To ensure stable operation of equipment with a total power of 20 kW in a three-phase 380V network, it is necessary to install a circuit breaker with a rated current of 32 Amperes. This basic value is obtained by converting the total power into current, taking into account the standard power factor and safety margin, however, simply buying a package marked C32 is not enough for safe operation. The wrong choice of a protective device can lead either to constant false shutdowns under load, or, what is much more dangerous, to overheating of the wiring and fire if the rating is selected without taking into account the cross-section of the cable cores.
In a single-phase 220V network, it is technically impossible to create such a load without catastrophic consequences for the home electrical network, since the current will increase to 90-100 Amperes, which requires industrial input and thick copper busbars. Therefore, we will conduct further analysis exclusively for three-phase circuit, which is the standard for private houses with electric heating, large workshops or cottages with powerful air conditioning and swimming pools. Understanding the physics of the process will help you avoid fatal mistakes when assembling the shield.
The choice of a specific module depends not only on the number in kilowatts, but also on the type of connected consumers, be it active heaters or motors with high starting currents. It is important to consider that the machine primarily protects the cable line, and secondarily the connected devices, so its characteristics must strictly correspond to the conductor’s throughput. Below we will analyze in detail the mathematical calculation, marking features and criteria for selecting a device for your case.
Physics of calculation: converting kilowatts to amperes
The first step in selecting switching equipment is to accurately calculate the operating current. Many people mistakenly divide power by voltage, forgetting about the presence of three phases and cosine phi. For a three-phase AC network, the formula is I = P / (√3 × U × cosφ), where P is the power in watts (20,000 W), U is the line voltage (380 V), and cosφ is the power factor. For modern systems with active load (lighting, heating) it is close to unity, but for motors and pumps it is about 0.8-0.85.
Substituting the values into the formula, we get: 20000 / (1.732 × 380 × 0.9) ≈ 33.7 Amperes. This is the estimated current that will flow through the line when fully loaded. However, the circuit breaker should not operate at its maximum capacity all the time. According to the operating rules, the long-term load should not exceed 80-85% of the nominal value of the machine, so that the thermal release does not operate falsely due to heating of the device body itself in a dense panel.
If we take the nearest standard rating of 32A, then at a current of 33.7A it will operate in the overload zone and will eventually turn off. Therefore, for a power of 20 kW, a 40 Amp circuit breaker is formally required. However, a critical rule comes into play here: The rating of the machine is selected according to the cable cross-section, and not just by load power. If the cable cannot withstand 40A, it is prohibited to install such a machine, even if the load requires that much.
⚠️ Attention: Never choose a machine “with a reserve”, just so as not to knock it out. If the wiring is designed for 32A, and you install a 50A circuit breaker, then in the event of a short circuit or overload, the cable will burn out before the protection operates, which will lead to a fire.
Thus, the cable-automatic bundle becomes the basic reference point. For 20 kW, the optimal solution is often a cable with a cross-section of 10 mm² over copper, which can withstand about 50-55A for a long time, which allows you to safely use the machine C40. If a 6 mm² cable is laid (the limit is about 40-42A), then you will have to limit consumption or install a 32A circuit breaker and put up with periodic shutdowns during peak loads.
Standard denominations and range of values
The industry produces circuit breakers in strictly defined series, and you will not find a 33 or 37 ampere device on sale. There is a standard range of rated currents that designers and electricians are guided by. Knowledge of this series is necessary to correctly interpret calculated data and select the nearest standard value.
The main denominations found in the household and semi-industrial sector come in increments that increase with increasing amperage. For the range of interest to us at a power of 20 kW, the following values are relevant:
- 🔹 25 Amps - corresponds to approximately 14-15 kW, not enough for 20 kW.
- 🔹 32 Amps - corresponds to approximately 18-19 kW, is on the verge of 20 kW.
- 🔹 40 Amps - corresponds to approximately 22-23 kW, the optimal choice with appropriate wiring.
