Cable Sizing Calculator
Size AC power cables to IEC 60364: design current, derating for ambient temperature and grouping, ampacity selection and a voltage-drop cross-check.
Inputs
0.87 for 40 °C on PVC
Formula
- Single phase: I_b = P / (V × cos φ); Three phase: I_b = P / (√3 × V × cos φ)
- Required ampacity I_z = I_b / (Ca × Cg)
- ΔV = k × ρ × L × I / A, k = 2 (1-ph) or √3 (3-ph)
- Selected size satisfies I_z(cable) ≥ I_b / (Ca·Cg) and ΔV% ≤ limit
IEC 60364-5-52 requires a cable to satisfy three independent checks: thermal capacity after derating, voltage drop, and short-circuit withstand. This calculator performs the first two. Derating factors reduce the tabulated ampacity for hot environments and for cables bunched together, so the cable actually installed must be sized on the corrected value, not the table value.
Step-by-step calculation
Design current
P / (√3 × V × cos φ)
49.101 A
Total derating
Ca × Cg
0.696
Required ampacity
I_b / (Ca × Cg)
70.55 A
Ampacity selection
first size with I_z ≥ required
16 mm²
Voltage drop check
√3 × ρ × L × I / A
5.485 V
Worked example
- 30 kW, 415 V three phase, pf 0.85, 60 m run in copper, Ca 0.87, Cg 0.8.
- I_b = 30 000 / (1.732 × 415 × 0.85) = 49.1 A → I_z required = 49.1 / 0.696 = 70.5 A.
- 16 mm² copper (72 A) meets ampacity; voltage drop = 1.732 × 0.0172 × 60 × 49.1 / 16 = 5.5 V = 1.32% — acceptable.
Assumptions
- Ampacity table is for PVC-insulated copper/aluminium in conduit at 30 °C ambient.
- Balanced load and steady-state operation; motor starting currents are not included.
Tips
- For long runs voltage drop, not heating, usually governs the size — check both as this tool does.
- Aluminium needs roughly one to two sizes larger than copper for the same duty.
Warnings
- Always verify the short-circuit withstand A ≥ I√t / k separately before finalising a size.
- Protective device rating must satisfy I_b ≤ I_n ≤ I_z, and I₂ ≤ 1.45 I_z.
Standards & references
- IEC 60364-5-52
- IS 732:2019
- IEC 60287
Frequently asked questions
What cable size do I need for a 30 kW three-phase load?
At 415 V and 0.85 power factor the design current is about 49 A. With typical derating you need 16 mm² copper, or 25 mm² aluminium, subject to a voltage-drop check on the route length.
What is a derating factor?
A multiplier below 1.0 that reduces a cable's tabulated current capacity for high ambient temperature, grouping with other cables, thermal insulation or burial conditions.
How much voltage drop is allowed?
IEC 60364 recommends 3% for lighting and 5% for power circuits, measured from the origin of the installation to the point of use.
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