Cable Sizing Calculator

What is Cable Sizing?

Cable sizing is the process of selecting the minimum conductor cross-section (in mm² for metric, or AWG for US) that safely carries the required current under the actual installation conditions — without overheating or causing an excessive voltage drop at the load.

Undersized cables are a leading cause of electrical fires. Oversized cables are safe but wasteful and expensive. The goal is to find the optimal minimum size that satisfies three criteria simultaneously:

• Current-carrying capacity — the cable's derated ampacity must exceed the design current
• Voltage drop — the drop along the cable run must not exceed 3–5% of supply voltage
• Short-circuit protection — the cable must withstand fault currents until the protective device disconnects

International Cable Sizing Standards

All international standards follow the same fundamental methodology: select a conductor whose tabulated current-carrying capacity (Iz), after applying applicable derating (correction) factors, remains at or above the design current (Ib).

How to Calculate Cable Size — Step by Step

1. Step 1 — Determine Design Current (Ib) For single-phase loads: Ib = P / (V × PF). For three-phase: Ib = P / (√3 × VLL × PF × η). For motors, use the full-load current from the motor nameplate.
2. Step 2 — Choose Installation Method (IEC Reference Method) Method A1/A2 (in conduit in wall), B1/B2 (in conduit on wall / trunking), C (clipped direct), D1 (direct buried), D2 (in duct in ground). The method determines which current-carrying capacity table to use.
3. Step 3 — Apply Derating Factors Ca = temperature correction factor, Cg = grouping/bunching factor, Ci = insulation factor (thermal insulation). Combined factor = Ca × Cg × Ci.
4. Step 4 — Calculate Required Tabulated Current (It)
   Required tabulated current

   It = Ib ÷ (Ca × Cg × Ci)
   Select the cable size whose tabulated rating Iz ≥ It from the relevant standard table.
5. Step 5 — Check Voltage Drop
   Voltage drop check (IEC/BS 7671)

   ΔU% = (Ib × L × mV/A/m) ÷ (1000 × Vn) × 100
   Must not exceed 3% for lighting or 5% for power (IEC/BS 7671), or 3% for branch circuits (NEC).
6. Step 6 — Verify Protection Coordination Confirm: In (protective device) ≤ Iz (derated cable ampacity). For short-circuit protection, check the cable's adiabatic limit: S ≥ √(I²t) / k.

IEC 60364-5-52 — Current-Carrying Capacity Reference Table

The values below are from IEC 60364-5-52 Table B.52.4 for copper conductors with XLPE/EPR insulation (90°C rated) at 30°C ambient, with no grouping. For PVC (70°C) cables, values are approximately 10–15% lower.

Source: IEC 60364-5-52:2009 Table B.52.4 — Copper conductors, XLPE insulation, 3-core cable, 30°C ambient. Actual values may vary by manufacturer.

Temperature Derating Factors (Ca)

When the ambient temperature differs from the standard 30°C reference, the cable's current-carrying capacity must be derated by the factor Ca from IEC 60364-5-52 Table B.52.14:

Worked Example — Cable Sizing for a 15kW Motor

Given: 15kW, 400V three-phase motor, PF = 0.85, η = 0.92. Cable installed in conduit on wall (Method B1), ambient 40°C, 3 circuits in the same conduit.

Step 1 — Design Current

Step 2 — Derating Factors

Ca (40°C, XLPE) = 0.87  |  Cg (3 circuits) = 0.73

Step 3 — Required Tabulated Current

Step 4 — Select Cable

From Method B1 table: 10mm² XLPE has Iz = 63A ≥ 43.6A ✅. The 6mm² (46A) also satisfies the requirement, so select 6mm² — no wait, 46A > 43.6A ✅ — 6mm² is the minimum cable size.

Step 5 — Voltage Drop Check (30m run)

Result: Select 6mm² copper XLPE 3-core cable. Verify the protective device (In ≤ 46A derated rating).

Worked Example 2 — Residential Sub-Main (BS 7671 / IEC)

Given: 230 V single-phase sub-main feeding a 12 kW domestic load (mixed lighting + sockets), cable run 22 m underground in conduit (Method D2), ambient soil 25°C, single circuit. Power factor assumed unity for resistive lighting + heating mix.

Step 1 — Design Current

Step 2 — Diversity (BS 7671 Appendix A)

For a domestic sub-main feeding a mixed final-circuit board, BS 7671 allows ~70 % diversity. Adjusted: 52.2 × 0.70 = 36.5 A.

Step 3 — Derating

Soil at 25 °C: Ca = 1.05 (cooler than reference 30 °C ground). Single circuit: Cg = 1.0. Combined factor = 1.05.

Step 4 — Required Tabulated Current

Step 5 — Cable Selection (Method D2, Cu PVC 70 °C)

From IEC Table B.52.4: 6 mm² = 41 A (Method D2) ≥ 34.8 A ✓. Voltage drop check: 36.5 × 22 m × 7.3 mV/A/m ÷ 1000 = 5.86 V → 2.55 % of 230 V → within the 5 % power limit ✓.

Result

Select 6 mm² copper PVC two-core SWA cable, protected by a 40 A or 50 A MCB. Always verify diversity assumption against actual maximum-demand calculation per BS 7671 Appendix A or NEC Article 220.

