3-Phase Cable Size Calculator
Enter your system values below. This tool estimates cable cross-sectional area (mm²) based on ampacity and voltage drop.
How this cable size 3 phase calculator works
This cable size 3 phase calculator is designed to give a practical first-pass cable recommendation for three-phase electrical systems. It combines two critical checks:
- Ampacity check: The cable must carry the design current safely.
- Voltage drop check: The cable must keep voltage drop within your allowed limit over the run length.
The tool then recommends the smallest standard cable size (in mm²) that satisfies both conditions. This approach is commonly used in preliminary electrical design for motors, panels, pumps, HVAC systems, and industrial feeders.
Core equations used
1) Three-phase current from power
If you provide load power in kW, current is estimated by:
I = P × 1000 / (√3 × V × PF × η)
Where:
- I = line current (A)
- P = power (kW)
- V = line-to-line voltage (V)
- PF = power factor
- η = efficiency (decimal)
2) Design current with margin
To allow future growth and avoid selecting a cable too close to limits:
Idesign = Iload × (1 + margin%)
3) Voltage drop estimate
Voltage drop is estimated from typical mV/A/m values:
Vdrop = (mV/A/m × I × L) / 1000
Then percentage drop:
Vdrop% = (Vdrop / V) × 100
Input guide
- Line Voltage: Typical three-phase low-voltage systems are 380V, 400V, or 415V depending on region.
- Load Power (kW): Total real power demand. You can skip this if you already know line current.
- Line Current Override (A): If entered, the calculator uses this value instead of kW-based current.
- Power Factor: Common range is 0.8 to 0.95 for many industrial loads.
- Efficiency: Use motor/drive efficiency if known; otherwise a reasonable estimate is often 90%–96%.
- Cable Length: One-way route length (not loop length).
- Max Voltage Drop: Many designs target 2% to 5% depending on feeder type and local standards.
- Design Margin: Additional capacity allowance; 10% to 30% is common depending on project needs.
- Material: Copper carries more current per size and generally has lower voltage drop than aluminum.
- Ambient + Installation: Used for simple derating of base ampacity.
Practical example
Suppose you have a 30 kW load at 415V, PF 0.90, efficiency 95%, and a 50 m run with a 3% voltage drop limit. Add a 25% design margin and use copper cable in conduit.
The calculator estimates load current, increases it by design margin, applies derating factors, and checks each standard cable size. It then selects the first cable that passes both ampacity and voltage-drop constraints.
In many similar cases, the selected cable is often in the 10–16 mm² range, but the exact value depends strongly on routing method, ambient temperature, and voltage-drop target.
Important engineering notes
1) This is a design aid, not a code replacement
Final cable selection must comply with your local code and standards (such as IEC, NEC, BS, AS/NZS, or local utility requirements).
2) Short-circuit withstand is not included here
Real designs also verify adiabatic short-circuit thermal limits and protective device coordination.
3) Grouping, soil thermal resistivity, and harmonics can change results
Where multiple circuits run together, where soil conditions are poor, or where non-linear loads create harmonics, ampacity can reduce significantly. Apply detailed derating and manufacturer data in final design.
When to choose a larger cable than the minimum
- Long feeder lengths where startup voltage dip is critical
- High inrush motor loads
- Future expansion plans
- Warm installation environments
- Low-loss / energy-efficiency optimization goals
FAQ
Is one-way or round-trip length required?
This calculator uses one-way length with standard three-phase mV/A/m values. Enter actual route length from source to load.
What if I know current but not kW?
Enter the line current in the override field. The tool will prioritize that value.
Can I use this for motor branch circuits?
Yes, for initial estimates. For final motor circuit design, include starting current, duty cycle, protection settings, and the applicable electrical code requirements.
If you want, you can run several scenarios (different PF, longer lengths, copper vs aluminum) to see how sensitive the result is. That is often the fastest way to make better cable decisions early in design.