Cable Size Calculator
Cable Size Calculator — quick estimate
Estimate the cable size (sqmm) you need so voltage drop stays within limits, for copper or aluminium conductors. Adjust the inputs, then get an engineered electrical design from our team.
Your circuit
Sizes the conductor on voltage-drop basis (assumed pf 0.85). A full design also checks current-carrying capacity, derating, short-circuit and earthing.
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Indicative estimates only — not a binding design. Actual cable selection must verify ampacity, derating factors, short-circuit withstand and protection coordination per IS/IEC.
How this calculator works — and where it stops
What it calculates
The cable size in mm² for an LT feeder from its load, length and permitted voltage drop, for copper or aluminium, with the resulting voltage drop.
Inputs
- Load in kW
- System — three-phase 415 V or single-phase 230 V
- Conductor — copper or aluminium
- Route length
- Permitted voltage drop
Method
- Current = load ÷ (√3 × V × 0.85) for three-phase, or load ÷ (V × 0.85) for single-phase, at an assumed 0.85 power factor.
- Steps through the standard sizes from 1.5 to 400 mm² against an approximate derated ampacity table for copper (aluminium at 80% of it), then checks the voltage drop over the route length from the conductor's resistivity, and selects the smallest size that satisfies both.
Assumptions
- Power factor 0.85 for every load.
- The ampacity table already carries a typical derating for grouping and ambient temperature; it is one typical installation condition, not yours.
- Reactance is ignored in the voltage-drop check, which is acceptable at these sizes and lengths but not for very large cables.
Limitations
- No short-circuit withstand check — the size must also survive the fault current for the protection's clearing time.
- No derating for the actual installation method, grouping, soil thermal resistivity or ambient; those can change the answer by a size or two.
- HT cables, bus ducts and parallel runs are outside the tool.
Worked example
Produced by running this calculator with the inputs below.
Inputs
- Load: 75 kW
- System: three-phase 415 V
- Conductor: copper
- Route length: 90 m
- Permitted voltage drop: 3%
Working
- Current = 75,000 ÷ (1.732 × 415 × 0.85) = 123 A
- Permitted drop = 3% of 415 V = 12.45 V
- CSA for voltage drop = (1.732 × 123 × 0.0175 × 90) ÷ 12.45 = 26.9 mm² → next standard size 35 mm²
- Ampacity: the first size in the derated table carrying 123 A is 70 mm²
- The larger of the two governs: 70 mm², selected on current capacity, not voltage drop
Result. 70 mm² copper, governed by current-carrying capacity; actual voltage drop 1.15%. Still to be checked for short-circuit withstand and the actual installation method.
How engineers use the result
To estimate cable sizes for a budget schedule and to check that a proposed size is not obviously undersized for the length.
When a professional design must replace it
Before the cable schedule is issued: sizes are fixed with the installation-method derating, the fault-level check and the protection coordination, to IS 732 and the manufacturer's data.
Got your number — what next?
A calculator gives you a first figure. These take it further.
The work itself
Read before you specify
The terms behind it
Delivered work with this scope
Need an exact BOQ, rate analysis or measurement sheet?
This is a quick engineering estimate. For tendering, billing or audit, our QS & estimation team prepares an item-wise BOQ, rate analysis and a verified measurement sheet to IS / CPWD norms — backed by 15+ years and ISO 9001:2015 quality processes.