Electrical Wire Size & Voltage Drop Calculator
Calculate single-phase, 3-phase, and DC voltage drop, recommended AWG/kcmil wire gauge, copper vs aluminum comparisons, and NEC Table 310.16 ampacity derating.
Electrical & Conduit Takeoff Worksheet
Date: September 3, 2026
Source: ProTrade Calculators
Circuit Parameters & Electrical Load
Calculate wire gauge, voltage drop, and NEC 310.16 ampacity for single-phase, 3-phase, and DC circuits.
Recommended Conductor & Voltage Drop Sizing
Delivers 235.08V at load terminal with 2.05% voltage drop (-4.92V drop).
Circuit Voltage Drop & Conductor Schematic
Conductor Candidate Sizing & Voltage Drop Comparison Table
COPPER Conductors| Conductor Size | Circular Mils | Base / Derated Ampacity | Voltage Drop | Voltage at Load | Max 3% Distance | Status |
|---|---|---|---|---|---|---|
| 14 AWG | 4,110 CM | 20A / 20A | 31.39V (13.08%) | 208.61V | 22.9 ft | FAIL |
| 12 AWG | 6,530 CM | 25A / 25A | 19.75V (8.23%) | 220.25V | 36.4 ft | FAIL |
| 10 AWG | 10,380 CM | 35A / 35A | 12.43V (5.18%) | 227.57V | 57.9 ft | FAIL |
| 8 AWG | 16,510 CM | 50A / 50A | 7.81V (3.25%) | 232.19V | 92.1 ft | WARNING |
| 6 AWGRecommended | 26,240 CM | 65A / 65A | 4.92V (2.05%) | 235.08V | 146.5 ft | RECOMMENDED |
| 4 AWG | 41,740 CM | 85A / 85A | 3.09V (1.29%) | 236.91V | 233 ft | PASS |
| 3 AWG | 52,620 CM | 100A / 100A | 2.45V (1.02%) | 237.55V | 293.7 ft | PASS |
| 2 AWG | 66,360 CM | 115A / 115A | 1.94V (0.81%) | 238.06V | 370.4 ft | PASS |
| 1 AWG | 83,690 CM | 130A / 130A | 1.54V (0.64%) | 238.46V | 467.1 ft | PASS |
| 1/0 AWG | 105,600 CM | 150A / 150A | 1.22V (0.51%) | 238.78V | 589.4 ft | PASS |
| 2/0 AWG | 133,100 CM | 175A / 175A | 0.97V (0.4%) | 239.03V | 742.9 ft | PASS |
| 3/0 AWG | 167,800 CM | 200A / 200A | 0.77V (0.32%) | 239.23V | 936.6 ft | PASS |
| 4/0 AWG | 211,600 CM | 230A / 230A | 0.61V (0.25%) | 239.39V | 1181 ft | PASS |
| 250 kcmil | 250,000 CM | 255A / 255A | 0.52V (0.22%) | 239.48V | 1395.3 ft | PASS |
| 300 kcmil | 300,000 CM | 285A / 285A | 0.43V (0.18%) | 239.57V | 1674.4 ft | PASS |
| 350 kcmil | 350,000 CM | 310A / 310A | 0.37V (0.15%) | 239.63V | 1953.5 ft | PASS |
| 400 kcmil | 400,000 CM | 335A / 335A | 0.32V (0.13%) | 239.68V | 2232.6 ft | PASS |
| 500 kcmil | 500,000 CM | 380A / 380A | 0.26V (0.11%) | 239.74V | 2790.7 ft | PASS |
| 600 kcmil | 600,000 CM | 420A / 420A | 0.22V (0.09%) | 239.78V | 3348.8 ft | PASS |
| 750 kcmil | 750,000 CM | 475A / 475A | 0.17V (0.07%) | 239.83V | 4186 ft | PASS |
| 1000 kcmil | 1,000,000 CM | 545A / 545A | 0.13V (0.05%) | 239.87V | 5581.4 ft | PASS |
Calculation Methodology & Mathematical Steps
1. How Voltage Drop is Calculated (Engineering Formulas)
Voltage drop occurs when electrical current passes through the inherent resistance of a conductor over distance. The standard IEEE and National Electrical Code formulas calculate voltage drop based on conductor resistivity (K), load current (I), one-way distance (D), and conductor cross-sectional area in circular mils (CM):
Single-Phase AC & DC Circuits:
Vdrop = (2 × K × I × D) ÷ CM
Three-Phase AC Circuits:
Vdrop = (√3 × K × I × D) ÷ CM
Where K represents conductor resistivity at 75°C (12.9 Ω·cmil/ft for Copper and 21.2 Ω·cmil/ft for Aluminum), I is load current in amperes, D is one-way distance in feet, and CM is circular mils.
