What is a three-phase cable? How to size it by load power?

Three-phase vs single-phase cables

For the same load, a three-phase cable carries about one third of the current that single-phase cable does. Each core can therefore be smaller, the cable is more compact and it uses less copper.

A three-phase cable usually has four parts, from the inside out: the conductor, the insulation, a protective layer (if any) and the outer sheath.

2.1 Conductor

The conductor is made of copper or aluminium. Copper conducts better, so for the same current a copper conductor can be smaller. Aluminium is lighter and cheaper, and is often used for large cables on long runs.

According to the IEC 60228 standard , conductors are classified into several classes:

• Class 1: a single solid wire.
• Class 2: several stranded wires, common in fixed installations.
• Class 5 and 6: many fine wires. Used on equipment that vibrates or has to bend often.

2.2 Insulation

The two most common materials are PVC and XLPE. The advantage of PVC is that it can withstand a maximum conductor temperature of 70°C during continuous operation, while XLPE can withstand 90°C. As a result, XLPE-insulated cables can carry a higher current for the same cross-sectional area. Learn more about XLPE electrical cables.

A three-phase cable is not one single cable type but a whole family of cables used for three-phase power. The simplest way to classify it is by the number of cores, which depends on how the system is earthed and what loads it feeds.

3.1 3-core three-phase cables

A 3-core three-phase cable (often just called 3-core cable) has only the three phase conductors, L1, L2 and L3, and no neutral. It is used when the circuit feeds only balanced three-phase loads, such as large motors or industrial machines. A balanced load draws almost no current through the neutral, so the neutral is not needed.

3.2 4-core three-phase cables

A 4-core three-phase cable (often just called 4-core cable) has the three phase conductors L1, L2 and L3, plus a fourth core. The fourth core can be a neutral, or a PEN conductor (combined neutral and protective earth) in a TN-C system. With a neutral, the cable can supply both three-phase and single-phase loads from the same circuit. That makes it a common choice for commercial buildings and distribution boards that feed mixed loads.

3.3 5-core three-phase cables

A 5-core cable has three phase conductors, a neutral (N) and a protective earth (PE). The PE carries no working current, so it is always free to carry fault current to earth. Keeping the neutral and the protective earth separate makes the system safer. This is the standard configuration for TN-S systems, typically used in:

• Industrial sites with strict safety requirements
• Modern factories and automation systems with many VFDs and electronic devices
• Projects with strict equipment protection requirements, such as data centres and hospitals

4.2 -J and -O suffixes: does the cable have an earth core?

On cables made to German standards, the suffix -J means the cable has a green/yellow protective earth (PE) core. The suffix -O means it does not. DIN VDE 0293-308 sets the core colors as follows:

From the table you can quickly read a three-phase cable's configuration:

• 4-core -J: three phases (brown, black, grey) and PE. No neutral.
• 4-core -O: three phases and neutral (blue). No PE.
• 5-core -J: three phases, neutral and PE, for TN-S systems.

For example, a cable marked 5G6, or a 5-core 6 mm² cable with the -J suffix, is a 3P + N + PE cable. The "G" in the size marking and the "-J" suffix say the same thing: the cable has a green/yellow core.

Sizing three-phase cable by load power takes three steps. Work out the current from the load power. Pick a cross-section with enough current-carrying capacity once you allow for the installation method, temperature and number of cables run together. Then check the voltage drop over the length of the run.

Step 1: Calculate the load current

The load current of a three-phase circuit is calculated using the following formula: I = P / (√3 × U × cosφ × η)

Here P is the power (W), U is the phase-to-phase voltage (380 V on Vietnam's low-voltage grid), and cosφ is the power factor shown on the nameplate. η is the efficiency, used only for motors, because the power on a motor nameplate is the mechanical output at the shaft. For other loads, take η = 1. If the motor nameplate gives a rated current, use that figure directly.

Example: a 15 kW motor, 380 V, cosφ = 0.85, η = 0.9. Load current I = 15,000 / (1.732 × 380 × 0.85 × 0.9) ≈ 29.8 A. Leave out the efficiency and you get only 26.8 A, about 10% below the real figure.

The table values must be reduced in two cases:

• Ambient temperature above 30°C: at 40°C, multiply by 0.87 for PVC cable or 0.91 for XLPE cable.
• Several cables run close together: for example, three circuits bunched on a wall or in a tray, multiply by 0.70.

Back to the example: load current 29.8 A, a 32 A breaker, cable clipped direct to a wall.

In the last case, you need to go up to PVC 10 mm² (≈ 34.7 A) or switch to XLPE 6 mm² (52 × 0.91 × 0.70 ≈ 33.1 A). Same motor, but the cable size changes from 4 to 10 mm² just because of temperature and how the cable is run.

On a factory floor, the right size is not enough. The cable also has to suit the load and its surroundings.

Besides, you should choose certified cable with a full datasheet (IEC, VDE, UL). Without a datasheet, you cannot look up the exact current-carrying capacity.

HELU supplies three-phase cables for a wide range of installations: power cable for fixed wiring, flexible cable for machinery, screened cable for VFDs and fire-resistant cable. Every product comes with a datasheet that sets out its construction, standards and technical data, so engineers can look up values and calculate accurately.