Cable current capacity chart for Pakistan — copper and aluminium

Read the assumptions before you read the amps

An ampacity without a stated installation method is not a number. The same 6 mm² conductor carries a different current clipped to a wall, buried in a conduit, or bunched with five other circuits in a tray, and a chart that does not say which one it means is guessing on your behalf. Every figure here carries the same four assumptions, printed in the caption of each table.

Read across the row, not down the page. A 6 mm² copper cable is rated 46 A with two loaded conductors and 41 A with three, clipped direct at 30 °C ambient under BS 7671 reference method C. That is the tabulated figure. Divide it by the correction factors for your own ambient temperature and for the number of circuits bunched with it, and the derated figure — never the table figure — is what the protective device has to sit under. In a Lahore roof void at 45 °C with two other circuits in the same bunch, that 46 A becomes 46 × 0.79 × 0.70 = 25 A.

Where a size is quoted on this site — on a standard cable datasheet, in the cable size calculator, or in a guide — it is read off these same tables, on these same assumptions.

Copper cable current capacity chart

Two loaded conductors is the single-phase case: live and neutral. Three loaded conductors is the three-phase case. The column counts conductors that carry current, not cores in the sheath — a 3 core cable feeding a single-phase load is read off the two-conductor column, because the earth is not loaded.

The sizes an ordinary Pakistani house lives on are the middle of that ladder: lighting on 1.5 mm², sockets on 2.5 mm², air conditioners on 4 mm², geysers and small motors on 6 mm², the submain from the meter on 10 mm² or 16 mm², and the incoming three-phase main on 25 mm².

Copper conductor data — resistance, stranding, weight

Resistance is the row that settles arguments. It is a measurable property of the conductor, fixed by IEC 60228 for a given size and class, and it is the honest way to check whether a coil is what it was sold as. Class 2 is the stiffer stranded conductor used in standard fixed-wiring cable; class 5 is the fine-strand flexible build.

The mass column is a buyer's tool, not a spec-sheet ornament. Copper in a coil = size × number of cores × 8.89 kg/km × length. A 90 m coil of 25 mm² 4 core holds 25 × 4 × 8.89 = 889 kg/km, which is 889 × 0.090 = 80 kg of copper before any insulation or sheath. Weigh your own coil, subtract nothing, and you have a floor to compare against. It is a method, not an accusation — coil lengths and sheath weights differ honestly between makers, so ask for the coil length with the rate.

Aluminium cable current capacity chart

Aluminium is the metal for service drops, long feeders and anything above roughly 25 mm² where the copper price stops making sense. In the market it is often sold as "silver cable", which is a trade nickname for the metal's colour, not a different material. It starts at 10 mm² because below that the terminals and the mechanical handling stop being worth it.

Two ratios fall straight out of the two charts and they are worth committing to memory. Aluminium carries about 77% of copper's current at the same nominal size — 48 against 63 at 10 mm², 87 against 112 at 25 mm², 230 against 299 at 120 mm². In practice that is one standard size step, so the same load that takes 25 mm² copper takes 35 mm² aluminium. And aluminium conductor weighs about 30% of copper at the same size — 67.5 against 222.3 kg/km at 25 mm² — which is the whole commercial reason it exists. The trade-off between the two metals, terminal by terminal, is set out in the copper versus aluminium guide; the sizes we draw are on the aluminium cable pages.

Derating: what the chart figure becomes on a real job

This is the step most Pakistani sizing charts skip, and it is the step that decides whether the cable or the breaker fails first. Two of the three corrections are numbers you multiply. The third is not a number at all.

Ambient temperature — the factor Ca

The 30 °C baseline is a European default. A cable clipped along a roof void, run across a flat roof, or chased into a west-facing wall in Lahore or Multan in June is not at 30 °C. BS 7671 Table 4B1 gives the correction for 70 °C thermoplastic cable, and at 45 °C it is 0.79 — a 21% cut, not the 15% that circulates on Pakistani wiring blogs.

