What size cable do I need for a 12000 BTU air conditioner in Sri Lanka?

A 12,000 BTU (1-ton) inverter AC draws roughly 6–7 A on a 230 V supply, so the design current sits well inside a 10 A circuit. On a normal run under 30 m, 1.5 mm² (7/0.53) is electrically adequate, but most SL electricians run 2.5 mm² (7/0.67) on a dedicated 16 A breaker for inrush and future headroom. Enter your exact BTU, run length and installation method above for the specific answer.

How many amps can a 2.5 mm² (7/0.67) cable carry?

A 2.5 mm² 70 °C PVC copper cable is rated 23 A enclosed in conduit and 27 A clipped direct (at 30 °C ambient), per the Kelani catalogue / IEC 60364-5-52. That rating drops with heat and grouping — at 40 °C it falls to about 20 A enclosed. The tool derates it for your ambient temperature and shows the result.

What does 7/0.85 mean on a cable?

It is the local strand notation: the conductor is made of 7 strands, each 0.85 mm in diameter. Seven strands of 0.85 mm give a cross-section of about 4 mm², so 7/0.85 = 4 mm². Sri Lankan hardware shops sell and price cable by this code, which is why this tool shows both mm² and the strand notation.

What cable size for a 40 A house main in Sri Lanka?

For a 40 A single-phase main on short tails in conduit at 35 °C, the required current after derating is about 42.6 A. 6 mm² (7/1.04) only reaches 38 A enclosed, so it fails; 10 mm² (7/1.35) at 52 A passes. Standard practice is 10 mm² tails with a 10 mm² earth conductor. Confirm the exact figure with the calculator and your CEB service conditions.

What is the maximum voltage drop allowed for a wiring circuit?

CEB-approved installations in Sri Lanka use a 4% limit from the origin to the point of use, which at 230 V is about 9.2 V. IEC 60364 quotes 3% for lighting and 5% for other circuits as guidance. This tool defaults to 4% and automatically steps the cable up one size if the drop over your run exceeds the limit.

Why does the tool sometimes recommend a bigger cable than the amps need?

Two reasons. First, voltage drop: a cable that carries the current fine can still drop too many volts over a long run, so the tool upsizes until the drop is within your limit. Second, derating: high ambient temperature and grouped circuits cut a cable's usable rating, pushing the required size up. The comparison table shows exactly which limit decided the pick.

Is aluminium cable okay for house wiring?

Aluminium is used mainly for service mains and larger feeders, not small final circuits, and it is not made below 6 mm². It carries about 78% of the current of the same-size copper and drops more voltage. The tool offers a basic aluminium mode from 6 mm² up; for the terminations and the exact catalogue rating, check with your electrician and the manufacturer.

Does this replace a licensed electrician?

No. This is a planning aid that sizes the cable, breaker and earth conductor from standard tables. Final-circuit design, earthing arrangement, RCD protection and certification must be done by a PUCSL-licensed electrician who inspects the actual installation, cable route and supply conditions. Treat the output as a starting point, not a certificate.

Which installation method should I choose?

Pick 'Enclosed in conduit' if the cable runs inside PVC conduit or trunking in the wall — the most common house case, and the lowest rating. Pick 'Clipped direct' for cable fixed on a surface with clips, which runs cooler and carries more. Pick 'Buried' for underground runs to a pump house or gate. The method changes the ampacity, so choose the one that matches your run.

Sri Lanka · Utility

Sri Lanka Cable Size Calculator — Wire Gauge & Amp Rating

Find the right copper or aluminium cable size for any load — in both mm² and the local strand code (7/0.67, 7/0.85…) your hardware shop uses. It sizes the breaker, derates for heat and grouping, and checks the 4% voltage drop over your run.

By Induwara Ashinsana— Executive Director, Ryzera TechnologiesUpdated Jul 18, 2026

Size your cable

IEC 60364 · Kelani V5

Supply

Conductor

Load entry

Appliance

Popular

Power factor

Only used when the load is entered as power (kW).

Cable run length (m)

One-way distance from board to load.

Installation method

Ambient temperature (°C)

°C

Use 35–40 for hot roof-space runs.

Grouped circuits

Cables bunched in the same conduit / tray.

Max voltage drop (%)

CEB default is 4%.

Recommended cable

1.5 mm²7/0.53

Design current (Ib)

9.7 A

Breaker (MCB) In

10 A

Voltage drop

2.68%

6.2 V over the run

Earth (CPC)

1.5 mm²

IEC 60364-5-54

Motor / AC load: use a Type-C (or Type-D) MCB to ride through start-up inrush, and consider one size up for long runs and repeated starts.

