What is the difference between AC and DC voltage drop calculation?

For pure resistive DC circuits, voltage drop = I × R (conductor resistance only). For AC circuits, impedance adds a small reactive component, but for typical building wire sizes up to AWG 2, the difference is negligible and the resistive formula is used in NEC calculations. Three-phase AC circuits multiply by √3 (1.732) instead of 2 because the three conductors share the return path.

How do I reduce voltage drop?

Four ways to reduce voltage drop: (1) Increase wire gauge (go to a lower AWG number — 10 AWG has less resistance than 12 AWG). (2) Reduce wire run length by moving the panel or subpanel closer to the load. (3) Increase system voltage (240V has 1/4 the drop of 120V for the same wattage load). (4) Use copper instead of aluminum (copper has lower resistivity). The wire size recommendation in this calculator shows the minimum AWG needed to stay within 3%.

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Voltage Drop Calculator (2026) – NEC, AWG, AC & DC

Q: What is the formula for voltage drop?

For single-phase AC and DC: Vdrop = 2 × K × I × L ÷ CM, where K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amps, L is one-way length in feet, and CM is the wire cross-section in circular mils. Simplified: Vdrop = I × R, where R = (resistance per 1000ft ÷ 1000) × 2 × length. For 3-phase: multiply by √3 (1.732) instead of 2.

Q: What is the maximum voltage drop allowed by NEC?

The NEC (National Electrical Code) recommends a maximum of 3% voltage drop for branch circuits (NEC 210.19 Informational Note) and 3% for feeders, with a combined maximum of 5% from the service entrance to the farthest outlet. These are recommendations, not hard code requirements — but 3% is the industry standard for design.

Q: What is the voltage drop per 100 feet for common wire sizes?

For copper wire carrying 15A on a 120V circuit: 14 AWG loses ~3.8V per 100ft (3.2%); 12 AWG loses ~2.4V per 100ft (2.0%); 10 AWG loses ~1.5V per 100ft (1.2%). Longer runs, higher currents, and smaller wire gauges all increase voltage drop.

Q: How do I calculate voltage drop across a resistor?

Use Ohm's Law: V = I × R. Multiply the current flowing through the resistor (in amps) by its resistance (in ohms). For example: 2A through a 47Ω resistor = 94V drop. For wire resistance, use the AWG resistance table — 12 AWG copper has 1.588Ω per 1000ft, so 50ft of wire has 0.0794Ω resistance per conductor.

Q: Is 4% voltage drop acceptable?

A 4% voltage drop on a branch circuit exceeds the NEC 3% recommendation. For most devices it is functional, but for sensitive electronics, motors (which draw more current at lower voltage), and LED drivers, staying at or below 3% is strongly recommended. Above 5% total drop can cause overheating, premature failure of motors, and code violations in inspected work.

Q: What AWG wire do I need for a long run?

A common rule of thumb: when a one-way run in feet exceeds the system voltage (e.g., over 120 feet on a 120V circuit), upgrade to the next wire size to stay within 3% voltage drop. For solar and DC systems at 12V, even a 20-foot run at 30A may require 6 AWG or larger. Use this calculator's wire size recommendation output to find the exact minimum gauge for your specific circuit.

Q: How is 3-phase voltage drop calculated differently?

For balanced 3-phase circuits: Vdrop = √3 × I × R × L ÷ 1000, where R is the resistance per 1000ft for your AWG size and L is the one-way length. The √3 (1.732) factor replaces the factor of 2 used in single-phase calculations, because in a balanced 3-phase system the three conductors share the return current. This means 3-phase runs can be significantly longer than single-phase runs with the same voltage drop.

Calculate voltage drop, percentage drop, volts at load, and get an instant NEC compliance check — for AC single-phase, AC 3-phase, and DC circuits. Includes wire size recommendations and a full AWG resistance reference.

For residential and commercial single-phase circuits — 120V, 240V outlets, lighting, appliances.

System Voltage (V)

120V or 240V residential · 208V commercial

Load Current (A)

Amps drawn by the connected load

One-Way Length (ft)

Panel to load — calculator doubles for round-trip

Wire Material

Wire Gauge (AWG)

Larger number = thinner wire = more resistance

For commercial and industrial 3-phase systems — 208V, 240V, 480V panels and motors.

Line-to-Line Voltage (V)

208V · 240V · 480V · 600V

Load Current (A)

Per-phase current (FLA from nameplate)

One-Way Length (ft)

Panel to load, one direction

Wire Material

Wire Gauge (AWG)

For solar arrays, battery banks, automotive, RV, and marine 12V/24V/48V DC systems.

