NEC Electrical Calculator
Voltage Drop Calculator
Calculate voltage drop for any wire run using the NEC K-factor method. Enter your wire size, load current, and run length to check compliance with the NEC 3% branch circuit and 5% total recommendations. The calculator also reverse-solves for the minimum wire size needed to meet the 3% threshold.
How to Use This Calculator
- Select your wire size — choose the AWG or kcmil conductor size you plan to use or want to check. If you are comparing options, start with what you have on the truck and see whether it meets the 3% recommendation.
- Enter the load current — use the slider or type the expected load in amps. For continuous loads, enter the actual load, not the 125% derated value. The voltage drop formula uses actual circuit current.
- Set the one-way length — measure or estimate the wire run distance from the panel or source to the load in feet. This is the one-way distance, not the total wire length. The formula accounts for the return path automatically.
- Choose voltage and phase — select the system voltage (120V, 208V, 240V, 277V, or 480V) and whether the circuit is single-phase or three-phase. Higher voltage systems have lower percentage drops for the same wattage.
- Select conductor material — copper (K=12.9) or aluminum (K=21.2). Aluminum has about 64% higher resistance per circular mil, so it drops more voltage for the same size conductor.
- Read the results — the right panel shows the voltage drop in volts and as a percentage, the voltage delivered at the load, and the minimum wire size needed to achieve 3% or less. The compliance badge turns green for 3% or under, amber for 3-5%, and red above 5%.
What Is Voltage Drop and Why It Matters
Voltage drop is the reduction in electrical potential as current flows through a conductor. Every wire has resistance, and when current passes through that resistance, some of the source voltage is consumed by the wire itself instead of being delivered to the load. The result is that the voltage at the outlet, motor terminal, or equipment connection is lower than the voltage at the panel. This difference is the voltage drop.
Voltage drop matters because electrical equipment is designed to operate within a specific voltage range. When voltage at the load falls too low, real problems occur. Incandescent lights dim noticeably. LED drivers may flicker or fail prematurely. Electric motors draw more current to compensate for the reduced voltage, which generates excess heat in the windings and shortens motor life. A motor rated for 240V that receives only 220V will draw roughly 9% more current, increasing I²R heating losses in both the motor and the circuit conductors.
Excessive voltage drop also wastes energy. The power consumed by the conductor resistance is pure waste heat — it does no useful work. On a commercial installation with long feeder runs, conductor losses of 5% or more translate directly into 5% higher electricity bills. Over the life of a building, upsizing wire by one or two gauges during initial installation often pays for itself through reduced energy costs.
Beyond equipment performance, excessive voltage drop can cause nuisance tripping of electronic circuit breakers and ground fault interrupters. When the voltage sags during motor startup or heavy load switching, sensitive protective devices may interpret the transient as a fault condition. This is especially problematic with long runs to well pumps, HVAC compressors, and workshop equipment where inrush currents are high.
NEC Voltage Drop Recommendations
The National Electrical Code addresses voltage drop in two key Informational Notes. NEC 210.19(A) Informational Note No. 4 states that conductors for branch circuits should be sized to prevent a voltage drop exceeding 3% at the farthest outlet of power, heating, and lighting loads. NEC 215.2(A) Informational Note No. 2 gives the same 3% recommendation for feeders. Both notes add that the maximum total voltage drop on both feeders and branch circuits to the farthest outlet should not exceed 5%.
Critically, these are Informational Notes, not enforceable code requirements. Per NEC 90.5(C), Informational Notes are explanatory material and are not mandatory. However, the practical reality is more nuanced. Many local jurisdictions adopt amendments that convert the 3% and 5% recommendations into enforceable requirements. Even where they remain advisory, most inspectors expect compliance and will flag circuits that exceed these thresholds. From a liability perspective, an electrician who installs a circuit with 10% voltage drop that subsequently damages a customer's equipment has a weak defense if the installation ignored the NEC recommendations.
For practical purposes, treat 3% as the target for branch circuits and 5% as the absolute maximum when you include the feeder. On critical installations — hospitals, data centers, fire alarm circuits, and motor loads — many engineers specify 2% or less for branch circuits.
How to Calculate Voltage Drop
The standard method for calculating voltage drop uses the K-factor (also called the circular mil method). This approach is based on the resistivity of the conductor material, expressed as the resistance in ohms of a conductor one circular mil in cross-section and one foot long. The formula is straightforward:
Single-phase: VD = (2 × K × I × L) / CM
Three-phase: VD = (1.732 × K × I × L) / CM
Where VD is the voltage drop in volts, K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C), I is the load current in amps, L is the one-way length of the conductor run in feet, and CM is the circular mil area of the conductor from NEC Chapter 9, Table 8.
