How to Size Cables for Motor Circuits the Right Way Using IEC 60364

Let me start with something I’ve seen too many times on site. A brand new motor trips within minutes of startup. The contactor is fine. The overload setting is correct. Protection coordination checks out. But the cable is undersized, the voltage drop is killing the starting torque, and nobody bothered to run the numbers properly.

Cable sizing is not guesswork. It’s not “just pick the next size up from the catalog.” And it’s definitely not something you should do based on a rule of thumb your supervisor taught you fifteen years ago. Standards exist for a reason, and IEC 60364-5-52 is the one you need to follow if you’re working outside North America.

I’ve spent over 18 years designing motor control panels, commissioning industrial systems, and troubleshooting problems in factories across multiple countries. The single most common electrical design error I encounter is incorrect cable sizing for motor circuits. This article will walk you through exactly how to do it right, step by step, with real numbers and practical advice you can use on your next project.

And if you want to skip the manual math, our free IEC cable sizing calculator at the top of this page does the heavy lifting for you. But you should still understand what’s happening behind the numbers.

Why Motor Circuits Are Different from General Loads

IEC 60364-5-52 covers the first two. Short circuit withstand falls under IEC 60364-4-43, but you need to consider it during the sizing process too. Ignore any one of these three, and you’ve got a problem waiting to happen.

The Three Checks You Must Perform for Every Motor Cable

Check 1: Current Carrying Capacity (Ampacity)

This is your starting point. You need the cable to carry the full load current of the motor continuously without the conductor temperature exceeding its insulation rating.

How to find the design current:

For a three phase motor, the formula is straightforward:

I = P / (√3 × V × PF × η)

Where:

• P is motor rated power in watts
• V is line voltage (typically 400V in most IEC countries)
• PF is power factor (usually 0.85 for standard induction motors)
• η is motor efficiency (typically 0.88 to 0.93 depending on the frame size)

Example: A 37 kW motor running with 400V; power factor 0.85; & efficiency 0.92;

I = 37,000 / (1.732 × 400 × 0.85 × 0.92) = 37,000 / 542.3 = 68.2 A

Now you go to the IEC 60364-5-52 tables. The table you Pick Depend on two things: your installation method and your insulation type.

The most common combinations That I seen in industries Like:

• PVC insulated, clipped direct to wall (Method C): Use Table B.52.4
• XLPE insulated, on perforated cable tray (Method E): Use Table B.52.6
• PVC insulated, in conduit on wall (Method B): Use Table B.52.3

For our 68.2 A example using XLPE copper cable clipped direct, you’d select a 16 mm² cable (rated for 85 A in Method C with XLPE) or a 25 mm² cable if using PVC (rated for 80 A).

But wait. You’re not done.

Derating Factors Change Everything

The ampacity values in those tables assume an ambient temperature of 30°C and a single circuit. In a real plant, neither condition is true.

Ambient temperature derating: If your cable runs through a boiler room at 45°C & you need to multiply the table value by the correction factor from Table B.52.14 then For XLPE at 45°C, that factor is 0.87. So your 85 A capacity for 16 mm² drops to 74 A. Still enough for our 68.2 A motor? Barely. And “barely” is not where I like to be.

Grouping derating: If you have six circuits bundled together on the same tray, use Table B.52.17. For six circuits in a single layer on a perforated tray, the factor is about 0.73. Now that 85 A becomes 62 A. That 16 mm² cable is officially undersized.

The corrected current carrying capacity formula:

I_required = I_design / (k_temp × k_group × k_other)

Or work it the other way: multiply the table rating by all derating factors and make sure the result exceeds your design current.

One common mistake I see is engineers applying derating factors to the design current instead of the cable rating, or mixing up which direction the correction goes. Be careful. The derating factors reduce the cable’s capacity. Your design current stays the same.

For our example with both derating factors applied, you’d need at least a 25 mm² XLPE copper cable. Possibly 35 mm² if you’re conservative, which for a motor feeder, I usually am.

Check 2: Voltage Drop Calculation

This is where most motor circuit problems actually show up on site.

