Three phases and 15 kW of power are the standard configuration for private homes, garages with workshops or small car repair shops. But when it comes to choosing circuit breaker, many are faced with confusion: how to convert kilowatts into amperes, take into account the power factor and not make a mistake with the rating? An error in calculations threatens not only constant power outages, but also a fire due to overheating of the cable.

In this article we will analyze exact algorithm for selecting a machine for a 3-phase network of 15 kW, taking into account all the nuances: from formulas to practical examples. We will pay special attention car workshops and garages, where in addition to household loads, welding machines, compressors and chargers are connected. You'll find out why The rating of the machine is not always equal to the rated current, how cable length affects the choice of protection, and what errors lead to false alarms.

First, let's look at the basics: 15 kW is the power that is allocated by the energy supply organization, but the machine protects not the power, but current. With a three-phase connection, the current is distributed over three wires, which changes the calculation formulas. If in a single-phase network everything is simple (power = voltage × current), then here you need to take into account power factor (cosφ), load asymmetry and even ambient temperature.

For example, in a garage with welding inverter and compressor starting currents can exceed the rated current by 3–5 times. This means that a 25A circuit breaker, calculated according to the standard formula, will trip every time the equipment is started. Read on to find out how to avoid such situations.

1. Formula for calculating current for a 3-phase 380V network

The basic formula relating power (P), current (I) and voltage (U) in a three-phase network:

I = P / (√3 Γ— U Γ— cosΟ†)

Where:

  • πŸ”Ή P β€” power (15,000 W for 15 kW);
  • πŸ”Ή U β€” line voltage (380V for 3 phases);
  • πŸ”Ή √3 β€” constant (~1.732), reflecting the features of a 3-phase system;
  • πŸ”Ή cosΟ† β€” power factor (depending on the type of load).

For purely resistive loads (heaters, incandescent lamps) cosΟ† = 1. For mixed loads (welding machines, electric motors) cosΟ† usually lies in the range 0.7–0.85. If the exact value is unknown, use cosΟ† = 0.75 is a safe middle option.

We substitute the values for 15 kW:

I = 15,000 / (1.732 Γ— 380 Γ— 0.75) β‰ˆ 28.5 A

This means that when the load is evenly distributed across the phases the estimated current will be ~28.5A. But this is not the final denomination of the machine!

πŸ“Š What type of load predominates in your network?
Household appliances (sockets, lighting)
Welding equipment
Electric motors (compressors, machine tools)
Mixed load

2. Why is the rating of the machine not equal to the rated current?

If, according to calculations, the current was 28.5A, why can’t you just install a 30A machine? There are two key mistakes here:

⚠️ Attention: The circuit breaker does not protect the equipment, but cable. Its denomination should be lessthan the permissible current for the wire, but morethan the operating load current taking into account starting currents.

Firstly, the machine does not operate when the rated current is reached, but when it is exceeded by 13–45% (depending on the class). For example, a class machine C25 will turn off when there is current 25 Γ— 1.45 = 36.25A, but can briefly skip large values. Secondly, the cable must withstand continuous current, not peak loads.

The second reason is phase unbalance. In real conditions, the load is rarely distributed perfectly evenly. If one phase is loaded by 90%, and the other two by 50%, the current in the β€œoverloaded” phase may exceed the calculated one by 30–50%. Therefore, always lay reserve 20–25%.

Load type Recommended stock Example of machine rating for 15 kW
Household (lighting, sockets) 15–20% 32A (class C)
Welding machines, compressors 25–30% 40A (class D)
Electric motors (machines, pumps) 30–40% 50A (Class D)
Mixed (garage, workshop) 20–25% 32–40A (class C/D)
πŸ’‘

If the garage is connected inductive consumers (for example, transformer chargers), use class machines D - they are less sensitive to inrush currents than class C

3. Selecting a machine according to cable cross-section

Even if the calculations showed a current of 28.5A, and you chose a 32A machine, this does not guarantee safety. The main thing is cable matching. If the wire is not designed to carry this current, it will overheat, causing a fire.

For a 3-phase network of 15 kW, the minimum cable cross-sections are:

  • πŸ”Œ Copper: 6 mmΒ² (permissible current 40A when laying in corrugated);
  • πŸ”Œ Aluminum: 10 mmΒ² (allowable current 32A).

