Cable Sizing Calculator

Parameters

This calculator exposes more inputs than most, because it runs four separate checks and each one needs its own information. Every field is documented below with the range the form accepts and the factor it actually applies inside the calculation.

Circuit

Phase configurationsingle phase, three phase, or DC

Sets the voltage drop multiplier and the phase voltage used by the earth fault loop check. Three phase uses 1.732, the square root of 3, and a phase voltage of the nominal divided by 1.732. Single phase and DC use a factor of 2 and the nominal voltage itself. Selecting DC also drops reactance from the calculation entirely and disables the loop check. Changing this selector resets the nominal voltage.

Nominal voltagevolts

Set by the phase selector: 230 volts single phase, 400 volts line to line three phase. The voltage drop percentage is measured against this figure.

Load currentamps, 0.1 to 500

The design current, normally written as Ib. This is the current the circuit actually carries, not the protective device rating. It drives the current capacity check, the voltage drop and the operating temperature estimate.

Cable run lengthmetres, 1 to 2000

The one-way route length. Do not double it for the return conductor; the phase multiplier already accounts for the return path. The same figure is compared against the maximum earth fault loop length when the loop check is enabled.

Protection device ratingamps, 1 to 800

The rating of the upstream circuit breaker or fuse, normally written as In. It is checked against the derated cable capacity so the cable is never protected by a device larger than it can carry, and when a device curve is selected it also sets the instantaneous trip current used by the loop check.

Cable construction and installation

Installation methodeight options

Applies a rating multiplier relative to clipped direct: free air spaced 1.18, perforated tray 1.10, free air touching 1.08, solid tray 1.00, clipped direct 1.00, enclosed in conduit in air 0.95, direct buried 0.92, and enclosed in conduit buried 0.88. Choosing either buried option also switches the whole derating calculation to the underground branch, which uses a 25 degree Celsius soil reference instead of a 40 degree Celsius air reference and reveals the burial depth and soil resistivity fields.

Conductor materialcopper or aluminium

Aluminium multiplies the base current rating by 0.78 and the conductor resistance by 1.6. Both effects push the selected size upward, so an aluminium run typically lands one to two standard sizes above the copper equivalent.

InsulationPVC 75, XLPE 90 or EPR 90

Sets the rated conductor temperature, 75 degrees Celsius for PVC and 90 for XLPE and EPR. That temperature feeds the ambient derating formula and the operating temperature estimate. It also selects the adiabatic constant used by the short-circuit check: 111.2 for copper with PVC, 143 for copper at 90 degrees, 73.6 for aluminium with PVC and 94 for aluminium at 90 degrees.

Cable typemulti-core or single-core

Single-core cables sit further apart than the cores of a multi-core cable, which raises inductive reactance. Selecting single-core multiplies the reactance by 1.4. It changes nothing else, so on small conductors where reactance is negligible the two options give nearly identical results.

Parallel cables per phasecount, 1 to 10

The number of cables running in parallel for each phase. The rating is multiplied by the count and the voltage drop is divided by it. Selecting more than one also forces a minimum conductor size of 4 square millimetres per cable, because very small conductors in parallel do not share current reliably.

Derating

Maximum voltage droppercent, 1 to 10, default 5

The compliance target for the voltage drop check. AS/NZS 3000:2018 Clause 3.6 uses 5 percent across the whole installation, so tighten this when the run being sized is only one part of the chain.

Ambient temperaturedegrees Celsius, 10 to 75

Air temperature for in-air installation methods, soil temperature for buried ones. The factor is the square root of the rated temperature minus the ambient, divided by the rated temperature minus the reference. The reference is 40 degrees in air and 25 degrees in soil. At the reference temperature the factor is exactly 1, so a PVC cable in 40 degree air gets no ambient penalty at all. Entering an ambient above the rated conductor temperature drives the factor to zero and no size will pass.

Number of circuits in groupcount, 1 to 20

How many loaded circuits share the same enclosure, tray or trench. The factor steps down: 1 gives 1.00, 2 gives 0.80, 3 gives 0.70, 4 to 6 gives 0.57, 7 to 9 gives 0.50, and 10 or more gives 0.44. This is often the largest single penalty in the whole calculation, so count the circuits honestly.

Arrangementtouching or spaced

Spaced cables shed heat better and recover 30 percent of the grouping penalty. With three circuits, touching gives 0.70 and spaced gives 0.79. It has no effect at all when the grouping count is 1, since there is no penalty to recover.

Burial depthmetres, 0.3 to 3, buried methods only

The factor is 1.00 at the 0.5 metre reference and falls by 0.06 for every metre deeper, with a floor of 0.85. Shallower than 0.5 metres gives no bonus. Deeper soil dissipates heat more slowly, which is why the penalty exists.

Soil thermal resistivitykelvin metres per watt, 0.5 to 3, buried methods only

The factor is 1.00 at the 1.2 reference and changes by 0.1 per unit either side, clamped between 0.7 and 1.2. Wet clay conducts heat away well and sits low; dry sand sits high and penalises the cable. If you have no soil thermal test, 1.2 is the usual default.

Optional deeper checks

Protective device curveblank, B, C or D

Leaving it blank disables the earth fault loop check entirely. Selecting a curve enables it and sets the instantaneous trip current as a multiple of the device rating: B is 4 times, C is 7.5 times and D is 12.5 times. The loop check is skipped on DC and skipped if the device rating is zero.

