NEC Electrical Calculator
Voltage Drop Calculator
Intry VerifiedA worked default reading, traceable end to end: what it was calculated from, what it was run with, how it was checked, and who has final say.
Where this number comes fromIntry Verified
- Calculated from
- NEC 210.19 Note, 3% guideline
- Run with
- Wire#12 AWG
- Load20 A
- One-way length100 ft
- System240 V, 1-phase
- MaterialCopper
- Checked
- Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the cited NEC section before each deploy. This is our own deterministic gate, not a third-party audit.
- Final say
- Your AHJ and local amendments have final say. Confirm before rough-in.
Intry Verified · NEC 2023 · Build AE2FC52 · 2026-08-21
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How to Use This Calculator
- Pick what you need. Choose Size the wire to get the smallest conductor for a run, or Check a run to test a size you already have. Sizing targets 3% (branch) or 5% (total) and returns the larger of the ampacity and voltage-drop minimums, so the result carries the load and holds the drop.
- Enter the load current. Use the slider or type the expected load in amps. For continuous loads, enter the actual running load here, not the 125% figure; voltage drop is computed on actual circuit current. Size the conductor's ampacity for 125% of a continuous load separately (the wire size calculator does that), since this tool's ampacity leg uses the running current.
- Set the one-way length. Measure the run from the panel or source to the load in feet. This is the one-way distance, not the total conductor length; the formula accounts for the return path automatically.
- Choose voltage and phase. Select the system voltage (120V, 208V, 240V, 277V, or 480V) and single- or three-phase. Higher voltage means a lower percentage drop for the same power.
- Set material and conduit. Copper or aluminum, and PVC, aluminum, or steel conduit. Conduit type matters: steel raises reactance, which adds drop on long runs and larger conductors. This is the part the K-factor shortcut cannot see.
- Set the power factor. Use 1.0 for resistive loads like heaters and most lighting, or 0.85 for motor loads, or enter a custom value. Power factor shifts the effective impedance, so a motor branch drops differently than a resistive one.
- Read the results. The right panel shows the voltage drop in volts and percent, the voltage delivered at the load, the effective impedance (R, reactance, and Z) behind the number, and, in Check mode, the minimum wire for 3%. The badge reads within 3%, 3 to 5%, or over 5%; those are NEC recommendations, not a code pass.
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 that 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 Informational Note 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)(2) 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 such as hospitals, data centers, fire alarm circuits, and motor loads, many engineers specify 2% or less for branch circuits.
How to Calculate Voltage Drop
The quickest hand method uses the K-factor (also called the circular-mil method). It 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, and it is the one worth memorizing for the field:
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 Impedance Method (What This Calculator Uses)
The K-factor method folds a conductor into a single resistance number and ignores two things that change the real drop: the reactance the conductor and its conduit add, and the power factor of the load. For small conductors on resistive loads that is a fair approximation, because resistance dominates and reactance is small. It is why the hand method has stayed useful.
This calculator uses the fuller method. It reads the alternating-current resistance (R) and reactance (XL) for the conductor and conduit type from NEC Chapter 9, Table 9, then combines them into an effective impedance at the load power factor using the formula in Table 9, Note 2: Ze = R × PF + XL × sin(arccos PF). That effective impedance drives the drop, so the result already reflects whether the run is in PVC, aluminum, or steel conduit and whether the load is resistive or a motor. The R, reactance, and Z values are shown with every result, so you can see what produced the number.
The two methods diverge most in three places. Steel conduit raises reactance noticeably, so a long run in steel drops more than the K method predicts. A motor or other low power-factor load shifts the effective impedance away from pure resistance. And larger conductors carry proportionally more reactance, so the gap widens as wire size grows. For a short branch circuit in PVC on a resistive load, the two methods land close together; for a 300 kcmil feeder in steel conduit, the difference is real. Table 9 covers 14 AWG through 1000 kcmil, which is the size range the calculator offers. Above that, a run moves to parallel conductor sets rather than a single larger wire. For a size and material the table does not list, such as 14 AWG aluminum, which is not a real building conductor, the calculator falls back to the K estimate for that entry.
