What is voltage drop?

Voltage drop is the reduction in electrical potential (voltage) as current flows through a conductor's resistance. When current flows through a wire, the wire's resistance converts some electrical energy to heat, reducing the voltage available at the load. Excessive voltage drop causes lights to dim, motors to overheat, and equipment to malfunction.

How do you calculate voltage drop?

For single-phase AC or DC circuits: Vdrop = 2 × K × I × L / CM. For three-phase: Vdrop = 1.732 × K × I × L / CM. Where K = conductor resistivity constant (12.9 for copper, 21.2 for aluminum at 75°C), I = current in amps, L = one-way conductor length in feet, CM = conductor cross-sectional area in circular mils. The result is in volts.

What is the maximum allowable voltage drop per NEC?

The NEC (National Electrical Code) recommends (not requires) that voltage drop not exceed 3% for individual branch circuits or feeders, and no more than 5% total from the service entrance to the last outlet. These are recommendations in NEC Article 210 and 215, not hard requirements, but they are widely used as design standards. Excessive voltage drop causes equipment damage and energy waste.

What is AWG wire gauge?

AWG (American Wire Gauge) is a standardized system for specifying wire conductor diameters. Counterintuitively, a higher AWG number means a smaller wire: 24 AWG is thin (used in communications), while 2/0 AWG is thick (used for high-current applications). Common residential wiring uses 14 AWG (15A circuits), 12 AWG (20A), and 10 AWG (30A). Lower gauge wires have lower resistance and can carry more current over longer distances.

Why is voltage drop worse on longer wire runs?

Resistance is directly proportional to conductor length (R = ρL/A). Doubling the wire length doubles the resistance and therefore doubles the voltage drop for the same current. This is why long wire runs (subpanels, outdoor lighting, EV chargers far from the panel) require larger wire gauge to stay within the 3–5% NEC voltage drop limit.

What causes high voltage drop?

High voltage drop is caused by: undersized wire (high resistance per foot), long wire runs (more total resistance), high current draw (more current × same resistance = more voltage drop), poor connections (added resistance at joints), and high ambient temperature (increases wire resistance). Solutions include larger wire gauge, shorter runs, reducing load current, and ensuring tight, clean connections.

How do I reduce voltage drop?

To reduce voltage drop: (1) Increase wire gauge (lower AWG number = larger wire = less resistance). (2) Reduce run length by placing the panel closer to loads. (3) Reduce current by distributing loads across multiple circuits. (4) Use copper instead of aluminum. (5) For three-phase systems, adding a phase reduces current per conductor. Use our calculator to find the minimum wire gauge needed to stay under 3% drop.

What is circular mil (CM)?

A circular mil (CM) is a unit of area used to measure conductor cross-sections. One circular mil equals the area of a circle with a diameter of one mil (1/1000 of an inch). It simplifies wire resistance calculations: resistance ∝ 1/CM. The CM of AWG wire can be calculated as: CM = (diameter in mils)². For example, 12 AWG wire has a diameter of 80.81 mils and a cross-section of 6,529.9 CM.

Can I use extension cords for long runs?

Extension cords are not recommended for permanent installations because they are typically made with small-gauge wire that causes significant voltage drop and overheating risk. For permanent long runs, always use appropriately sized wire in conduit or cable installed per NEC. If an extension cord must be used temporarily, choose a heavy-duty cord (12 AWG or larger) rated for the load and keep it as short as possible.

DC · Single-Phase · Three-Phase · NEC

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Q: What is the difference between copper and aluminum wiring?

Copper is the standard for residential wiring. It has lower resistivity (K=12.9 vs 21.2 for aluminum at 75°C), is more flexible, and doesn't require special connectors. Aluminum is used for larger feeders and service entrance conductors because it's lighter and less expensive per foot, despite having higher resistance. For the same current-carrying capacity, aluminum wire needs to be 1–2 gauge sizes larger than copper.

Calculate voltage drop for DC, single-phase, and three-phase circuits. Uses AWG wire gauge, checks NEC 3% and 5% limits, and recommends minimum wire size.

Circuit types

AWG

Wire sizes

NEC

Compliance

Cu/Al

Materials

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NEC-compliant · AWG · Cu/Al

Source Voltage e.g. 12, 24, 120, 240

Load Current amps

One-Way Wire Length feet

Wire Gauge (AWG)

Conductor Material

Wire Reference

AWG Wire Gauge — Resistance & Ampacity

Resistance values at 75°C (167°F) per NEC. Ampacity for copper in conduit.

