Voltage Drop Calculator

Voltage Drop Calculator for copper or aluminum wire on DC, single phase or 3-phase circuits. It gives volts lost, percent drop, voltage at the load and the wire size that meets 3%.

What the Voltage Drop Calculator Works Out

The Voltage Drop Calculator takes a wire size, a voltage, a load current and a one-way distance. It returns the volts lost in the wire, that loss as a percent, and the voltage left at the load. The return conductor is already counted, so the distance is panel to load, not there and back.

The default circuit is a common one. A 15 A load on 12 AWG copper, 100 ft from a 120 V panel, loses 5.19 V. That is 4.32%, and the load receives 114.81 V. The wire turns 90 W into heat for as long as the load runs.

The result cards go past that single number. One gives the longest run that stays at 3%, which is 69.4 ft here, and the largest load the 100 ft run can carry at 3%, which is 10.4 A. Another names the wire size that solves it, 10 AWG at 3.14 V and 2.62%.

Voltage Drop Formula

How do you calculate a voltage drop? Multiply the current by the resistance of the whole circuit. Wire tables list resistance per 1,000 ft, and a single phase or DC circuit has two conductors, one out and one back. That gives the standard formula.

$$V_{drop}=\frac{2\times I\times R\times L}{1000}$$

I is the load current in amps, R is ohms per 1,000 ft and L is the one-way length in feet. Percent drop is the result divided by the source voltage, times 100. The line under the main result in the Voltage Drop Calculator prints this formula with your own numbers filled in.

Mike Holt works two examples in his voltage drop calculations article. A 16 A load on 12 AWG at 100 ft drops 6.4 V, and a 44 A load on 6 AWG at 160 ft drops 6.9 V. With Power Factor set to 1, the Voltage Drop Calculator returns 6.40 V and 6.90 V for the same two circuits.

Holt also notes that the current in the formula is the real load at 100 percent. The 125 percent figure used to size conductors for motors and continuous loads does not belong in a voltage drop check, so enter what the load draws.

How to calculate 3 phase voltage drop

A three phase circuit swaps the 2 for the square root of 3, which is 1.732. The voltage you enter is line to line, and the drop that comes back is line to line as well. Everything else in the formula stays the same.

$$V_{drop}=\frac{1.732\times I\times R\times L}{1000}$$

Holt's three phase example is a 100 A, 208 V load on 1 AWG aluminum, 80 ft from the panel, and his answer is 3.5 V. Selecting AC 3-Phase and aluminum in the Voltage Drop Calculator, with a power factor of 1, gives 3.46 V, which is 1.67% of the supply.

Why Two Calculators Disagree

Run the same 12 AWG circuit through several tools and the answers differ by more than a volt. The formula is the same everywhere. The resistance figure is not, and the Resistance Data setting in this Voltage Drop Calculator shows each version side by side.

NEC data reads Chapter 9 of the code. DC uses Table 8, the resistance of stranded uncoated copper or aluminum at 75 °C. AC uses Table 9, which adds reactance for three conductors in PVC, aluminum or steel conduit.

On AC the Voltage Drop Calculator blends resistance and reactance with the effective impedance formula given in Note 2 of Table 9. The result is the ohms figure shown on the first card, 1.728 for the default circuit.

$$Z=R\times PF+X\times \sin (\arccos (PF))$$

Table 9 prints its own effective impedance column at 0.85 power factor, where 12 AWG lists 1.7 ohms. A heater or other resistive load runs at a power factor of 1, so R alone applies and the same wire reads 2.0. Set Power Factor to 1 for those loads, or the result comes out low.

Large conductors flip that effect. A 300 A, 480 V three phase feeder on 500 kcmil copper in steel conduit, 400 ft long, drops 6.03 V at a power factor of 1. At 0.85 it drops 10.38 V, because reactance now outweighs resistance.

Estimated From Wire Size computes resistance from the resistivity of the metal at 20 °C, the method several simple online tools use. Wire in service runs warmer, so this setting reads about 20% under the NEC figure for copper. It covers what the NEC tables leave out, such as 16 AWG and 18 AWG down to 40 AWG, plus silver and gold. The Resistivity Calculator and Wire Resistance Calculator show that math on its own.

Voltage Drop per 100 Feet

How much voltage drop there is in 100 feet depends on the wire and the load, so no single number answers it. The table shows a 15 A load at 120 V on NEC data at 0.85 power factor. Drop grows in a straight line with distance, so 200 feet is double and 300 feet is triple.

Only 10 AWG stays under 3% at 100 ft, and none of the three does at 200 ft. On 12 AWG the loss works out to about 0.35 V for every amp of load per 100 ft, so a full 20 A load loses 6.91 V over that distance.

The chart under the result cards of the Voltage Drop Calculator runs this comparison at your own load and distance. It lists every size from 14 AWG to 1000 kcmil with its drop, its percent and the longest run that size can make at the limit.

