Voltage drop calculator

Chapter 1

Wire is a long, thin resistor

Voltage drop is Ohm's law, V = I × R, applied to the wire itself. A conductor's resistance grows with its length and shrinks with its cross-sectional area. So three things set the drop: how long the run is, how much current it carries, and how big the conductor is.

The bench below holds the same circuit as the calculator. The source on the left holds its voltage steady. The reel holds the whole one-way run, so cranking out more wire makes the circuit longer, and the footage counter reads it. The load on the right draws a set current, and the meter reads the difference as a share of the source voltage. Crank the reel and the calculator above changes with it.

• Length. Double the run and you double the drop. It is the input people underestimate, because a circuit's length is the route the wire takes, not the straight line on the plan.
• Current. Double the amps and you double the drop. The heat in the wire goes up four times, since it is I²R.
• Size and material. Every three AWG sizes roughly doubles the conductor's area and halves its resistance. Aluminum conducts about 61% as well as copper, so it takes about two sizes larger to match: Table 8 lists #10 aluminum at 2.00 Ω per 1,000 ft, close to #12 copper's 1.98 Ω.

Voltage matters because the drop is judged as a percentage. The same 3.66 V lost on a 240 V circuit is 1.5% instead of 3.05%. And a load that needs the same power at twice the voltage draws half the current, so its percentage drop falls to a quarter. That is why long runs and big loads move to 208, 240 or 480 V.

Chapter 2

Voltage leaks away one foot at a time

The drop is linear in length: every foot of the same wire gives up the same sliver of voltage. Walk along your run below and read the voltage at each point. Halfway out, you have lost exactly half of the total.

Chapter 3

Out and back: why the 2, and why √3

On a single-phase or DC circuit the current goes out on one conductor and comes back on the other. Both are in its path, so a run of L feet puts 2L feet of wire in the circuit. That is the 2 in the formula, and why you enter the one-way length.

A balanced three-phase load sends no return current down the neutral, because the three phase currents cancel there. Each phase loses I × Z, and √3 converts that line-to-neutral drop into the line-to-line drop you would measure between two phases, which is how 208 V and 480 V are quoted. With the same wire and current, a three-phase circuit loses √3 ÷ 2, about 87%, of the volts a single-phase circuit would. The formula assumes a balanced load.

Chapter 4

How far can each wire size go?

Turn the formula around and it tells you the longest run each size can make before it crosses your target: allowed volts × 1,000 ÷ (M × I × Z). Here it is for every modeled size at your load and voltage. Ampacity is a separate check, so a size can reach a long way and still be too small for the current.

Chapter 5

AC adds reactance: steel (EMT) vs PVC conduit

For DC, resistance is the whole story. For AC, Table 9 adds two things. Its AC resistance is a little higher than the DC value on large conductors, because alternating current crowds toward the outside of the conductor (skin effect). It also adds inductive reactance, X, from the magnetic field around each conductor. A steel raceway like EMT strengthens that field, so Table 9 lists higher reactance in steel than in PVC.

How much reactance counts depends on the load's power factor. Note 2 to Table 9 gives the effective impedance Z = R × PF + X × sin(arccos PF), a close approximation while the drop is a few percent. At a power factor of 1 only R counts. On small wire R is so much larger than X that the raceway barely changes anything; on 500 kcmil copper in EMT, X (0.048 Ω) is bigger than R (0.029 Ω), which is why upsizing past a point stops helping.

NEC Chapter 9 Table 9: steel (EMT) vs PVC

Here is the same copper conductor in each raceway at a 0.9 power factor. Steel adds 0.33% to #12 and 13.86% to 500 kcmil, so the raceway choice matters most on feeders.

Power factor and voltage drop

Power factor is the share of the current that does real work. Heaters and incandescent lamps run near 1.0; motors and many electronic drivers run lower. The calculator starts at 0.9 and takes any value from 0.1 to 1. DC has no power factor.

Table 9's Note 2 formula weights resistance by the power factor and reactance by the rest, so a lower power factor moves the drop in opposite directions on small and large wire. On #12, where resistance dominates, it lowers the drop, and 1.0 gives the largest answer. On #4/0, where reactance is close to resistance, it raises it.