Why this size — candidate comparison

Size	Strand	Derated A	Drop %	Verdict
1 mm²	1/1.13	12.2	4.07%	too much drop
1.5 mm²pick	7/0.53	15.5	2.68%	OK
2.5 mm²	7/0.67	21.6	1.66%	OK
4 mm²	7/0.85	28.2	1.02%	OK
6 mm²	7/1.04	35.7	0.67%	OK
10 mm²	7/1.35	48.9	0.41%	OK
16 mm²	7/1.70	64.9	0.26%	OK
25 mm²	7/2.14	84.6	0.16%	OK
35 mm²	7/2.52	104.3	0.12%	OK

Sources: Kelani Cables Catalogue V5 (strand codes & local ratings), IEC 60364-5-52 (ampacity, derating & mV/A/m), IEC 60364-5-54 (earth conductor), CEB / SLS 733 (4% drop limit). Guidance only — a PUCSL-licensed electrician must certify the installation.

How it works

This calculator follows the cable-selection procedure in IEC 60364-5-52, the wiring-systems standard that Sri Lanka's installation practice adopts, and reads its current ratings and strand codes from the Kelani Cables Catalogue V5. It works in five steps.

Design current (Ib). For a single-phase load, Ib = P / (V × pf); for three-phase, Ib = P / (√3 × V × pf). Enter a current directly and it is used as-is. Appliance presets carry a typical power and power factor (motors ≈ 0.8, resistive heaters = 1.0).
Protective device (In). The breaker is the smallest standard BS EN 60898 rating — 6, 10, 16, 20, 25, 32, 40, 50, 63 A — that is at least Ib, satisfying the IEC 60364-4-43 rule Ib ≤ In ≤ Iz.
Required tabulated rating (It).The cable's table rating must clear the breaker after derating: It ≥ In / (Ca × Cg). Ca is the ambient-temperature factor (IEC Table B.52.14 — e.g. 0.94 at 35 °C, 0.87 at 40 °C for 70 °C PVC) and Cg is the grouping factor (Table B.52.17 — 1.0 for a single circuit, 0.80 for two bunched, and so on).
Cable selection. The tool takes the smallest size whose method-specific rating — enclosed in conduit (B), clipped direct (C) or buried (D) — meets It. A 1.0 mm² conductor is treated as lighting-only, so power circuits start at 1.5 mm².
Voltage-drop check. Vd = (mV/A/m × Ib × L) / 1000, and %Vd = Vd / V × 100. If the drop exceeds your limit (4% by default under CEB rules), the size steps up until it passes. Finally the earth conductor is sized by IEC 60364-5-54 Table 54.7: equal to the phase up to 16 mm², 16 mm² between 16 and 35 mm², and half the phase above that.

The mV/A/m figures are cross-checked against first principles using the conductor resistance R = ρ × L / A at operating temperature, which reproduces the catalogue values to within about 3% — confirmation the voltage-drop side is grounded in physics, not a lookup alone.

Worked examples

1.5-ton AC, 22 m in conduit

Load: 2.0 kW, pf 0.9, single-phase 230 V
Ib = 2000 / (230 × 0.9) = 9.66 A → breaker In = 10 A
Ambient 35 °C → Ca = 0.94, so It ≥ 10 / 0.94 = 10.64 A
1.5 mm² (7/0.53) enclosed = 16.5 A ≥ 10.64 A ✓
Voltage drop = 29 × 9.66 × 22 / 1000 = 6.16 V = 2.68% < 4% ✓
Result: 1.5 mm² (7/0.53) — an existing 2.5 mm² run is more than enough.

40 A house main, 8 m tails in conduit

Main rating In = 40 A, single-phase 230 V
Ambient 35 °C → Ca = 0.94, so It ≥ 40 / 0.94 = 42.55 A
6 mm² (7/1.04) enclosed = 38 A ✗ (below 42.55 A)
10 mm² (7/1.35) enclosed = 52 A ≥ 42.55 A ✓
Voltage drop = 4.4 × 40 × 8 / 1000 = 1.41 V = 0.61% < 4% ✓
Result: 10 mm² (7/1.35) tails with a 10 mm² earth (CPC).

1-ton AC on a very long 120 m run (voltage-drop driven)

Load: 1.4 kW, pf 0.9, single-phase 230 V → Ib = 6.76 A, In = 10 A
Ampacity alone allows 1.5 mm² (16.5 A ≥ 10 A) ✓
But drop on 1.5 mm² = 29 × 6.76 × 120 / 1000 = 23.5 V = 10.2% ✗
2.5 mm² → 6.35% ✗; step up again
4 mm² (7/0.85) → 11 × 6.76 × 120 / 1000 = 8.92 V = 3.88% < 4% ✓
Result: 4 mm² (7/0.85) — the long run, not the current, sets the size.

Frequently asked questions

Sources & references

The ampacity, mV/A/m and derating tables on this page were last cross-checked against the Kelani Catalogue V5 and IEC 60364-5-52 on 2026-07-18. This tool covers 70 °C PVC copper (and basic aluminium) house wiring — AC final circuits and main tails. It is guidance only: a PUCSL-licensed electrician must design and certify the actual installation.

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