- 🔹 50 Amps - corresponds to 28-30 kW, may be redundant if there is no cable reserve.
As can be seen from the list, the 32A machine is a borderline solution. When all consumers are fully loaded up to 20 kW, it will heat up and may turn off after 20-40 minutes of operation. A 40A automatic machine provides the necessary technological reserve. However, the choice between them is dictated solely wire cross-section. If you have a 40A machine, the input cable must have a copper cross-section of at least 6 mm² (preferably 10 mm²).
It is also important to pay attention to the tripping characteristic, indicated by a Latin letter before the rating number. For introductory machines and powerful loads, the characteristics are most often used B, C and D. In everyday life, the de facto standard is the characteristic C, which allows a short-term overload of up to 5-10 nominal values (for starting the engines of refrigerators, pumps, compressors) without instantaneous operation of the electromagnetic release.
Influence of load type on the choice of machine
Not all 20kW are created equal. The nature of energy consumption greatly affects the operation of the circuit breaker. The load is divided into active (resistive) and reactive (inductive). Active ones include electric heaters, stoves, incandescent lamps, boilers. Reactive - electric motors, transformers, powerful pumps, welding machines.
For an active load, the calculation is simple: the current is stable, inrush surges are minimal. Here the 40A machine will operate normally. It's more difficult with an inductive load. The electric motor at the moment of starting consumes a current 5-7 times higher than the rated current. If you have a powerful machine or a 20 kW pumping station, the usual automatic characteristics C can perceive the inrush current as a short circuit and knock it out instantly.
In such cases, the following solutions apply:
- 🔸 Using machines with characteristics D, which hold the starting current up to 10-12 nominal values.
- 🔸 Use of soft starters (SPD) or frequency converters that reduce the starting current.
- 🔸Increasing the machine's rating (if the cable allows) to go through the starting mode.
It is also worth considering the simultaneity factor. It is unlikely that you will simultaneously turn on the oven, hob, three air conditioners, pool pump and welding machine. If 20 kW is the total power of all appliances in the house, but only 12-14 kW actually work at the same time, then a 32A automatic machine may be quite sufficient. This allows you to save on allocated power limits if they are limited by an agreement with the energy retailer.
Cable cross-section: main limiter
As already mentioned, the circuit breaker is the guardian of the cable. Its main task is to turn off the current before the wire insulation begins to melt. Therefore, choosing a machine rating for 20 kW is impossible without knowing the cross-section and material of the input cable cores.
Below is a table of correspondence between the cross-section of the copper cable, the permissible current and the recommended rating of the machine for a three-phase 380V network:
| Cable cross-section (mm²) | Material | Continuous current (A) | Recommended machine | Max. power (kW) |
|---|---|---|---|---|
| 4.0 | Copper | ~36-39 A | 32 A | ~18-19 kW |
| 6.0 | Copper | ~46-50 A | 40 A | ~22-23 kW |
| 10.0 | Copper | ~60-65 A | 50 A | ~28-30 kW |
| 16.0 | Copper | ~80-85 A | 63 A | ~35-40 kW |
The table shows that for the full implementation of a power of 20 kW with a safety margin, a cable with a cross-section of 6 mm² or 10 mm². The 4mm² cable is already at its limit and will not allow the full 20kW power to be used without risk of overheating. Aluminum cables with the same cross-section have a lower carrying capacity, so for them the cross-section needs to be taken one step higher (for example, instead of 6 mm² copper, take 10 mm² aluminum).
When installing, it is also important to consider the installation method. A cable lying in the ground or in a pipe cools worse than an open wire. In such cases, the load capacity is reduced by 10-20%. If your 6 mm² cable runs in a tight bundle with other cables or in a thermally insulated wall, its real ability to carry current drops, and installing a 40A circuit breaker on it is already risky.