System Voltage (V)

12V · 24V · 48V · custom

Load Current (A)

Amps drawn by DC load

One-Way Length (ft)

Battery/source to load — one way

Wire Material

Wire Gauge (AWG)

For 12V systems, wire sizing is critical — even short runs at high current drop quickly

Results

Adjust inputs and click Calculate

2.38 V Voltage Drop

1.98% % Voltage Drop

117.62 V Volts at Load

0.1588 Ω Total Wire Resistance

35.7 W Power Lost in Wire

✅

NEC Compliant — Under 3% Meets NEC 210.19 recommendation for branch circuits. No wire size upgrade needed for voltage drop compliance.

Wire Size Recommendation

Current wire12 AWG copper

Min. for 3% NEC limit12 AWG ✓

Recommended (2% headroom)10 AWG

*NEC 210.19 and 215.2 recommendations. Always verify with a licensed electrician for inspected work.

How to Use This Voltage Drop Calculator

Three modes cover every common US electrical circuit type. Pick your mode, enter five values, and get an instant NEC compliance check with a wire size upgrade recommendation.

AC Single-Phase — for all standard US residential circuits: 120V outlets, 240V dryers, ranges, HVAC, sub-panels.
AC 3-Phase — for commercial and industrial panels running 208V, 480V, or 600V motors, HVAC units, and feeder runs.
DC Circuit — for solar arrays, battery banks, RV/marine 12V/24V/48V systems, and automotive wiring where even short runs at high current can drop voltage critically.

What Each Input Means

System Voltage — the supply voltage at the source (panel, battery, inverter). Common: 120V, 240V, 480V AC; 12V, 24V, 48V DC.
Load Current (Amps) — the current the connected device draws. For motors, use the FLA (Full Load Amps) from the nameplate. For resistive loads, divide watts by voltage.
One-Way Length — the distance from panel to load in feet. The calculator automatically doubles this for the complete round-trip conductor path.
Wire Material — copper has lower resistance and is preferred. Aluminum is used for large feeders and service entrances where cost and weight matter.
AWG Gauge — smaller number = thicker wire = less resistance. 14 AWG is minimum for 15A circuits; 12 AWG for 20A; 10 AWG for 30A under NEC 310.

💡 DC systems drop voltage fast — a 12V system loses the same absolute voltage as a 120V system, but it's 10× more impactful as a percentage. A 1.2V drop on 12V is 10%; the same drop on 120V is 1%. Always oversize wire on low-voltage DC runs.

Voltage Drop Formula Explained

There are two ways to calculate voltage drop. The NEC standard method uses conductor circular mils (CM); the simplified practical method uses resistance per 1000ft from AWG tables. Both give the same result.

Single-Phase AC and DC Formula

Vdrop = 2 × I × R_per_ft × L where R_per_ft = AWG resistance (Ω/1000ft) ÷ 1000 L = one-way length in feet Factor 2 = round-trip path (hot + neutral/return)

Example — 120V, 15A, 12 AWG copper, 50 ft one-way:

R = (1.588 ÷ 1000) × 50 × 2 = 0.1588 Ω Vdrop = 15 × 0.1588 = 2.38 V Drop% = (2.38 ÷ 120) × 100 = 1.98% ✓ Under 3%

Three-Phase Formula

Vdrop = √3 × I × R_per_ft × L √3 = 1.7321 (replaces the factor of 2 in single-phase) This is the line-to-line voltage drop

The √3 factor accounts for the 120° phase angle between conductors in a balanced 3-phase system. Because three conductors share the return path, 3-phase circuits have inherently lower voltage drop than single-phase for the same wire size and current.

NEC K-Factor Method (Alternative)

Vdrop = (2 × K × I × L) ÷ CM K = 12.9 for copper · K = 21.2 for aluminum CM = circular mils of the conductor (Use 1.732 instead of 2 for 3-phase)

NEC Voltage Drop Limits & Code Requirements

The National Electrical Code (NEC) addresses voltage drop in two places — as recommendations, not hard requirements. However, these recommendations are treated as design standards by utilities, engineers, and inspectors.

