Cable Size Calculator
Smallest cable for voltage drop and short circuit — and which check governs.
Criteria
| Criterion | Minimum size | Detail |
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How it was calculated
Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Cable Size Calculator
A cable has to pass three checks: it must carry the design current without overheating, keep the voltage drop within the limit, and survive a short circuit until the protective device clears it. This calculator finds the smallest standard conductor — IEC 60228 mm² sizes or AWG/kcmil — that meets your voltage-drop limit (IEC 60364-5-52 Annex G) and the adiabatic short-circuit check S ≥ √(I²t) ÷ k, and tells you which one governs.
The third check, current-carrying capacity, depends on the cable type, how it is installed and your country’s rules, and the tables are part of copyrighted wiring codes — so the tool does not guess it. It works out the tabulated rating you need to look up (I_t ≥ I_n ÷ correction factors), and you enter the smallest size your own table or cable datasheet allows. Only then is the selection complete.
How to use it
- Circuit: choose DC, single-phase or three-phase, enter the voltage, the design current I_B (or the load in kW and its power factor) and the one-way route length.
- Cable: choose copper or aluminium, mm² or AWG sizes and the insulation. The insulation sets k for the short-circuit check and the full-load conductor temperature for the voltage drop.
- Voltage-drop limit: pick IEC 60364 / BS 7671 or the NEC recommendation, or enter your own. The IEC and BS 7671 figures cover the whole installation, from its origin to the load: for a final circuit fed from a sub-main, enter what is left after the sub-main’s drop as your own limit.
- Short circuit: enter the prospective fault current and the disconnection time from the device’s time–current curve, or — for MCBs and fuses — the let-through I²t from the maker’s data. Choose Skip only if you will check it another way.
- Current-carrying capacity: enter the device rating and your correction factors, look up a cable with at least the I_t shown in your table, and choose that size. Read the minimum size and which criterion governs.
Examples
PF 0.85, λ = 0.08 mΩ/m, 5% limit, fault 10 kA cleared in 0.2 s
Voltage drop needs 25 mm² (4.38%) · short circuit: √(10,000² × 0.2) ÷ 143 = 31.3 mm² → 35 mm² · minimum 35 mm², governed by short-circuit withstand — then confirm 35 mm² carries 160 A in your table
230 V, PF 1, 70 °C, 5% limit, MCB let-through 50,000 A²s
Voltage drop: 4 mm² (4.31%; 2.5 mm² would be 6.89%) · short circuit: √50,000 ÷ 115 = 1.94 mm² → 2.5 mm² · minimum 4 mm², governed by voltage drop
Copper at 75 °C, NEC 3% branch-circuit recommendation
10 AWG drops 3.24%, so voltage drop needs 8 AWG (2.04%)
The three checks
- Overload / current-carrying capacity — IEC 60364-4-43 §433.1: I_B ≤ I_n ≤ I_z, where I_B is the design current, I_n the protective device rating and I_z the cable’s current-carrying capacity in its installed conditions (and I₂ ≤ 1.45 × I_z for the device’s conventional operating current).
- Voltage drop — u = b × (ρ × L ÷ S × cos φ + λ × L × sin φ) × I_B from IEC 60364-5-52 Annex G, with b = 2 for single-phase and DC and 1 for three-phase, compared with U₀. The voltage drop calculator explains it in detail.
- Short-circuit withstand — the conductor must not overheat before the fault is cleared: S ≥ √(I²t) ÷ k.
The largest of the three sizes is the minimum. Long runs are usually governed by voltage drop, heavily loaded short runs by current-carrying capacity, and cables close to a large supply with slow protection by the short-circuit check.
The adiabatic short-circuit equation
For faults cleared within 5 s, the heat cannot escape in time, so the conductor’s own heat capacity must absorb it: S ≥ √(I²t) ÷ k, with S in mm², I the fault current in amperes (RMS) and t the disconnection time in seconds (IEC 60364-4-43 §434.5.2 for line conductors; IEC 60364-5-54 §543.1.2 and BS 7671 Regulation 543.1.3 for protective conductors). For times under 0.1 s and for current-limiting devices, use the let-through energy I²t from the device maker instead of I² × t.
k depends on the conductor and on the temperatures before and after the fault (IEC 60364-4-43 Table 43A, the same as BS 7671 Table 43.1):
- PVC 70 °C (70 → 160 °C): copper 115, aluminium 76 — above 300 mm² (to 140 °C): 103 and 68
- PVC 90 °C (90 → 160 °C): copper 100, aluminium 66 — above 300 mm²: 86 and 57
- XLPE or EPR 90 °C (90 → 250 °C): copper 143, aluminium 94
- Rubber 60 °C (60 → 200 °C): copper 141, aluminium 93
They come from k = √(Q_c(β + 20) ÷ ρ₂₀ × ln(1 + (θ_f − θ_i) ÷ (β + θ_i))) in IEC 60364-5-54 Annex A. The same k values apply to a protective conductor that is a core of the cable; a separate protective conductor starts cooler and has different k values.
