How to Size Three-Phase Power Cables for Long Distances

Why Long Distance Runs Demand Different Sizing

Most sizing errors happen when engineers apply a short-run rule to an extended circuit. A cable that carries a 100A load comfortably over 50 meters can fail the voltage drop test over 500 meters. The conductor resistance multiplies with distance, and the heating effect changes as the run length increases.

For three phase systems, you must look at both the current capacity of the wire and the voltage loss across the entire path. The correct approach uses a two-part check. First, confirm the conductor can handle the continuous current. Second, confirm the system voltage stays within acceptable limits at the load center.

In short runs, the resistance drop is often negligible. The conductor size is chosen almost entirely by ampacity. In long runs, the resistance becomes the dominant variable. Even a 0.5 ohm drop per kilometer can accumulate to several volts over a distance, causing equipment to operate outside its optimal range. This is why a 95 mm² cable might pass the current check for a 200 meter run but fail for a 2 km run.

What You Need Before You Start

Before selecting a size, gather these four items:

1. The continuous load current in amperes.
2. The supply voltage and the voltage drop limit.
3. The total length of the run, including both outgoing and returning conductors.
4. The installation method and ambient temperature.

If you do not have the ambient temperature, use the site average for the hottest month. If the installation method is buried in concrete or packed earth, note the thermal mass. These details change the derating factor you apply to the manufacturer ampacity table.

You also need the power factor of the load. A motor at full load typically operates around 0.85 to 0.9, while a transformer at no load may have a very low power factor. Using the wrong power factor leads to an incorrect design current, which cascades into every subsequent calculation.

Step 1: Determine the Design Current

Start with the nameplate rating of the equipment. For a motor, use the full-load current. For a transformer, use the rated primary current. For a general load, use the total connected power divided by the voltage and power factor.

For a three phase system, the design current is calculated from the total kVA and power factor. If the load is 100 kVA with a 0.85 power factor, the current is approximately 165 amperes. Round this up to the next standard size if the equipment nameplate is not available.

Do not rely on the nameplate full-load current if the equipment will operate at partial load for extended periods. In those cases, calculate the continuous current based on the expected duty cycle. If the motor runs at 80% of its rated power continuously, use 0.8 times the nameplate current. This prevents over-sizing the cable, which increases material costs without improving reliability.

Step 2: Select a Starting Conductor Size

Use a standard ampacity table for the conductor type you plan to use. Copper and aluminum are the two most common materials. Aluminum carries a higher current for the same diameter than copper, but it has higher resistance per unit length.

Start with a size that meets the ampacity requirement at your ambient temperature. Do not start with a size that is too small. You want enough margin for the voltage drop calculation to work.

If the calculated starting size is between two standard diameters, choose the larger one. For example, if the ampacity check suggests 120 mm² but 120 mm² is not a standard size, select 150 mm². This gives you a safety margin for the voltage drop check and accounts for potential variations in ambient temperature or installation conditions.

Step 3: Calculate the Voltage Drop

This is the step that controls long distance cable run projects. The voltage drop formula for a three phase system is:

V_drop = (1.732 x R x I x L) / 1000

Where R is the resistance of the conductor in ohms per kilometer, I is the current in amperes, and L is the length in kilometers.

Use the resistance value from the manufacturer data sheet for the selected conductor size. If you use a 100 mm² copper cable and the run is 1 km, calculate the drop. If the drop exceeds your limit, increase the conductor size and repeat the calculation.

For a 100 mm² copper cable, the resistance is typically around 0.72 ohms per kilometer. If the design current is 165 A and the run is 1 km, the voltage drop is approximately 19.8 V. If your limit is 20 V, the cable passes. If the run is 1.5 km, the drop becomes 29.7 V, which fails. In this case, you must increase the conductor size to 150 mm² or 240 mm² to meet the limit.

Step 4: Check the Total System Voltage

Confirm that the voltage at the load center remains within the acceptable band. Most industrial loads accept a drop of 3 to 5 percent from the supply voltage. If the drop is higher, the equipment may run inefficiently or trip protection devices.

For a 400V supply, a 5 percent drop allows a maximum of 20V loss. If your calculated drop is 25V, the current size is too small. Increase the diameter and recalculate.

Some sensitive equipment, such as variable frequency drives or precision manufacturing tools, may require a tighter tolerance, such as 2 to 3 percent. Always check the equipment manufacturer’s specifications. If the equipment requires a tighter tolerance, you may need to oversize the cable significantly to keep the drop within range.

Step 5: Verify the Insulation Rating

Check the rated voltage of the cable. A cable rated for 1 kV may not be suitable for a 6.6 kV feeder. The insulation thickness and material must match the system voltage class.

Also check the temperature rating of the insulation. PVC insulation typically handles 70 degrees Celsius. XLPE insulation handles 90 degrees Celsius. If your ambient temperature is high, use the lower rating to be safe.

If you use a 70 degree PVC cable in an environment where the ambient temperature is 45 degrees Celsius, the cable will be derated further. The insulation cannot exceed its temperature rating, so the current capacity must be reduced. This is why ambient temperature is a critical input in the sizing calculation.

Step 6: Apply Derating Factors

Derating accounts for the way cables share heat in a group. If you place four cables in one tray, the heat from each cable raises the temperature of the others. The manufacturer provides a derating factor for each number of cores in a bundle.