Chapter 6

Where 3% and 5% come from

For most circuits the NEC's voltage-drop numbers are recommendations, not rules. Informational notes in 210.19 (branch circuits) and 215.2 (feeders) say that conductors sized for no more than 3% drop to the farthest outlet, and no more than 5% for the feeder and branch circuit combined, "provide reasonable efficiency of operation." Informational notes are not enforceable on their own (90.5(C)), but engineers, project specifications and energy codes adopt them widely.

Read them as one 5% budget. Whatever the feeder uses, the branch gets the rest:

A few articles do set mandatory limits for specific systems. Under 647.4(D), sensitive electronic equipment on a separately derived 120 V system with 60 V to ground is held to 1.5% on fixed-equipment branch circuits and 2.5% combined, and receptacle circuits to 1% and 2%. For fire pumps, 695.7 limits the voltage at the controller to 15% below normal while the motor starts, and to 5% below the motor's rating while it runs. The reasons behind the recommendation are practical: motors at low voltage draw more current and run hotter, lamps dim, electronics brown out, and every volt lost is power you pay for as heat in the walls.

Chapter 7

The route is longer than the drawing looks

The most common voltage-drop mistake is measuring the straight line on the plan. Real runs go up the wall, across the ceiling, around the stair and back down to the device. That routed path is the length to enter, and it is often half again as long.

A worked example

Take the starting circuit: 20 A at 120 V single-phase, on #12 copper in EMT, 50 ft from the panel, at a power factor of 0.9. Table 9 gives #12 copper 2.0 Ω of resistance and 0.068 Ω of reactance in steel, per 1,000 ft. So Z = 2.0 × 0.9 + 0.068 × 0.436 = 1.830 Ω. The drop is 2 × 20 A × 1.830 Ω × 50 ft ÷ 1,000 = 3.66 V. That is 3.05% of 120 V, just over 3%; #12 reaches 49 ft before it crosses. Step up to #10 (Z = 1.107 Ω) and the same run drops 2.21 V, or 1.85%.

Voltage drop examples: single-phase 120 V and 240 V, 277 V lighting, three-phase 208 V and 480 V

Common circuits at each system voltage, for copper in EMT at a 0.9 power factor. The last column is the smallest copper size that keeps the run within 3% and still carries the load in the 75 °C column of Table 310.16.

Enter your own voltage, load, raceway and power factor in the calculator above for any other circuit.

What to check when the result is high

1. Verify the route. Confirm the one-way length follows the intended pathway, including bends, offsets and routing constraints.
2. Verify the current. Use the expected design load for the question you are answering; the breaker rating and the actual load are not automatically the same input.
3. Review the assumptions. Phase, material, raceway and power factor can change an AC result. Make sure the defaults describe the installation.
4. Compare conductor options. A larger conductor lowers the drop, but also affects cost, raceway fill, terminal compatibility and the equipment grounding conductor.

What this calculator leaves out

• Modeled scope. Copper and aluminum from #14 through 600 kcmil (aluminum from #12). AC raceways are EMT and PVC Schedule 40; DC uses Table 8 resistance at 75 °C with no reactance or power factor. Lengths are one-way route lengths in feet.
• The ampacity check is a simple one. It uses the 75 °C column of Table 310.16 at 30 °C ambient and the small-conductor limits of 240.4(D). It does not apply 60 °C terminal ratings (110.14(C)(1)(a)), other ambient temperatures, more than three current-carrying conductors, the 125% continuous-load factor, or wiring-method limits such as NM cable's 60 °C ampacity.
• Equipment grounding conductors. When circuit conductors are upsized for voltage drop, 250.122(B) increases the equipment grounding conductor in proportion. The calculator sizes only the circuit conductors.
• Load behavior. The model holds the current constant as voltage sags. A heater draws a little less current at low voltage, and a motor draws more.
• Table conditions. Table 9 assumes 60 Hz, conductors at 75 °C and three single conductors in one conduit. A lightly loaded wire runs cooler, so the tables tend to overstate the drop.
• Loads along the way. A circuit with several outlets carries less current past each one. Using the full load to the farthest outlet is the simple, conservative answer.
• Other conditions. Parallel conductors, harmonics, service and feeder specifics, and jurisdiction-specific rules may need additional review.

This page and ZapSketch's checks use NEC 2023. Follow equipment instructions and all applicable codes and regulations.