Step 1: Establish the design current
Start with the actual load. Calculate the design current (Ib) from the connected load, applying diversity where it genuinely applies and accounting for power factor and starting current on motor circuits.
The protective device rating (In) must then be at least equal to the design current, and the cable's rated capacity (Iz) must be at least equal to the protective device rating. That ordering — Ib ≤ In ≤ Iz — is the foundation of the whole exercise.
Step 2: Read the base current carrying capacity
Manufacturer tables give a base current rating for each size, but that figure is tied to a specific reference condition: a stated ambient temperature, a stated installation method and a single circuit with no adjacent cables.
Your installation will almost never match those reference conditions, which is what Step 3 corrects for.
Step 3: Apply derating factors
Each factor below is a multiplier applied to the base rating. They compound — a cable in a hot plant room bunched with five others can end up with well under half its table value.
| Factor | What drives it |
|---|---|
| Ambient temperature | Air or ground temperature above the reference value |
| Grouping | Number of circuits bunched together in a tray, duct or conduit |
| Installation method | Clipped direct, tray, conduit, buried or in free air |
| Soil thermal resistivity | Ability of the surrounding soil to carry heat away (buried runs) |
| Depth of laying | Burial depth for direct-buried cable |
Derating factors multiply. Two factors of 0.80 and 0.75 give 0.60, not 0.775. This is the single most common error in hand-calculated sizing.
Step 4: Check voltage drop
A cable can be thermally adequate and still unusable. On long runs, voltage drop rather than current rating sets the size.
Calculate the drop over the full route length at design current and check it against the project limit. IS 732 recommends the drop from the origin of the installation does not exceed 3% for lighting circuits and 5% for other uses — confirm the figure your specification actually calls for, as project requirements are often tighter.
Motor circuits deserve a second look here. Starting current is several times running current, and the momentary drop during start must still leave enough terminal voltage for the motor to accelerate its load.
Step 5: Verify short-circuit withstand
During a fault the conductor must carry prospective fault current for the protective device's disconnection time without the insulation exceeding its short-circuit temperature limit.
The adiabatic check is S ≥ √(I²t) / k, where S is the cross-sectional area, I the fault current, t the disconnection time and k a constant set by the conductor and insulation combination. XLPE tolerates a higher short-circuit temperature than PVC, so it gives a more favourable k value for the same conductor.
On systems with high fault levels this check occasionally sets a larger size than the current rating did — particularly for short, heavily loaded feeders close to the transformer.
Step 6: Confirm the earth fault loop
Finally, verify that the earth fault loop impedance is low enough for the protective device to operate within the required disconnection time. On long circuits this can require increasing the protective conductor size even when the phase conductors are adequate.
Reference standards: IS 3961 for current ratings, IS 7098 for XLPE cable construction, IS 732 for installation practice and IEC 60364-5-52 for wiring system selection. Always size against the current edition and the manufacturer data sheet for the specific cable.