AWG	Area (CM)	Ω/1000ft (Cu)	Ω/1000ft (Al)	Max Ampacity (Cu)	Typical Use
14 AWG	2,580	3.14	—	15 A	General lighting/outlets
12 AWG	6,530	1.98	—	20 A	Kitchen, bathroom circuits
10 AWG	10,380	1.24	2.04	30 A	Dryers, A/C units
8 AWG	16,510	0.778	1.28	40 A	EV chargers (Level 2), ranges
6 AWG	26,240	0.491	0.808	55 A	Sub-panels, large A/C
4 AWG	41,740	0.308	0.508	70 A	Sub-panels, hot tubs
2 AWG	66,360	0.194	0.319	95 A	Service entrance, feeders
1/0 AWG	105,600	0.122	0.201	125 A	Large feeders
2/0 AWG	133,100	0.0967	0.159	145 A	Service entrance
4/0 AWG	211,600	0.0608	0.100	195 A	Large service entrance

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NEC Recommends Max 3% Drop on Branch Circuits

The National Electrical Code recommends no more than 3% voltage drop on individual branch circuits and feeders, and 5% total from service entrance to load. This is a recommendation (not a code requirement), but exceeding it causes equipment inefficiency, overheating, and shortened equipment life. For motor circuits and sensitive electronics, stay under 3%.

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Single-Phase vs Three-Phase

Single-phase (1Ø): Two conductors carry current. Current flows out on one wire and returns on the other — doubling the effective wire length. Formula uses multiplier 2. Used in homes (120V, 240V) and small commercial applications.

Three-phase (3Ø): Three conductors, 120° apart. Uses √3 (≈1.732) multiplier instead of 2, because the phases partially cancel. More efficient for long runs and heavy loads. Used in commercial and industrial buildings (208V, 480V).

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Copper vs Aluminum Wiring

Copper (K=12.9): Standard for residential. Lower resistivity, more flexible, more reliable connections, compatible with all devices. Preferred for everything under 1 AWG.

Aluminum (K=21.2): About 64% higher resistance than copper. Used for large feeders (1/0 AWG and up) and service entrance where its lighter weight and lower cost justify use. Requires anti-oxidant compound and compatible connectors rated AL/CU. Always 1–2 gauge larger than copper for same ampacity.

How to Calculate Voltage Drop

Voltage drop is calculated using Ohm's Law applied to the conductor resistance. The standard NEC-based formula uses the K-constant method, which incorporates conductor resistivity, current, and length per circular mil area.

DC and Single-Phase AC Formula

Vdrop = (2 × K × I × L) / CM Where: K = conductor constant (Cu: 12.9, Al: 21.2 at 75°C) I = current in amperes (A) L = one-way conductor length in feet CM = conductor cross-section in circular mils The 2× multiplier accounts for both conductors (out + return). Voltage at load = Source voltage − Vdrop % Voltage drop = (Vdrop / Source voltage) × 100 Example: 120V, 15A, 50 ft, 12 AWG copper (CM = 6,530): Vdrop = (2 × 12.9 × 15 × 50) / 6,530 Vdrop = 19,350 / 6,530 = 2.96 V % drop = (2.96 / 120) × 100 = 2.47% ✓ (under 3%)

Three-Phase Formula

Vdrop = (1.732 × K × I × L) / CM The √3 (1.732) replaces 2× because three phases partially offset each other, reducing the effective return path voltage. Example: 208V 3Ø, 30A, 100 ft, 10 AWG copper (CM = 10,380): Vdrop = (1.732 × 12.9 × 30 × 100) / 10,380 Vdrop = 66,988 / 10,380 = 6.45 V % drop = (6.45 / 208) × 100 = 3.10% (just over 3% — use 8 AWG)

Finding Minimum Wire Size for Target % Drop

Rearranging for CM: CM = (2 × K × I × L) / Vdrop_max (1Ø/DC) CM = (1.732 × K × I × L) / Vdrop_max (3Ø) Where Vdrop_max = Source voltage × (max % drop / 100) Example: Stay under 3% on 120V, 20A, 75 ft copper (1Ø): Vdrop_max = 120 × 0.03 = 3.6 V CM = (2 × 12.9 × 20 × 75) / 3.6 CM = 38,700 / 3.6 = 10,750 CM → 10 AWG (10,380 CM) just fits

When Does Voltage Drop Matter?

Voltage drop becomes a practical concern whenever wire runs are long relative to the load current, or when the load is sensitive to voltage variation.

Common Problem Scenarios

EV Chargers: A Level 2 charger at 48A on a 240V circuit, located 80 feet from the panel, can easily exceed 5% drop on 6 AWG wire. Many EV chargers will throttle charge rate or fault if supply voltage drops below 208V on a 240V circuit.

