Cable Sizing Calculator

How cable sizing actually works

A cable is sized by four independent checks, and the answer is the largest size any of them demands. Skipping one is how undersized cable gets installed.

1. Design current

The current the circuit will actually draw. For a three-phase load:

Ib = P / (√3 × V × cos φ × η)

The efficiency term η matters. A motor rated 45 kW at the shaft with 92 % efficiency draws current for roughly 49 kW at the terminals. Leaving efficiency out understates the current by about 8 %, which is often a full cable size.

2. Derating

Published current ratings assume one cable, in free air, at a reference ambient temperature. Real installations are hotter, bunched, and enclosed. Three factors apply:

• Ambient temperature — a 45 °C plant room costs you 5 % against a 40 °C reference.
• Grouping — the big one. Four circuits bunched and touching removes a quarter of the capacity.
• Installation method — conduit and duct trap heat; free air does not.

These multiply. A 45 °C ambient, four bunched circuits on a tray gives 0.95 × 0.75 × 0.95 = 0.677. The cable has lost a third of its capacity before you have chosen it.

Iz ≥ Ib / (Ca × Cg × Ci)

3. Volt drop

Every cable has a published drop in millivolts per ampere per metre. IS 732 permits 3 % for lighting and 5 % for power circuits, measured from the origin of the installation.

Vd = (mV/A/m × Ib × L) / 1000

Volt drop governs the selection more often than people expect on runs past about 100 metres. When it does, this calculator tells you so explicitly, because it changes how you would fix it — a bigger cable helps volt drop, but so does moving the distribution board closer.

4. Short-circuit withstand

The check almost everyone skips. During a fault, the cable must survive the let-through energy until the protective device clears:

S ≥ √(I² t) / k

where k is 115 for PVC-insulated copper and 76 for PVC-insulated aluminium, I is the prospective fault current, and t is the clearing time. A cable that passes all three checks above can still be destroyed by a fault it has to hold for a tenth of a second.

This calculator does not run that check, because it depends on your board fault level and breaker curve rather than on the cable alone. Take the result here to your protection study.

Why aluminium is not simply cheaper

Aluminium costs less per metre but carries roughly 78 % of the current of the same copper size, so you typically move up two sizes. Larger cable means larger glands, larger bending radius, more tray width and bigger terminations. On short runs the copper often wins once you price the accessories; on long runs at high current, aluminium usually still wins.

About these tables

The current ratings used here are representative values for PVC insulated armoured cable in air at 40 °C. Real ratings vary between manufacturers by 5–10 % for the same nominal size. Use this for preliminary sizing and estimation; confirm against the datasheet of the cable you are actually buying before issuing a design.

What this calculator does not check

It runs the two conditions that depend only on the cable and its surroundings: derated current capacity, and volt drop. Two more decide real installations, and both need information from outside this page.

• Breaker coordination. The protective device has to land between the design current and the cable’s derated capacity. Where only one standard rating fits, and barely, the cable is marginal even though nothing has formally failed.
• Short-circuit withstand. The cable must survive the fault current for as long as the device takes to clear it. That needs your board fault level and the device curve — and on short runs at high fault levels it is often the check that picks the size.

Getting the design current right

Everything downstream depends on the number you put in, and two adjustments get missed.

Motor circuits are continuous duty, so the cable is sized at 1.25 times full load current before any derating. Enter 1.25 × FLC, not FLC.

Efficiency belongs in the current calculation. A motor nameplate gives shaft output; the supply provides more than that. Leaving efficiency out understates the current by 8–10 %.

Cores, armour and the neutral

• 3.5-core has a reduced neutral. Fine for balanced three-phase load, wrong wherever harmonic current from drives or LED lighting flows — triplen harmonics add in the neutral instead of cancelling, and it can carry more than the lines.
• Never use steel armour on single-core AC cable. The alternating field induces eddy currents in the steel, which heats and derates the cable badly. Use aluminium wire armour, or none.
• Trefoil or flat spaced changes the rating of single-core runs, and the drawing has to say which — otherwise site does whatever is easiest and the rating you designed to no longer applies.