IEC 60364-5-52 recommends a maximum voltage drop of 4% from the origin of the installation to the load. Many companies use an internal standard of 3% for motor feeders, because motors are sensitive to voltage during starting.

Here’s the thing about voltage drop that trips people up (pun intended): it depends on the cable length, the current, and the cable cross section. You can have a perfectly sized cable for ampacity that fails the voltage drop test because the motor is 200 meters away from the MCC.

Voltage drop formula for three phase circuits:

ΔV = (√3 × I × L × (R × cosφ + X × sinφ)) / 1000

Where:

• I = design current in amps
• L = one-way cable length in meters
• R = resistance of conductor in mΩ/m (from manufacturer data or IEC tables)
• X = reactance of conductor in mΩ/m (typically 0.08 mΩ/m for most sizes)
• cosφ = power factor of the load

Voltage drop percentage:

ΔV% = (ΔV / V_nominal) × 100

Example; Our 37 kW motor, 68.2 A, cable length 120 meters, 25 mm² copper cable.

THe resistance of 25 mm² copper at operating temperature (almost 70C for PVC) is roughly 0.868 mΩ/m; & also Reactance is about 0.08 mΩ/m.

ΔV = 1.732 × 68.2 × 120 × (0.868 × 0.85 + 0.08 × 0.527) / 1000

ΔV = 1.732 × 68.2 × 120 × (0.738 + 0.042) / 1000

ΔV = 14,175 × 0.780 / 1000

ΔV = 11.06 V

ΔV% = 11.06 / 400 × 100 = 2.77%

That passes the 4% limit, and even the stricter 3% internal limit.; Good.

But what if the cable run was 250 meters? You’d get 5.76%, which fails badly. You’d need to jump to 50 mm² or even 70 mm² to bring the voltage drop under control.

This is the exact scenario where our IEC cable sizing calculator saves you twenty minutes of manual calculation. Enter the load, length, installation method, and derating factors, and it gives you the recommended cable cross section with voltage drop percentage and a clear PASS or FAIL result.

Check 3: Short Circuit Withstand

I won’t go deep into this one because it deserves its own article, but here’s what you need to know.

OK, Your cable must withstand the prospective short circuit current at its location for the time it takes the upstream protection device (MCCB or fuse) to clear the fault. The formula from IEC 60364-4-43 is;

S = (I × √t) / k

Where S is the minimum cross section in mm², I is the short circuit current, t is the disconnection time, and k is a constant depending on conductor and insulation material (115 for copper PVC, 143 for copper XLPE).

In practice, for most motor circuits fed from an MCC with properly coordinated MCCBs, the cable size selected for ampacity and voltage drop usually satisfies the short circuit requirement too. But always verify. Never assume.

Installation Methods That Matter Most in Industrial Settings

From my experience working in food processing plants, automotive factories, and water treatment facilities, here are the installation methods you’ll actually use:

Clipped direct (Method C): Single core or multicore cables clipped to a wall or ceiling. Very common for individual motor drops from a cable ladder down to the motor terminal box. Good current carrying capacity because the cable is exposed to air.

Perforated cable tray (Method E): This is your main route from the MCC room to the plant floor. THE Good airflow, good ratings, but grouping derating hits hard when you stack 20 motor circuits on the same tray.

Enclosed conduit on wall (Method B): Often used in hazardous areas or where mechanical protection is required. Lower current ratings because heat can’t escape as easily.

Cable duct in floor (Method B): Common in older plants. Terrible for heat dissipation. Always oversize when you’re pulling cables through floor ducts.

Pick the wrong installation method in your calculation, and you could be off by 20% or more on the current rating.

Copper vs Aluminium: A Practical Perspective

For motor circuits up to about 95 mm², I almost always specify copper. The reasons are practical, not theoretical:

• Copper terminations are standard on motor terminal boxes, contactors, and MCCBs
• Aluminium requires special anti-oxidant compound at every termination
• Aluminium connections loosen over time due to cold flow and need retorquing
• For the same current rating, aluminium cable is about 1.6 times the cross section of copper

For main distribution feeders above 150 mm², aluminium starts making economic sense. But for your typical motor feeder between 2.5 mm² and 70 mm², stick with copper unless budget absolutely forces the issue.

