Generator Wire Size Calculator
Use this calculator to determine the correct wire gauge for your generator installation based on distance, amperage, and voltage drop requirements.
Generator Wire Size Inputs
List of the Top 10 Best Generator Cord:
About Generator Wire Sizing
Choosing the correct wire size for your generator is critical to ensure both safety and efficiency. Proper wire sizing prevents excessive voltage drop, overheating, and potential fire hazards. This calculator takes into account key factors such as amperage load, wire length, phase type, voltage, and conductor material to recommend the ideal wire gauge for your generator installation.
Wire Size Selection Table
| Amps | Copper Wire (AWG) | Aluminum Wire (AWG) |
|---|---|---|
| 20A | 12 AWG | 10 AWG |
| 30A | 10 AWG | 8 AWG |
| 50A | 6 AWG | 4 AWG |
| 70A | 4 AWG | 2 AWG |
| 100A | 3 AWG | 1 AWG |
| 125A | 1 AWG | 2/0 AWG |
| 150A | 1/0 AWG | 3/0 AWG |
| 200A | 3/0 AWG | 250 kcmil |
| 300A | 500 kcmil | 700 kcmil |
| 400A | 750 kcmil | 1000 kcmil |
Understanding Voltage Drop
Voltage drop occurs when current flows through a conductor, causing the voltage at the load end to be lower than the source. Excessive voltage drop can lead to inefficient operation, motor overheating, and equipment failure. NEC recommends keeping voltage drop below 3% for branch circuits and 5% total for feeders and branch circuits combined.
Key Factors That Influence Wire Size
- Current Load (Amperage): The higher the amps, the larger the wire size required.
- Wire Length: The longer the distance, the more resistance, increasing the required wire size.
- Voltage: Higher voltage systems typically require smaller conductors for the same load due to lower current.
- Conductor Material: Copper has better conductivity than aluminum, requiring smaller wire size for same load.
- Phase Type: Single-phase vs three-phase systems affect current and wire sizing formulas.
- Allowable Voltage Drop: Lower allowed drop requires thicker wires to maintain voltage stability.
Wire Size Calculation Formula
Wire size is determined using the following formula:
Wire Size = (2 × Length × Current × Resistivity) / Allowable Voltage Drop
- For 3-phase systems, use
√3 × Length × Currentinstead of 2 × Length - Resistivity values:
- Copper: 11.2 ohms per mil foot
- Aluminum: 17 ohms per mil foot
NEC and Code Compliance
Always refer to National Electrical Code (NEC) Article 310 for conductors for general wiring. While this calculator provides estimates, always verify calculations against the NEC, local building codes, and consult a licensed electrician for critical installations.
FAQs About Generator Wire Sizing
Why is wire size so important for generators?
Undersized wires may overheat, damage insulation, or cause fires. Oversized wires add unnecessary cost but improve safety margins. Correct sizing balances safety, efficiency, and cost.
Is copper always better than aluminum?
Copper conducts electricity better but is more expensive. Aluminum is cheaper but needs larger gauge to carry same current. Both are safe when properly installed and sized.
Can I use this calculator for buried cables?
Yes, but for buried cables consider additional factors such as soil temperature, conduit fill, and insulation type which may affect ampacity. NEC Table 310.16 applies for adjustments.
How does temperature affect wire sizing?
Higher ambient temperatures reduce conductor ampacity. Consult NEC correction factors for temperatures above 86°F (30°C).
Do I include starting surge for generator sizing?
No. Wire size depends on steady-state amperage, not generator surge or starting load.
Should I oversize wires for future expansion?
Yes, consider upsizing conductors if you plan future generator upgrades or longer cable runs to new subpanels.
Does conduit type affect wire size?
Indirectly. Certain conduits may require derating depending on fill capacity, heat buildup, and number of conductors.
Is voltage drop more important at longer distances?
Yes. Longer runs increase resistance and cause higher voltage drop. That’s why distance is a major input in wire size calculations.
How much voltage drop is acceptable?
Generally 3% or less for feeders. Total voltage drop (feeder + branch) should stay under 5% to protect appliances and motors.
Can I mix aluminum and copper wiring?
Only with proper approved connectors rated for mixed metals. Improper connections between copper and aluminum may cause corrosion and heating.