Cable Size Calculator — Wire Gauge for Load & Distance
Min Cross-Section Area
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Suggested AWG
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Actual Voltage Drop
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How Cable Sizing Works
Cable sizing is the process of selecting the smallest standard conductor gauge that safely carries the required current without exceeding temperature limits or voltage drop thresholds. An undersized cable carries too much current for its cross-sectional area, causing excessive heat that degrades insulation, increases fire risk, and shortens the cable's lifespan. An oversized cable wastes money on unnecessary conductor material. The goal is to find the smallest standard wire gauge that satisfies two requirements simultaneously: it must carry the required current without overheating (ampacity), and it must deliver voltage to the load with acceptably low voltage drop.
Voltage drop is the reduction in voltage between the power source and the load, caused by the resistance of the conductors. Every wire has resistance that depends on its material, length, and cross-sectional area. As current flows through this resistance, some electrical energy is converted to heat, and the voltage at the load end is lower than at the source. The National Electrical Code (NEC/NFPA 70) recommends that voltage drop on branch circuits should not exceed 3%, and the combined drop from the service entrance through the feeder and branch circuit to the furthest outlet should not exceed 5%.
Ampacity is the maximum current a conductor can carry continuously without exceeding its insulation temperature rating. Ampacity depends on the conductor material (copper or aluminum), the insulation type (60C, 75C, or 90C rated), the number of conductors in the conduit or cable assembly, and the ambient temperature. NEC Table 310.16 is the primary reference for conductor ampacity under standard conditions, with adjustment factors applied for elevated temperatures and bundled conductors.
Cable Sizing Formula
The standard voltage drop formula for single-phase and DC circuits is:
VD = (2 x L x I x R) / 1000
Where:
- VD = voltage drop in volts
- 2 = accounts for the round-trip distance (current flows out to the load and back)
- L = one-way cable length in meters (or feet, depending on units)
- I = load current in amperes
- R = conductor resistance per unit length (ohms per km or per 1000 ft)
Worked Example
A 20-amp circuit runs 30 meters one way on copper wire in a 240V single-phase system. Using the cross-sectional area approach with copper resistivity of 0.0172 ohm-mm^2/m:
- Maximum allowed voltage drop at 3%: 240 x 0.03 = 7.2V
- Minimum cross-section: A = (2 x 0.0172 x 30 x 20) / 7.2 = 2.87 mm^2
- Next standard size up: 3.31 mm^2 (AWG 12)
- Actual voltage drop with AWG 12: (2 x 0.0172 x 30 x 20) / 3.31 = 6.24V (2.6%)
The AWG 12 cable passes both the voltage drop test (2.6% is below 3%) and the ampacity test (AWG 12 copper is rated for 20A at 60C). If the run were longer, say 50 meters, the minimum area would increase to 4.78 mm^2, requiring an upgrade to AWG 10 (5.26 mm^2).
Key Cable Sizing Terms
| Term | Definition |
|---|---|
| AWG (American Wire Gauge) | A standardized wire gauge system used in North America. Lower numbers indicate thicker wire: AWG 14 has a diameter of 1.63 mm, while AWG 4/0 (0000) has a diameter of 11.68 mm. Each step down in gauge number increases the cross-sectional area by about 26%. |
| Ampacity | The maximum continuous current a conductor can carry without exceeding its insulation temperature rating. Determined by conductor size, material, insulation type, ambient temperature, and installation method. |
| Voltage Drop | The difference in voltage between the source and load ends of a circuit, caused by conductor resistance. Expressed as a percentage of the source voltage. NEC recommends 3% max for branch circuits. |
| Conductor Material | Copper (resistivity 1.72 x 10^-8 ohm-m) or aluminum (resistivity 2.82 x 10^-8 ohm-m). Copper is more conductive but more expensive. Aluminum is lighter and cheaper per ampere of capacity. |
| Conduit Fill | The percentage of a conduit's cross-sectional area occupied by conductors. NEC limits fill to 40% for three or more conductors to allow heat dissipation and ease of pulling. Exceeding fill limits causes overheating. |
| Derating | Reducing the allowed ampacity when conductors operate in elevated ambient temperatures or when multiple current-carrying conductors are bundled together. NEC Tables 310.15(B)(1) and 310.15(C)(1) provide derating factors. |
AWG Wire Gauge Reference Table
The table below shows common AWG sizes with their cross-sectional area, ampacity ratings at three insulation temperature levels (for copper), and DC resistance. Use this as a quick reference when checking calculator results against NEC standards.