Grouping and bundling — the factor Cg

Cables bunched together heat each other. The correction depends on how many loaded circuits are in the bunch, and a flat "20% for bundling" is only right when there are exactly two. Six circuits in one conduit takes the figure down by 43%.

Conduit — a different column, not a multiplier

Conduit is where most charts go wrong. Putting a cable in a conduit does not apply a percentage to the method C figure — it moves the cable to a different reference method with its own tabulated values. Method C, clipped direct, is the most generous of the common methods, so every figure on this page is an upper bound for a conduit run. Do not invent a multiplier for it. Take the method A or method B column from BS 7671 for the cable you are actually installing, or size the conduit run in the cable size calculator, which asks for the method.

Buried cable is a fourth case again, with soil thermal resistivity and burial depth of its own; the practical points for a buried run are in the underground cable guide.

How to use the chart in six steps

Worked example: a 55 A house submain in a 45 °C roof void

A single-phase submain from the meter to the distribution board. Design current 55 A. The route is 22 m through a roof void that reaches 45 °C in June, and the cable is bunched with two other loaded circuits along the way — three circuits in the bunch.

Step 1. Ca at 45 °C is 0.79. Cg for three circuits is 0.70. Multiplied: 0.79 × 0.70 = 0.553.

Step 2. Required tabulated capacity = 55 ÷ 0.553 = 99.5 A. That is the number to look up, not 55 A.

Step 3. Read down the two-loaded-conductor column. 10 mm² is 63 A — short. 16 mm² is 85 A — still short. 25 mm² is 112 A and clears it. Checked the other way round: 112 × 0.79 × 0.70 = 61.9 A, comfortably over the 55 A load.

Step 4. Voltage drop on 25 mm² is 1.75 mV/A/m: 1.75 × 55 × 22 ÷ 1000 = 2.12 V, which is 0.92% of 230 V. Inside the budget with room to spare, so heat decided this size, not length.

Read the same job off the raw chart with no correction and you land on 10 mm², because 63 A looks like it covers 55 A. That is a two-size error, and it is the error that makes a submain warm to the touch in June. Check your own numbers in the cable size calculator and the voltage drop calculator.

Worked example: a 95 A three-phase feeder, copper against aluminium

A 400 V three-phase workshop feeder, design current 95 A, 4 core cable clipped direct in a ventilated riser at 40 °C, running on its own. Ca is 0.87, Cg is 1.00, so the required tabulated capacity is 95 ÷ 0.87 = 109.2 A.

Read the three-loaded-conductor column, because all three lines are loaded. In copper, 25 mm² is 96 A — short of 109.2. 35 mm² is 119 A and takes it. In aluminium, 35 mm² is 94 A — short. 50 mm² is 113 A and takes it. One size step between the metals, exactly as the 77% ratio predicts.

Weigh the two: 35 mm² copper carries 311.2 kg of conductor per kilometre, 50 mm² aluminium carries 135.0 kg — 43% of the mass for the same duty. That is the trade the aluminium buyer is making, before terminals, lugs and the extra diameter are counted. Three-phase sizing end to end, including the neutral and the earth, is worked through on the 3 phase cable size page; the single-phase equivalent is on single phase cable size.

Trade codes: what 7/.029 and 7/.064 are in mm²

Ask for a size in mm² in a Pakistani shop and you get quoted a strand code. Seven strands of 0.036 inch is 7/.036. The codes are imperial, so the area they compute to does not land on the IEC nominal size the cable is sold as — one wire, two names, and the gap between them is where most of the confusion in this market lives. The arithmetic is simple enough to check: strand diameter in inches × 25.4 gives millimetres, area per strand is π/4 × d², multiplied by the strand count.

The codes have their own pages here: 3/.029, 7/.029, 7/.036, 7/.044, and in aluminium 7/.052 at 10 mm² and 7/.064 at 16 mm². One warning worth repeating: 25 mm² and 2.5 mm² are not the same cable. 25 mm² carries 112 A, 2.5 mm² carries 27 A, and a written order that drops the decimal point is a safety problem, not a typing one.

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