3-phase cable size in Pakistan: kW → amps → mm²

kW to cable size: the one sum that does it

On a 415 V three-phase supply at a power factor of 0.85, the design current in amps is the load in watts divided by 611 — that is √3 × 415 × 0.85 = 611. A 30 kW load therefore draws 49 A, a 50 kW load 82 A, and a 100 kW load 164 A. Those currents are then read against a cable's tabulated capacity for the way it is actually installed: every figure on this page is BS 7671 reference method C, clipped direct, 30°C ambient, PVC-insulated copper, and each one must be reduced by the correction factors for a hotter ambient or a grouped tray before it is compared with the load.

Two things follow. First, a 415 V cable size is not a 400 V one — the higher the volts, the lower the current for the same kW, so a chart built for 400 V reads about 4% high. Second, the power factor matters as much as the kW. At 0.85 a 30 kW load is 49 A; correct the same plant to unity and it is 42 A. Use the figure from your own plate or bill, not ours, if you know it.

Two readings of the same chart. The small end gives the electrical minimum and nothing more — 1.5 mm² satisfies a 10 kW three-phase load on paper, and no plant runs a feeder that thin, because terminations, mechanical strength and the next machine on the board all argue for heavier copper. The top end runs out: past 120 mm² the answer is parallel runs or a larger conductor than the range on this page, and that is a conversation, not a chart lookup.

What the 30°C column is worth on a plant floor at 45°C

Every ampacity figure published here is a 30°C, clipped-direct, single-cable number. A shed roof in Multan in June is not 30°C, and a feeder in a loaded tray is not on its own. Both pull the same lever: the tabulated capacity is multiplied by correction factors before it is compared with the load current.

We do not publish a correction-factor table, and that is deliberate: the correct factor depends on your insulation, your installation method, your ambient and how many circuits share the tray, and a factor pulled off the wrong row is worse than no factor at all. Take them from BS 7671 or IEC 60364-5-52 for your own case. What this page can give you is the shape of the arithmetic, which almost no local sizing page shows: a 30°C rating of 57 A with factors multiplying to 0.80 is a 45.6 A cable, because 57 × 0.80 = 45.6. Write it as base × factor = result every time, so the next person can audit it.

Current capacity and voltage drop by size

Read the three-or-four-loaded-conductor column for a three-phase feeder. The number of loaded cores sets the rating, not the number of cores in the sheath: a four-core cable feeding a balanced motor with no neutral still has three conductors carrying current.

Worked example: 30 kW plant load, 60 m, hot and grouped

A workshop board carrying a mixed 30 kW three-phase load, 60 m from the main panel, run on a tray with other loaded feeders in a shed that gets well past 30°C in summer.

Current. 30,000 ÷ 611 = 49.1 A.

Capacity. 10 mm² is rated 57 A with three loaded conductors. Take the ambient and grouping factors as multiplying to 0.80 for this shed — your own factors come out of BS 7671 — and 57 × 0.80 = 45.6 A, which is below 49.1 A. 10 mm² fails. 16 mm² is 76 A, and 76 × 0.80 = 60.8 A, which clears 49.1 A with 11.7 A to spare.

Voltage drop. On 16 mm² three-phase: 2.4 × 49.1 × 60 ÷ 1000 = 7.07 V. Against a 2.5% feeder budget of 10.4 V that passes. Had 10 mm² survived the derating it would have dropped 3.8 × 49.1 × 60 ÷ 1000 = 11.19 V and failed anyway.

Answer: 16 mm², four core. Both checks land on the same size, which is the comfortable case. When they disagree, the larger size wins — that is the next example.

Worked example: a 200 m feeder at 80 A, where the drop decides

Long feeder runs are ordinary in Pakistani plants — transformer yard to a shed at the back of the compound, 150 to 300 m. On those runs the current stops being the constraint.

Take 80 A over 200 m. On capacity alone, 25 mm² at 96 A looks right; apply 0.80 and it is 76.8 A, just under the load, so 35 mm² at 119 × 0.80 = 95.2 A is the first size that passes on current. Now the drop, on a 2.5% budget of 10.4 V:

Answer: 70 mm² — two sizes above what the current alone allowed, and the money is in the copper, not the calculation. If the load is motors and the budget is the looser 5% (20.8 V), 35 mm² passes and the bill halves; that choice of budget is the single biggest lever on a long feeder, so make it deliberately and write it on the drawing.

The general shape: on a 415 V feeder the drop starts binding somewhere near 100 m, whatever the size. This is how far each size reaches at its own full 30°C rating before it eats a 2.5% budget.

Check your own run in the voltage drop calculator, or start from the load in the cable size calculator.

415V par 3 phase cable ka size kaise nikalte hain?

Load ko watt mein le kar 611 se taqseem kar dein — yeh √3 × 415 × 0.85 hai. 30 kW ka matlab 49 amp. Ab chart se woh size lein jiska rating 30°C wala figure hai, aur us par garmi aur grouping ke factor laga kar dekhein ke woh current se ooper rehta hai ya nahi. Factor zarb hote hain, jama nahi.

Aur ek baat jo aksar chhoot jati hai: 100 meter se lambi run par size voltage drop se tay hota hai, current se nahi. Machine ki plate par likha full-load current har chart se behtar hai — jahan plate mil jaye, wahan chart chhor dein.

7/.052, 7/.064: what the trade codes mean at feeder sizes

Panel drawings are in mm²; the shop still quotes strand codes. The codes are imperial and do not land exactly on the IEC 60228 nominal areas, which is why one cable has two names and why a purchase order and a delivery note can disagree without anybody lying.

Where this page stops and another one starts

• A single motor — nameplate full-load current, starting current and long tray runs are their own problem. That is motor and plant feeder sizing.
• You do not have the kW yet — adding up a building's connected load and applying diversity to get maximum demand comes first: commercial load calculation.
• The run goes in the ground — burial changes the rating and the cable type. See cable for an underground run and armoured vs unarmoured.
• Single-phase supply — 220 V circuits use the two-loaded-core column and a different sum: single-phase cable size.
• The earth conductor — sized from the phase conductor, not guessed: earth cable sizes.
• Aluminium instead of copper — the size steps up for the same current; per-size figures on aluminium cable, and the trade-off in copper vs aluminium.

The cable for a three-phase feeder

Fixed three-phase feeders are class 2 stranded copper, PVC insulated and sheathed, 450/750 V — the standard cable range, made in 4-core for a line-neutral-earth feeder and 3-core where a balanced load needs no neutral. These are the sizes that leave the Lahore line most often for panel and motor work.

Cable is priced off the day's copper rate, so we quote per metre and per coil on WhatsApp rather than publish a number that goes stale. Send the sizes and total metres — a bill of quantities is fine as a photo — with your city: Punjab in 1–2 days, rest of Pakistan in 2–4.

Frequently asked