For a buried installation, check the thermal resistance of the soil. Dense clay holds heat longer than sandy soil. Use the derating factor for the worst case scenario. This protects the insulation from long term thermal aging.

If you are installing multiple cables in a single tray, apply the derating factor for the number of cables in the tray. For example, if you install four cables in a tray, the derating factor might be 0.8 or 0.85, depending on the cable type and spacing. Multiply the standard ampacity by this factor to get the reduced current capacity.

Step 7: Select the Final Cable Diameter

Once the ampacity, voltage drop, and insulation checks pass, select the final cable diameter. For three phase power cables, you will use three single core cables or one three core cable.

Three single core cables offer better heat dissipation and easier routing. One three core cable is more compact and cheaper for short runs. For long distance runs, single core cables are often preferred because they heat more evenly.

Single core cables are easier to install in long trenches or ducts. They can be pulled individually, which reduces the risk of damage. Three core cables are easier to manage in tight spaces because they move as a unit. However, three core cables generate more heat in the center of the bundle, which can reduce the ampacity of the inner cores.

Step 8: Confirm the Installation Space

Check the conduit, tray, or duct size. A 150 mm cable may not fit in a 200 mm duct if you add the spacing requirements. The standard spacing between cables is usually 2 to 3 times the cable diameter.

If the space is tight, use a larger tray or a different routing path. Do not force cables into a space where they cannot expand thermally. This causes mechanical stress on the insulation.

When calculating the space, include the clearance for the cable insulation and the spacing between cables. For example, if you are installing four 150 mm cables, the total width required is 4 times 150 mm, plus the spacing between them. If the spacing is 3 times the cable diameter, the total width is 4 x 150 + 3 x 3 x 150 = 2250 mm. This is a large space, so you may need to use a larger tray or a different routing path.

Common Mistakes in Sizing

1. Ignoring the return path. The voltage drop calculation must include both the phase and neutral conductors.
2. Using a standard table without derating. The table values assume a specific ambient temperature and installation method.
3. Forgetting the power factor. A low power factor increases the current for the same power load.
4. Selecting a cable based only on cost. A cheaper cable may fail the voltage drop test.
5. Not checking the joint connections. The joints must be rated for the same current and voltage as the cable.

A common mistake is to calculate the voltage drop using only the length of the outgoing conductor. In a three phase system, the current flows in both the phase and neutral conductors, so the total length is twice the one-way distance. If you ignore this, your voltage drop calculation will be off by a factor of two.

Another mistake is to use the standard ampacity table without considering the installation method. A cable installed in a tray in a hot environment will have a lower ampacity than the same cable installed in a cool underground trench. If you ignore this, your cable may overheat and fail prematurely.

Final Verification Step

Before you order the cable, run the numbers one last time. Use the final selected size and the actual measured length from the site survey. Confirm the voltage drop is within limit. Confirm the ampacity is above the design current with a safety margin.

Keep a copy of the calculation sheet with the as built drawings. If you change the load or the route later, you can recalculate quickly without guessing.

A safety margin of 10 to 20 percent is typical. If your calculated ampacity is 150 A and your design current is 165 A, you need a cable with an ampacity of at least 198 A. This margin accounts for variations in ambient temperature, installation conditions, and future load increases.

Comparison of Conductor Sizes for a 1 kV System

Conductor Size (mm squared)	Approx. Current (Ampere)	Voltage Drop per 1000m (V)
50	125	6.5
95	175	3.2
150	235	2.1
240	320	1.3
353	400	0.9

The table shows the general trend. A larger conductor has a lower resistance and a smaller voltage drop. The exact values depend on the material and insulation type.

For example, a 95 mm² cable has an ampacity of 175 A and a voltage drop of 3.2 V per kilometer. If your design current is 165 A and your run is 1 km, the voltage drop is 3.2 V, which is well within the 5 percent limit for a 400V system. However, if your run is 2 km, the voltage drop is 6.4 V, which is 1.6 percent of 400V. This is still within the limit, but it is getting close. If you increase the conductor size to 150 mm², the voltage drop drops to 2.1 V per kilometer, which is 0.525 percent for a 2 km run. This gives you a larger safety margin.

When to Use Larger Conductors

Use a larger conductor than the minimum requirement when:

1. The load may increase in the future.
2. The run is in a hot environment.
3. The cable is buried in a trench with high thermal resistance.
4. The voltage drop limit is strict, such as for sensitive equipment.
5. You want to reduce the heating effect in the cable tray.

If you expect the load to increase in the future, use a larger conductor now. This avoids the cost of replacing the cable later. For example, if you are installing a 100 kVA transformer now but expect to add another 50 kVA in five years, use a 150 mm² or 240 mm² cable instead of a 95 mm² cable.

If the run is in a hot environment, use a larger conductor to compensate for the reduced ampacity. For example, if the ambient temperature is 50 degrees Celsius, the ampacity of a 95 mm² cable may be reduced from 175 A to 140 A. If your design current is 165 A, you need a larger cable, such as a 150 mm² or 240 mm² cable.

Final Thoughts

Three phase cable sizing is a practical task. It requires accurate data and careful calculation. Follow the steps in order. Check the ampacity, then the voltage drop, then the insulation, and finally the space. This method keeps your long distance cable run reliable and within budget.

A well-sized cable reduces energy losses, improves equipment performance, and extends the life of the system. A poorly sized cable can lead to overheating, voltage instability, and premature failure. Take the time to do the calculations correctly, and the results will be reliable and cost-effective.