Voltage Drop Calculator - Wire Gauge for Long Runs

Voltage drop and the wire gauge that keeps it in check

A 16 A load at the end of 100 ft of 12 AWG copper sees 113.66 V instead of 120. That is a 5.28% drop, and 12 AWG is the gauge most people would pick for a 20 A circuit. This calculator finds the smallest copper or aluminum gauge that keeps the drop under the limit you choose and still carries the current, or checks a gauge you already own. For that same run it answers 8 AWG, with a drop of 2.07%.

It works for single-phase and three-phase AC and for DC, takes the load in amps or watts and the length in feet or meters, and lists the sizes on either side so you can see what one step up or down would do.

The nine boxes, from supply to limit

What you want - a suggested gauge, or a check of the gauge you already have.
Supply - single-phase AC (a 120 V or 240 V house circuit), three-phase AC (208 V or 480 V commercial), or DC (12 V, 24 V or 48 V battery and solar wiring).
Voltage - the nominal supply voltage, for example 120, 240, 208, 480 or 12.
Load and its unit - the current in amps from the nameplate, or the power in watts.
Power factor - only for a load in watts on AC: 1 for heaters and incandescent lamps, about 0.8 to 0.9 for motors.
Length and its unit - the one-way distance from the panel to the load, not the total length of wire.
Conductor - copper or aluminum.
Voltage drop limit - 3% for a branch circuit is the usual choice, 5% for a less sensitive load, 1% or 2% for low-voltage lighting and electronics.
Read the result - the gauge, the drop in volts and percent, the voltage left at the load, the heat in the wire and the longest run that gauge allows.

Sizing a run in four moves

Under the form, the calculator does the same four things an electrician does on paper. Here they are for a 40 A EV charger circuit, 240 V, 120 ft of copper, 3% limit.

Turn the limit into volts.
3% of 240 V is 7.2 V. That is the whole budget for the wire out and the wire back.

Find the most resistance you can afford.
R = 7.2 x 1,000 / (2 x 40 x 120) = 0.75 ohm per 1,000 ft. The 2 is there because the current travels both ways.

Pick the first gauge under that figure.
8 AWG copper is 0.778, just over. 6 AWG is 0.491, so 6 AWG it is: 4.71 V, or 1.96%.

Check it can carry the current.
6 AWG copper has a usable ampacity of 55 A, above 40 A, so both checks agree. On a shorter run the ampacity check is usually the one that wins.

Copper against aluminum on one 150 ft feeder

The same load, 50 A at 240 V over 150 ft with a 3% limit, sized in both metals. Aluminum needs one size larger here and runs a little warmer, which is the usual trade for a cheaper, lighter cable.

What you get	Copper	Aluminum	Who should choose it
Smallest gauge that passes	4 AWG	3 AWG	Aluminum if the conduit has room for one size up
Resistance, ohm per 1,000 ft	0.308	0.403	From NEC Chapter 9, Table 8
Voltage drop	4.62 V (1.93%)	6.05 V (2.52%)	Copper for motors that start hard
Heat in the wire	231 W	302 W	Copper for loads that run all day
Usable ampacity	70 A	65 A	Both leave headroom over 50 A
Longest run at 3%	233 ft	178 ft	Copper if the load may move farther later
Verdict	Both pass; aluminum costs one gauge		Terminals must be rated AL/CU

The formula, and three runs checked against it

Voltage drop is Ohm's law applied to the wire itself: VD = k x I x R x L / 1,000, with I in amps, R in ohms per 1,000 ft and L the one-way length in feet. On single-phase and DC, k = 2, because the current flows out on one conductor and back on the other. On a balanced three-phase circuit, k = 1.732, the square root of 3.

Workshop outlet, 16 A at 120 V, 100 ft, copper, 3%.
12 AWG: 2 x 16 x 1.98 x 100 / 1,000 = 6.34 V, which is 5.28%. 10 AWG gives 3.31%, still over. 8 AWG: 2.49 V, 2.07%, with 117.51 V left at the tool. Distance decided it.

Three-phase motor feeder, 100 A at 480 V, 300 ft, copper, 3%.
3 AWG would already keep the drop under 3%, but its usable ampacity is 85 A. 1 AWG carries 110 A and drops 1.732 x 100 x 0.154 x 300 / 1,000 = 8.00 V, 1.67%. Current decided it.