| AWG | Area (mm^2) | Diameter (mm) | 60C (A) | 75C (A) | 90C (A) | Resistance (ohm/km) |
|---|---|---|---|---|---|---|
| 14 | 2.08 | 1.63 | 15 | 20 | 25 | 8.29 |
| 12 | 3.31 | 2.05 | 20 | 25 | 30 | 5.21 |
| 10 | 5.26 | 2.59 | 30 | 35 | 40 | 3.28 |
| 8 | 8.37 | 3.26 | 40 | 50 | 55 | 2.06 |
| 6 | 13.3 | 4.11 | 55 | 65 | 75 | 1.30 |
| 4 | 21.2 | 5.19 | 70 | 85 | 95 | 0.815 |
| 2 | 33.6 | 6.54 | 95 | 115 | 130 | 0.513 |
| 1/0 | 53.5 | 8.25 | 125 | 150 | 170 | 0.323 |
| 2/0 | 67.4 | 9.27 | 145 | 175 | 195 | 0.256 |
| 3/0 | 85.0 | 10.40 | 165 | 200 | 225 | 0.203 |
| 4/0 | 107.2 | 11.68 | 195 | 230 | 260 | 0.161 |
Practical Cable Sizing Examples
Example 1: Residential Kitchen Circuit
A 20-amp kitchen small-appliance branch circuit runs 15 meters from the panel to the first outlet at 120V. Using copper conductors:
- Max voltage drop at 3%: 120 x 0.03 = 3.6V
- Min cross-section: (2 x 0.0172 x 15 x 20) / 3.6 = 2.87 mm^2
- AWG 12 (3.31 mm^2) satisfies both voltage drop and ampacity (20A at 60C)
- Actual drop: (2 x 0.0172 x 15 x 20) / 3.31 = 3.12V (2.6%) -- passes
Example 2: Long Run to a Detached Shed
A subpanel in a shed 60 meters from the main panel needs a 40-amp feeder at 240V single-phase, copper conductors:
- Max voltage drop at 3%: 240 x 0.03 = 7.2V
- Min cross-section: (2 x 0.0172 x 60 x 40) / 7.2 = 11.47 mm^2
- AWG 6 (13.3 mm^2) passes voltage drop but has 55A ampacity at 60C (adequate for 40A)
- Actual drop: (2 x 0.0172 x 60 x 40) / 13.3 = 6.20V (2.6%) -- passes
Example 3: Commercial Installation
A 100-amp three-phase feeder runs 45 meters in a commercial building at 480V using aluminum conductors:
- Three-phase formula: VD = (1.732 x L x I x rho) / A
- Max voltage drop at 3%: 480 x 0.03 = 14.4V
- Min cross-section: (1.732 x 0.0282 x 45 x 100) / 14.4 = 15.27 mm^2
- AWG 4 aluminum (21.2 mm^2) provides adequate area with 65A ampacity at 75C per conductor -- but since the load per phase is 100A, you need AWG 1/0 aluminum (53.5 mm^2, 120A at 75C)
- Ampacity governs in this case, not voltage drop
Copper vs Aluminum Comparison
Both copper and aluminum are used as electrical conductors, but they have distinct properties that affect cable sizing, cost, and installation. The table below compares the two materials across key metrics to help you choose the right conductor for your project.
