Cable sizing calculator

What the calculator does

A cable has to pass three tests: it must carry its load current continuously without its insulation overheating in the place it is installed, it must be protected by a device that trips before the cable is damaged, and it must deliver an acceptable voltage at the far end. The calculator does the first two together, the way IEC 60364 frames them, and the third as a separate check. It takes the design current and the rating of the circuit breaker, derates the tabulated current-carrying capacities of IEC 60364-5-52 for the installation method, the insulation, the ambient temperature and the number of grouped circuits, walks up the standard copper sizes until the derated capacity covers the breaker rating, then works out the voltage drop over the route at the conductor's rated temperature and steps up again if the limit is exceeded. It reports which of the two checks decided the size, so you can see whether a cooler route, a smaller group or a shorter run would have allowed a smaller cable.

Enter 0 for the protective device if you only want the cable checked against the load current; the result is then a cable that still needs a breaker chosen to fit it. The power and current calculator converts a load in kW or hp to its full-load current, and the voltage drop calculator explores the drop alone for any size and temperature.

Formula

where I_b is the design current of the circuit, I_n the rated current of the protective device, I_t the current-carrying capacity tabulated in Annex B of IEC 60364-5-52 for the reference method and number of loaded conductors, C_a the temperature correction factor (Table B.52.14 for cables in air, B.52.15 for ducts in the ground), C_g the grouping reduction factor (Table B.52.17), L the one-way route length in metres and R₂₀ the IEC 60228 class 2 conductor resistance at 20 °C, the same values the voltage drop calculator uses. The size is the first standard conductor whose I_z reaches I_n (or I_b when no device is entered), or the first whose drop is within the limit, whichever is larger.

I₂ is the current that makes the protective device operate within its conventional time. For a miniature circuit breaker to IEC 60898-1 it is 1.45 × I_n, and for a moulded-case breaker to IEC 60947-2 it is 1.3 × I_n, so with either device I_n ≤ I_z satisfies the 1.45 condition on its own and the calculator does not test it separately. A gG fuse to IEC 60269 has I₂ = 1.6 × I_n and therefore needs I_n ≤ 0.9 × I_z; if the circuit is fused, enter a rating 10 % above the fuse's.

Worked example

A 40 A single-phase load at 230 V, 40 m from the distribution board, wired with PVC-insulated single-core conductors in conduit on a wall (method B1) through a ceiling void that reaches 40 °C in summer, one circuit in the conduit, a 40 A MCB, power factor 0.85, voltage drop limit 5 %.

1. Correction factors: C_a = 0.87 (PVC in air at 40 °C), C_g = 1.00 (a single circuit). The table value needed is I_t ≥ 40 ÷ (0.87 × 1.00) = 46.0 A.
2. Table B.52.2, column B1: 6 mm² is rated 41 A, too small; 10 mm² is rated 57 A. Derated, I_z = 57 × 0.87 = 49.6 A, and 40 A ≤ 40 A ≤ 49.6 A, so the coordination holds.
3. Voltage drop with 10 mm² at 70 °C: r = 1.83 ÷ 1000 × (1 + 0.00393 × 50) = 0.00219 Ω/m; ΔV = 2 × 40 × 40 × 0.00219 × 0.85 = 5.96 V, which is 2.59 % of 230 V and within the 5 % limit.
4. Voltage drop on its own would have accepted 6 mm² (4.36 %), so the answer is 10 mm² copper, governed by current-carrying capacity.

Put the same circuit in a conduit that already carries two other circuits and C_g falls to 0.70: the table value needed becomes 40 ÷ (0.87 × 0.70) = 65.7 A, 10 mm² (57 A) no longer passes and 16 mm² (76 A) is the answer. At 45 °C as well, the factors multiply to 0.553, the required table value rises to 72.3 A and 16 mm² clears it with 42.0 A of derated capacity against the 40 A breaker.

Current-carrying capacity used: PVC 70 °C, copper (IEC 60364-5-52 Tables B.52.2 and B.52.4)

Amperes for one circuit at 30 °C in air (A1, B1, C) or 20 °C in the ground (D1). Each cell gives two loaded conductors (single-phase, DC) / three loaded conductors (three-phase). A1: insulated conductors in conduit in a thermally insulated wall. B1: insulated conductors in conduit, trunking or ducting on a wall. C: single-core or multicore cable clipped direct to a wall. D1: multicore cable in conduit or duct in the ground.

Current-carrying capacity used: XLPE 90 °C, copper (IEC 60364-5-52 Tables B.52.3 and B.52.5)

Same conditions and layout as the PVC table. XLPE and EPR share these values. The 90 °C rating may only be used where the terminals and equipment the cable connects to are rated for it; many are limited to 70 °C.