☑️ Checking readiness for installation
Number of poles and connection diagram
For a power of 20 kW we are definitely talking about a three-phase network. This means that the circuit breaker must be at least three-pole (3P) for switching phases L1, L2, L3. However, four-pole circuit breakers (4P) are more often used as an input device, which also breaks the neutral conductor (N).
The use of a 4-pole circuit breaker at the input is necessary for several reasons:
- 🔹 Safety: when repairing the shield, the zero must also be disconnected, since if the phases are misaligned, a dangerous potential may appear on it.
- 🔹 Operation of the RCD: if the input device is combined with differential protection (diffautomatic), a zero break is required for the correct operation of the mechanism.
- 🔹 PUE requirements: in some grounding schemes (for example, TN-C-S), a zero break at the input is mandatory.
The connection diagram is as follows: three phase wires are connected to terminals 1, 3, 5 (or A, B, C), and the neutral wire is connected to terminal N (usually it is on the far right and marked in blue). It is important not to confuse phases and zero, especially if you are using a machine with an electromagnetic release only on the phase poles (although in 4-pole releases, the releases are usually located on all poles).
When assembling a 20 kW panel, it is also recommended to use Surge protection device (SPD) or phase control relay. Voltage surges in rural networks or a zero break can instantly disable expensive equipment. The circuit breaker does not protect against high voltage, it only reacts to current. Therefore, the combination “Automatic + Voltage Relay” is the standard for high-quality electrical assembly.
⚠️ Attention: Tightening contacts. For currents of 32-40A and higher, the quality of screw tightening is critically important. A weak contact will begin to heat up, the plastic will melt, and the machine will fail. Be sure to re-tighten the screws 24 hours after switching on.
Brands and quality of workmanship
The market offers a huge selection of circuit breakers, but you cannot skimp on safety. Cheap Chinese analogues of nameless brands often have a real operating current that differs by 20-30% from what is written on the case. For a load of 20 kW, where the currents are high, this is unacceptable.
It is recommended to choose products from trusted manufacturers, such as ABB, Legrand, Schneider Electric, IEK (Professional series), EKF (PROxima series). These brands provide the declared time-current characteristics and have certificates of conformity.
What to look for when purchasing:
- 🔸 Breaking capacity: for a private house at least 6 kA (6000 Amperes). This is the current that the machine can break without destroying the case during a short circuit.
- 🔸 Current limiting class: It is better to choose 2 or 3 class. They extinguish the arc faster, prolonging the life of the wiring.
- 🔸 Switching resistance: number of on/off cycles. High-quality machines can withstand thousands of cycles.
Frequently asked questions (FAQ)
Is it possible to connect 20 kW through one machine in a single-phase 220V network?
Theoretically, the current will be about 91 Amperes. To do this, you will need a cable with a cross-section of at least 25-35 mm² and a 100A circuit breaker. However, in ordinary houses, single-phase input rarely allows such currents (usually the limit is 10-15 kW). In addition, the phase imbalance in the general network will be huge. For 20 kW, a three-phase connection is required.
Why does a 32A machine heat up at a load of 18 kW?
Heating up to 40-50 degrees is considered normal when operating under a load of 80-90%. If the machine burns your hand hotly, the contact in the terminal may be loose (needs tightening) or there may be poor ventilation in the panel. It is also possible that the actual current exceeds the rating due to voltage surges.
Which machine to choose: 32A or 40A for 20 kW?
The choice depends on the cable. For copper 6 mm² and above, feel free to set it to 40A (characteristic C). For copper 4 mm² - only 32A, but then you will have to ensure that the total power of the devices does not exceed 16-17 kW, otherwise there will be constant outages.
Do I need to change the meter when installing a 20 kW machine?
Yes, the old single-tariff 5-40A meter may not be suitable. A modern three-phase direct-connection meter with a rated current of at least 60A (for example, 5-60A or 10-100A) is needed so that it can correctly account for all the power without overloading the internal circuits.