2. Copper vs. Aluminum Conductors: Performance & Sizing
Copper is a superior conductor with 64% lower electrical resistance than aluminum, but modern 8000-series aluminum alloy provides substantial cost and weight savings for heavy feeders:
| Conductor Material | Resistivity (K @ 75°C) | Standard Sizing Advantage | Typical Application |
|---|---|---|---|
| Copper (Cu) | 12.9 Ω·cmil/ft | Smaller conduit fill, higher ampacity per gauge | Branch circuits (15A–50A), EV chargers, tight conduits |
| Aluminum (Al) | 21.2 Ω·cmil/ft | Lower material cost, lighter cable pulling weight | Main electrical services, 100A/200A subpanels, long exterior feeders |
3. 120V vs. 240V Circuits: Why 120V Drops Twice as Fast
Because voltage drop percentage is relative to system voltage (Drop % = Vdrop ÷ Vsystem), a 120V circuit suffers twice the percentage drop of a 240V circuit carrying the exact same current over the same distance:
120V Circuit (15A @ 100 ft, 14 AWG)
Voltage drop is 9.42V, which equals a severe 7.85% drop (delivers only 110.58V). Requires upsizing to 10 AWG or 8 AWG to maintain 3% or lower drop.
240V Circuit (15A @ 100 ft, 14 AWG)
Voltage drop is still 9.42V, but represents only a 3.92% drop (delivers 230.58V).
4. NEC Voltage Drop Recommendations (3% Branch / 5% Total)
While the National Electrical Code does not mandate voltage drop limits as a hard violation in all residential installations, it specifies critical engineering recommendations:
- NEC 210.19(A) Informational Note 4: Branch circuits should be sized for a maximum voltage drop of 3% at the furthest outlet.
- NEC 215.2(A)(1) Informational Note 2: Feeder conductors should not exceed 3% drop, and total drop across both feeder and branch conductors should not exceed 5%.
- Equipment Protection: Motors subjected to low voltage draw excessive current and overheat; sensitive electronics and LED drivers flicker or shut down when voltage drops below 95% of nominal.
5. Level 2 EV Charger Wire Sizing & Continuous Loads
Electric vehicle charging stations are classified as continuous loads under NEC Article 625, requiring circuit conductors and overcurrent devices to be sized at 125% of the charger's rated amperage:
| EV Charger Continuous Load | 125% Design Current | Breaker Size | Min Copper Wire (Up to 75 ft) | Long Run Copper (100–150 ft) |
|---|---|---|---|---|
| 32A Charger | 40A | 40A Breaker | 8 AWG Copper | 6 AWG Copper |
| 40A Charger | 50A | 50A Breaker | 6 AWG Copper | 4 AWG Copper |
| 48A Charger (Hardwired) | 60A | 60A Breaker | 6 AWG (90°C) or 4 AWG | 4 AWG or 2 AWG Copper |
6. Sizing 100A & 200A Subpanel Feeders for Workshops & Garages
When running electrical subpanels to detached garages, workshops, or outbuildings over distances of 100 to 300 feet, voltage drop almost always dictates conductor sizing before thermal ampacity:
- 100A Subpanel (50 ft run): 4 AWG Copper or 2 AWG Aluminum (0.8% drop).
