Building / DIY

Voltage Drop Calculator for Copper and Aluminum Wire

This voltage drop calculator estimates voltage drop and percent drop for copper or aluminum wire from AWG gauge, one-way length, load amps, and system volts.

Wire size

Circular mils come from the table below.

One-way length (ft)

Distance from the source to the load, one way. The formula accounts for the return path.

Load (amps)

Current the load draws.

System voltage

120 V and 240 V are common on single-phase branch circuits.

Phase

Single-phaseThree-phase

Single-phase uses a factor of 2. Three-phase uses √3, about 1.732, the same factor as the watts to amps calculator.

Conductor

CopperAluminum

K is 12.9 for copper and 21.2 for aluminum. Aluminum needs a larger size for the same drop.

Circular-mil table

Size	Circular mils
18 AWG	1,620
16 AWG	2,580
14 AWG	4,110
12 AWG	6,530
10 AWG	10,380
8 AWG	16,510
6 AWG	26,240
4 AWG	41,740
3 AWG	52,620
2 AWG	66,360
1 AWG	83,690
1/0 AWG	105,600
2/0 AWG	133,100
3/0 AWG	167,800
4/0 AWG	211,600
250 kcmil	250,000
300 kcmil	300,000
350 kcmil	350,000
400 kcmil	400,000
500 kcmil	500,000

Examples

Each example is single-phase copper. The 12 AWG run is over the 3% guideline. The 10 AWG run is under it.

Voltage drop

7.902 V

6.58% drop · 112.098 V at the load

Above the 3% branch-circuit guideline.

VD = 2 × K × I × L / CM = 2 × 12.9 × 20 × 100 / 6,530 = 7.902 V

This is an estimate only. Follow local code and have a licensed electrician design critical circuits.

How to use

Choose the AWG or kcmil size. Circular mils for that size come from the table on the form.
Enter the one-way length in feet, the load in amps, and the system voltage.
Choose single-phase or three-phase, and copper or aluminum.
Read voltage drop, percent drop, and voltage at the load. A result above 3% is flagged.

Formula

Single-phase voltage drop is VD = 2 × K × I × L / CM. Three-phase voltage drop is VD = √3 × K × I × L / CM, where √3 is about 1.732. K is 12.9 for copper and 21.2 for aluminum. I is load amps, L is one-way length in feet, and CM is circular mils. Percent drop is VD ÷ system volts × 100. Voltage at the load is system volts − VD.

Example. 12 AWG copper (6,530 circular mils), 100 ft one way, 20 A, 120 V, single-phase: VD = 2 × 12.9 × 20 × 100 / 6,530 = 7.902 V, which is 6.58% and leaves 112.098 V at the load.

Example. The same run on three-phase uses √3 instead of 2, so the drop is 6.843 V (5.70%).

Example. The same single-phase run in aluminum (K = 21.2) drops 12.986 V (10.82%).

Branch-circuit guideline

NEC informational notes suggest no more than 3% voltage drop on a branch circuit and 5% total for the feeder plus the branch. Those notes are not a sizing method by themselves. Conductor size also depends on ampacity, temperature, and how the wire is installed. This is an estimate only. Follow local code and have a licensed electrician design critical circuits.

Frequently asked questions

What is the voltage drop formula?

For single-phase: VD = 2 x K x I x L / CM, where K is about 12.9 for copper (21.2 for aluminum), I is amps, L is one-way length in feet, and CM is the wire's circular mils.

How much voltage drop is acceptable?

NEC informational notes suggest no more than 3% on a branch circuit and 5% total for feeder plus branch.

How do I reduce voltage drop?

Use a larger wire (smaller AWG number), shorten the run, raise the voltage, or reduce the load.

Is aluminum wire worse?

Aluminum has higher resistance than copper, so it needs a larger size for the same drop.

Is this a substitute for an electrician?

No. It is an estimate; follow local code and have a licensed electrician design critical circuits.