But this minimum meanings! If the cable is laid in the wall (and not in the open air), its cooling deteriorates and the cross-section needs to be increased by 20–30%. For garages with high humidity or dust, it is recommended to use a cable VVGng-LS 3Γ—6 + 1Γ—4 (3 phases + neutral).

⚠️ Attention: In automobile workshops where they are used mobile welding stations, the cable must be flexible (for example, KG 3Γ—10) and protected from mechanical damage. In this case, the machine is selected taking into account maximum cable length - with a length of more than 20 meters, the short circuit current decreases, and the machine may not work!

β˜‘οΈ Check the cable before connecting

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4. Machine class: C, D or B?

The class of a circuit breaker determines how quickly it will trip when the rated current is exceeded. For 3-phase networks of 15 kW, three classes are relevant:

  • πŸ”₯ Class B: Triggered by current 3–5 Γ— In (for example, 32A Γ— 3 = 96A). Suitable for purely active loads (heaters, lighting), but not recommended for garages.
  • ⚑ Class C: Response range 5–10 Γ— In. Optimal for mixed loads (sockets + small motors).
  • πŸ”§ Class D: Withstands short-term overloads up to 10–20 Γ— In. Required for welding machines, compressors and other equipment with high inrush currents.

Example: if installed in a garage compressor 3 kW with starting current 45A, automatic class C25 may trigger falsely at startup. The solution is either increase the rating to 32A, or use the class D25.

For car workshops with lifts and machines A combination scheme is often used:

  • πŸ”Ή Input machine: 40A, class D;
  • πŸ”Ή Group machines: 16–25A, class C (for sockets and lighting).
What happens if you put a class B machine on a welding machine?

A class B machine will work the first time you start the welding inverter due to the high starting current (can exceed the rated current by 7–10 times). This will result in the equipment being unable to operate. In the worst case, the contacts of the machine will burn out and it will have to be replaced.

5. Common mistakes when choosing a 15 kW machine

Even experienced electricians sometimes make mistakes. Here are the most critical of them:

  1. 🚫 Ignoring power factor. If we use in calculations cosΟ† = 1 for a network with a welding machine, the current will be reduced by 20–30%. The result is that the machine will not withstand the real load.
  2. 🚫 Selecting a machine "butt-to-end". For example, with a current of 28.5A, they install a 30A machine. This is dangerous: if the phases are unbalanced or the voltage increases, the current can reach 35A, and the machine will not turn off, which will lead to overheating of the cable.
  3. 🚫 Failure to take into account starting currents. Electric motors consume 5–7 times more current at startup. A 25A class C circuit breaker will turn off compressor 2.2 kW every time you turn it on.
  4. 🚫 Using a machine as a switch. Frequent manual shutdown of the machine (for example, for repairs) leads to wear of the mechanism. For these purposes you need to install additional switch.

A special error for car services - connecting powerful equipment (for example, washing plant) per phase. This leads to phase imbalance and false alarms of the machine. The solution is uniform load distribution or installation three-phase RCD.

πŸ’‘

For garages with variable load (for example, seasonal use of heaters), an automatic machine with adjustable denomination (for example, ABB DS201 C 16–40A). This allows you to adapt protection to current needs.

6. Practical examples of machine selection

Let's consider three real-life scenarios for 15 kW 3 phase:

πŸ”§ Scenario 1: Private house with electric heating

Load:

  • πŸ”₯ Electric boiler 12 kW;
  • πŸ’‘ Lighting and sockets 3 kW;
  • πŸ“Ί Household appliances (refrigerator, washing machine).

Solution:

  • πŸ”Ή Rated current: 15,000 / (1.732 Γ— 380 Γ— 0.95) β‰ˆ 25A;
  • πŸ”Ή Automatic: 32A, class C (with a margin of 25%);
  • πŸ”Ή Cable: VVGng 3Γ—6 mmΒ².

πŸš— Scenario 2: Garage with a welding machine and compressor

Load:

  • ⚑ Welding inverter 5 kW (starting current 35A);
  • πŸ”§ Compressor 2.2 kW (starting current 20A);
  • πŸ”Œ Sockets and lighting 1 kW.

