Wire Size Calculator Pakistan
A 3,000 W single-phase load over a 20 m run in conduit needs a 2.5 mm² copper cable (7/29 in Pakistani sizes), rated 24 A, on a 20 A MCB, with a 2.6% voltage drop. The cable must carry the design current of 16.3 A times a 1.25 safety margin and keep the drop under 5%.
How this result was calculated
Design current = 3,000 W ÷ (1 × 230 V × 0.8 power factor) = 16.3 A (the first factor is one for single phase and DC, root three for three phase). MCB = next standard size above 16.3 A = 20 A. The cable must be rated at least 20.4 A, the larger of 16.3 A × 1.25 and the MCB, and be at least 2.5 mm² for this circuit type: 2.5 mm² (7/29) is rated 24 A. Voltage drop over the 20 m run = 5.9 V, which is 2.6% of 230 V, within the 5% limit.
Key facts
- Wire size = the smallest copper cable whose current rating covers the design current × 1.25 (and the MCB) and whose voltage drop over the run stays under 5% for power circuits or 3% for lighting.
- Pakistani wire sizes are strands/diameter: 7/29 is rated as a 2.5 mm² cable and carries 24 A in conduit; 7/36 as 4 mm² at 32 A; 7/44 as 6 mm² at 41 A (IEC 60364-5-52, PVC copper, in conduit).
- A 3,000 W load at 230 V draws 16.3 A and fits on 7/29 with a 20 A MCB; a 5,000 W load draws 27.2 A and needs 6 mm² (7/44) with a 32 A MCB.
- Long runs are sized by voltage drop, not current: 3,000 W over 60 m needs 4 mm² although 2.5 mm² would carry the current.
Formula
I_b = P ÷ (k × V × PF); I_n = next standard ≥ I_b; I_z ≥ max(I_b × margin, I_n); ΔV% = k_v × L × I_b × ρ ÷ A ÷ V × 100
- I_b = design current; k = 1 for single phase and DC, √3 for three phase
- I_n = MCB rating; I_z = cable current rating for the installation method
- k_v = 2 for single phase and DC (there and back), √3 for three phase
- L = one-way run in metres; ρ = resistivity of copper; A = cable area in mm²
- Minimum sizes: lighting circuits and power circuits each have a floor whatever the current
How does this wire gauge and cable size calculator pick a size?
It works the way an electrician does, in three steps. First the design current: watts ÷ (volts × power factor), or ÷ (root three × volts × power factor) on three phase. Second the MCB: the next standard size above the design current, with a 1.25 margin first for motors, compressors and ACs. Third the cable: the smallest copper size whose rating for the installation method covers the design current × 1.25 and the MCB, and whose voltage drop over your run stays under the limit.
For the default 3,000 W at 20 m that is 16.3 A, a 20 A MCB and a 2.5 mm² cable (7/29) rated 24 A, with a 2.6% drop. The MCB is always rated at or below the cable, so the breaker protects the cable.
What is the 7/29 wire ampere rating (2.5 mm² wire amps)?
7/29 copper wire is rated as a 2.5 mm² cable here and carries 24 A in conduit inside a wall, a little more when clipped on the surface. With the 1.25 safety margin that means a design current of about 16.3 A, the default 3,000 W load, fits on 7/29 with a 20 A MCB and room to spare (20.4 A needed against 24 A).
A 4,400 W load on the same supply draws 23.9 A and needs a cable rated 29.9 A, above what 7/29 can carry, so the calculator moves to 7/36 (4 mm²), rated 32 A, with a 25 A MCB. 7/29 is also the minimum size for any socket or AC circuit.
How many amps can 4 mm² wire carry?
4 mm² copper, sold as 7/36 in Pakistan, carries 32 A in conduit. It is the usual choice for a kitchen socket ring, a geyser plus sockets, or an AC on a long run. In the example below a 4,400 W load at 10 m gets 7/36 with a 25 A MCB and a 1.2% drop. The next size, 7/44 (6 mm²), carries 41 A.
What is copper wire ampacity and why does the installation method matter?
Ampacity is the current a cable can carry continuously without its insulation getting too hot. The ratings here are for PVC-insulated copper from IEC 60364-5-52: method B1 is a cable in conduit inside a wall, which is most Pakistani house wiring and traps heat, so it is rated lower than method C, a cable clipped to a surface in free air. Pick the method that matches your installation.
Several cables in one conduit, a roof space that bakes in summer or an aluminium conductor all reduce the rating further. The calculator uses one cable at normal room temperature; an electrician derates for the rest.
Is there a cable size chart for Pakistani wire sizes?
The table below is one: common single-phase loads at 20 m with the mm² size, the Pakistani name, the current rating, the MCB and the voltage drop. The names count the strands and their diameter in thousandths of an inch: 3/29 is three strands and is rated as 1 mm² for lighting points, 7/29 is seven strands rated as 2.5 mm² for sockets and ACs, 7/36 as 4 mm², 7/44 as 6 mm², 7/52 as 10 mm² and 7/64 as 16 mm² for main feeds.
A 7/29 is actually a little larger than 2.5 mm², but it is rated as 2.5 mm² here to stay on the safe side. Always buy cable with the size printed on the sheath from a known maker; thin or copper-clad wire sold as 7/29 does not carry 7/29 current.
What wire size for a 5 kW load?
