A generator cable size calculator converts your generator rating, voltage, phase, and cable length into a practical conductor recommendation using ampacity, voltage drop, and a safety margin.
The maximum continuous current a conductor can carry without exceeding its temperature rating.
The reduction in voltage along the cable length. For long runs, voltage drop often determines the minimum cable size.
Continuous loads require a 125% safety factor so the conductor is not operating at its absolute limit.
The calculator combines the full‑load current, a 125% safety margin, the conductor's ampacity, and the voltage drop over the cable length. The result is a recommended cable size in mm² and AWG, plus a matching breaker rating.
Select the conductor material and see the size difference side by side.
Supports single‑phase and three‑phase systems at 50 Hz and 60 Hz.
Includes actual voltage drop percentage and a recommended breaker rating for the selected cable.
Use the result as the basis for a factory‑direct generator and cable package quotation.
Enter your generator rating, system voltage, phase, cable length, and conductor material. The calculator returns the minimum safe cable size in mm² and AWG, plus voltage drop and breaker rating.
All fields are required. The result updates when you click "Calculate Cable Size".
Quick reference for three‑phase, 415 V generators at 0.8 PF using XLPE‑insulated copper cable. Use this as a starting point, then confirm with the calculator for your exact voltage, length, and material.
| Generator Rating | Full‑Load Current | Recommended Copper Cable |
|---|---|---|
| 15 kVA | 20 A | 4 mm² |
| 25 kVA | 35 A | 6 mm² |
| 40 kVA | 55 A | 10 mm² |
| 62.5 kVA | 87 A | 16 mm² |
| 82.5 kVA | 115 A | 25 mm² |
| 100 kVA | 140 A | 35 mm² |
| 125 kVA | 175 A | 50 mm² |
| 160 kVA | 225 A | 70 mm² |
| 200 kVA | 280 A | 95 mm² |
| 250 kVA | 350 A | 120 mm² |
Follow this four‑step method to move from your generator rating to a practical cable size. The calculator automates the process, but understanding the method helps you verify the result.
For three‑phase: A = (kVA × 1000) ÷ (1.732 × V). For single‑phase: A = (kVA × 1000) ÷ V. If your generator is rated in kW, convert: kVA = kW ÷ PF.
Continuous loads require a margin. Per NEC, size at 125% of full‑load current. For a 250 kVA, 415 V three‑phase set, 347.7 A × 1.25 = 434.6 A.
Choose the smallest standard conductor whose ampacity meets or exceeds the adjusted current. For 435 A, a copper 240 mm² cable (490 A) is suitable.
For long runs, voltage drop often dictates the size. Use A = (k × ρ × I × L) ÷ VDallowed where k = 2 (1‑phase) or 1.732 (3‑phase), ρ = resistivity, and VDallowed = V × allowed %.
The final cable size is the larger of the ampacity result and the voltage‑drop result, rounded up to the next standard IEC size: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240 mm².
Full‑load current = 347.7 A. With 125% margin = 434.6 A. Ampacity points to 240 mm² copper. Voltage drop with 240 mm² at 30 m is only 0.35%, so ampacity governs. Final size = 240 mm² (4/0 AWG).
A cable's current‑carrying capacity is not fixed. These variables can force you to select a larger conductor than the base chart suggests.
As surrounding air temperature rises, the cable dissipates less heat. A cable rated at 25 °C carries noticeably less current at 40 °C or 45 °C, common in hot climates and enclosed plant rooms.
Free‑air cable on tray or ladder carries the most current. The same conductor in conduit, in a duct, or directly buried carries less. Always match the installation method to the correct ampacity table.
When several circuits run together in one conduit or trench, cables heat each other. Bundled cables require derating, sometimes by 20% or more.
Motor‑starting and non‑linear loads draw inrush currents and harmonics that add heat beyond the simple full‑load figure. Generator feeders serving large motors often warrant an extra margin.
Copper carries more current per cross‑section and terminates more reliably. Aluminum is lighter and cheaper but needs a larger size for the same voltage drop and requires careful termination.
Long runs increase voltage drop. Beyond a certain distance, the voltage‑drop requirement, not ampacity, dictates a larger cable. A cable that works for 20 m may be undersized at 100 m.
Most sizing problems come from a small number of recurring assumptions. Avoiding these errors helps prevent both undersized conductors and unnecessary overspending.
Ampacity is not the only constraint. For long cable runs, voltage drop often determines the minimum size. A conductor that works at 20 m may be undersized at 100 m.
Continuous loads require a margin. Sizing exactly to the full‑load current means the conductor operates at its absolute limit and can overheat under normal conditions.
Copper and aluminum have different resistivities. Using copper values for aluminum (or vice versa) produces an incorrect voltage‑drop calculation and the wrong size.
A cable rated in free air at 25 °C carries less current at 40 °C, in conduit, or bundled with other circuits. Apply derating factors before selecting the final size.
The cable must also be coordinated with the breaker and the load. A cable that matches the generator's output may still be unprotected if the breaker is incorrectly selected.
Final cable selection should be checked against the complete load schedule, installation conditions, local electrical codes, and the generator's output breaker rating before equipment is ordered.
A correctly sized cable is only half the equation. As a factory‑direct manufacturer, Shandong Huali delivers the complete package — generator set, connection guidance, and engineering support — without the markup of intermediaries. 8 kVA to 4,000 kVA, prime and standby, with Cummins, Perkins, Weichai, and Yuchai engines and Stamford alternators.
A: The cable size depends on the generator’s full‑load current, cable length, conductor material, and allowable voltage drop. For example, a 100 kVA, 415 V three‑phase generator (≈140 A) typically uses a 35 mm² copper cable for short runs, stepping up for longer distances. Use the calculator above for your exact figures.
A: Calculate the full‑load current (kVA × 1000 ÷ voltage ÷ 1.732 for three‑phase), apply a 125% safety margin, then size the conductor to meet both ampacity and voltage‑drop limits. The final size is the larger of the two results, rounded up to the next standard IEC size.
A: Copper carries more current per cross‑section and terminates more reliably, but costs more. Aluminum is lighter and cheaper but needs a larger size for the same voltage drop and requires careful termination with anti‑oxidation compound. For critical or short runs, choose copper; for long, budget‑sensitive industrial runs, aluminum XLPE is common.
A: The breaker must protect the cable, so its rating should not exceed the conductor’s ampacity. Size the cable first, then select an overcurrent device rated at or below the cable’s safe current, typically matching the generator’s output breaker rating. The calculator recommends a breaker rating alongside the cable size.
A: Yes. Long runs increase voltage drop, and beyond a certain distance the voltage‑drop requirement, not ampacity, dictates a larger cable. A cable that works for 20 metres may be undersized at 100 metres.
