Cable Size Calculator
// CALCULATE OPTIMAL CABLE SIZE BASED ON LOAD REQUIREMENTS //
The National Electrical Code provides recommendations for maximum voltage drop to ensure efficient operation of electrical equipment. While these are recommendations (not requirements), they represent best practices for electrical installations.
3%
Branch Circuits
From panel to individual loads (outlets, lights, equipment)
2%
Feeders
From service entrance to subpanels
5%
Total Combined
Maximum from source to farthest outlet
What is Electrical Cable Sizing?
Cable sizing is the process of selecting the appropriate conductor size for an electrical circuit based on multiple factors including current capacity (ampacity), voltage drop limitations, and installation conditions. Proper cable sizing ensures safety, efficiency, and compliance with the National Electrical Code (NEC).
The Two Critical Sizing Criteria
1. Ampacity (Current Capacity)
The wire must be able to carry the required current without overheating. NEC Table 310.16 provides ampacity values based on conductor size, material, and insulation temperature rating.
2. Voltage Drop
The wire must be large enough to limit voltage drop to acceptable levels. Excessive voltage drop causes equipment malfunction, reduced efficiency, and can damage sensitive electronics.
Important: You must satisfy BOTH criteria. The final wire size is always the larger of the two requirements. A wire that has adequate ampacity but excessive voltage drop is undersized, and vice versa.
Voltage Drop Calculation
Single-Phase Voltage Drop Formula
VD = (2 × I × R × L) / 1000
Three-Phase Voltage Drop Formula
VD = (√3 × I × R × L) / 1000
| Variable | Description | Unit |
|---|---|---|
| VD | Voltage drop | Volts |
| I | Load current | Amperes |
| R | Wire resistance | Ohms per 1000 feet |
| L | One-way cable length | Feet |
Ampacity and Resistance Reference (75°C Insulation)
This table shows ampacity values from NEC Table 310.16 and resistance values at 75°C for common wire sizes. Use this as a reference for quick cable sizing decisions.
| AWG/kcmil | Copper Amps | Copper Ω/1000ft | Aluminum Amps | Aluminum Ω/1000ft |
|---|---|---|---|---|
| 14 | 15 | 3.070 | - | - |
| 12 | 20 | 1.930 | 15 | 3.180 |
| 10 | 30 | 1.210 | 25 | 2.000 |
| 8 | 40 | 0.764 | 35 | 1.260 |
| 6 | 55 | 0.491 | 45 | 0.808 |
| 4 | 70 | 0.308 | 55 | 0.508 |
| 3 | 85 | 0.245 | 65 | 0.403 |
| 2 | 95 | 0.194 | 75 | 0.319 |
| 1 | 110 | 0.154 | 85 | 0.253 |
| 1/0 | 125 | 0.122 | 100 | 0.201 |
| 2/0 | 145 | 0.097 | 115 | 0.159 |
| 3/0 | 165 | 0.077 | 130 | 0.126 |
| 4/0 | 195 | 0.061 | 150 | 0.100 |
Values based on NEC Table 310.16 for 75°C rated insulation (THWN, XHHW). Derate for higher ambient temperatures or bundled conductors.
Real-World Cable Sizing Examples
Example 1: Residential Subpanel Feeder
Scenario: 100A subpanel feeder, 150 feet from main panel, 240V single-phase, copper wire, 3% maximum voltage drop.
Ampacity Requirement
1 AWG (110A)
VD with 1 AWG
3.85%
VD Requirement
1/0 AWG (2.29%)
Final Selection
1/0 AWG
Result: Although 1 AWG meets ampacity, it fails voltage drop. Use 1/0 AWG copper for both requirements.
Example 2: Commercial HVAC Unit
Scenario: 45A HVAC unit, 200 feet from panel, 208V three-phase, aluminum wire, 3% maximum voltage drop.
Ampacity Requirement
6 AWG (45A min)
VD with 6 AWG
5.42%
VD Requirement
2 AWG (2.31%)
Final Selection
2 AWG Aluminum
Result: Voltage drop is the limiting factor. 2 AWG aluminum provides 75A ampacity with acceptable voltage drop.
Example 3: Industrial Motor Circuit
Scenario: 60A motor, 50 feet from MCC, 480V three-phase, copper wire, 3% maximum voltage drop.
Ampacity Requirement
4 AWG (70A)
VD with 4 AWG
0.33%
VD Requirement
Any (all pass)
Final Selection
4 AWG Copper
Result: Short distance and high voltage make ampacity the only limiting factor. 4 AWG copper is adequate.
Key Factors in Cable Sizing Decisions
Distance
Longer distances increase voltage drop proportionally. For every doubling of distance, voltage drop doubles. Long runs often require wire much larger than ampacity alone would suggest.
Voltage Level
Higher voltages allow longer distances with the same wire size. A 480V circuit can run 4× farther than 120V with the same percentage voltage drop, which is why industrial facilities use higher voltages.
Conductor Material
Aluminum has ~61% the conductivity of copper. For equivalent ampacity, aluminum must be 2 sizes larger (e.g., 2 AWG aluminum ≈ 4 AWG copper). However, aluminum is lighter and less expensive.
Ambient Temperature
Higher ambient temperatures reduce ampacity. In hot locations (attics, rooftops, hot climates), apply derating factors from NEC Table 310.15(B)(1). At 40°C ambient, 75°C wire is derated to 88%.
Conductor Bundling
When multiple current-carrying conductors share a raceway, ampacity must be derated per NEC Table 310.15(C)(1). 4-6 conductors = 80%, 7-9 = 70%, 10-20 = 50% of listed ampacity.
Common Cable Sizing Mistakes
Sizing for Ampacity Only
Many people select wire based solely on ampacity from NEC tables, ignoring voltage drop. For runs over 50-100 feet, voltage drop often requires larger wire than ampacity alone.
Using Wrong Distance
Distance should be measured as the actual cable run, not straight-line distance. Remember to account for vertical runs, routing around obstacles, and extra length for terminations.
Forgetting Derating Factors
Ampacity must be derated for high ambient temperatures and conductor bundling. A wire rated for 30A in ideal conditions may only carry 21A in a hot attic with multiple circuits.
Mismatching Copper/Aluminum
When switching between copper and aluminum, don't use the same size. Aluminum requires larger conductors and special terminations rated AL/CU or CO/ALR.
Using Nameplate Current Only
For motors, use NEC Table 430.248 (FLA values), not nameplate current. Also apply the 125% multiplier for continuous loads when sizing overcurrent protection.