Quick answer: To calculate cable size, work in four steps: (1) find the design current — for three-phase use I = P ÷ (√3 × V × pf), for single-phase use I = P ÷ (V × pf); (2) apply derating factors for ambient temperature and grouping; (3) select the cable whose derated current-carrying capacity is greater than the design current; (4) verify voltage drop stays within 3% and that the cable can withstand the short-circuit current.
Why correct cable sizing matters
An undersized cable overheats, degrades its insulation, wastes energy as heat and becomes a fire risk. An oversized cable is safe but expensive and hard to terminate. Correct sizing is the balance between the four constraints below: ampacity, derating, voltage drop and short-circuit withstand.
Step 1 — Calculate the design (load) current
Three-phase: I = P / (√3 × V × pf) (V = line-to-line voltage)
Single-phase: I = P / (V × pf)
Motors: use the rated full-load current (FLC) from the nameplateWhere P is power in watts, V is voltage, and pf is the power factor (typically 0.8–0.9 for motors).
Step 2 — Apply derating factors
Published ampacity assumes ideal conditions. Multiply it by correction factors for high ambient temperature, for cables grouped/bunched together, and for the installation method (buried, in conduit, on tray). The result is the derated ampacity.
Step 3 — Select from the ampacity table
| Copper cable (mm²) | Approx. ampacity in air (A) | Typical use |
|---|---|---|
| 1.5 | 19 | Lighting circuits |
| 2.5 | 26 | Socket / small power |
| 4 | 35 | Power circuits |
| 6 | 45 | Sub-mains, small motors |
| 10 | 62 | Feeders |
| 16 | 82 | Motor feeders |
| 25 | 108 | Distribution mains |
| 35 | 135 | Distribution mains |
| 50 | 168 | Large feeders |
Indicative values only — always confirm against the manufacturer datasheet and local code (IS 732 / IEC 60364 / NEC).
Enter load, voltage, length and installation conditions — the calculator handles derating and voltage drop for you.
Open the Cable Size Calculator →Worked example — 15 kW three-phase motor
- Design current: I = 15,000 ÷ (√3 × 415 × 0.85) ≈ 24.6 A
- Grouping + ambient derating factor ≈ 0.8 → required ampacity = 24.6 ÷ 0.8 ≈ 30.8 A
- From the table, 4 mm² (35 A) clears it — but check voltage drop over the run length.
- If the cable is long, step up to 6 mm² to keep voltage drop under 3%.
Step 4 — Verify voltage drop and short circuit
A cable can be big enough for current yet still fail on voltage drop over a long run. Always check both: keep steady-state voltage drop ≤3% for feeders (≤5% overall), and confirm the conductor withstands the prospective fault current for the breaker's clearing time.
Related MEP calculators
Pair this with the Voltage Drop Calculator, the Motor Starter Sizing Calculator, and the Earthing & Grounding Calculator.
Frequently Asked Questions
What is the formula for cable size calculation?
First find the design current: three-phase I = P / (root3 x V x pf), single-phase I = P / (V x pf). Then select a cable whose derated ampacity exceeds that current and check voltage drop.
What cable size do I need for a 15 kW motor?
A 15 kW three-phase 415 V motor draws about 24-25 A. After derating, 4 mm2 copper is usually sufficient, but longer runs may require 6 mm2 to keep voltage drop within limits.
What is derating in cable sizing?
Derating reduces a cable rated ampacity to account for real conditions such as high ambient temperature, cables bunched together, and the installation method. You size against the derated value.
How much voltage drop is allowed?
Keep steady-state voltage drop within 3% for feeders and 5% total from source to final load, per IEC and NEC recommendations.