Real World Application Scenarios

Motor control panels (DOL, Star-Delta, Soft Starter, VFD): Size the cable for the motor’s full load current, not the starter’s output current. The cable between the starter and the motor sees the same current regardless of starting method.

Pump stations: Long cable runs are normal here. Voltage drop almost always governs the cable size, not ampacity. I’ve seen 7.5 kW pump motors fed with 35 mm² cable because the pump was 400 meters from the MCC.

HVAC systems: Multiple compressors on a single distribution board create grouping issues. Don’t forget to derate.

Solar inverter AC cables: THe current is steady state; but ambient temperatures on rooftops can exceed 50°C. Use XLPE, derate aggressively, and verify voltage drop on long string runs.

EV charger installations: These run at high load factors (sometimes close to 100% for hours). IEC 60364-7-722 has specific requirements that go beyond the standard cable sizing process.

Common Mistakes That Cost Time and Money

Mistake 1: Ignoring the starting voltage drop; Your cable might pass at full load current, but what about 6 times full load current during DOL starting? If the voltage drop exceeds 10 to 15% during starting; the motor may not develop enough torque to accelerate the load. This is Critical for the High inertia load like crushers, large fans, & loaded conveyors.

Mistake 2: Using ambient temperature of 30°C for a factory in the Middle East. I’ve reviewed designs for plants in Saudi Arabia where the cable tray runs through an unconditioned warehouse at 50°C ambient. The derating factor for PVC insulation at 50°C is 0.71; That’s a 29% reduction in capacity. Miss that, and you’re buying new cables after the first summer.

Mistake 3: Not accounting for future load growth. If the plant is likely to add more circuits to the same cable tray, your grouping derating will get worse. Size the cables for the final installation, not just what’s going in today.

Mistake 4: Mixing up single phase and three phase voltage drop formulas. The √3 factor makes a real difference. Double check which formula you’re using.

How to Use the Free IEC Cable Sizing Calculator

Our calculator on this page follows IEC 60364-5-52 and handles all the heavy math for you. Here that how to use it;

1. Select the standard as IEC 60364 (it’s the default).
2. Choose conductor material; (copper or aluminium) & insulation type (PVC or XLPE)..
3. Pick your installation method – Clipped direct (C) is the most common for motor circuits.
4. Enter load parameters: Select three phase or single phase, enter your voltage (400V is standard), choose general load or motor load, and fill in the power in kW, power factor, and cable length in meters.
5. Set the derating factors: Ambient temperature and grouping. Be honest with these numbers. Optimistic derating factors cause real problems.
6. Click Calculate.

The results show you the recommended cable cross section (for example, 70 mm²); & the actual current, voltage drop percentage, power loss in watts, and a clear PASS or FAIL status. It also detailed calculation breakdown so you can verify the math and include it in yours design documentation.

Key Takeaways For Cable Sizing

Cable sizing for motor circuits under IEC 60364 comes down to three checks; ampacity with derating; voltage drop at full load (ideally at starting current too); & short circuit withstand. Skipping any one of them, and you’re rolling the dice.

Always use the correct installation method from the standard. Always apply derating factors for real site conditions, not laboratory conditions. And always verify voltage drop, especially for cable runs over 50 meters.

Use XLPE insulation when you can, because it gives you better current ratings at the same cross section compared to PVC. Use copper for motor feeders. And when in doubt, go one size up. The cost difference between a 25 mm² and 35 mm² cable is nothing compared to the cost of replacing an undersized cable after the plant is running.

The free cable sizing calculator on this page is built to handle these calculations quickly and accurately. Use it. But understand the principles behind it; because a calculator is only as good as the inputs you give it.

Size your cables right the first time. Your future self, and the maintenance team that inherits your work, will thank you.

IEC Cable Sizing Calculator FAQ

For More Online Tools;

Motor Full Load Current Calculator: FLA Calculator For Motor

Free 3-Phase Power Calculator: Free 3-Phase Power Calculator

IEC Official Standard: IEC 60364-5-52