The factor of 2 in the single-phase formula accounts for the complete circuit — current flows through the hot conductor to the load and returns through the neutral or second hot. Both conductors have resistance, so the total conductor length is twice the one-way distance. For three-phase circuits, the factor of 1.732 (the square root of 3) replaces the factor of 2 because the return current in a balanced three-phase system is distributed among all three phases rather than flowing through a single return conductor.
To convert voltage drop to a percentage, divide VD by the source voltage and multiply by 100. To reverse-solve for the minimum wire size that meets a target percentage, rearrange the formula: CM = (2 × K × I × L) / VD_max, where VD_max is the source voltage times the target percentage (for example, 240V × 0.03 = 7.2V for a 3% target on a 240V circuit). Then find the smallest standard wire size whose circular mil area meets or exceeds the calculated value.
K-Factor vs AC Impedance Method
The K-factor method assumes that conductor resistance is the dominant component of impedance. For conductors smaller than 250 kcmil, this assumption is accurate enough for practical field calculations. The resistance component dominates, and the reactance component (caused by the magnetic field around the conductor) is negligibly small.
For larger conductors (250 kcmil and above), AC effects become significant. Skin effect causes current to concentrate near the outer surface of the conductor, effectively reducing the usable cross-section and increasing AC resistance above the DC value. Additionally, the reactance component of impedance grows with conductor size and spacing. For these larger conductors, NEC Chapter 9, Table 9 provides AC impedance values (resistance plus reactance) for different conduit types and conductor configurations. Using these values with the standard impedance-based voltage drop formula gives more accurate results.
This calculator uses the K-factor method, which is appropriate for the vast majority of branch circuit and feeder calculations that electricians encounter daily. For conductors 250 kcmil and larger, the calculator displays a note reminding the user to verify with the AC impedance method when precision matters. For most residential and light commercial work where conductors rarely exceed 4/0, the K-factor method is the standard and accepted approach.
When to Upsize Wire for Voltage Drop
The most common scenario requiring wire upsizing for voltage drop is long runs. Any branch circuit over 100 feet on a 120V system should be checked, and circuits over 150 feet will almost always require upsizing. On 240V systems, the threshold is roughly double — but long runs to detached garages, workshops, barns, and outbuildings frequently exceed it.
Motor circuits deserve special attention. NEC 430.72 addresses motor branch circuit conductors, and motors are particularly sensitive to low voltage. A 5% voltage reduction at a motor terminal reduces available torque by roughly 10% (torque varies as the square of voltage). For well pumps, compressor motors, and any motor that starts under load, excessive voltage drop during startup can prevent the motor from reaching full speed, causing thermal overload trips.
Critical equipment installations also warrant conservative sizing. Medical imaging equipment, server rooms, precision CNC machinery, and fire alarm circuits all benefit from voltage drop well below 3%. The incremental cost of one wire size larger during installation is trivial compared to the cost of equipment malfunction or callback.
Rules of thumb for field estimation: on 120V copper circuits, #12 AWG is good for about 35 feet at 20A before exceeding 3%. Double the distance for 240V. Going up one wire size roughly increases the allowable distance by 60%. Going up two sizes roughly doubles it. For aluminum, multiply the required circular mils by about 1.6 compared to copper.
Maximum Wire Run Distance at 3% Drop
How far can you run each wire size before exceeding the NEC-recommended 3% voltage drop? These values are for copper conductors at 75°C. Actual distances depend on your specific load and conditions.
| Wire Size | 15A / 120V | 20A / 120V | 20A / 240V | 30A / 240V | 50A / 240V |
|---|---|---|---|---|---|
| #14 AWG | 38 ft | 28 ft | 57 ft | 38 ft | 22 ft |
| #12 AWG | 60 ft | 45 ft | 91 ft | 60 ft | 36 ft |
| #10 AWG | 96 ft | 72 ft | 144 ft | 96 ft | 57 ft |
| #8 AWG | 153 ft | 114 ft | 229 ft | 153 ft | 91 ft |
| #6 AWG | 243 ft | 182 ft | 365 ft | 243 ft | 146 ft |
| #4 AWG | 387 ft | 290 ft | 580 ft | 387 ft | 232 ft |
| #2 AWG | 615 ft | 461 ft | 923 ft | 615 ft | 369 ft |
Formula: Max Distance = (CM × VD_max) / (2 × K × I). Values rounded down to nearest foot. Use the calculator above for exact results with your specific parameters.
