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Wire Size Calculator

The copper wire size in sq mm for one home circuit, from the load, the MCB and the run length, with the voltage drop.

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Quick Answer

Divide the load in watts by 230 V (and by the power factor for motors and ACs) to get the current. Then pick the smallest wire whose rated current, after corrections, is at least your MCB's rating. Under IS 3961 (Part 5), single-core copper wire in conduit, with an MCB, at 40 °C, carries about 16.0 A in 1.5 sq mm, 22.1 A in 2.5 sq mm and 29.5 A in 4 sq mm. Hotter rooms and more circuits sharing a conduit lower these figures.

Figures checked Oct 2026. Source: IS 3961 (Part 5):1968, Recommended current ratings for cables, Part 5: PVC insulated light duty cables (reaffirmed): clause 2.1, clause 4.2, Table 1 (columns 3 and 5) and Table 8 (a), (b), (c).

15.99 A1.5 sq mm with an MCB, in conduit
× 1.23MCB factor (IS 3961 Table 8)
0.693 circuits in one conduit
0.725largest fuse as a share of the wire's rating

Guides: AC nameplate to MCB, 2BHK wiring plan checklist.

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The Formula

Wire size = IB = W ÷ (V × PF); IZ = Table 1 rating × 1.23 (MCB) × ambient factor × grouping factor; answer = smallest size with IB ≤ In ≤ IZ (MCB) or In ≤ 0.725 × IZ (rewirable fuse); voltage drop = 2 × L × IB × (R20 × 1.2) ÷ 1,000

The table ratings are IS 3961 (Part 5):1968 Table 1 for single-core PVC copper wire at 40 °C. The factors are Table 8 of the same standard: 1.23 for MCB protection, the room-temperature row and the grouping row. The rule linking load, breaker and wire is IS 732:1989 clause 5.3.3.2. Resistance is IS 8130:1984 Table 3 at 20 °C, raised by 1.2 for a wire running at 70 °C.

How the wire, the MCB and the load must line up (2,000 W geyser)2,000 W at 230 V is 8.70 A; the 16 A MCB is above that; 1.5 sq mm carries 15.99 A, below 16 A; 2.5 sq mm carries 22.14 A, so it passes.1Load current IB2,000 W ÷ 230 V = 8.70 A2MCB In must be at least IB16 A ≥ 8.70 A31.5 sq mm: 13 A × 1.23= 15.99 A, below 16 A, fails42.5 sq mm: 18 A × 1.23= 22.14 A, at least 16 A, passes
How the wire, the MCB and the load must line up (2,000 W geyser). The calculator's check for worked example 1, step by step.

Assumptions

  • Single-core PVC-insulated copper wire to IS 694, two wires per circuit (phase and neutral), 230 V single-phase. Aluminium wire is not covered.
  • Table 1 of IS 3961 (Part 5) assumes 40 °C air and a conductor that may reach 70 °C. Below 40 °C the wire may carry more, above it less.
  • An MCB counts as close protection (× 1.23). A rewirable fuse counts as coarse protection: no 1.23, and its rating must be at most 0.725 times the wire's rating.
  • Grouping factors are the single-core row of IS 3961 Table 8(c). Count every circuit that runs through the same conduit, including this one.
  • The earth wire is not sized here. Your electrician sizes it with the earthing.
  • We quote the editions on the free public copies: IS 3961 (Part 5):1968 (still current, reviewed 2021), IS 732:1989 and IS 8130:1984. IS 732 was revised in 2019 and IS 8130 in 2013; check the current edition before you rely on a clause.
How heat and a shared conduit cut a 2.5 sq mm wire's ratingAlone at 40 °C 22.14 A; alone at 45 °C 20.15 A; alone at 50 °C 18.15 A; 3 circuits at 40 °C 15.28 A; 3 circuits at 45 °C 13.90 A; reference line at 16 A.Alone, 40 °C22.14 AAlone, 45 °C20.15 AAlone, 50 °C18.15 A3 circuits, 40 °C15.28 A3 circuits, 45 °C13.90 A16 A MCB
How heat and a shared conduit cut a 2.5 sq mm wire's rating. 2.5 sq mm in conduit with an MCB. Three circuits in one conduit take it below a 16 A MCB, even at 40 °C.

