What is an acceptable voltage drop percentage?

The NEC recommends keeping voltage drop at or below 3% on any individual branch circuit. For the combined voltage drop across a feeder and branch circuit together, the total should not exceed 5%. Most residential circuits stay well under 3% when properly sized, but long runs to outbuildings, garages, or subpanels often require upsizing the wire gauge to meet this limit.

What is the voltage drop formula?

For single-phase circuits: V_drop = 2 × L × I × R, where L is the one-way wire length in feet, I is the load current in amps, and R is the conductor resistance per foot (Ω/ft). For three-phase circuits: V_drop = √3 × L × I × R. The resistance values come from NEC Chapter 9, Table 9 (e.g., 12 AWG copper = 1.98 Ω/1,000 ft).

How does wire gauge (AWG) affect voltage drop?

Larger wire (smaller AWG number) has lower resistance per foot, which directly reduces voltage drop. Going from 12 AWG to 10 AWG copper roughly cuts resistance by 37% (from 1.98 to 1.24 Ω/1,000 ft). For long runs where 12 AWG would produce excessive drop, upgrading to 10 AWG or 8 AWG is the most cost-effective fix — wire is far cheaper than the efficiency losses from sustained high voltage drop.

Does copper or aluminum wire have more voltage drop?

Aluminum wire has about 60–65% more resistance per foot than copper of the same AWG. For example, 2 AWG copper has 0.194 Ω/1,000 ft while 2 AWG aluminum has 0.319 Ω/1,000 ft. When using aluminum wiring — common for service entrance conductors and large feeders — you typically need to upsize by one or two AWG to achieve the same voltage drop as copper.

What happens if voltage drop is too high?

Excessive voltage drop reduces the voltage delivered to the load, which causes motors to run hot and lose torque, lights to appear dim, and sensitive electronics to malfunction or fault. A sustained 10% drop on a 120V circuit delivers only 108V to the load, which can exceed motor nameplate tolerances and void equipment warranties. The NEC 3% and 5% recommendations are practical thresholds that prevent these effects under normal operating conditions.

How do I reduce voltage drop on a long wire run?

The three practical options are: (1) increase the wire gauge — upsizing one or two AWG sizes is usually the simplest fix; (2) reduce the one-way distance by locating panels or subpanels closer to the load; or (3) increase the circuit voltage from 120V to 240V, which cuts current in half for the same wattage and reduces drop proportionally. For very long runs to detached garages or agricultural buildings, installing a subpanel at the load end fed by larger conductors is the standard professional approach.

What is the difference between single-phase and three-phase voltage drop?

In a single-phase circuit, current travels out on one conductor and returns on the neutral, so the total conductor length in the formula is doubled (2 × one-way distance). In a balanced three-phase circuit, the return currents cancel each other in the neutral, and the effective multiplier is √3 ≈ 1.732 instead of 2. This means three-phase systems have inherently lower voltage drop for the same wire gauge, current, and run length — which is one reason large commercial and industrial loads use three-phase power.

What is the acceptable voltage drop per NEC?

The NEC recommends (in informational notes to sections 210.19(A)(1) and 215.2(A)(1)) that voltage drop on any branch circuit not exceed 3%, and that the combined drop across a feeder plus branch circuit not exceed 5%. These are recommendations, not enforceable requirements — but most AHJs and equipment manufacturers cite the same thresholds. For critical or sensitive loads (motors, medical equipment, data center UPS), targeting 2% or less is better practice.

Voltage Drop Calculator — V, %, & NEC Check

Calculates voltage drop, load voltage, and NEC compliance for single-phase and three-phase circuits.

How to Use This Voltage Drop Calculator

This voltage drop calculator checks NEC compliance for any wire run — enter your system voltage, load current, one-way wire length, wire gauge, and conductor material. The calculator returns the voltage drop in volts, the percentage drop relative to source voltage, and the actual voltage delivered to the load. A color-coded NEC compliance badge shows whether the result is within the 3% branch circuit or 5% total system recommendation. Use this tool before pulling wire on any long circuit run — a few seconds of planning can save you from re-pulling undersized conductors.

Voltage drop occurs along the wire run. Enter one-way distance — the formula accounts for the return conductor automatically.

What Is Voltage Drop?

Voltage drop is the reduction in electrical potential that occurs as current flows through the resistance of a wire. Every conductor has resistance; when current passes through it, some of the electrical energy is converted to heat, and the voltage at the far end of the wire is lower than at the source. The longer the wire and the higher the current, the greater the drop. In residential wiring, a 120V circuit with 5% voltage drop delivers only 114V to the outlet — enough to dim lights noticeably and stress motors.