Worked Example 3 — Solar PV DC String Cable (the Cs factor everyone forgets)

Given: 12-panel string of 400 W modules, max DC current Imp = 10.2 A, open-circuit Voc = 480 V, cable run 18 m on roof in sun-exposed conduit, ambient design 60 °C (rooftop), 4 strings grouped in same conduit.

Step 1 — Design Current per IEC 62548 / NEC 690.8

For PV strings, design current must include the irradiance-and-temperature safety factor: Ib = 1.25 × Isc × 1.25 (NEC) = 1.56 × Isc. Or per IEC: 1.25 × Isc.

Step 2 — Solar-Specific Derating

Rooftop ambient 60 °C is far above the reference 30 °C. For XLPE solar cable: Ca ≈ 0.65 at 60 °C. Four strings grouped in conduit: Cg = 0.65. Combined: 0.65 × 0.65 = 0.42.

Step 3 — Required Tabulated Current

Step 4 — Select PV-Rated Cable

For 39.8 A in solar cable (TÜV-rated H1Z2Z2-K): 4 mm² single-core PV cable rated 55 A at reference conditions ≥ 39.8 A ✓. The 2.5 mm² (40 A reference) would only barely satisfy after derating; engineers always select one size larger for solar to allow for cable aging in UV exposure.

Result

Select 4 mm² double-insulated PV cable (red + black, TÜV/IEC 62930 rated) with UV-stable jacket. Voltage drop on a 480 V DC string is negligible (under 0.5 %), so cable choice is dominated by ampacity and rooftop derating, not voltage drop. Lesson: rooftop ambient and string-grouping derating combined often double the required cable size compared to the same load in a cool indoor conduit.

Common Mistakes — Eight Cable-Sizing Errors That Cause Real Failures

1. Using the cable's "free air" rating in conduit installations. Free-air ampacity (Method E/F) can be 30–50 % higher than in-conduit rating (Method B1). Always pick the table that matches your actual installation method.
2. Forgetting to apply both Ca AND Cg. If the cable is in a 40 °C plant room AND grouped with three others in a tray, both factors apply: total derating = Ca × Cg, often as low as 0.6 of nominal ampacity.
3. Sizing only for current and not voltage drop. A 2.5 mm² cable comfortably carries 25 A at the panel — but at 50 m distance the voltage drop exceeds 5 %, browning out lights and burning out motor windings. Always check both criteria.
4. Using copper-cable ampacity tables for aluminium. Aluminium has roughly 60 % the conductivity of copper; for the same ampacity you need ~1.6 × the cross-section. Most installations also require special bi-metallic connectors to prevent galvanic corrosion at terminations.
5. Ignoring the protective device coordination. The cable must protect the device and the device must protect the cable: In (breaker rating) ≤ Iz (derated cable ampacity) ≤ I2 (breaker conventional tripping current ÷ 1.45). Per IEC 60364-4-43, all three inequalities must hold.
6. Specifying single-core instead of multi-core for parallel runs. Three single-core cables of same phase grouped together can have circulating-current and skin-effect issues at high currents; multi-core or proper-spaced single-core arrangements per IEC 60364-5-523 are required for circuits above ~150 A.
7. Underrating for short-circuit (k²S² requirement). A correctly-sized cable for normal load can still be destroyed by a fault current if the protective device's let-through energy (I²t) exceeds the cable's withstand. Verify k² × S² ≥ I² × t per IEC 60364-4-434.
8. Ignoring harmonics. Modern LED drivers, VFDs, and switching power supplies inject 3rd-harmonic currents that flow in the neutral conductor of 3-phase systems. The neutral can carry as much (or more) current than the phases. Per IEC 60364-5-52 Annex E, derate phase conductors when 3rd-harmonic content exceeds 15 %, and never undersize the neutral.

Where Cable Sizing Matters — 8 Real-World Project Types

1. Residential and commercial wiring — every distribution board's outgoing cables must be sized to its connected load with derating for grouping and ambient.
2. Industrial motor circuits — see our motor starting current calculator for in-rush analysis; cable must carry 1.25 × FLC continuously per NEC 430.22.
3. Data centres — busbars and tap-off cables sized with extreme-temperature derating (data centres often run 25–35 °C cold-aisle, 45 °C hot-aisle).
4. Solar PV installations — DC string cables (worked example 3 above), AC inverter output cables, and combiner-box-to-disconnect cables each have their own derating layers per IEC 62548 and NEC 690.
5. EV charging infrastructure — Level 2 and DC fast-charge installations require cable sizing per NEC 625 / IEC 61851. EV charging is treated as continuous load (1.25 × rating).
6. Marine and offshore — corrosive environment + grouping in conduit + variable ambient → use IEC 60092 instead of standard IEC 60364, with additional safety factors.
7. Mining and hazardous areas — explosion-protection requirements per IEC 60079; cable selection includes mechanical-protection criteria (armoured, lead-sheathed) beyond pure ampacity.
8. Building services — HVAC chiller feeders, lift motors, fire-pump circuits all use specialised codes (NFPA, BS 9999) with prescribed margins above standard sizing.

Frequently Asked Questions