Lighting Circuits

LED drivers are generally tolerant of 5–10% voltage variation, but incandescent and fluorescent lights show visible dimming above 3% drop. Long runs for landscape lighting or string lights (often low-voltage 12V or 24V DC) suffer dramatically from voltage drop — even a small resistance causes a large percentage drop on low-voltage circuits.

Motor Circuits

Motors are the most sensitive loads to voltage drop. A 10% voltage reduction causes approximately a 19% reduction in torque (torque ∝ V²), forcing the motor to draw more current to maintain load. This increases winding temperature and dramatically shortens motor life. NEC recommends staying under 3% for motor branch circuits.

Low-Voltage DC Systems

Voltage drop is proportionally much worse at low voltages. A 1V drop on a 120V circuit is 0.83%. The same 1V drop on a 12V DC system is 8.3% — a significant problem. Solar and battery systems, automotive wiring, and LED lighting on 12V/24V DC require very careful wire sizing to keep percentage drop under 3%.

Frequently Asked Questions

Voltage drop is the reduction in electrical potential as current flows through a conductor's resistance. The wire converts some electrical energy to heat, leaving less voltage at the load end. A 120V circuit with 3.6V drop delivers only 116.4V to the load. Excessive drop causes dimming lights, overheating motors, and malfunctioning electronics.

Single-phase/DC: Vdrop = (2 × K × I × L) / CM. Three-phase: Vdrop = (1.732 × K × I × L) / CM. K=12.9 for copper, 21.2 for aluminum at 75°C. I=amps, L=one-way length in feet, CM=conductor circular mils. Enter your values above and the calculator shows voltage drop, end voltage, and NEC compliance automatically.

NEC recommends (NEC 210.19, 215.2) no more than 3% voltage drop on branch circuits or feeders, and no more than 5% total (feeder + branch circuit combined). These are recommendations, not hard requirements, but widely used as design standards. For motors and sensitive equipment, 3% is typically the target.

AWG (American Wire Gauge) is a standardized wire sizing system where higher numbers = thinner wire. 14 AWG (15A, typical outlets) is thinner than 10 AWG (30A, dryers). For voltage drop, smaller wire = more resistance = more drop. Going down two AWG sizes approximately halves the resistance — so switching from 14 AWG to 10 AWG roughly halves your voltage drop.

Wire resistance = resistivity × length / area. Doubling length doubles resistance, doubling voltage drop for the same current. A 20A circuit on 12 AWG at 25 ft has ~0.6% drop; at 100 ft it has ~2.5%; at 150 ft it exceeds 3.7%. This is why sub-panels, detached garages, EV chargers far from the main panel, and landscape lighting all need larger wire than a typical short run.

Copper K=12.9, aluminum K=21.2 — aluminum has 64% higher resistivity per circular mil-foot. For the same current and length, you need about 1.65× more cross-section area in aluminum (roughly 2 AWG sizes larger). Copper is standard for residential branch circuits. Aluminum is used for 1/0 AWG and larger feeders and service entrance conductors where cost and weight savings justify it.

Main causes: undersized wire (too small gauge), long wire runs, high current draw, loose/corroded connections (add significant resistance), and high ambient temperature (increases wire resistance). Solutions: increase wire gauge, reduce run length, split loads across circuits, ensure tight clean connections, and use copper instead of aluminum where possible.

Best options: (1) Increase wire gauge — going from 12 AWG to 10 AWG cuts resistance by 37%. (2) Move panel closer to load. (3) Distribute loads across multiple circuits. (4) Use copper instead of aluminum. (5) For 3-phase systems, the √3 multiplier makes 3-phase inherently more efficient per conductor. Use our calculator to find the minimum AWG needed to stay under 3%.

A circular mil is a unit of conductor cross-section area. 1 CM = area of a circle 1/1000 inch in diameter. CM = diameter(mils)². Wire resistance scales inversely with CM: doubling CM halves resistance. The K-constant formula uses CM directly. 12 AWG = 6,530 CM, 10 AWG = 10,380 CM, 8 AWG = 16,510 CM. Our AWG dropdown shows CM for each size.

Not for permanent installations. Extension cord wire is typically 16–18 AWG — very thin, high resistance, and rated for temporary use only. A 50ft 16 AWG extension cord at 15A has over 8% voltage drop. For any permanent run, install properly sized wire in conduit per NEC. If a temporary extension cord is unavoidable, use the shortest heavy-duty (12 AWG) cord rated for the load.

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