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 430.22 sizes motor branch circuit conductors at 125% of the full-load current, 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 45 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.
Allowable Voltage Drop by Application
How much voltage drop is acceptable depends on the circuit. The NEC gives two numbers as recommendations, not enforceable requirements: 3% for a branch circuit and 3% for a feeder, with 5% for the two combined to the farthest outlet. Sensitive and motor loads are usually held tighter, but that comes from engineering practice and equipment listings, not from the NEC itself.
| Application | Recommended max drop | Basis |
|---|---|---|
| Branch circuit | 3% | NEC 210.19 Informational Note (recommendation) |
| Feeder | 3% | NEC 215.2(A)(2) Informational Note No. 2 (recommendation) |
| Feeder plus branch, total | 5% | NEC 210.19 / 215.2 Informational Notes (recommendation) |
| Sensitive electronics, data, imaging | 2% or less | Engineering practice, not NEC |
| Motor branch circuits | 3% or tighter | Practice; NEC sets no motor-specific drop limit |
| Fire alarm and life-safety | Per NFPA 72 and the device listing | Device operating voltage, not the NEC notes |
The 3% and 5% figures are NEC Informational Notes (explanatory material under NEC 90.5(C)), so they are recommendations at the federal-model level. Many jurisdictions adopt them as enforceable limits by local amendment, and most inspectors expect compliance. The tighter targets for sensitive and life-safety loads come from the equipment listing and the designer, so confirm them against the equipment documentation and NFPA 72 where it applies.
Maximum Wire Run Distance at 3% Drop
How far can you run each copper wire size before exceeding the NEC-recommended 3% voltage drop? These are quick estimates from the K-factor (resistance-only) method, so they ignore conduit type and power factor. The calculator above uses the fuller NEC Chapter 9 impedance method and will differ, most on long runs, steel conduit, and motor loads. An "n/a" means the conductor cannot carry that load: the pairing exceeds the 75°C ampacity or the 240.4(D) small-conductor limit, so a distance would be meaningless.
| Wire Size | 15A / 120V | 20A / 120V | 20A / 240V | 30A / 240V | 50A / 240V |
|---|---|---|---|---|---|
| #14 AWG | 38 ft | n/a | n/a | n/a | n/a |
| #12 AWG | 60 ft | 45 ft | 91 ft | n/a | n/a |
| #10 AWG | 96 ft | 72 ft | 144 ft | 96 ft | n/a |
| #8 AWG | 153 ft | 115 ft | 230 ft | 153 ft | 92 ft |
| #6 AWG | 244 ft | 183 ft | 366 ft | 244 ft | 146 ft |
| #4 AWG | 388 ft | 291 ft | 582 ft | 388 ft | 232 ft |
| #2 AWG | 617 ft | 462 ft | 925 ft | 617 ft | 370 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.
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 = 4.97V or 4.14%. That is 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 = 4.64V or 1.93%, which is comfortably within limits and leaves headroom for the feeder contribution to total drop. (#6 is rated 65A at 75°C, so carrying 50A is fine; this is a drop question, not an ampacity one.)
The same run by the impedance method the calculator uses lands differently. At 0.85 power factor in PVC conduit, NEC Chapter 9 Table 9 gives 2.77% (6.65V); in steel conduit, 2.81% (6.75V). The K estimate reads 3.07% because it treats the load as purely resistive; at a real 0.85 power factor the resistive term shrinks faster than the small reactance of #6 adds back, so the drop falls and #6 lands inside the 3% guide. At power factor 1.0 the impedance method lands right on the K estimate. The direction reverses on large conductors in steel conduit, where reactance dominates and a lower power factor raises the drop instead. This is why the tool asks for conduit and power factor rather than assuming.
Example 3: 200A Feeder, Three-Phase 208V (ampacity first)
A 200-amp three-phase feeder runs 200 feet from the service entrance to a subpanel at 208V. A common instinct is to reach for 4/0 aluminum. Check ampacity before drop.