Circuit Type	NEC Reference	Recommended Max Drop	Notes
Branch Circuits	210.19(A) Info Note 4	3%	From panel to outlet/load
Feeders	215.2(A) Info Note 2	3%	From service to sub-panel
Combined Maximum	Both above combined	5%	Service entrance to final outlet
Sensitive Electronics	Industry standard	1–2%	Computers, PLCs, medical
Motor Circuits	430.52 + 215.2	3%	Excess drop causes overheating

Important: NEC voltage drop limits are informational notes, not mandatory code violations. However, a circuit with excessive voltage drop that causes equipment damage or fire could expose the installer to liability. Many AHJs (Authorities Having Jurisdiction) treat 3%/5% as enforceable during inspections. Always design to stay within 3% for branch circuits.

Why Voltage Drop Matters Beyond Compliance

Motors — draw higher current at lower voltage (torque is proportional to V²), causing overheating and premature failure.
LED drivers and smart devices — voltage-sensitive electronics behave erratically or fail below rated voltage.
HVAC and refrigeration — compressors start harder at low voltage, increasing wear and energy consumption.
Solar/battery systems — excessive DC drop means your panels or batteries deliver less usable power to your inverter.

AWG Wire Resistance Reference Table

This table shows the DC resistance per 1,000 feet for copper and aluminum conductors at 75°C (standard NEC calculation temperature). Use these values to verify calculator outputs or perform manual calculations.

AWG Size	Copper (Ω/1000ft)	Aluminum (Ω/1000ft)	Typical Use	Max Amps (60°C)
14 AWG	2.525	4.150	15A branch circuits, lighting	15A
12 AWG	1.588	2.630	20A outlets, kitchen circuits	20A
10 AWG	0.999	1.650	30A circuits, dryers, HVAC	30A
8 AWG	0.628	1.050	40A circuits, ranges, sub-feeds	40A
6 AWG	0.395	0.661	55A circuits, small sub-panels	55A
4 AWG	0.249	0.415	70A circuits, large sub-panels	70A
3 AWG	0.197	0.329	85A feeders	85A
2 AWG	0.156	0.261	95A feeders, service entrances	95A
1 AWG	0.124	0.207	110A feeders	110A
1/0 AWG	0.098	0.164	125A feeders	125A
2/0 AWG	0.078	0.130	145A feeders, service entrances	145A
3/0 AWG	0.062	0.103	165A service entrances	165A
4/0 AWG	0.049	0.082	195A service entrances, main feeds	195A

Source: NEC Table 9, resistance at 75°C for stranded conductors. Ampacity ratings from NEC Table 310.16 at 60°C in conduit. Always verify with the current NEC edition for inspected installations.

DC vs. AC Voltage Drop — Key Differences

DC Systems (Solar, Automotive, Marine)

DC voltage drop is purely resistive — V = I × R, no reactive component. The challenge with DC systems is that low voltages (12V, 24V) amplify the percentage impact of any drop. A 1.2V drop on a 12V system is 10%; on a 120V AC circuit it's only 1%. This is why solar installers and marine electricians oversize wire aggressively — staying under 3% on a 12V system with 30A of current and 20 feet of wire requires 6 AWG or larger.

AC Single-Phase Systems

For building wire sizes (14 AWG through 4/0 AWG), AC voltage drop is calculated with the same resistive formula as DC. The reactive (inductive) component of impedance is negligible for conductors smaller than 4/0 in typical building wiring runs. The NEC uses resistance values from Table 9 for all practical voltage drop calculations.

AC Three-Phase Systems

Three-phase systems are inherently more efficient. The √3 factor instead of 2 means 3-phase drops only 86.6% of the voltage that single-phase would for the same wire, current, and distance. A 480V 3-phase feeder can run much farther than a 240V single-phase circuit with the same voltage drop percentage — which is why industrial facilities standardize on 480V 3-phase distribution.

System	Formula Factor	Same Wire/Current/Distance	Best For
DC	× 2 (round trip)	Baseline	Solar, battery, automotive
AC Single-Phase	× 2 (round trip)	= DC	Residential, small commercial
AC 3-Phase	× √3 (1.732)	13.4% less drop	Commercial, industrial, motors

How to Reduce Voltage Drop

When your calculation exceeds 3%, you have four practical options:

1. Increase Wire Gauge (Lower AWG Number)

The most direct fix. Going from 12 AWG to 10 AWG reduces resistance by 37%. Going from 12 to 8 AWG reduces it by 60%. Use the wire size recommendation in this calculator to find the minimum AWG for 3% compliance. For critical circuits, size for 2% to leave headroom.

