Electrical Cable Size Calculator: Wire Gauge by Amperage and Distance
Find the correct copper wire gauge (mm²) for any circuit based on current (amps), run length (m), and voltage. Applies IEC 60364 voltage-drop and ampacity criteria. Includes reference table.
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Reviewed by: Martín Rodríguez (editorial policy ) · Last reviewed:
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Required cable size
2.5 mm² — Actual voltage drop: 4.29 V (1.95%) ✓
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Selecting the wrong cable size is one of the most common — and most dangerous — electrical mistakes. A cable that is too thin for the current load overheats, melts its insulation, and can start a fire. A cable that is too thin for the run length causes excessive voltage drop: appliances receive less voltage than rated, motors overheat, and sensitive electronics malfunction. This calculator evaluates both IEC 60364 criteria — the thermal limit (ampacity) and the voltage-drop limit — and returns the smallest commercially-available copper conductor size that satisfies both simultaneously, plus the appropriate breaker rating.
When to use this calculator
- Sizing cables for a new circuit to an air conditioner, water heater, or electric oven.
- Verifying that existing wiring is adequate before adding a new load.
- Designing electrical installations that comply with IEC 60364 or local wiring regulations.
- Quick job-site reference for electricians selecting cable from stock.
- Students learning IEC 60364 / NEC electrical design principles.
Copper Conductor Ampacity by Cross-Section — IEC 60364-5-52 (Single-Core in Conduit, 30 °C Ambient)
| Cross-section (mm²) | Max. current (A) | Typical application | Standard breaker (A) |
|---|---|---|---|
| 1.0 | 10 | Lighting | 10 |
| 1.5 | 14 | General outlets, TV | 13–16 |
| 2.5 | 21 | Refrigerator, washing machine, small A/C | 20 |
| 4.0 | 28 | Water heater, electric oven | 25 |
| 6.0 | 36 | Air conditioner, larger loads | 32 |
| 10.0 | 50 | Sub-panel, three-phase pump | 50 |
| 16.0 | 68 | Workshop, industrial equipment | 63 |
| 25.0 | 89 | Large residential service | — |
| 35.0 | 108 | Industrial, main panel | — |
| 50.0 | 131 | Heavy industry | — |
Fuente: IEC 60364-5-52 — Electrical Installations of Buildings: Selection and Erection of Electrical Equipment — Wiring Systems. Copper conductors only; γ = 56 m/(Ω·mm²) at 20 °C (IEC 60228). For aluminium conductors (γ ≈ 34 m/(Ω·mm²)) or bundled/buried cables, apply derating factors per IEC 60364-5-52.
How it works
How to Calculate Electrical Cable Size
Correct cable sizing follows two independent criteria under IEC 60364-5-52. The required conductor cross-section is the larger of both.
Criterion 1 — Voltage-drop sizing (IEC 60364)
For a single-phase circuit (220–240 V):
S = (2 × L × I) / (γ × ΔU)For a three-phase circuit (380–415 V):
S = (√3 × L × I) / (γ × ΔU)Where:
Example: 220 V × 3% = 6.6 V maximum drop.
Criterion 2 — Thermal ampacity (current-carrying capacity)
Every cable cross-section has a maximum current it can carry before overheating. Standard values for single-core copper in conduit, 30 °C ambient (IEC 60364-5-52):
| Cross-section (mm²) | Max. current (A) | Typical application |
|---|---|---|
| 1.0 | 10 A | Lighting |
| 1.5 | 14 A | General outlets, TV |
| 2.5 | 21 A | Refrigerator, washing machine, small A/C |
| 4.0 | 28 A | Water heater, electric oven |
| 6.0 | 36 A | Air conditioner, larger loads |
| 10.0 | 50 A | Sub-panel, three-phase pump |
| 16.0 | 68 A | Workshop, industrial equipment |
| 25.0 | 89 A | Large residential service |
| 35.0 | 108 A | Industrial, main panel |
| 50.0 | 131 A | Heavy industry |
Quick-reference sizing table — Single-phase 220 V, 3% voltage-drop limit
| Current | 10 m run | 20 m run | 30 m run | 50 m run |
|---|---|---|---|---|
| 10 A | 1.5 mm² | 1.5 mm² | 1.5 mm² | 2.5 mm² |
| 16 A | 1.5 mm² | 2.5 mm² | 2.5 mm² | 4 mm² |
| 20 A | 2.5 mm² | 2.5 mm² | 4 mm² | 6 mm² |
| 25 A | 2.5 mm² | 4 mm² | 4 mm² | 6 mm² |
| 32 A | 4 mm² | 4 mm² | 6 mm² | 10 mm² |
| 40 A | 4 mm² | 6 mm² | 10 mm² | 10 mm² |
| 63 A | 10 mm² | 10 mm² | 16 mm² | 25 mm² |
Sizes shown are the larger of voltage-drop and thermal criteria. Always round up to the next commercial size.
Standard commercial cross-sections (IEC 60228)
After computing the theoretical minimum, always select the next commercial size up: 1.0 – 1.5 – 2.5 – 4 – 6 – 10 – 16 – 25 – 35 – 50 – 70 – 95 – 120 mm².
Recommended breaker (circuit breaker) rating
Select the commercial breaker value equal to or just above the circuit current. Standard values: 6 – 10 – 13 – 16 – 20 – 25 – 32 – 40 – 50 – 63 A.