- 100A Subpanel (150 ft run): 1 AWG Copper or 2/0 Aluminum (2.3% drop).
- 100A Subpanel (300 ft run): 3/0 Copper or 250 kcmil Aluminum (2.8% drop).
- 200A Service (200 ft run): 4/0 Copper or 350 kcmil Aluminum (2.4% drop).
7. Common Electrical Wire Sizing Mistakes to Avoid
Critical Safety & Code Traps
- Ignoring Terminal Temperature Ratings: Breakers and lugs rated for 75°C must use the 75°C column of NEC Table 310.16, even if using 90°C THHN wire.
- Forgetting Rooftop Solar Conduit Derating: Exterior conduits on hot rooftops can reach 140°F+, requiring up to 50% ampacity derating.
- Using Small Aluminum Conductors: Aluminum smaller than 8 AWG is restricted in residential branch wiring due to oxidation and thermal expansion hazards.
Complete Your Electrical Feeder & Raceway Planning
Upsizing conductors for distance affects conduit fill and panel capacity. Use our related electrical instruments to complete your takeoff:
Comprehensive guide on how breaker size, copper vs aluminum, and 3% voltage drop interact on 50–300 ft runs.
Verify EMT, PVC, and RMC conduit trade sizes for upsized wire gauges per NEC Chapter 9 tables.
Calculate total home electrical load in Amps and VA per NEC 220.82 for 100A, 200A, or 400A panels.
9. Frequently Asked Questions
What is the maximum acceptable voltage drop according to the National Electrical Code (NEC)?
The NEC recommends a maximum voltage drop of 3% on branch circuits (NEC 210.19(A) Informational Note 4) and a maximum total voltage drop of 5% across both the feeder and branch circuit combined (NEC 215.2(A)(1) Informational Note 2) to ensure reasonable efficiency of operation.
What is the difference between single-phase and 3-phase voltage drop formulas?
Single-phase circuits require current to travel out along the hot conductor and return via the neutral (a 2-wire round trip), using the multiplier 2.0: Vdrop = (2 × K × I × D) / CM. Three-phase circuits share return currents among phases 120° apart, reducing effective resistance by using the multiplier √3 (~1.732): Vdrop = (√3 × K × I × D) / CM.
When should I use aluminum wire instead of copper?
Aluminum (typically 8000-series alloy such as XHHW-2 or USE-2) is commonly used for heavy feeder circuits (100A, 150A, 200A subpanels and main services) because it is significantly lighter and less expensive than copper. However, because aluminum has higher resistivity (K = 21.2 vs 12.9 for copper), it requires upsizing by approximately one to two AWG gauge sizes.
Why does voltage drop matter for EV chargers and subpanels?
Level 2 EV chargers draw heavy continuous loads (e.g. 48A on a 60A circuit) for 4 to 10 hours continuously. Excessive voltage drop results in power wasted as heat in walls or conduit, slower vehicle charging rates, and potential thermal tripping of breakers.
How does ambient temperature affect conductor ampacity?
Conductor insulation ratings (60°C, 75°C, 90°C) are calibrated for a 30°C (86°F) ambient baseline. When cables run through hot attics, commercial roofs, or exterior conduit in direct sunlight (temperatures of 104°F to 140°F), NEC Table 310.15(B)(1) requires derating conductor ampacity by 10% to 50% to prevent insulation degradation.
What is circular mils (CM) and why is it used to size wire?
Circular mils (CM) is a standard unit of cross-sectional area for electrical conductors. One circular mil equals the area of a circle with a diameter of 1 mil (0.001 inch). Larger conductors have higher CM values, which reduces electrical resistance and lowers voltage drop.