Your First Cable Sizing Calculation: A Tutorial for Junior Engineers

I have supervised hundreds of cable sizing calculations over 18 years in mining. The ones that go wrong almost always share the same root cause: the engineer skipped a step, or applied steps in the wrong order. Cable sizing is not complicated. It is sequential. Miss one step and the result is either unsafe or unnecessarily expensive.

This tutorial walks through a complete cable sizing calculation for a real circuit. Every number is shown. Every step is explained — not just what to do, but why it matters.

The Circuit We Are Sizing

Here is the load data, taken straight from an equipment schedule:

• Motor: 22 kW, three-phase, 415 V, power factor 0.85, efficiency 0.91
• Cable run: 45 metres on perforated cable tray
• Grouping: 4 circuits on the same tray
• Ambient temperature: 40 degrees C (standard Australian reference)
• Cable type: 4-core XLPE/PVC copper (X-90 insulation, 90 degrees C maximum)
• Standard: AS/NZS 3008.1.1:2017

Let us work through this step by step.

Step 1: Calculate Design Current (Ib)

The design current is what the cable must carry continuously under normal operation. For a three-phase motor, the formula is:

Plugging in our values:

Our design current is 39.5 A.

Why this step matters: Every subsequent step depends on getting the design current right. An error here propagates through the entire calculation.

Step 2: Select Protective Device Rating (In)

The protective device must have a nominal rating equal to or greater than the design current:

Our Ib is 39.5 A. The next standard MCCB/MCB rating above this is 40 A. We select a 40 A device.

For a motor circuit, we also need to consider starting current. The protective device must not trip during motor start. This is why motor circuits typically use Type D MCBs (10-20x instantaneous trip) or MCCBs with adjustable magnetic trip settings. But for cable sizing purposes, the continuous rating In = 40 A is what matters.

Why this step matters: The protective device rating sets the minimum cable capacity. The cable must carry at least In continuously without the protective device tripping.

Step 3: Find the Tabulated Current Rating (It)

Now we look up the current carrying capacity from AS/NZS 3008.1.1:2017, Table 13. But first, we need to identify which column applies.

Our cable is 4-core XLPE copper installed on perforated cable tray. From Table 3 of AS/NZS 3008.1.1, this corresponds to Installation Method 13 (unenclosed on a perforated cable tray).

AS/NZS 3008.1.1, Table 3 — Description of installation methods

Looking at Table 13 for X-90 insulation, 4-core cable, Method 13:

The tabulated current is the rating at reference conditions (40 degrees C ambient in air for Australia). We will apply derating factors next to adjust for our actual conditions.

Why this step matters: The tabulated value assumes ideal conditions. Your installation is almost never at reference conditions.

Step 4: Apply Derating Factors

Derating factors reduce the cable's effective current carrying capacity to account for conditions that impair heat dissipation. We need two derating factors for this circuit.

Factor 1: Ambient Temperature (Ci)

Our ambient temperature is 40 degrees C. For X-90 insulation in Australia, 40 degrees C is the reference ambient temperature. From AS/NZS 3008.1.1:2017, Table 22, the derating factor is:

AS/NZS 3008.1.1, Table 22 — Rating factors for ambient air temperature

Ci = 1.00 (no derating needed — we are at the reference temperature)

If the ambient were 45 degrees C, the factor would drop to approximately 0.93, and at 50 degrees C, to approximately 0.87. Each 5 degrees above reference costs you roughly 7% of cable capacity.

Factor 2: Grouping (Cg)

We have 4 circuits grouped on the same tray. From AS/NZS 3008.1.1:2017, Table 21, for cables on a perforated tray in a single layer with touching arrangement:

AS/NZS 3008.1.1, Table 21 — Rating factors for groups of more than one circuit

For 4 circuits: Cg = 0.72

This is the factor that catches most junior engineers. Four cables touching on a tray means each cable can only dissipate 72% of the heat it could dissipate in isolation.

Combined Derating

Why this step matters: Grouping derating alone reduced our cable capacity by 28%. Ignoring it means the cable runs hotter than its insulation can tolerate, leading to accelerated ageing and eventual failure.

Step 5: Verify Cable Size — Current Capacity Check

The derated current capacity of the selected cable must be equal to or greater than the protective device rating:

Let us check 6 mm2 first: 47 x 0.72 = 33.8 A. This is less than 40 A. Fails.

Try 10 mm2: 65 x 0.72 = 46.8 A. This is greater than 40 A. Passes.

We select 10 mm2 cable based on current carrying capacity.

Step 6: Check Voltage Drop

Current capacity is satisfied. Now we verify that the voltage arriving at the motor terminals is adequate. AS/NZS 3000:2018, Clause 3.6.2 limits voltage drop to 5% from the point of supply to the point of use.

AS/NZS 3000, Clause 3.6.2 — Voltage drop

We use the mV/A/m method from AS/NZS 3008.1.1:2017, Table 42. For 10 mm2 4-core XLPE copper at 75 degrees C conductor temperature, three-phase:

AS/NZS 3008.1.1, Table 42 — Voltage drop — three-phase

The combined mV/A/m value (at the table's reference power factor) is approximately 4.09 mV/A/m for 10 mm2 three-phase.

As a percentage of supply voltage:

This is within the 5% limit. Passes.

If this circuit is part of a larger installation, remember to add the upstream voltage drop (from the transformer to the distribution board) to get the total. The 5% limit applies to the entire path, not just this cable run.

Step 7: Verify Protective Device Operates Within Required Time

The final step is confirming that the selected cable and protective device combination provides automatic disconnection of supply within the time required by the standard. For a TN system with a 415 V three-phase circuit, AS/NZS 3000 Clause 2.5.5.2 requires disconnection within 0.4 seconds for final subcircuits.

AS/NZS 3000, Clause 2.5.5.2 — Automatic disconnection — TN systems

This requires the earth fault loop impedance (Zs) to be low enough that the fault current exceeds the protective device's threshold for 0.4 second operation. For a 40 A Type D MCB, the instantaneous trip range is 10-20 times rated current (400-800 A). The maximum loop impedance that ensures this fault current at 230 V phase voltage must be verified against the actual loop impedance of the circuit.

For this tutorial, we flag this as a verification step that must be completed using the actual loop impedance of the installed cable and the upstream network.

Summary of Results

The final selection is 10 mm2 4-core XLPE copper cable, protected by a 40 A device.

What Junior Engineers Get Wrong

After reviewing hundreds of these calculations, the most common errors I see are:

1. Forgetting grouping derating. This is by far the most frequent mistake. On a busy cable tray, grouping can reduce capacity by 30-50%.
2. Checking voltage drop at In instead of Ib. Voltage drop should be checked at the actual design current, not the protective device rating. The cable operates at Ib, not In.
3. Not accounting for the full voltage drop path. The 5% limit is from the supply point to the load, not just for your cable run.
4. Using calculated motor current instead of nameplate FLC. Always prefer the manufacturer's measured value.
5. Selecting cable based on design current alone. The cable must satisfy In after derating, not just Ib.

Cable sizing is methodical. Follow the steps in order, show your intermediate numbers, and the calculation practically does itself. Skip a step, and you either buy cable you do not need or install cable that will fail.