Cable Sizing Calculator: Ampacity, Voltage Drop and Insulation Limits

Every cable size is really a temperature limit. The conductor can carry current. The insulation is what sets how much. PVC is rated for a lower continuous temperature than XLPE, and it also softens sooner under fault current. Pick the wrong one, or derate it badly, and the insulation ages fast. That usually shows up years later as a low IR reading.

This tool sizes a cable against three limits at once: current-carrying capacity, voltage drop, and short-circuit heating. It follows the AS/NZS 3008 method. The logic carries over to IEC work.

The four checks, in order

1. Load current. You enter the load as A, kW, kVA or hp. The tool converts it to line current. For kW and hp it needs a power factor. For three-phase it uses √3 × V.

2. Derated capacity. It takes the base rating for your material, insulation and medium, then multiplies by each derating factor:

• Ambient temperature (air or soil)
• Installation method (spaced, touching, tray, conduit, buried, duct bank)
• Grouping (circuits × cables per enclosure)
• Soil thermal resistivity, against a 1.2 m·K/W reference
• Burial depth
• Cable type and flexible cable

The smallest size whose derated capacity covers the load passes this step.

3. Voltage drop. The tool checks the drop with k·I·L·(R·cosφ + X·sinφ)/V, where k is 2 for single-phase and DC, and √3 for three-phase. If the size from step 2 fails, it steps up until the drop fits.

4. Short circuit and loop impedance (optional). It checks whether the cable survives the fault energy, whether the device breaks the fault current, and whether the loop impedance is low enough for a fast trip.

Why the insulation choice changes the answer

Same copper, same route, different insulation, different cable. XLPE runs at 90 °C continuous, while PVC runs lower. So XLPE carries more current for the same mm².

The short-circuit check shows it more clearly. The tool uses the adiabatic equation, which assumes all fault heat stays in the conductor for the fault time:

I = k × S / √t

The k factor depends on conductor and insulation:

Conductor	PVC	XLPE
Copper	115	143
Aluminium	76	94

A 16 mm² copper XLPE cable can handle about 2.3 kA for 1 s. The same cable in PVC handles about 1.8 kA. The difference is the insulation’s temperature limit, not the copper.

If a fault goes over that limit, the insulation gets damaged even if the protection clears. That’s why a cable that survived a fault should get an IR test before it goes back in service.

Inputs that people get wrong

Number of circuits. Count every loaded circuit in the group, not just yours. Five circuits on one tray gives a grouping factor of 0.60. That alone can push you up two sizes.

Ambient default. Part 1 (Australia) assumes 40 °C air and 25 °C soil. Part 2 (New Zealand) assumes 30 °C air and 15 °C soil. In a hot plant room or on a roof, switch to custom and enter the real value.

Soil resistivity. Dry sand or backfill can be well above 1.2 m·K/W. Higher resistivity means less heat escapes, so the rating drops.

Earth core size. For the loop impedance check, enter the real earth conductor size. A small earth core raises loop impedance and can fail the trip check.

Conductor temperature mode. “Maximum” uses the full insulation rating for resistance. It gives a more conservative voltage drop. Use it when the cable will run near full load.

Reading the result

The result shows:

• Recommended size and line current
• Adjusted capacity and margin
• Voltage drop against your limit
• The full chain of derating factors
• Loop and short-circuit results, if enabled

A red banner lists every check that failed. You can download the result as a text file for your design notes.

This is a first-pass tool. Final sizes should be checked against the full tables in the standard you work to and against the cable maker’s data.

FAQ

Does this replace the AS/NZS 3008 tables?
No. It uses simplified base ratings and factors. Use it to get close fast, then confirm with the full tables.

Why did the tool pick a bigger size than ampacity needs?
Voltage drop. On long runs, drop decides the size, not current.

Can I use it for IEC 60364 work?
The method is the same: derate, check drop, check fault withstand. The base ratings and factors differ, so treat results as an estimate.

Why does short-circuit withstand matter for insulation?
Fault current heats the conductor in milliseconds. If the temperature goes past the insulation’s short-time limit, the insulation degrades even when the breaker clears in time.

What test should follow a fault on a cable?
An insulation resistance test at minimum. On MV cables, a VLF withstand or tan δ test gives a better picture.