Voltage Drop Calculator

This voltage drop calculator works out how many volts a circuit loses between the source and the load, as a number and as a percent of the source voltage. It covers copper and aluminum conductors on single-phase, three-phase and DC circuits. For AC it uses the NEC Chapter 9, Table 9 impedance method, so the conduit type and your load’s power factor are part of the answer. It also gives the smallest conductor size that meets your target and still carries the load, the longest run the size you picked can reach, and a warning when the load current is more than the conductors’ NEC Table 310.16 ampacity.

How to calculate voltage drop

Voltage drop is the current multiplied by the impedance of the conductors that carry it. Longer runs, higher current and smaller conductors all increase it. The NEC gives the data in Chapter 9: Table 8 lists the DC resistance of each conductor size, and Table 9 lists the AC resistance and reactance for conductors in PVC, aluminum and steel conduit.

The calculator uses these formulas, with the length L in feet one way, the current I in amperes and the table values in ohms per 1,000 feet:

• Three-phase: VD = 1.732 × I × L × Ze ÷ 1,000. The result is the line-to-line drop for a balanced load.
• Single-phase: VD = 2 × I × L × Ze ÷ 1,000. The factor 2 counts the current going out on one conductor and back on the other.
• DC: VD = 2 × I × L × R ÷ 1,000, with R from Table 8.
• Effective impedance: Ze = R × PF + XL × sin θ, from Table 9, Note 2, where PF is the power factor and sin θ = √(1 − PF²).
• Percent drop: VD ÷ source voltage × 100.

With several conductors in parallel per phase, divide the drop by the number of sets. The table values are at 75 °C. The calculator can correct the resistance to 60 °C or 90 °C with the temperature coefficients in Table 8, Note 2.

The quick method with K

Many electricians use a shorter formula: VD = 2 × K × I × L ÷ CM for single-phase, or 1.732 × K × I × L ÷ CM for three-phase. CM is the conductor’s area in circular mils, and K is the resistance of a 1-circular-mil, 1-foot conductor: about 12.9 for copper and 21.2 for aluminum at 75 °C. The K method uses resistance only. It works well for small conductors and for loads near unity power factor. On large conductors at a lagging power factor it reads low, because reactance is then a large part of the impedance.

Why power factor matters

Motors, transformers and most mixed loads draw lagging current. The reactance of the conductors then adds to the drop, in proportion to sin θ. Small conductors are mostly resistance, so power factor changes their drop very little. Large conductors are a different story.

Take a 400 A three-phase load at 480 V, 300 ft away on one set of 500 kcmil copper in steel conduit. At power factor 1.0 the drop is 6.03 V, or 1.26%. At 0.8 lagging it is 10.81 V, or 2.25%: almost twice as much, from the same wire. In PVC conduit, where reactance is lower, the 0.8 power factor drop is 9.35 V (1.95%).

Enter the power factor of the load you are checking. Use 1.0 for resistive loads such as heaters. Running motors and mixed commercial loads are usually 0.8 to 0.9 lagging. A motor starting draws several times its running current at a low power factor, often 0.3 to 0.4. Check starting drop separately with the starting current and that power factor. A leading power factor, from capacitor banks or lightly loaded long cables, reduces the drop and can turn it into a voltage rise.

Worked examples

Three-phase feeder at 208 V

A 60 A three-phase load at 208 V sits 300 ft from the panel, on 6 AWG copper in PVC conduit, at power factor 0.9 lagging. Table 9 gives R = 0.49 Ω and XL = 0.051 Ω per 1,000 ft. Ze = 0.49 × 0.9 + 0.051 × 0.436 = 0.463 Ω per 1,000 ft. VD = 1.732 × 60 × 300 × 0.463 ÷ 1,000 = 14.44 V, or 6.94% of 208 V. The load sees about 193.6 V.

That is more than twice the 3% guideline. 4 AWG brings it to 4.50%, and 2 AWG to 2.86% (5.94 V). Load the example in the calculator above to see each step.

Single-phase branch circuit at 120 V

A 16 A load at 120 V, 100 ft from the panel on 12 AWG copper: at power factor 1.0, VD = 2 × 16 × 100 × 2.0 ÷ 1,000 = 6.40 V, or 5.33%. 10 AWG gives 3.84 V (3.20%) and 8 AWG gives 2.50 V (2.08%). The K method gives 6.32 V for the 12 AWG case, close to the Table 9 result.

48 V DC run

30 A at 48 V DC over 50 ft of 8 AWG copper: VD = 2 × 30 × 50 × 0.778 ÷ 1,000 = 2.33 V, or 4.86%. On low-voltage DC systems a few volts is a large share, so these circuits often need much larger conductors than their ampacity alone would suggest. Here 6 AWG gives 3.07% and 4 AWG gives 1.93%.

What the NEC says about voltage drop

For most circuits, the National Electrical Code recommends voltage drop limits but does not require them. Informational notes in 210.19 (branch circuits) and 215.2 (feeders) say that conductors sized for a drop of no more than 3% on a feeder or branch circuit, and no more than 5% on feeder and branch circuit combined, give reasonable efficiency of operation. Informational notes are explanatory material, not enforceable rules.

