Electrical & Instrumentation · Free tool

Cable sizing calculator

Size a low-voltage copper or aluminium cable the way an engineer does: check voltage drop and short-circuit withstand, then confirm the current rating. Every step of the working is shown.

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The circuit

Supply
V

Nominal voltage at the source

Load given as
A

Full-load value

m

One way, source to load

Conductor
Insulation
°C

Check the datasheet

%

Typical 3%

Short circuit (optional)
kA

Prospective, at the source

s

From the protection

Minimum cable size

16 mm²

copper, XLPE

Set by voltage drop, before the current rating check below.

Design current
40 A
Min. rating
50 A
Voltage drop
2.02%
Drop (volts)
8.1 V

Minimum area each check needs

Voltage drop10.77 mm²

Full bar = the chosen standard size (16 mm²).

For your installation method and conditions. Must be at least 50 A.

A
  • Short-circuit capability not checked. Enter the prospective fault current and clearing time to include it.

The working

Equation numbers refer to IEEE Std 525-2007, Annex C.

  1. Input

    Design current Ib

    Load entered in amperes

    40 A
  2. C.2.2

    Required cable ampacity

    1.25 × 40 A

    50 A
  3. C.3

    Allowed voltage drop

    3% × 400 V

    12 V
  4. C.5

    Minimum area for voltage drop

    ρ × √3 × 80 m × 40 A / 12 V × [1 + α(90 − 20)] × 1.02 × 1.04, copper ρ = 0.017241 Ω·mm²/m, α = 0.00393/°C

    10.771 mm²
  5. C.4

    Resistance of 16 mm² at 90 °C

    0.017241 / 16 × [1 + 0.00393(90 − 20)] × 1.02 × 1.04

    1.4575 mΩ/m
  6. C.2b

    Voltage drop with 16 mm²

    √3 × 80 m × 40 A × R

    8.08 V (2.02%)

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The method

How this calculator works

Three checks
A cable must carry the load current without overheating (ampacity), deliver enough voltage at the far end (voltage drop), and survive a fault until the protection clears it (short circuit). The largest size any check needs is the answer. Method: IEEE Std 525-2007, Annex C.
Voltage drop
Conductor resistance is calculated from the metal's resistivity, corrected to operating temperature and for stranding (Eq. C.4). The drop counts both conductors on single phase and DC, and √3 on three phase. Typical limits: 3% for a feeder or a final circuit, 5% in total.
Short circuit
During a fault the conductor heats from its operating temperature to the insulation's limit: 250 °C for XLPE, 150 °C for PVC. Eq. C.15 gives the minimum area for the fault current and clearing time.
Current rating
The cable's current rating must be at least 1.25 times the load (C.2.2). Ratings depend on how the cable is installed, so look up the recommended size in ZS 772-1 (The wiring of premises) and enter it to complete the check. Built-in ratings are coming.
Standard sizes
Results step through the conductor sizes in ZS 688, the cable standard made compulsory in Zambia by Statutory Instrument No. 59 of 2013.
Limits
Low voltage only. Reactance is left out, which is accurate below about 107 mm²; above that the tool warns you. Motor starting, harmonics and parallel cables need a full design.

Tools on this site are for guidance and learning. Results and examples must be checked by a qualified engineer against the applicable standards before any installation or design decision.

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