12 V DC pump, 10 A, 20 ft from the battery, copper, 3%.
The budget is only 0.36 V. 14 AWG would drop 10.47%. 8 AWG: 2 x 10 x 0.778 x 20 / 1,000 = 0.31 V, 2.59%. Low voltage is where distance hurts most.

How far each copper gauge reaches at 15 A

Longest one-way run before the drop passes 3% with a 15 A load. Doubling the voltage doubles the reach, because the same volts lost become half the percentage. For other currents, scale inversely: twice the amps, half the distance.

Copper gauge	Ohm per 1,000 ft	Usable ampacity	120 V, 15 A	240 V, 15 A
14 AWG	3.14	15 A	38 ft	76 ft
12 AWG	1.98	20 A	60 ft	121 ft
10 AWG	1.24	30 A	96 ft	193 ft
8 AWG	0.778	40 A	154 ft	308 ft
6 AWG	0.491	55 A	244 ft	488 ft
4 AWG	0.308	70 A	389 ft	779 ft
2 AWG	0.194	95 A	618 ft	1,237 ft
1/0 AWG	0.122	150 A	983 ft	1,967 ft

Usable ampacity is the Table 310.16 figure you can actually load the wire to: the 60 °C column up to 1 AWG and the 75 °C column from 1/0, with the 15, 20 and 30 A limits NEC 240.4(D) puts on 14, 12 and 10 AWG copper.

Four mistakes that shrink the wire too far

Entering the total wire length. The length box wants the one-way distance. Typing 200 ft for a run that is 100 ft away doubles the answer a second time, because the formula already counts the return conductor.

Sizing for ampacity alone. 12 AWG is fine for 16 A over 30 ft. Over 100 ft at 120 V it drops 5.28%, and a motor on the end starts slowly and runs hot.

Using the 90 °C column. The insulation may be rated 90 °C, but the breaker and receptacle terminals rarely are. NEC 110.14(C) holds most circuits to the 60 °C or 75 °C figures, which is what the calculator uses.

Forgetting continuous loads. A load that runs three hours or more, such as an EV charger or a water heater, is sized at 125% of its current. Enter 50 A for a 40 A charger if you are sizing for the breaker too.

Voltage drop questions from real runs

Is the 3% voltage drop limit required by code?

No. The 3% for a branch circuit and 5% for feeder plus branch appear as an informational note in NEC 210.19(A), which is advice rather than a rule. Some cities and states adopt them as mandatory amendments, and a few equipment makers require them for the warranty, so check locally.

What size wire for 100 ft at 20 A on 120 V?

At a 3% limit, 8 AWG copper: 2 x 20 x 0.778 x 100 / 1,000 = 3.11 V, or 2.59%. 10 AWG would drop 4.13%. If you want margin, 6 AWG brings it down to 1.96 V, 1.64%, and the table under the result shows both.

Why does 240 V reach so much farther than 120 V?

At the same current the wire loses the same number of volts, but 3% of 240 V is 7.2 V against 3.6 V at 120 V, so the reach doubles. For the same wattage it goes further still, because 240 V draws half the current: a 1,800 W heater on 12 AWG reaches 60 ft at 120 V and 242 ft at 240 V, four times as far.

Does this include the neutral on a three-phase circuit?

On a balanced three-phase load the neutral carries almost no current, so the drop is line to line with the factor 1.732. For a single-phase load connected line to neutral on a three-phase system, choose single-phase and enter the line-to-neutral voltage, for example 120 V or 277 V.

Why is there no 14 AWG aluminum?

Table 310.16 gives no ampacity for 14 AWG aluminum, and building wire in aluminum starts at 12 AWG. In practice aluminum is used for feeders of 6 AWG and larger, where its lower price per ampere pays for the bigger size.

How accurate is it on very large wire?

The calculator uses DC resistance at 75 °C and leaves out reactance. Up to about 1/0 that changes the answer very little. On 250 kcmil and larger in steel conduit, with a motor load at a low power factor, the real drop can be several percent higher, and NEC Chapter 9, Table 9 is the reference to use.

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