Circular Mil Reference Table
Circular mils (CM) measure the cross-sectional area of a conductor. One circular mil equals the area of a circle with a diameter of one mil (one thousandth of an inch). Larger CM values mean lower resistance and less voltage drop.
| Wire Size | Circular Mils |
|---|---|
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 3 AWG | 52,620 |
| 2 AWG | 66,360 |
| 1 AWG | 83,690 |
| 1/0 AWG | 105,600 |
| 2/0 AWG | 133,100 |
| 3/0 AWG | 167,800 |
| 4/0 AWG | 211,600 |
| 250 kcmil | 250,000 |
| 300 kcmil | 300,000 |
| 350 kcmil | 350,000 |
| 400 kcmil | 400,000 |
| 500 kcmil | 500,000 |
| 750 kcmil | 750,000 |
| 1000 kcmil | 1,000,000 |
Worked Examples
Example 1: 20A Branch Circuit, 120V
A 20-amp general-purpose branch circuit using #12 copper THHN runs 100 feet from the panel to the farthest outlet. System voltage is 120V single-phase.
VD = (2 × 12.9 × 20 × 100) / 6,530 = 7.90V
Drop percentage: 7.90 / 120 × 100 = 6.58%. This exceeds both the 3% and 5% NEC recommendations. The voltage at the load would be only 112.1V. The solution: upsize to #10 AWG (10,380 CM), which gives VD = (2 × 12.9 × 20 × 100) / 10,380 = 4.96V or 4.13% — still above 3% but within 5%. For full 3% compliance, #8 AWG would be needed at this distance.
Example 2: 50A Circuit to Workshop, 240V
A 50-amp circuit feeding a detached workshop uses #6 copper, running 150 feet from the main panel. System voltage is 240V single-phase.
VD = (2 × 12.9 × 50 × 150) / 26,240 = 7.37V
Drop percentage: 7.37 / 240 × 100 = 3.07%. This barely exceeds 3%. The voltage at the load is 232.6V, which is acceptable for most equipment but technically above the NEC recommendation. Upsizing to #4 AWG (41,740 CM) gives VD = 4.63V or 1.93%, which is comfortably within limits and leaves headroom for the feeder contribution to total drop.
Example 3: 200A Feeder, Three-Phase 208V
A 200-amp three-phase feeder uses 4/0 aluminum conductors running 200 feet from the service entrance to a subpanel. System voltage is 208V.
VD = (1.732 × 21.2 × 200 × 200) / 211,600 = 6.94V
Drop percentage: 6.94 / 208 × 100 = 3.34%. This exceeds the 3% feeder recommendation. Voltage at the subpanel is 201.1V. Considering that branch circuits from the subpanel will add their own drop, the total at the farthest outlet could easily exceed 5%. Upsizing to 250 kcmil aluminum (250,000 CM) gives VD = 5.87V or 2.82%, leaving 2.18% budget for branch circuit drop. Alternatively, using 300 kcmil (300,000 CM) gives 2.35%, providing more headroom for longer branch runs.
Frequently Asked Questions
What is an acceptable voltage drop?
NEC 210.19(A) Informational Note No. 4 recommends a maximum of 3% for branch circuits and 5% total for feeders plus branch circuits combined. These are recommendations, not hard requirements, but most inspectors treat them as practical requirements. For sensitive equipment, target 2% or less.
How far can I run 12 gauge wire?
For 20A on 120V single-phase copper #12 AWG at 3% max drop, the maximum one-way distance is about 45 feet. At 240V with the same load, approximately 91 feet. At 5% tolerance those distances roughly double. Always calculate for your specific load and voltage.
Does voltage drop change with three-phase?
Yes. Three-phase uses a 1.732 multiplier instead of 2 for single-phase, resulting in approximately 13.4% less voltage drop for the same wire size, length, and current. The formula is VD = (1.732 × K × I × L) / CM.
Is voltage drop a code requirement or recommendation?
Strictly speaking, the NEC 3% and 5% limits are recommendations (Informational Notes per NEC 90.5(C)), not enforceable requirements. However, many jurisdictions adopt them as requirements via local amendments, and most inspectors expect compliance.
How do I reduce voltage drop on a long run?
Four strategies: increase wire size, increase supply voltage (240V has half the drop of 120V at the same power), shorten the run by relocating the panel, or distribute load across multiple circuits. For very long runs, a subpanel near the loads is often most cost-effective.
Does wire temperature affect voltage drop?
Yes. The standard K-factors (12.9 copper, 21.2 aluminum) assume 75°C conductor temperature. At lower temperatures, resistance decreases and drop is less. At higher temperatures, both increase. For most branch circuits at normal operating temperatures, the K-factor method is sufficiently accurate.
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