2. Increase System Voltage

Voltage drop percentage = Vdrop ÷ V_source. Doubling the voltage on the same circuit cuts the voltage drop percentage in half. This is why 240V circuits can run twice as far as 120V circuits with the same wire size and load. For DC systems, upgrading from 12V to 24V or 48V is the most cost-effective long-run solution.

3. Reduce Wire Run Length

Install a sub-panel or distribution point closer to the loads. In a large commercial building or farm, a strategically located sub-panel can cut average run lengths by 50–70%, dramatically reducing voltage drop across all branch circuits without upsizing wire.

4. Use Copper Instead of Aluminum

Aluminum has 64% higher resistivity than copper. Switching from aluminum to copper for the same AWG size reduces voltage drop by ~38%. For feeders where aluminum is used for cost savings, upsize by two AWG gauges (e.g., use 2/0 aluminum instead of 2 AWG copper) to approximate equivalent performance.

💡 Quick rule of thumb: When your one-way run in feet exceeds the system voltage (e.g., a 120-foot run on 120V, or a 240-foot run on 240V), voltage drop becomes significant and you should verify your wire size. For 12V DC systems, this threshold is only 12 feet — size wire carefully on all DC runs.

Frequently Asked Questions

What is the formula for voltage drop?

For single-phase AC and DC: Vdrop = 2 × I × (R/1000) × L, where I is current in amps, R is resistance in Ω per 1000ft (from AWG table), and L is the one-way length in feet. The factor of 2 accounts for the round-trip conductor path. For 3-phase AC, replace 2 with √3 (1.732). Percentage drop = (Vdrop ÷ source voltage) × 100.

What is the maximum voltage drop allowed by NEC?

The NEC recommends a maximum of 3% drop on branch circuits (NEC 210.19 Informational Note 4) and 3% on feeders (NEC 215.2 Informational Note 2), with a combined maximum of 5% from service entrance to the final outlet. These are recommendations, not mandatory violations — but are treated as design standards by inspectors and engineers. Always design to 3% or less for branch circuits.

How do I calculate voltage drop across a resistor?

Use Ohm's Law: V = I × R. Multiply the current through the resistor (amps) by its resistance (ohms). For example: 2A through a 47Ω resistor = 94V drop. For wire resistance, the AWG table in this page gives Ω per 1000ft — divide by 1000 to get Ω per foot, multiply by total wire length (both conductors), then multiply by current.

Is 4% voltage drop acceptable?

A 4% drop on a branch circuit exceeds the NEC 3% recommendation. For most resistive loads (heaters, incandescent lights), it is functionally acceptable. For motors, HVAC compressors, LED drivers, and electronics, 4% can cause overheating, premature failure, and erratic behavior. For inspected work, staying at or below 3% is strongly recommended. Above 5% combined is considered excessive by virtually all codes and standards.

What is the voltage drop per 100 feet for common wire sizes?

For copper wire carrying 15A on a 120V circuit (one-way 100ft): 14 AWG: 7.58V (6.3%) — fails NEC; 12 AWG: 4.76V (4.0%) — exceeds 3%; 10 AWG: 3.00V (2.5%) — passes; 8 AWG: 1.88V (1.6%) — well within. At 20A on 240V over 100ft: 12 AWG drops 3.2% (borderline); 10 AWG drops 2.0% (passes).

Why is DC voltage drop more critical than AC?

Because low-voltage DC systems (12V, 24V) have the same absolute voltage drop as higher-voltage AC systems, but that drop represents a much higher percentage. A 1.5V drop on 12V = 12.5%; the same drop on 120V = 1.25%. DC loads also typically have tighter voltage tolerances — a 12V motor or LED driver may function poorly below 11V. Always oversize wire on DC runs, especially at 12V.

How is 3-phase voltage drop calculated differently?

For balanced 3-phase circuits, use √3 (1.732) instead of 2 as the multiplier: Vdrop = 1.732 × I × (R/1000) × L. This accounts for the 120° phase offset between conductors sharing the return path. Three-phase drops approximately 13.4% less voltage than single-phase for identical wire, current, and distance — which is why 3-phase distribution is preferred for long industrial runs.

What AWG wire do I need for a long run?

Use this calculator's wire size recommendation output. As a rule of thumb: when your one-way run in feet exceeds your system voltage (e.g., 120ft on 120V, 240ft on 240V), step up to the next larger wire gauge. For 12V DC systems, even a 12-foot run at 30A requires 6 AWG to stay under 3%. The minimum NEC-compliant gauge is shown in the results above after you calculate.