| Property | Copper | Aluminum |
|---|---|---|
| Resistivity (ohm-m) | 1.72 x 10^-8 | 2.82 x 10^-8 |
| Conductivity (relative) | 100% (reference) | 61% |
| Weight (per unit length) | Heavier (8.96 g/cm^3) | Lighter (2.70 g/cm^3) |
| Cost per ampere | Higher | Lower (typically 40-60% of copper) |
| Typical AWG for 100A | AWG 1 (75C) | AWG 1/0 (75C) |
| Thermal Expansion | Lower | Higher (can loosen connections) |
| Oxidation | Oxide is conductive | Oxide is resistive (requires anti-oxidant compound) |
| Best Use | Branch circuits, receptacles, short runs | Service entrance, feeders, long runs, utility lines |
Aluminum connections require special attention: always use anti-oxidant compound, use connectors rated for aluminum (marked AL or AL/CU), and torque terminations to manufacturer specifications. The higher thermal expansion of aluminum can cause connections to loosen over time, which was a major factor in residential wiring fires in the 1960s and 1970s when aluminum branch wiring was common.
This calculator is for informational purposes only and does not constitute electrical engineering, safety, or legal advice. Always consult a licensed electrician and follow local electrical codes for permanent installations.
Frequently Asked Questions
Why does voltage drop matter in cable sizing?
Excessive voltage drop causes motors to overheat, lights to dim, and sensitive electronics to malfunction or shut down. The National Electrical Code (NEC) recommends no more than 3% voltage drop on branch circuits and 5% total from the service entrance to the furthest outlet. Under-voltage can also void equipment warranties and, in extreme cases, damage compressor motors in refrigerators and air conditioners by causing them to draw excessive current.
What is the difference between copper and aluminum wire for cable sizing?
Copper has a resistivity of 1.72 x 10^-8 ohm-meters, while aluminum is 2.82 x 10^-8 ohm-meters, making copper about 61% more conductive per cross-sectional area. However, aluminum costs significantly less and weighs about 30% as much as copper, making it the standard choice for large feeders and service entrance cables. For the same ampacity, aluminum wire must be approximately two AWG sizes larger than copper (e.g., AWG 1/0 aluminum instead of AWG 2 copper for a 100A circuit).
How do I calculate cable size for a three-phase circuit?
For three-phase circuits, replace the factor of 2 in the voltage drop formula with 1.732 (the square root of 3). The formula becomes VD = (1.732 x L x I x R) / 1000. Three-phase circuits are more efficient because the power is distributed across three conductors, and each conductor carries less current for the same total power, often resulting in smaller cable sizes compared to single-phase equivalents at the same wattage.
What AWG wire size do I need for a 20-amp circuit?
For a 20-amp circuit at standard residential distances (up to about 50 feet or 15 meters one way), 12 AWG copper wire is typically sufficient and is the NEC minimum for 20-amp branch circuits. For longer runs, you may need to upsize to 10 AWG to keep voltage drop below 3%. Use this calculator with your specific cable length and voltage to verify the correct size.
Does wire temperature rating affect cable sizing?
Yes. Wire insulation is rated at 60C, 75C, or 90C, and the ampacity of a given gauge increases with higher temperature ratings. For example, 12 AWG copper is rated for 20A at 60C but 25A at 75C and 30A at 90C. However, most residential terminations (breakers, receptacles) are rated for only 60C or 75C, so the lower ampacity rating of the termination applies regardless of the wire's insulation rating.
How does conduit fill affect cable size selection?
When multiple current-carrying conductors share a conduit, heat dissipation is reduced because each conductor heats the others. The NEC requires derating ampacity when more than three current-carrying conductors are in the same conduit: 4-6 conductors derated to 80%, 7-9 conductors to 70%, 10-20 conductors to 50%, and 21-30 conductors to 45% of the listed ampacity. This derating often means selecting the next larger cable size to maintain safe operating temperatures.