Current ratings and resistance by wire size

The current each size carries under IS 3961 (Part 5) Table 1, alone in its conduit at 40 °C, with the MCB factor applied, and its resistance at 20 °C from IS 8130 Table 3.

Size (sq mm)Conduit, Table 1 (A)Conduit with MCB (A)Surface, Table 1 (A)Surface with MCB (A)Resistance at 20 °C (Ω/km)
11113.531214.7619.50
1.51315.991619.6813.30
2.51822.142227.067.98
42429.522935.674.95
63138.133745.513.30
104251.665162.731.91
165770.116883.641.21
257187.3386105.780.78
3591111.93110135.300.554
50120147.60145178.350.386

With MCB = Table 1 value × 1.23 (IS 3961 Table 8a). Read from the standards on 9 October 2026.

What each size carries in conduit with an MCB, at 40 °C1 sq mm 13.53 A, 1.5 sq mm 15.99 A, 2.5 sq mm 22.14 A, 4 sq mm 29.52 A, 6 sq mm 38.13 A; reference line at 16 A.1 sq mm13.53 A1.5 sq mm15.99 A2.5 sq mm22.14 A4 sq mm29.52 A6 sq mm38.13 A16 A MCB
What each size carries in conduit with an MCB, at 40 °C. Table 1 conduit column × 1.23. The line marks a 16 A MCB: 1.5 sq mm falls just short.

Worked Examples

Voltage drop over a 20 m run at 10 A1 sq mm 9.36 V, 1.5 sq mm 6.38 V, 2.5 sq mm 3.83 V, 4 sq mm 2.38 V.1 sq mm9.36 V (4.07%)1.5 sq mm6.38 V (2.78%)2.5 sq mm3.83 V (1.67%)4 sq mm2.38 V (1.03%)
Voltage drop over a 20 m run at 10 A. 2 × 20 m × 10 A × resistance at 70 °C. Thicker wire loses less.

2,000 W geyser, 15 m in conduit, 16 A MCB, 40 °C, alone in the conduit

  1. IB = 2,000 ÷ 230 = 8.70 A
  2. Factor k = 1.23 (MCB, 40 °C, one circuit)
  3. 1.5 sq mm: 13 × 1.23 = 15.99 A, below 16 A, so it fails
  4. 2.5 sq mm: 18 × 1.23 = 22.14 A, passes
  5. Voltage drop: 2 × 15 × 8.70 × (7.98 × 1.2) ÷ 1,000 = 2.50 V (1.09%)
  6. Answer: 2.5 sq mm

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3,000 W at power factor 0.9, 25 m, 3 circuits in the conduit, 45 °C, 16 A MCB

  1. IB = 3,000 ÷ (230 × 0.9) = 14.49 A
  2. k = 1.23 × 0.91 × 0.69 = 0.772
  3. 2.5 sq mm: 18 × 0.772 = 13.90 A, fails
  4. 4 sq mm: 24 × 0.772 = 18.54 A, passes
  5. Voltage drop on 4 sq mm: 2 × 25 × 14.49 × (4.95 × 1.2) ÷ 1,000 = 4.30 V (1.87%)
  6. Answer: 4 sq mm. The same circuit alone at 40 °C would pass on 2.5 sq mm; heat and the shared conduit cost a whole size.

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800 W lighting circuit, 20 m clipped on the surface, 6 A rewirable fuse, 35 °C

  1. IB = 800 ÷ 230 = 3.48 A
  2. k = 1.03 (rewirable fuse row at 35 °C)
  3. The wire must carry at least 6 ÷ 0.725 = 8.28 A
  4. 1 sq mm: 12 × 1.03 = 12.36 A, passes
  5. Voltage drop: 2 × 20 × 3.48 × (19.5 × 1.2) ÷ 1,000 = 3.26 V (1.42%)
  6. Answer: 1 sq mm meets the table; the tips below say why many electricians still run 1.5

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Tools on Amazon.in

Tools for checking the circuit

For measuring the run and checking the load.