Correction factors used (IEC 60364-5-52 Tables B.52.14, B.52.15 and B.52.17)

Ambient factor Ca, reference 30 °C in air and 20 °C in the ground; the calculator interpolates linearly between rows. Grouping factor Cg for circuits bunched in air, on a surface, embedded or enclosed (Table B.52.17, item 1): 1 circuit 1.00, 2: 0.80, 3: 0.70, 4: 0.65, 5: 0.60, 6: 0.57, 7: 0.54, 8: 0.52, 9: 0.50, 12: 0.45, 16: 0.41, 20: 0.38; a count between tabulated values takes the next tabulated count.

Assumptions and limitations

• Copper conductors, 1.5 to 300 mm², reference methods A1, B1, C and D1 only. Aluminium, multicore cables in conduit (A2, B2), cables on trays, ladders or in free air (E, F, G) and cables buried direct (D2) have their own columns in the standard and are not offered. For a multicore cable in conduit the B2 values are roughly 5–15 % below B1.
• One circuit per table value, 50/60 Hz, balanced loads. Three-phase circuits are taken as balanced with no neutral current, so the neutral is not a loaded conductor. Harmonic (triplen) currents from drives, LED drivers and IT loads add neutral current and require the reduction factors of IEC 60364-5-52 Annex E, which are not applied.
• Grouping uses the bunched factors for every method. Table B.52.17 item 1 is the most severe air arrangement; single layers on walls or trays are treated more generously by the standard, and for ducts in the ground its Table B.52.19 applies instead. Check grouped buried runs against that table.
• No soil correction for D1. The buried values assume a soil thermal resistivity of 2.5 K·m/W and a depth of about 0.7 m; Table B.52.16 corrects for other resistivities and is not applied. Dry sandy soil is worse, moist clay better.
• No short-circuit or earth-fault check. The conductor must also withstand the let-through energy of the protective device under fault: S = √(I²t) ÷ k with k = 115 for PVC-insulated copper and 143 for XLPE-insulated copper (IEC 60364-4-43, 434.5.2). That calculation, and the earth-fault loop impedance needed for disconnection in time, are not done here.
• The 1.45 condition assumes a circuit breaker. The coordination check relies on an MCB to IEC 60898-1 or an MCCB to IEC 60947-2, whose conventional tripping current is at most 1.45 × I_n. Fuses need the 0.9 allowance described under Formula.
• Voltage drop is resistive, at rated conductor temperature. Cable reactance is ignored, which understates the drop for large conductors at low power factor, and the conductor is assumed to be at 70 °C or 90 °C, which overstates it for a lightly loaded cable. The two errors go in opposite directions; the tabulated mV/A/m values of your wiring rules are the reference where it matters.
• Tables are for 30 °C air and 20 °C ground. The correction factors run from 10 °C to 60 °C. Above 60 °C PVC has no tabulated factor and the cable should not be there.
• Not a design review. Mechanical protection, minimum sizes, the sizing of neutral and protective conductors, selectivity and the rules of the local authority (in Pakistan NEPRA's Distribution Code and Consumer Service Manual) are separate matters.

References

• IEC 60364-5-52:2009, Low-voltage electrical installations — Part 5-52: Selection and erection of electrical equipment — Wiring systems — Annex B Tables B.52.1–B.52.5 (current-carrying capacities), B.52.14 and B.52.15 (ambient and ground temperature), B.52.17 (grouping); Annex G (voltage drop)
• IEC 60364-4-43:2008, Part 4-43: Protection for safety — Protection against overcurrent — 433.1 coordination Ib ≤ In ≤ Iz and I2 ≤ 1.45 Iz; 434.5.2 adiabatic equation and k values
• IEC 60228:2004, Conductors of insulated cables — class 2 maximum conductor resistance at 20 °C
• IEC 60898-1:2015, Circuit-breakers for overcurrent protection for household and similar installations — Part 1: Circuit-breakers for a.c. operation — conventional tripping current 1.45 In
• Top Cable, Methods of installation and current-carrying capacities based on IEC 60364-5-52 Ed.3 — manufacturer's reproduction of Tables A.52.3 and B.52.1–B.52.19, used to verify the data above
• asutpp.com, Continuous current-carrying capacity of a conductor — independent reproduction of Tables B.52.2–B.52.5, cross-checked cell by cell
• ECalPro, IEC 60364-5-52 correction factors (B.52.14–B.52.21) — cross-check of the ambient and grouping factors
• CableApp, Low voltage power cable technical data — cross-check of the air and ground temperature factors (2001 edition numbering A.52-14, A.52-15)
• Eland Cables, BS 7671:2018 Table 4D4A — BS 7671 carries the same data; used to cross-check the D1 columns
• CSE Distributors, BS 7671 Tables 4D1A, 4E1A, 4E2A and 4E4A — cross-check of the A1, B1 and C columns

Last reviewed 2026-09-20.