4/0 aluminum carries only 200A? No: it is rated 180A at 75°C (NEC 310.16), so it cannot serve a 200A feeder at all, before voltage drop even enters. The 310.12 allowance that lets 4/0 Al feed a 200A single-phase dwelling service does not apply to this three-phase commercial feeder. The smallest aluminum that carries 200A is 250 kcmil (205A at 75°C).
At 250 kcmil: VD = (1.732 × 21.2 × 200 × 200) / 250,000 = 5.87V
Drop percentage: 5.87 / 208 × 100 = 2.82%. So 250 kcmil aluminum, the ampacity minimum, also clears the 3% feeder recommendation on its own, leaving about 2.18% for the branch circuits downstream. Going to 300 kcmil gives 2.35% for more headroom on long branch runs. For contrast, 4/0 at this run would compute 3.34%, but that number is moot: the conductor is already too small to carry the load. Size for ampacity, then confirm the drop, and take the larger. That is what the calculator does.
Frequently Asked Questions
Is the Intry Voltage Drop Calculator accurate and NEC compliant?
Every figure an Intry calculator computes comes from a locked source module, not hand-typed. 21026 automated checks re-derive and source-check the numbers against the cited NEC section before each deploy. This is our own deterministic gate, not a third-party audit. The per-tool receipt is public at https://www.intrysys.com/verified.
How do I calculate voltage drop for a circuit?
Use VD = (2 × K × I × L) / CM for single-phase, where K is the conductor resistivity (12.9 for copper, 21.2 for aluminum at 75°C), I is the load current in amps, L is the one-way run length in feet, and CM is the wire's circular-mil area from NEC Chapter 9 Table 8. For three-phase, replace the 2 with 1.732. Example: 20A of copper on #12 AWG (6530 CM) over 50 ft one-way = (2 × 12.9 × 20 × 50) / 6530 = 3.95 V, or 3.3% on a 120V circuit. Percent drop = VD / source voltage × 100.
How far can a 240V, 50A, three-phase load be from the panel on 3 AWG copper?
About 339 feet one-way, at the 3% branch-circuit recommendation. Using VD = (1.732 × K × I × L) / CM with K = 12.9 (copper), I = 50A, and CM = 52,620 for #3 AWG (NEC Chapter 9 Table 8): the 3% budget on 240V is 7.2V, so L = 7.2 × 52,620 / (1.732 × 12.9 × 50) ≈ 339 ft. At the 5% feeder limit it is about 565 ft. Aluminum #3 (K = 21.2) drops those to roughly 206 ft and 344 ft. Confirm ampacity separately: #3 copper is good for 100A at 75°C, so 50A is well within its rating.
What is an acceptable voltage drop?
NEC 210.19 Informational Note 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.
Does conduit type affect voltage drop?
Yes, on AC circuits. Steel conduit is magnetic, so it raises the reactance of the conductors inside it, which adds to the impedance and therefore the drop. PVC and aluminum conduit have lower reactance. NEC Chapter 9 Table 9 lists separate reactance values for steel versus PVC/aluminum conduit, and this calculator uses them, so switching the conduit type changes the result. The effect is small on short branch circuits and grows on long runs and larger conductors. The K-factor hand method ignores this entirely.
Does power factor affect voltage drop?
Yes, on AC circuits with reactance. Voltage drop depends on the effective impedance, which NEC Chapter 9 Table 9 Note 2 computes as Z = R × PF + XL × sin(arccos PF). A resistive load (power factor 1.0, like heaters and most lighting) sees mostly resistance. A motor load nearer 0.85 shifts the balance toward reactance, so the same conductor and run can drop a different amount. This calculator lets you set the power factor; the K-factor method assumes a purely resistive load and cannot account for it.
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Drop checks out? Confirm the ampacity side.
Voltage drop and ampacity are separate checks, and a gauge that passes one can fail the other. The wire size calculator runs NEC 310.16 with derating and terminal temperature limits so the whole circuit holds up.