Hire a licensed electrician for any wiring work.

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  • Measures the cable run for the calculator.Stanley 5 m measuring tape (4.5★, 27,100 ratings) · See on Amazon.in

Or search Amazon.in yourself. Type what you need, or tap a common search.

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As an Amazon Associate we earn from qualifying purchases. Ratings checked October 2026. We don't show prices; Amazon has the current one.

How to Use This Calculator

  1. Pick the load. Choose a preset, enter the appliance's watts, or enter the amps on the nameplate for an AC or a motor.
  2. Enter the MCB rating. Use the rating your electrician has fixed for this circuit. If you do not know it yet, ask first: the wire follows the breaker.
  3. Measure the run. Measure from the distribution board to the farthest point along the real route, up and down the walls.
  4. Set how it is laid. Choose conduit or surface, the hottest temperature the run sees, and how many circuits share the conduit.
  5. Read the size. The first size that meets the rule is the answer. Check the voltage drop line and the table of all sizes under it.

Tips and tricks

  • Size the wire to the MCB, not to today's load. If someone later swaps a 10 A MCB for a 16 A one, a 1.5 sq mm conduit run is no longer protected (15.99 A against 16 A in example 1).
  • Measure the run along the actual route: up the wall, along the ceiling, down again. That's often double the straight-line distance, and voltage drop grows with length.
  • Count the circuits in each conduit before you pull wire. A fourth circuit in the same pipe cuts every wire's rating to 0.62.
  • Roof slabs and kitchens run hot. If the conduit runs through a sun-baked roof slab, use the 45 °C setting.
  • Read the coil label before you buy. It should name the size in sq mm and the standard the wire is made to; if either is missing, pick another coil.

What we would do

For a new flat, we'd plan the circuits first: one per AC, geyser and kitchen appliance, lights grouped separately. Then fix each MCB to its load, run every circuit through this calculator at 45 °C with the real number of circuits per conduit, and take the answer or one size up where you may add load later. Then hand the sheet to your electrician to check before anything is bought.

Frequently Asked Questions

Which wire size for a 16 A MCB?
In conduit, with nothing else in the pipe and at 40 °C, 2.5 sq mm copper (22.1 A under IS 3961 with an MCB). 1.5 sq mm comes to 15.99 A, just under 16 A, so it does not meet the rule.
Is 1.5 sq mm enough for a geyser?
It depends on the MCB, not only the geyser. A 2,000 W geyser draws 8.7 A, but if the circuit has a 16 A MCB, 1.5 sq mm in conduit falls short. Use 2.5 sq mm or the MCB rating your electrician fixes for that load.
How do I calculate amps from watts?
Amps = watts ÷ (volts × power factor). At 230 V a 2,000 W heater draws 8.7 A. A motor or AC has a power factor below 1, so use the amps on its nameplate if you have it.
Does wire length change the size?
Not for heat, which depends on current. Length changes the voltage drop. On long runs the calculator may show a drop you think is too high, and then you go up a size.
What is the difference between MCB and fuse in wire sizing?
IS 3961 lets wire protected by an MCB carry 1.23 times its table rating. A rewirable fuse needs the fuse rating to be at most 0.725 times the wire's capacity (IS 732), so the same wire takes a smaller fuse.

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Sources & Limitations

What this calculator does not model

  • It sizes one circuit at a time. It does not add up a whole house's load or size the main cable from the meter.
  • Voltage drop uses resistance at 70 °C and ignores reactance, so it reads slightly high for lightly loaded runs.
  • Flexible cords, armoured cables, three-phase supply and aluminium are out of scope.
  • Motor starting current and short-circuit ratings are not checked. Those are the electrician's job.