Generator Cable Size Calculator

How to Use Generator Cable Size Calculator

Sizing power cables for diesel, natural gas, or propane generators is critical for electrical system safety and operational efficiency. Undersized cables cause high resistance, resulting in voltage drops and dangerous heat buildup. Follow these step-by-step instructions to calculate the required conductor size:

1. 1
   Enter generator capacity. Input the nominal rating of your generator.
2. 2
   Select kVA or kW. Choose the appropriate capacity unit from the dropdown.
3. 3
   Enter voltage and phase. Input the system operating voltage (e.g. 415 V or 230 V) and choose Single Phase or Three Phase configuration.
4. 4
   Specify cable length. Input the physical distance of the cable run in meters from the generator terminals to the transfer switch or distribution panel.
5. 5
   Choose copper or aluminum conductor. Select the conductor material compound used for the installation.
6. 6
   Select allowable voltage drop. Select the target maximum voltage drop (typically 3% for power feeders, or up to 5% per code).
7. 7
   Click Calculate. Press the Calculate Cable Size button to run the engineering math model.
8. 8
   Review current and recommended cable size. Check the computed generator load current, the recommended minimum cross-sectional area (mm²), and actual voltage drop percentage.

In addition to standard ampacity, the calculator integrates the physical voltage drop limit to determine the final conductor dimensions. This ensures that your generator installation satisfies code criteria for both safety and performance.

How to Calculate Generator Cable Size

Determining the correct cable size involves two distinct constraints: Conductor Ampacity and Voltage Drop. First, the cable must have sufficient thermal capacity to handle the generator's full-load current under continuous operating conditions. Second, the cable resistance must be low enough to prevent excessive voltage drop over the run length.

Step 1 — Calculate Generator Rated Current (Amps)

We first convert active power (kW) to apparent power (kVA) if necessary, and then compute the nominal line current based on system phase type.

Step 2 — Apply the Safety Margin

To prevent overload conditions and satisfy code requirements for continuous loads, apply the selected safety margin (such as 20% or 25%).

Step 3 — Size Conductor by Ampacity Reference

Find the smallest standard conductor cross-sectional area (mm²) that has a rating greater than or equal to the adjusted current. Refer to the standard ratings for Copper (minimum 16 mm², max 300 mm²) and Aluminum (minimum 25 mm², max 300 mm²).

Step 4 — Calculate Size by Voltage Drop Constraint

Calculate the minimum required conductor cross-sectional area (A_vd) that will restrict voltage drop within allowable limits. Conductor resistivity constants used are 0.0175 Ω·mm²/m for copper and 0.0282 Ω·mm²/m for aluminum.

Select the standard commercial size that is greater than or equal to A_vd. The final recommended cable size is the larger of the ampacity size and the voltage drop size.

Step-by-Step Engineering Worked Example

Input Parameters:

• Generator Capacity: 250 kVA
• System Voltage: 415 V
• Power Factor: 0.8
• Phase: Three Phase
• Conductor Material: Copper
• Cable Length: 50 Meters
• Allowable Voltage Drop: 3%
• Safety Margin: 20%

Step 1 — Calculate Rated Current

Current (A) = 250 × 1000 ÷ (1.732 × 415) = 347.7 A

Step 2 — Apply the 20% Safety Margin

Adjusted Current = 347.7 × 1.20 = 417.2 A

Step 3 — Size by Ampacity

By comparing the adjusted current of 417.2 A against standard copper cable ampacity ratings, we find that a 240 mm² copper cable (rated at 415 A) handles the load within a very close margin, whereas a 185 mm² cable (rated at 353 A) is too small. Thus, the minimum size by ampacity is 240 mm².

Step 4 — Verify Voltage Drop Sizing

Allowable Voltage Drop Limit (V) = 415 V × 3% = 12.45 V. The minimum conductor cross-sectional area (A_vd) for a 3% drop over 50 meters is:

A_vd = (1.732 × 50 × 347.7 × 0.0175) ÷ 12.45 = 527.05 ÷ 12.45 = 42.3 mm². Since 42.3 mm² is much smaller than 240 mm², the ampacity constraint governs this installation.

Final Verified Answer

A 250 kVA, 415 V three-phase generator typically requires a 240 mm² copper cable under standard installation conditions. The actual voltage drop will be 0.53%, which is well within the 3% target limit.

Generator Cable Size Chart

This reference chart displays generator rated currents and recommended cable sizes for common generator ratings. The values are calculated assuming a three-phase system at 415 V, a power factor of 0.8, and a 20% safety margin applied to the load current. Cable lengths are assumed to be short, with negligible voltage drop impact.

Note: Actual cable size may vary depending on ambient temperature, installation method and applicable standards.

Generator Cable Size Calculator Frequently Asked Questions