Why Voltage Drop Matters

Motors are the most voltage-sensitive common load. A motor rated for 115V but receiving 105V draws more current to maintain torque, which heats the windings and shortens motor life. HVAC systems, power tools, and well pumps are especially susceptible. Sensitive electronics and LED drivers can also malfunction or produce flickering when supply voltage falls outside rated tolerances. Keeping drop within NEC recommendations protects equipment and ensures circuits perform as designed.

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The Voltage Drop Formula

The NEC Chapter 9, Table 9 method gives the industry-standard formula for copper and aluminum conductors at 75°C operating temperature.

DC and AC Single-Phase Voltage Drop

V_drop = 2 × L × I × R_per_ft

Where L is the one-way wire length in feet, I is the load current in amps, and R_per_ft is the conductor resistance per foot (Ω/ft). The factor of 2 accounts for both the outgoing and return conductors — current must travel out and back. This formula applies equally to DC circuits and AC single-phase circuits. For example: 100 ft of 12 AWG copper carrying 15 A at 120V — R = 1.98 Ω/1,000 ft = 0.00198 Ω/ft. V_drop = 2 × 100 × 15 × 0.00198 = 5.94 V, or 4.95%.

AC Three-Phase Voltage Drop

V_drop = √3 × L × I × R_per_ft

The multiplier drops from 2 to √3 ≈ 1.732 because in a balanced three-phase circuit the return currents cancel each other in the neutral, reducing the effective conductor length in the calculation. Three-phase systems therefore have lower voltage drop for the same wire gauge and current. This formula assumes unity power factor (resistive loads); inductive loads such as motors have a slightly higher effective impedance.

NEC Voltage Drop Limits and Compliance

The National Electrical Code does not mandate specific voltage drop limits — instead it provides informational notes recommending that conductors be sized so that maximum voltage drop on any branch circuit or feeder does not exceed 3%, and that the combined drop across a feeder plus branch circuit does not exceed 5%. These recommendations appear in NEC 2023 Informational Notes to sections 210.19(A)(1) and 215.2(A)(1). While informational notes are not enforceable requirements, most AHJs (Authorities Having Jurisdiction) treat them as best practice, and many utility companies and equipment manufacturers cite the same thresholds in their specifications.

This calculator flags results with a PASS badge (≤3%), a CAUTION badge (3–5%, acceptable only if feeder drop keeps the total under 5%), or a FAIL badge (>5%, exceeds all NEC recommendations). For critical loads — motors, medical equipment, data centers — target 2% or less.

How to Reduce Voltage Drop in Long Wire Runs

When a calculation shows excessive voltage drop, you have three practical options:

Increase wire gauge — upsizing one or two AWG sizes is usually the fastest fix. Going from 12 AWG to 10 AWG copper reduces resistance by 37%, cutting drop proportionally.
Add a subpanel — for runs longer than 100–150 ft to a detached structure (garage, barn, workshop), installing a subpanel at the load end fed by larger feeder conductors is the most cost-effective long-term solution. Check with your Electrical Calculators for wire sizing guidance.
Increase circuit voltage — converting a 120V circuit to 240V cuts current in half for the same wattage, which halves the voltage drop. This requires a double-pole breaker and a 240V appliance or a step-down transformer at the load.

For underground feeder runs to outbuildings, consult a licensed electrician and your local building department — burial depth, conduit type, and GFCI protection requirements vary by jurisdiction.

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Copper vs. Aluminum Conductors

Aluminum wire is roughly 60–65% more resistive per foot than copper of the same AWG. This means an aluminum conductor produces significantly more voltage drop for the same run length and current. However, aluminum costs considerably less per foot and is standard for service entrance conductors (the wires from the utility pole to your panel) and large feeders to subpanels.

When to Use Aluminum

Aluminum makes sense for circuits of 4 AWG and larger where the weight and cost savings are significant — large HVAC feeders, service conductors, and inter-panel feeders. To match the ampacity and voltage drop of copper, aluminum is typically upsized by one or two AWG. For branch circuits in walls and conduit, copper remains the standard because it is easier to terminate and is not subject to the oxidation concerns that apply to aluminum in branch circuit receptacles.

Use the material toggle in the calculator above to compare copper vs. aluminum results for your specific run. For sizing help, see our wire size calculator once available.

Electrical Disclaimer

This calculator provides estimates for informational purposes only and is not a substitute for the advice of a licensed electrician or electrical engineer. Results are based on NEC Chapter 9 formulas and assume ideal conditions — actual voltage drop depends on installation method, ambient temperature, conduit type, conductor bundling, and local code amendments. Always have electrical work designed and inspected by a qualified professional and your local Authority Having Jurisdiction (AHJ). The National Electrical Code is a model code; local jurisdictions may adopt amendments that differ from the values used here.

Sources & References

NFPA 70: National Electrical Code (NEC) 2023 Edition — National Fire Protection Association

Voltage Drop Calculator

NEC Voltage Drop Limits

Common System Voltages

Copper Wire Resistance