Input machine for an apartment: how many amperes are needed for safe operation of the network?

Choosing an introductory machine for an apartment is a task that requires a balance between safety, functionality and compliance with standards. An error in the calculations can lead to two extremes: either the machine will work when the kettle and microwave are turned on at the same time, or it will not turn off if there is a short circuit, creating a risk of fire. In this article we will look at how correctly calculate the rating of the input machine taking into account the power of the devices, type of wiring and requirements PUE 7th edition (current for 2026).

Many people mistakenly believe that it is enough to take a machine gun “with a reserve” - for example, 50A instead of calculated 32A. However, this approach is fraught with overheating of the cable: if the wiring is designed for 25A, then the machine is on 50A It just won't work if the load is exceeded until the wires start to melt. We will analyze real cases from the practice of electricians and present power and rating tables, and we will also tell you what nuances to consider when replacing a machine in old houses with aluminum wiring.

Why is it important to choose the correct rating of the input machine?

Input machine (or input switch) is the first level of protection for the apartment’s electrical network. His tasks:

• 🔌 Power off in case of short circuit (SC) or overload to prevent fire.
• ⚡ Power limitation, consumed by the apartment, according to the allocated energy supply organization.
• 🛡️ Counter protection from damage in emergency situations.

If the rating of the machine is too high, it will not work if overloaded, and the wires will begin to heat up. For example, in Soviet-built houses with aluminum wiring cross-section 2.5 mm² maximum permissible current - 19A. Installing the machine on 25A in this case, it will lead to overheating of the cable at a load close to the maximum. On the other hand, an underrated rating will cause false alarms when turning on powerful devices (for example, heat pump or instantaneous water heater).

⚠️ Attention: In apartment buildings with a common grounding system TN-C (where the neutral and protective conductors are combined) installation of an input circuit breaker with a higher rating 40A requires approval from the management company. This is due to the risk of phase imbalance in the riser.

How to calculate the nominal value of an introductory machine: step-by-step instructions

The main parameter for calculation is total power of all electrical appliances in the apartment. We use the formula:

I = P / (U × cosφ)

where:

• I — current in amperes (machine rating);
• P — total power of devices in watts;
• U — mains voltage (220V for single-phase, 380V for three-phase);
• cosφ — power factor (for household appliances they take 0.95).

Calculation example for an apartment with a load 10 kW (typical case for a family with an electric stove, washing machine and air conditioner):

I = 10,000 W / (220V × 0.95) ≈ 47.6 A

However You can’t install a 50A circuit breaker - must be taken into account permissible wiring current and allocated power under an agreement with the energy supply organization. In most cases, apartments are allocated 7–10 kW, which corresponds to the machine on 32–40A.

Sum up the power of all devices (including backup ones)

Check the cross-section and material of the wiring (copper/aluminium)

Specify the allocated capacity in the contract with Energosbyt

Assess the possibility of simultaneously turning on all devices

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Table: Correspondence between power and rating of the input circuit breaker

Distributed power, kW	Rating of the machine, A (single-phase network)	Recommended cable cross-section, mm² (copper)
3–5	16–25	2.5–4
5–7	25–32	4–6
7–10	32–40	6–10
10–15	40–50*	10–16

*For automatic machines 50A and above, coordination with the energy supply organization and checking the cross-section of the input cable in the riser is required.

3–5 kW

5–7 kW

7–10 kW

More than 10 kW

I don't know

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Typical mistakes when choosing an introductory machine

Even experienced electricians sometimes make mistakes when selecting an input machine. Let's look at the most common ones:

• 🔥 Ignoring wire cross-section. Automatic on 40A protects the cable cross-section 6 mm², but if it is laid in the wall 2.5 mm², then at current 30A the wires will overheat and the machine will not work.
• ⚡ Installation of the machine without taking into account starting currents. For example, refrigerator or air conditioner upon startup, they consume 3–5 times the rated current. Automatic machine with characteristics B may trigger falsely, and with C or D - no.
• 🏢 Inconsistency with the energy supply organization. In homes with power limitations (e.g. 5 kW for an apartment) installing a machine on 40A will lead to the activation of the limiter in the panel on the landing.

⚠️ Attention: In homes with gas stoves, the allocated power is often limited 3–5 kW. Installing the machine on 32A in this case, it will lead to a power outage when several devices are turned on (for example, microwaves + iron). Before replacing the machine, check the allocated power in the contract or with the management company.

What happens if you install a machine with a lower nominal value?

With a lower rating (for example, 16A instead of the required 32A), the machine will operate when powerful appliances are turned on: an electric stove, a water heater or a washing machine in heating mode. This will cause discomfort but will not create a fire risk. However, frequent operations reduce the service life of the machine mechanism (the contacts burn out, the bimetallic plate loses sensitivity).