Loop methodestimate, calculate or measured

Estimate assumes 80 percent of the source voltage is available to drive the fault and needs no extra input. Calculate derives the source impedance from a prospective earth fault current you supply. Measured uses a loop impedance you have actually tested at the board. Estimate is the right default when you have nothing measured.

Prospective earth fault currentkiloamps, 0 to 100, calculate method only

The fault current available at the origin of the circuit. Used only when the loop method is set to calculate. Leave at zero if you are not using that method.

Measured loop impedanceohms, 0 to 10, measured method only

The loop impedance measured at the origin of the circuit with a loop tester. Used only when the loop method is set to measured.

Prospective fault currentkiloamps, 0 to 100

The prospective short-circuit current at the cable. This field and the clearing time below must both be greater than zero for the short-circuit withstand check to run. Leave both at zero to skip it.

Fault clearing timeseconds, 0 to 5

How long the protective device takes to clear that short circuit. Together with the fault current it gives the let-through energy the conductor has to survive.

Load power factornot exposed on this form

The engine can use a load power factor to compute voltage drop as resistance times cosine phi plus reactance times sine phi. The web form does not collect it, so the calculator falls back to the worst-case impedance magnitude, the square root of resistance squared plus reactance squared. That is conservative and will read slightly higher than a power-factor-aware calculation.

Assumptions and limits

The selection method here follows the structure of a real AS/NZS 3008.1.1 cable calculation: derate the rating, check voltage drop, check the loop, check short-circuit withstand, and take the smallest size that survives all of it. The method is complete. The numbers behind it are not yet authoritative, and that distinction matters.

What the calculator assumes

• Every lookup table is indicative, not transcribed. The base current ratings, the conductor resistance and reactance values, the installation method factors, the grouping, depth and soil resistivity factors, and the adiabatic constants are all working figures. They have not been taken from a licensed copy of AS/NZS 3008.1.1 and they have not been validated by a chartered professional engineer. Expect the selected size to be broadly sensible and the individual amperages to differ from the standard.
• One rating table for every construction. The standard selects a different rating column for each installation method and cable construction. This calculator starts from a single base table and applies the installation method as a flat multiplier instead. That is a structural simplification, not just a data one.
• Resistance is fixed at 75 degrees Celsius. It does not vary with insulation type and it is not corrected for the actual operating temperature. The operating temperature shown in the results is reported for information and is not fed back into the voltage drop.
• Aluminium resistance is a flat 1.6 times copper, and aluminium current rating a flat 0.78 of copper. Real aluminium tables are not a constant ratio across the size range.
• Voltage drop uses worst-case impedance, because the form does not collect load power factor. This overstates the drop slightly on most real loads.
• The neutral is reported as the same size as the active. There is no reduced-neutral logic and no allowance for triplen harmonic current in the neutral of a three phase circuit feeding non-linear loads, which in a heavily loaded office or data installation can exceed the phase current.
• The earth size uses a simplified rule: equal to the active up to 16 square millimetres, 16 square millimetres for 25 and 35, and half the active above that, snapped up to a standard size. The reduced-earth columns of AS/NZS 3000 Table 5.1 are not implemented, so this is conservative.
• The loop check uses the instantaneous trip band midpoint, 4, 7.5 or 12.5 times the device rating, and for the estimate method an 80 percent source voltage allowance. It does not use the maximum Zs values in AS/NZS 3000 Table 8.1, it does not model fuse time-current curves, and it takes no account of RCD protection, which changes the disconnection argument entirely.
• The short-circuit check compares the adiabatic constant squared times area squared against current squared times time. It assumes the full prospective current flows for the whole clearing time and takes no credit for current limiting by the protective device, which for a modern current-limiting breaker is a significant conservatism.
• Voltage drop percent is rounded to one decimal before it is compared, so a true 5.04 percent displays as 5.0 percent and reads as a pass. Treat anything within a tenth of a percent of the limit as needing a second look.
• Selection returns the smallest standard size that satisfies every enabled check. If no size satisfies them all, the largest size is shown along with a warning that there is no solution. The per-size selection table shows which check each size failed on.

What this should not be used for

• The design of record for any installation. Verify the selected size against the current edition of AS/NZS 3008.1.1 using licensed tables before it goes on a drawing.
• High voltage cable, which is covered by AS/NZS 3008.1.1 Part 2 and its own rating and screening rules.
• Solar photovoltaic DC string circuits, where voltage rise rather than drop governs. Use the voltage rise calculator.
• Busbar, busway or bare conductor systems, which are not cables and do not use these rating tables.
• Fire-rated, mineral-insulated or circuit-integrity cable, which carries its own manufacturer ratings and its own installation rules.
• Hazardous area installations, which add AS/NZS 60079 requirements on top of everything here.
• Any circuit protected by an RCD where you are relying on the loop check for disconnection, since RCD operation is not modelled.

Results should be checked against the current edition of AS/NZS 3008.1.1 and AS/NZS 3000, and the final design signed off by the responsible person for the installation. Nothing on this page is a certification.

Worked examples

Three complete selections showing every factor applied in order, covering single phase copper, three phase aluminium buried with full derating, and a case where the short-circuit check rather than current rating decides the answer.