On single phase at 230 V a 5,000 W mixed load draws 27.2 A, so it needs a 32 A MCB and a 6 mm² cable (7/44) rated 41 A, with a 1.8% drop over 20 m. This is the size of a typical main feed for a small home or a sub-main to a floor. The same five kW on three phase splits over three live wires and needs a smaller cable.
Worked examples
Five kW mixed load, 20 m, single phase
5,000 W ÷ (230 V × 0.8) = 27.2 A, so a 32 A MCB. The cable must be rated 34 A: 6 mm² (7/44) at 41 A. Voltage drop 4.1 V, or 1.8%.
Long 60 m run: voltage drop decides
3,000 W draws 16.3 A and 2.5 mm² would carry it, but over 60 m the drop would pass the 5% limit, so the calculator moves to 4 mm² (7/36), which drops 11 V, or 4.8%. MCB 20 A.
Ten kW three-phase motor, 20 m
10,000 W ÷ (root three × 400 V × 0.85) = 17 A per phase. Motor loads take the margin on the breaker: 21.2 A, so a 25 A MCB. Cable 4 mm² (7/36) rated 32 A per core; drop 0.8%.
48 V battery to inverter cable, 3,000 W, 3 m
DC has no power factor: 3,000 W ÷ 48 V = 62.5 A. The cable must be rated 78.1 A, so 25 mm² (19/52) at 101 A, protected by a 63 A DC-rated breaker or fuse. Even at 3 m the drop is 0.7%, which is why battery cables are kept short and thick.
4,400 W socket circuit, 10 m
4,400 W ÷ (230 V × 0.8) = 23.9 A, so a 25 A MCB. With the 1.25 margin the cable must be rated 29.9 A, above 7/29, so 4 mm² (7/36) at 32 A. Voltage drop 1.2%.
Cable size chart: single-phase loads at 230 V over 20 m in conduit, power factor 0.8
| Load | Design current | Cable | Pakistani size | Cable rating | MCB | Voltage drop |
|---|---|---|---|---|---|---|
| 1,000 W | 5.4 A | 2.5 mm² | 7/29 | 24 A | 6 A | 0.9% |
| 2,000 W | 10.9 A | 2.5 mm² | 7/29 | 24 A | 16 A | 1.7% |
| 3,000 W | 16.3 A | 2.5 mm² | 7/29 | 24 A | 20 A | 2.6% |
| 5,000 W | 27.2 A | 6 mm² | 7/44 | 41 A | 32 A | 1.8% |
| 7,000 W | 38 A | 10 mm² | 7/52 | 57 A | 40 A | 1.5% |
| 10,000 W | 54.3 A | 16 mm² | 7/64 | 76 A | 63 A | 1.3% |
Common mistakes
- Sizing the cable to the load alone. The cable must also be rated above the MCB that protects it, otherwise the breaker lets the cable overheat before it trips.
- Measuring the run across the room. Cable follows walls, ceilings and the distribution board, so the real run and the voltage drop are longer than the straight line.
- Ignoring voltage drop on long runs. A cable that carries the current comfortably can still lose too much voltage over 60 m; motors and compressors suffer most.
- Treating copper-clad or aluminium wire as copper. Both need a larger size and different terminals; the ratings here are for copper only.
- Trusting the name on the roll. A cheap 7/29 with thin strands is not 2.5 mm²; buy from a known maker with the size printed on the sheath.
Safety: These sizes are planning values for one copper cable at normal room temperature; a licensed electrician must confirm the cable, MCB and earth on site and derate for grouped cables, hot roof spaces and long runs.
Frequently asked questions
Does this wire size calculator use Pakistani wire sizes?
Yes. Every result shows the mm² size and the Pakistani name (3/29, 7/29, 7/36, 7/44 and up), using the copper current ratings of IEC 60364-5-52 for cable in conduit or clipped on a surface.
What is the maximum load on 7/29 wire?
7/29 carries 24 A in conduit. With the 1.25 planning margin a 3,000 W single-phase load (16.3 A) fits with a 20 A MCB; a 4,400 W load (23.9 A) does not and moves up to 7/36.
What size wire for a five kW load at 230 V?
6 mm² copper (7/44) with a 32 A MCB: the load draws 27.2 A and the cable is rated 41 A.
Which wire for a 32 A MCB?
At least 6 mm² (7/44), rated 41 A in conduit, so that the cable rating stays above the breaker and the breaker protects the cable.
Does cable length change the wire size?
Yes, through voltage drop. 3,000 W at 60 m needs 4 mm² to keep the drop at 4.8%, under the 5% limit, while the same load at 20 m fits on 2.5 mm².
What cable for a three-phase motor?
The current divides over three phases: a ten kW motor at 400 V draws 17 A per phase and takes a 25 A MCB on 4 mm² (7/36) cable.
Which wire between the battery and the inverter?
DC current is high because the voltage is low: 3,000 W at 48 V is 62.5 A, needing 25 mm² (19/52) over a 3 m run with a DC-rated breaker or fuse. Keep battery cables as short as possible.
What do 3/29, 7/29 and 7/36 mean?
The number of copper strands and each strand's diameter in thousandths of an inch: 7/29 is seven strands of twenty-nine thousandths of an inch. The calculator converts each to its area in mm² and rates it as the IEC size it is sold as.
Sources
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