Features of choosing a machine for old houses (Khrushchev, Brezhnevka)

In Soviet-built houses (Khrushchev buildings, Brezhnevka, stalinkas) electrical wiring is often made with aluminum cable cross-section 2.5 mm². For such wiring, the maximum permissible current is 19A, and the recommended rating of the input machine is 16A. However, this creates a problem: with modern loads (refrigerator, TV, computer, microwave), the total power easily exceeds 3.5 kW, and the machine is triggered.

Solutions:

1. Replacing wiring to copper (section 4 mm²) with installation of the machine on 25A.
2. Power limitation by distributing the load across different outlet groups.
3. Setting the priority relay, which turns off non-critical devices (for example, boiler) when the load is exceeded.

Important: in houses with aluminum wiring It is prohibited to install circuit breakers with a rating higher than 25A without replacing the cable. This requirement is specified in PUE 7.1.34 and is associated with a risk of fire due to the high resistance of aluminum.

Three-phase input: when is it needed and how to calculate the machine

Three-phase input (voltage 380V) is installed in apartments with a power higher 10 kW or in the presence of three-phase consumers (for example, electric boiler, industrial equipment). To calculate the nominal value, use the same formula, but taking into account three phases:

I = P / (√3 × U × cosφ)

where √3 ≈ 1.73, and U = 380V.

Example: for an apartment with power 15 kW:

I = 15,000 / (1.73 × 380 × 0.95) ≈ 23.7 A

Round up and select the machine on 25A. However, for three-phase input it is required three-pole or four-pole circuit breaker (depending on the grounding system). You also need:

• 🔌 Install RCD or differential machine for each phase.
• 📊 Distribute the load evenly among the phases (the difference is no more than 20%).
• 🛡️ Use a cable with a cross section of at least 6 mm² for copper.

Which brand of machine to choose: review of reliable manufacturers

The quality of the introductory machine directly affects safety. Cheap Chinese machines (for example, IEK or EKF budget line) can have a range of characteristics up to ±20%, that is, the machine on 32A will actually work when 25–40A. This is dangerous due to both false alarms and non-operation due to overload.

Recommended brands (price/quality ratio):

Brand	Series	Features	Price (2026), ₽
ABB	SH200, S200	High accuracy, resistance to temperature changes	1 200–2 500
Schneider Electric	Acti9, Easy9	Compact dimensions, reliable contacts	800–2 000
Legrand	TX3, DX3	Long service life, low heat generation	1 000–2 200
Hager	MCB	Impact-resistant housing, suitable for wet areas	1 500–3 000

The best choice for an apartment is Schneider Electric Acti9 or ABB SH200. They are certified for Russian networks and have a minimal variation in characteristics. Budget options you can consider KEAZ OptiDin (Russia), but only if the denomination is selected with a reserve (for example, 40A instead of calculated 32A).

FAQ: Frequently asked questions about introductory machines

Is it possible to install a 50A machine if 15 kW is allocated?

No. For 15 kW in a single-phase network, an automatic circuit breaker is required 70A (15,000 / 220 ≈ 68A), but this exceeds the capabilities of standard residential wiring. In most cases, 15 kW is installed three-phase input with a machine gun 25A (see calculation above). If the energy supplying organization has allocated 15 kW per phase, it is necessary to coordinate the project and replace the input cable with a cross-section of at least 10 mm².

What should you do if the machine keeps crashing for no apparent reason?

The reasons may be as follows:

1. The total power of the devices exceeds the rating of the machine (check the calculation).
2. Short circuit in the wiring or one of the devices (need to check with a multimeter).
3. Malfunction of the machine itself (the contacts are burnt, the bimetallic plate is jammed).
4. High inrush currents (for example, from refrigerator compressor). The solution is to replace the machine with the characteristic B on C.

If the problem persists, call an electrician to diagnose using megohmmeter and current clamps.

Do I need to change the input machine when replacing the wiring?

Yes, if:

• The cable cross-section has increased (for example, from 2.5 mm² up to 6 mm²).
• The allocated power has increased (for example, from 5 kW up to 10 kW).
• The old machine has signs of wear (burnt contacts, unclear operation).

If the wiring and power remain the same, replacement is not required, but it is recommended to check the condition of the contacts.

Is it possible to install two machines instead of one introductory one?

Technically possible, but it violates PUE 7.1.22, where it is said that the input device must provide shutdown all apartment network. An exception is if the second machine is installed as a backup (for example, for emergency lighting) and is agreed upon with the energy supply organization. In domestic conditions, such a scheme should be avoided.

Which machine should I put in the garage or country house?

For a garage or cottage with a power of up to 5 kW a machine gun is enough for 25A (with cable cross-section 4 mm²). If you plan to use welding machine or compressor, it is better to choose an automatic machine with the characteristic D (for example, ABB S201 D25), since these devices have high inrush currents. Also be sure to install RCD on 30 mA for protection against electric shock.