Cable Size Calculator
How the formula works
The calculator picks the smallest standard cable that passes three checks. The largest of the three sizes is the recommended size.
1. Design current
3-phase: Ib = P × 1000 / (√3 × V × pf)
1-phase: Ib = P × 1000 / (V × pf)
For kVA input, remove pf from the formula.
2. Current-carrying capacity
Required tabulated rating = Ib / (Kt × Kg)
Where:
- Ib = design current (A)
- Kt = ambient temperature factor
- Kg = grouping factor
3. Voltage drop
3-phase: Vd = √3 × Ib × L × (R cosφ + X sinφ) / 1000
1-phase: Vd = 2 × Ib × L × (R cosφ + X sinφ) / 1000
R is conductor resistance at operating temperature (ohm/km), X is taken as 0.08 ohm/km, L is one-way length in metres.
4. Short-circuit withstand (optional)
S min = I × √t / k, with I in amps and t in seconds. Typical k: Cu PVC 115, Cu XLPE 143, Al PVC 76, Al XLPE 94.
Typical values: ambient factors for 35, 40, 45, 50, 55 °C are 0.94, 0.87, 0.79, 0.71, 0.61 (PVC) and 0.96, 0.91, 0.87, 0.82, 0.76 (XLPE). Grouping factors for 2 to 6 circuits are 0.80, 0.70, 0.65, 0.60, 0.57. These follow IEC 60364-5-52 practice. The local code and the cable maker’s data are final.
How to use the calculator: select the options, enter load and length, click Calculate, then download the PDF if needed.
Important notes: ratings are typical for multicore cables on a tray or clipped direct. Buried and conduit installations need different tables.
Worked example
Example 1: 50 kW pump, 3-phase
415 V, pf 0.85, Cu XLPE, 40 °C (0.91), 2 circuits grouped (0.80), 80 m, limit 4 %.
Ib = 50 × 1000 / (1.732 × 415 × 0.85) = 81.8 A. Total derating = 0.728. Current needs 25 mm² (rating 82.3 A). Voltage drop alone needs 16 mm². Result: 25 mm² Cu, Vd = 8.95 V (2.16 %).
Example 2: 100 A feeder
415 V, pf 0.9, Cu PVC, 30 °C, single circuit, 50 m, limit 3 %.
Current needs 35 mm² (rating 111 A). Result: 35 mm² Cu, Vd = 4.90 V (1.18 %).
Example 3: 3 kW single-phase load
230 V, pf 0.9, Cu PVC, 40 °C (0.87), 40 m, limit 3 %.
Ib = 3000 / (230 × 0.9) = 14.5 A. Current needs only 2.5 mm², but voltage drop needs more. Result: 4 mm² Cu, Vd = 5.42 V (2.36 %), governed by voltage drop.
Example 4: 200 kVA, Al cable with fault check
415 V, pf 0.8, Al XLPE, 30 °C, 3 circuits (0.70), 150 m, limit 5 %, fault 25 kA for 1 s.
Ib = 200 × 1000 / (1.732 × 415) = 278.2 A. S min = 25000 × 1 / 94 = 266 mm². No single cable passes, so two parallel cables are used. Result: 2 × 150 mm² Al per phase, rating 352.7 A, Vd = 8.71 V (2.10 %), governed by short-circuit.
Common mistakes
- Sizing only by current and skipping the voltage drop check on long runs.
- Using the cable’s full tabulated rating without ambient temperature and grouping derating.
- Entering the total round-trip length instead of the one-way length.
- Using kW as if it were kVA and forgetting the power factor.
- Choosing aluminium and forgetting its higher resistance and larger size.
- Ignoring the short-circuit withstand check when the fault level is high.
- Using tray-installation ratings for cables buried in the ground or in conduit.
- Selecting the cable first and the breaker later, so the cable does not match the protection.
- Not allowing for motor starting current, which can cause a large temporary voltage drop.
Frequently asked questions
How do I choose a cable size?
Find the design current, apply derating factors, then pick the smallest size that also meets the voltage drop and short-circuit limits.
What is the usual voltage drop limit?
Many codes suggest about 3 % for lighting and 5 % in total. Some projects use 4 % for feeders. Your local code and specification decide.
Why is the cable size different from the current-only size?
Long cables lose more voltage. The voltage drop check often needs a larger size than the current check.
Copper or aluminium?
Copper carries more current for the same size. Aluminium is lighter and cheaper but needs a larger cross-section and proper terminations.
Why is the minimum Al size 16 mm²?
Small aluminium conductors are mechanically weak. Many codes set a minimum size for aluminium cables.
PVC or XLPE?
XLPE runs hotter (90 °C) and carries more current than PVC (70 °C) for the same size.
What is the grouping factor?
Cables laid close together heat each other, so each one must carry less current.
What is the short-circuit check?
The conductor must survive the fault for the time the protection takes to clear it. The formula is S = I × √t / k.
When are parallel cables used?
When one cable of the largest size cannot carry the load or meet the limits. This calculator tries 2 to 6 cables per phase.
Does the result apply to buried cables?
No. The ratings are typical for cables on a tray or clipped direct. Use manufacturer tables for buried or conduit installations.
What power factor should I use?
Use the real value if known. Typical values are 0.8 to 0.9 for motors and 0.95 to 1.0 for heaters and lighting.
Is this calculator a final design?
No. It gives a preliminary size. Check the final design against the local code and the cable data sheet.