Some articles do set mandatory limits:

• Sensitive electronic equipment (Article 647): 647.4(D) limits the drop to 1.5% on a branch circuit and 2.5% for feeder and branch circuit combined.
• Fire pumps (Article 695): 695.7 limits the drop at the controller line terminals to 15% below normal during motor starting. With the motor at 115% of full-load current, the voltage at the controller load terminals must stay within 5% of the motor’s rated voltage.

Energy codes, such as ASHRAE 90.1 and the state energy codes based on it, can also limit voltage drop in commercial buildings. Project specifications often set tighter limits than the NEC notes. Check the code editions your jurisdiction has adopted.

Choosing a wire size for voltage drop

1. Size for ampacity first. Pick the conductor from NEC 310.16 with the terminal temperature limits of 110.14(C) and any adjustment or correction factors, and coordinate it with the overcurrent device. Voltage drop never makes a conductor smaller than ampacity requires.
2. Check the drop. Enter that size in the calculator with the real load current, length and power factor.
3. Upsize if needed. If the drop is over your target, use the smallest size the calculator suggests, or consider parallel sets, a shorter route or a higher distribution voltage.
4. Upsize the equipment grounding conductor too. When the circuit conductors are increased beyond the size ampacity requires, NEC 250.122(B) requires a wire-type equipment grounding conductor to be increased in proportion to the circular-mil area.
5. Check the terminations. Larger conductors must fit the lugs on the breaker and the equipment. Reducing splices or larger lugs may be needed.

Conductors in parallel are generally permitted only at 1/0 AWG and larger. Every set must have the same length, material, size, insulation and termination so the current divides evenly.

What too much voltage drop does

• Motors: starting torque falls with roughly the square of the voltage. A motor at low voltage draws more current, runs hotter and may stall or fail to start.
• Lighting: lamps dim or flicker, and some LED drivers drop out at low voltage.
• Electronics and controls: drives, PLCs and IT equipment can trip or reset during dips.
• EV chargers and heat pumps: long runs to detached garages and outdoor units are common trouble spots.
• Energy: the lost voltage is lost power, I²R, turned into heat in the conductors for as long as the load runs.

Limits of this calculator

The Table 9 values are for 600 V cables, three single conductors in conduit, at 60 Hz and 75 °C. They are a good approximation for cable assemblies such as MC and NM in the smaller sizes. They are not right for medium-voltage cable, conductors in cable tray or spaced apart, or circuits with heavy harmonic current. The Ze × I formula is the standard approximation from Table 9, Note 2. It is accurate for drops of a few percent. For a full power system, motor starting studies or harmonic loads, a load flow model in ETAP or SKM gives a better answer.

The calculator checks one run at a time. Add the feeder and branch circuit drops yourself to compare against the 5% combined guideline.

Frequently asked questions

What is an acceptable voltage drop?

The common design target is 3% for a feeder or branch circuit and 5% for the feeder and branch circuit together, from the NEC informational notes in 210.19 and 215.2. Sensitive electronic equipment under Article 647 is held to 1.5% and 2.5%, and fire pumps have their own limits in 695.7. Many engineers design to tighter numbers, such as 2% on feeders.

Is a 3% voltage drop a code requirement?

Not in general. The 3% and 5% figures are informational notes, which the NEC treats as recommendations. They become requirements only where another article, an adopted energy code, the authority having jurisdiction or the project specification makes them mandatory.

How do I reduce voltage drop?

Use a larger conductor, run conductors in parallel, shorten the route, or serve the load at a higher voltage. For example, a three-phase load at 480 V draws less current than the same load at 208 V. Copper has less drop than aluminum of the same size, and PVC conduit gives slightly less drop than steel for large conductors at a lagging power factor.

Does the type of conduit matter?

Yes, for AC circuits with larger conductors. Steel conduit is magnetic, which raises the conductors’ reactance. Steel and aluminum conduit both raise the AC resistance of the largest sizes. Small conductors are mostly resistance, so the conduit type changes their drop very little.

Why does this calculator give a different answer from others?

Many calculators use the K method or Table 8 DC resistance and ignore power factor and reactance. This calculator uses the Table 9 effective impedance at the power factor you enter. For small conductors the answers are close; for large conductors at a lagging power factor this method gives a higher, more realistic drop. Differences in conductor temperature and in single-phase versus three-phase formulas also matter.

Can I use it for DC and solar circuits?

Yes. Choose DC and the calculator uses the Table 8 DC resistance of stranded conductors with the out-and-back length. For PV strings, designers often use the module maximum-power current and design to 1% to 2% to limit energy loss. Check the requirements of NEC Article 690 and the equipment manufacturer separately.

When to bring in an engineer

A single run is easy to check here. A facility with long feeders, large motors, sensitive equipment or an upcoming expansion needs the whole system checked together. Dreiym’s licensed Professional Engineers size feeders and branch circuits, run load analyses and model voltage drop across complete systems in ETAP and SKM, PE-stamped where required. Related tools: the available fault current calculator and the arc flash calculator. For a free quote, contact us. We typically respond within one business day.