3% or 5%: Which Limit Applies

The NEC recommends holding a branch circuit to 3%, and the feeder plus the branch circuit together to 5%. A feeder checked on its own also gets the 3% figure. The Voltage Drop Limit setting switches between the two, and the fourth card shows the volts allowed and how far over or under the circuit sits.

Use 3% when you are checking one branch circuit or one feeder. Use 5% when the distance covers the whole path from the service to the last outlet. Electricians on the Mike Holt forum put it as 3% on either part and 5% overall.

These figures sit in informational notes to NEC 210.19 and 215.2, so they are recommendations and not enforceable limits. Holt points to the rule that does bind, which is that equipment has to run within its rated voltage. A few circuits carry mandatory limits, among them fire pumps and the 60/120 V systems that feed sensitive audio and video equipment.

Is 4% voltage drop acceptable?

A 4% drop on a branch circuit is over the 3% recommendation, but it still fits the 5% total when the feeder ahead of it loses 1% or less. The default circuit in the Voltage Drop Calculator sits at 4.32%, and the card reads Over the 3% Limit by 1.59 V. Switch the limit to 5% and the same circuit shows 0.81 V of room.

Is 6% voltage drop OK?

A 6% drop is past both NEC recommendations. It breaks no rule by itself, but the load on a 120 V circuit now sees 112.8 V, and motors or electronics may be outside their rated range. Holt says his own 5.3% example is acceptable only if the load can run on the 113.6 V it receives.

How much voltage drop is acceptable at 220 V or 240 V?

The percentages are the same at any voltage, so the allowed volts scale with the supply. At 240 V the 3% limit is 7.2 V and the 5% limit is 12 V. At 220 V they are 6.6 V and 11 V. A 240 V load also draws half the current of a 120 V load of the same wattage, which is why these circuits reach much farther.

How to Fix Voltage Drop

Voltage drop is caused by the resistance of the wire, plus its reactance on AC. It rises with load current and run length and falls as the conductor gets larger. Those are the levers, and the Voltage Drop Calculator lets you test each one before buying wire.

Does thicker wire reduce voltage drop? Each step up in size cuts resistance. Going from 12 AWG to 10 AWG takes the NEC resistance from 2.0 to 1.2 ohms per 1,000 ft, and the default drop falls from 5.19 V to 3.14 V. The third card names the first size that gets under the limit, so the answer is ready when a wire fails.

Parallel conductors divide the drop by the number of sets, and a shorter route lowers it in proportion. Aluminum needs about two sizes more than copper. On a 100 A, 240 V feeder 150 ft long, 3 AWG copper drops 7.12 V and 1 AWG aluminum drops 7.10 V.

How to size wire for voltage drop

Size the wire for ampacity first, then check the drop. The size named here covers voltage drop only, and the conductor must still carry the load under NEC Table 310.16. The Wire Ampacity Calculator and Wire Size Calculator handle that side, and whichever check asks for the larger wire decides.

One code rule follows from upsizing. When circuit conductors are increased to offset voltage drop, NEC 250.122(B) requires the equipment grounding conductor to be increased as well, a point Holt lists among the binding rules.

Low Voltage, DC and 2.5 mm Cable

Low voltage circuits are where drop does the most damage, because each lost volt is a large share of the supply. A 12 V DC load drawing 10 A through 25 ft of 12 AWG loses 0.99 V, which is 8.25%. Choosing DC under Phase in the Voltage Drop Calculator and stepping up the size shows 6 AWG is the first one under 3%, at 0.25 V.

What is the voltage drop for 2.5 mm cable?

Metric cable goes into the Voltage Drop Calculator through Custom Resistance. IEC 60228 sets the maximum resistance of a 2.5 mm² copper conductor at 7.41 ohms per kilometer at 20 °C, a figure cable makers such as Bayka publish. Enter 7.41, pick ohms per kilometer, and switch the distance unit to meters.

A 16 A load at 230 V over 20 m of 2.5 mm² cable then drops 4.74 V, or 2.06%. The maximum distance at 3% is 29.1 m, and at 5% it is 48.5 m. In AWG terms 2.5 mm² falls between 14 AWG and 12 AWG, a conversion the AWG to mm² Calculator handles.

Which Voltage to Enter

Enter the voltage between the conductors that feed the load. In US homes single phase is 120 V from hot to neutral, or 240 V across both hots. Three phase systems are named by their line to line voltage, most often 208 V or 480 V, and that is the number the Voltage Drop Calculator expects.

Phase voltage is the line voltage divided by 1.732. A 208 V system measures 120 V from each phase to neutral, and a 480 V system measures 277 V. The same ratio explains 230 V supplies abroad, where a 400 V three phase system gives 230 V on each phase.

Checking Voltage Drop With a Multimeter

A calculation predicts the drop and a meter confirms it. Measure the voltage at the panel, then at the load while the load is running, and subtract one from the other. With no current flowing the two readings match, so a test with the load switched off shows nothing.

Compare the reading with the Voltage Drop Calculator result for the same current. A measured drop well above the calculated one means resistance has been added somewhere other than the wire itself, and connections are the first place to look.