IEC 60364 voltage-drop limits
Applicability notes
Worked example: 12 A air conditioner, 25 m run, 220 V single-phase
Inputs: 12 A load, 25 m one-way run, 220 V single-phase, 3% max voltage drop = 6.6 V.
Voltage-drop criterion: S = (2 × 25 × 12) / (56 × 6.6) = 600 / 369.6 = 1.62 mm² → next commercial size: 2.5 mm².
Thermal criterion: 2.5 mm² copper carries up to 21 A; 12 A < 21 A → thermal criterion satisfied.
Actual voltage drop at 2.5 mm²: ΔV = (2 × 25 × 12) / (56 × 2.5) = 4.29 V → 1.95% ✓ within the 3% limit.
Result: 2.5 mm² copper cable + 16 A breaker.
2.5 mm² — Actual voltage drop: 4.29 V (1.95%) ✓
Frequently asked questions
What size wire do I need for 20 amps over 30 metres?
For 20 A over 30 m on a 220 V single-phase circuit with a 3% voltage-drop limit: S = (2 × 30 × 20) / (56 × 6.6) = 1200 / 369.6 = 3.25 mm². The next commercial size is 4 mm², which also satisfies the 28 A thermal limit. Use 4 mm² copper cable + a 20 A breaker.
What size wire do I need for 30 amps over 50 metres?
For 30 A over 50 m at 220 V single-phase (3% drop): S = (2 × 50 × 30) / (56 × 6.6) = 3000 / 369.6 = 8.12 mm². The next commercial size is 10 mm² (ampacity 50 A). Actual voltage drop: (2 × 50 × 30) / (56 × 10) = 5.36 V = 2.44% ✓. Use 10 mm² copper + a 32 A breaker.
Why does cable size depend on distance, not just amperage?
Cable resistance is proportional to length (R = ρ × L / S). A longer run creates more resistance, causing a larger voltage drop (ΔV = R × I). When voltage drop exceeds 3% of the supply voltage (IEC 60364), appliances run below their rated voltage: motors overheat, lamps dim, and electronics malfunction. The same current can therefore require different cable sizes at different distances.
What is the difference between voltage-drop sizing and thermal sizing?
Thermal sizing ensures the cable can carry the current without overheating and melting its insulation. Voltage-drop sizing ensures the resistance of the cable does not reduce the voltage at the load by more than the allowed limit (3–5% per IEC 60364). A cable meeting the thermal requirement can still fail the voltage-drop requirement on long runs. The final size must satisfy both: always pick the larger of the two.
Why is there a factor of 2 for single-phase but √3 for three-phase?
In a single-phase circuit, current flows through the live conductor and returns through the neutral — it travels twice the physical run length, hence the factor 2. In a balanced three-phase circuit, the return current is shared across all three phases with 120° phase displacement. The net effect is a factor of √3 ≈ 1.732, which is why three-phase transmission is more efficient for transporting power over long distances.
What is copper conductivity and why does this calculator use 56 m/(Ω·mm²)?
Copper conductivity γ = 56 m/(Ω·mm²) at 20 °C is the value specified in IEC 60228 for annealed copper conductors used in insulated cables. Some references use 57 or 58 for higher-purity copper, but the difference in the final result is under 4% — well within the margin covered by selecting the next commercial size.
How do I calculate current if I only know the power in watts?
For 220 V single-phase: I (A) = W ÷ 220. For 380 V three-phase with power factor cosφ = 0.9: I = W ÷ (380 × √3 × 0.9). Example: a 2,000 W water heater at 220 V draws 2000 ÷ 220 = 9.1 A. A 3,000 W air conditioner: 3000 ÷ 220 = 13.6 A. For horsepower ratings: 1 HP = 746 W.
Is the length I enter one-way or total round-trip?
Enter the one-way distance from the panel to the load. The calculator automatically applies the factor 2 (for single-phase) or √3 (for three-phase) to account for the return path. If the panel is 30 m from the socket, enter 30, not 60.
Does this calculator work for AWG wire sizes?
This calculator outputs cross-sections in mm², which is the IEC international standard. To convert to AWG for North American applications: 2.5 mm² ≈ 14 AWG; 4 mm² ≈ 12 AWG; 6 mm² ≈ 10 AWG; 10 mm² ≈ 8 AWG; 16 mm² ≈ 6 AWG. Note that AWG ampacity tables follow NEC, which may differ slightly from IEC 60364 values.
What if the existing cable does not meet the calculated requirement?
If the cable is undersized for ampacity (thermal), the risk is immediate: overheating, insulation damage, and potential fire. If it only fails the voltage-drop criterion by a small margin (e.g. 3.5% vs 3%), the main consequence is reduced appliance performance. In either case, the correct action is to replace the cable or reduce the circuit load. Always consult a licensed electrician before making changes.
Sources & references
Methodology & trust
Editorial
Calculadora de construcción revisada por el equipo editorial de Hacé Cuentas, contrastada con IEC 60364-5-52 — Electrical Installations of Buildings: Selection and Erection of Electrical Equipment — Wiring Systems, según nuestra política editorial y metodología.
Updates
Última revisión: June 20, 2026. Los parámetros se verifican periódicamente con las fuentes citadas.
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Limitations
Indicative results. For critical decisions, consult a professional.
📌 How to cite this calculator
Rodríguez, M. (2026). Electrical Cable Size Calculator: Wire Gauge by Amperage and Distance. Hacé Cuentas. https://hacecuentas.com/electrical-cable-gauge-amperage-distance
Contenido bajo licencia CC-BY 4.0 — reutilizable citando la fuente con enlace a Hacé Cuentas.