Solution:

  • πŸ”Ή Rated current: 15,000 / (1.732 Γ— 380 Γ— 0.75) β‰ˆ 28.5A;
  • πŸ”Ή Automatic: 40A, class D (we take into account starting currents);
  • πŸ”Ή Cable: KG 3Γ—10 mmΒ² (flexible, for mobile equipment).

🏭 Scenario 3: Car service with a lift and a washing unit

Load:

  • πŸš— Lift 3 phases, 4 kW;
  • πŸ’¦ Washing unit 5.5 kW;
  • πŸ”§ Machines and tools 5.5 kW.

Solution:

  • πŸ”Ή Estimated current: 28.5A (as in scenario 2);
  • πŸ”Ή Automatic: 50A, class D + three-phase RCD 63A/30mA;
  • πŸ”Ή Cable: VVGng 3Γ—16 mmΒ² (we take into account the length of the route 30m).

7. How to check whether the machine is selected correctly?

After installing the machine, three tests must be carried out:

  1. πŸ” Triggering check. Connect a load close to the maximum (for example, turn on all heaters and a welding machine). The machine should not turn off during stable operation, but must work when short circuit (for example, if you short a phase to the machine body).
  2. 🌑️ Heating control. After 1–2 hours of operation under load, check the temperature of the machine and cable. If they are hot (more than 50Β°C), it means that the rating is too low or the cable cross-section is insufficient.
  3. βš–οΈ Phase balance. Use current clampsto measure the current in each phase. The difference between the phases should not exceed 20%. If one phase is loaded at 30A and the others at 10A, the load needs to be redistributed.

For automobile workshops, an additional test is checking operation at reduced voltage. If the network often happens drawdown to 340–360V, the current will increase by 10–15%, and the machine may trigger falsely. In this case, you need to either increase its nominal value or set voltage stabilizer.

πŸ’‘

If used in a garage generator as a backup power source, the machine must be selective (with delay). This will prevent tripping during momentary power surges during transfer to the generator.

8. Frequently asked questions (FAQ)

❓ Is it possible to install a 25A machine for 15 kW 3 phases?

No, it's dangerous. With a uniform load, the current will be ~28.5A, and the 25A circuit breaker will constantly turn off. In addition, a cable rated for 25A (for example, 4 mmΒ²) will overheat. Minimum rating for 15 kW - 32A (for household load) or 40A (if engines are available).

❓ Which machine should I install if the power is 15 kW, but the load is only on one phase?

In this case, the calculation is carried out as for a single-phase network: I = P / U, where U = 220V. For 15 kW the current will be 68A, but connect like this 15 kW prohibited by the rules β€” the maximum power per phase in household networks is limited 7–10 kW. Solution: distribute the load across three phases or reduce power.

❓ Is it necessary to install an RCD together with a 15 kW machine?

Yes, especially in garages and car repair shops. RCD protects against current leakage (for example, in case of breakdown of insulation on the body of a machine tool or welding machine). For a 3-phase network, select three-phase RCD with leakage current 30mA and a rated current one step higher than the circuit breaker (for example, RCD 40A + circuit breaker 32A).

❓ Why does a 40A automatic machine operate at a load of 12 kW?

There are several reasons:

  • πŸ”Ή Starting currents (for example, starting a compressor);
  • πŸ”Ή Phase unbalance (one phase is overloaded);
  • πŸ”Ή Poor contact in a machine or terminal block (leads to heating and false alarms);
  • πŸ”Ή Machine defect (check with a different load).

For diagnostics, measure the current in each phase and check the temperature of the machine after 30 minutes of operation.

❓ Is it possible to replace a three-phase machine with three single-phase ones?

Technically possible, but not recommended. Three single-phase circuit breakers will not provide:

  • πŸ”Ή Protection from phase-to-phase faults;
  • πŸ”Ή Synchronous shutdown of all phases (if there is a short circuit in one phase, the others will remain energized);
  • πŸ”Ή Protection from phase imbalance.

The exception is temporary circuits (for example, for testing equipment), but even in this case, be sure to use three-phase RCD.