.88 volts. On a 120 volt circuit that is 6.57 percent. The same run in 8 AWG drops 3.12 volts (2.60 percent) and in 10 AWG 4.96 volts (4.13 percent).

What is the rule of thumb for voltage drop?

A common target is 3 percent. The U.S. Department of Energy cites a 3 percent limit in its motor systems tip sheet, and Southwire shows 3 and 5 percent targets in its voltage drop guide, which is written for the Canadian code. This calculator starts at 3 percent and lets you change it. Your local code, the equipment maker and the project specification decide what applies to your job.

What is the voltage drop on 18 gauge wire?

Stranded 18 AWG copper has about 6.66 ohms per 1,000 feet at 25 °C. Carrying 1 amp over 50 feet one way (100 feet of wire out and back), it drops 0.67 volts. On a 12 volt circuit that is 5.55 percent, which is why low-voltage runs need thicker wire than most people expect. Enter your own current and length above.

Does this calculator work for three-phase circuits?

Yes. Choose three-phase and type the line-to-line voltage, such as 208 or 480. The calculator uses the √3 factor and assumes the three phases are balanced. For a load wired line to neutral, use the single-phase setting with the line-to-neutral voltage.

Can I use it for aluminum wire?

No. The calculator covers copper only. We did not find an open source for the resistance of aluminum building wire that we could check, so we left it out instead of guessing. For aluminum, use the resistance in the conductor maker's data sheet and the same formula.

Does the smallest size shown mean the wire is safe to use?

No. The smallest size is the first one whose voltage drop falls within your target, and nothing else. It does not check whether the wire can carry the current without overheating (ampacity), and it does not consider the breaker, the insulation or the installation conditions. Those depend on the National Electrical Code and your local rules, and a licensed electrician must confirm them.