ParrotNotes

Voltage Drop Calculator

Find the voltage drop and percent drop on a copper or aluminum wire run by AWG size, length, load and phase, against the 3% and 5% guides. Enter the wire, the one-way length, the load and the voltage to get the drop in volts and percent, the voltage at the load, and the smallest size that keeps the drop at 3% or less. Uses NEC resistance values at 75°C. An estimate: a licensed electrician and the local code decide.

Your numbers

Source to load, along the route.

A
V

120, 208, 240, 277, 480; 12, 24 or 48 for DC.

Conductors per phase run in parallel. Usually 1; parallel runs are 1/0 AWG and larger.

Voltage drop

–

Percent drop
–
Voltage at the load
–
3% branch-circuit guide
–
5% feeder plus branch guide
–
Smallest copper size for 3% or less
–
Ampacity of 12 AWG, 75°C column
25 A; breaker limited to 20 A by 240.4(D)
Resistance used
1.98 Ω per 1,000 ft

The 3% branch-circuit and 5% total figures come from Informational Notes in NEC 210.19 and 215.2. They are recommendations for good performance, not code requirements.

An estimate: a licensed electrician and the local code decide.

The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.

How the voltage drop is calculated

The calculator uses the resistance of the wire from NEC Chapter 9, Table 8: the DC resistance of stranded conductors at 75°C in ohms per 1,000 feet. Voltage drop = 2 × current × resistance × one-way length ÷ 1,000 for DC and single-phase circuits, and 1.732 × current × resistance × length ÷ 1,000 for a balanced three-phase circuit. With parallel sets, the resistance is divided by the number of sets. The percent is the drop divided by the source voltage.

Worked example: a 16 A load at 120 V, 100 feet from the panel on 12 AWG copper. 2 × 16 × 1.98 × 100 ÷ 1,000 = 6.34 V, or 5.28%, leaving 113.7 V at the load. That is over the 3% guide. 10 AWG gives 3.31%, still over; 8 AWG gives 2.49 V, or 2.07%, so 8 AWG is the smallest size that meets 3%.

You may have seen the circular-mil form, VD = 2 × K × I × L ÷ CM, with K about 12.9 for copper and 21.2 for aluminum. It is the same formula: K is the resistance per 1,000 feet times the circular mils, divided by 1,000. Using the table resistance for each size is a little more exact than one K for every size.

The 3% and 5% guides are recommendations

NEC 210.19 (branch circuits) and 215.2 (feeders) carry Informational Notes recommending a voltage drop of no more than 3% on the branch circuit, or on the feeder, and no more than 5% on the feeder and branch circuit combined. On a 120 V circuit, 3% is 3.6 V; on 240 V it is 7.2 V; on 480 V it is 14.4 V.

Informational Notes explain; they do not require. A design that drops 4% on a branch circuit is not a code violation by itself. It can still be a problem: motors run hotter and start harder at low voltage, LED drivers and electronics have a minimum input voltage, and an EV charger or heater delivers less power. Some specifications, local amendments and equipment instructions do set a limit, and where they do, that limit applies.

What the number does and does not include

Table 8 resistance is for direct current at a conductor temperature of 75°C, the usual basis for voltage drop. For AC circuits up to about 4/0 AWG the result is close. On large conductors at a low power factor, inductive reactance adds to the drop, and NEC Chapter 9, Table 9 gives the AC impedance for steel and PVC conduit; an engineer's calculation will then show a somewhat higher figure.

The drop shown is at full load. A conductor running cooler than 75°C has a little less resistance, so the real drop is usually slightly lower. Voltage drop is not the same check as ampacity: a wire can meet 3% and still be too small to carry the current. The Wire Size Calculator checks both.

For electricians and solar installers on a job walk

The run lengths, loads and panel locations that decide the wire size come up on the walk-through: 140 feet to the detached garage, a 48 A charger, a subpanel in the barn, the array on the far roof. They change as the customer changes their mind, and the quote has to follow.

Run each circuit here while you are still on site and note the size that meets the drop. A long run is often where a quote is won or lost, because the wire for it can cost more than the labor.

An estimate: a licensed electrician and the local code decide. Ambient temperature, conductors bundled in one raceway, terminal ratings and the equipment's instructions all affect the final size.

Sources, read on 2026-10-23:

Need the size for a new circuit? The wire size calculator checks ampacity and voltage drop together. Sizing the load first? Try the BTU calculator for heating and cooling, or the solar calculator for an array.

More for electrical and solar trades: Solar sales, Electrical distributor sales, Construction daily report, HVAC sales.

The run length gets said on site. Keep it.

On a job walk the numbers are said out loud: 140 feet to the detached garage, a 48 A charger, which panel has room. Record the walk-through in ParrotNotes and get a structured note, a summary and the action items, so the quote uses the run you measured. Free for 100 minutes of recording a month.

Get Started Free

Related tools

Voltage drop calculator FAQ