Current-carrying capacity: what to look up
Ampacity tables depend on the installation method (clipped direct, in conduit, in a tray, buried, in thermal insulation …), the cable construction and the number of loaded conductors, and they are published in copyrighted codes — IEC 60364-5-52 Annex B, BS 7671 Appendix 4, NEC Table 310.16 and national equivalents — or in cable makers’ datasheets.
With IEC and BS 7671 tables, divide the device rating by the correction factors that apply — ambient temperature C_a, grouping C_g, thermal insulation C_i and any other factor C_c your rules give — to get the tabulated rating you need: I_t ≥ I_n ÷ (C_a × C_g × C_i × C_c). Choose the smallest cable whose tabulated value is at least that. With the NEC, apply the 310.15 correction and adjustment factors, size continuous loads at 125% and respect the terminal temperature ratings of 110.14(C).
Sources
- IEC 60228, Conductors of insulated cables — nominal cross-sections
- IEC 60364-4-43, Protection against overcurrent — §433.1 and §434.5.2, Table 43A
- IEC 60364-5-52:2009, Wiring systems — Annex G (voltage drop)
- IEC 60364-5-54, Earthing arrangements and protective conductors — §543.1.2 and Annex A (k formula)
- BS 7671:2018 (IET Wiring Regulations, as amended) — Regulations 434.5.2 and 543.1.3, Table 43.1, Appendix 4
- NFPA 70 (NEC) — 110.14(C), 210.19, 310.15 and Table 310.16
Limitations
- No current-carrying-capacity tables are included: until you enter the size your own table allows, the result covers voltage drop and short circuit only.
- One conductor per phase. Conductors in parallel need the extra checks of IEC 60364-4-43 (§433.4 and §434.4).
- The voltage-drop check covers this one cable. The limits apply from the origin of the installation, so the drop in any sub-main feeding it counts too.
- It does not check earth-fault loop impedance and automatic disconnection times, protective-conductor size, harmonic currents in the neutral, motor starting, mechanical protection or the minimum sizes your code sets.
- The voltage drop uses resistivity-based conductor resistance; real stranded conductors may have slightly more (see the voltage drop calculator for datasheet values).
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Frequently asked questions
What size cable do I need for 32 A?
It depends on the length, the installation method and your rules, which is why this calculator asks for your table’s answer. For 32 A over 30 m at 230 V with a 5% limit, voltage drop alone needs 4 mm² copper (4.31%); then check that 4 mm², installed your way, has a tabulated rating of at least 32 A after correction factors.
Why doesn’t the calculator give the current rating of each cable?
Current-carrying capacity depends on how the cable is installed, its insulation, the surroundings and grouping, and the tables belong to copyrighted codes (IEC 60364-5-52, BS 7671, NEC). A single built-in table would be wrong for most installations, so the tool works out the value to look up and lets you enter your table’s result.
What is the k factor?
k (in A·√s/mm²) combines the conductor’s heat capacity and resistivity with the temperature it may reach during a fault. Copper with PVC insulation is 115; copper with XLPE is 143, because XLPE may reach 250 °C instead of 160 °C. Larger k means a smaller conductor survives the same fault.
Where do I find the fault current and disconnection time?
The prospective fault current comes from the supply (your network operator states it at the origin, or it is calculated from the transformer and cable impedances). The disconnection time comes from the protective device’s time–current curve at that current. For MCBs and fuses, which clear large faults in a few milliseconds, use the maker’s let-through I²t chart instead.
What are I_t and the correction factors?
I_t is the current rating printed in the table for the reference conditions. Correction factors reduce it for hotter surroundings (C_a), several circuits grouped together (C_g) or thermal insulation (C_i). Dividing the device rating by the factors gives the I_t to look for — 160 A with C_a = 0.87 and C_g = 0.8 needs I_t ≥ 230 A.
Which check usually decides the size?
Long runs are usually governed by voltage drop, heavily loaded short runs by current-carrying capacity, and cables near a large transformer with time-delayed protection by the short-circuit check. The calculator shows all three and marks the one that governs.