Which introductory machine should be installed in an apartment: calculation of amperes based on power and cable cross-section

Choice input machine for an apartment is a task that requires taking into account several factors at once: the total power of household appliances, the cross-section of the power cable, the type of electrical wiring and even the rules of the energy supply organization. An error in the calculations can lead to constant protection triggering when the kettle or washing machine is turned on - or, worse, to overheating of the wiring and fire due to an underrated rating.

Many apartment owners mistakenly believe that the more powerful the machine, the better. However excess of nominal value in relation to the cable cross-section, this is a direct safety hazard. For example, if the input is a machine on 50A, and the wiring in the house is aluminum and is designed for 16A, then if overloaded, the cable will begin to melt and the protection will not work. On the other hand, a machine that is too “weak” will break down at the slightest load, making life in the apartment uncomfortable.

In this article we will look at how correctly calculate the rating of the input machine for an apartment taking into account:

- single-phase or three-phase network,

- total power of the equipment,

- cable cross-section and material,

- requirements PUE 7.1.34 and local power grids.

We will also provide real examples of calculations and tables for quick selection.

Why you can’t install the introductory machine “by eye”

Selecting a circuit breaker based on the "my neighbor has the same one" or "the store recommended" is one of the most common mistakes. The point is that rating of the input machine depends on:

• 🔌 Network type: single-phase (220V) or three-phase (380V). In apartments, single-phase is most often used, but in new houses or when connecting powerful equipment (for example, an electric stove), three-phase can also be used.
• ⚡ Allocated power: how many kilowatts are allocated to the apartment under an agreement with the energy sales company. The standard for old buildings is 3–5 kW, for new houses – 7–15 kW.
• 📏 Cable cross-section and material: an aluminum cable with a cross section of 2.5 mm² can withstand 16–19A, and a copper cable of the same cross section can withstand 21–25A. The input circuit breaker must protect the cable, not the load!
• 🏢 Management company requirements: some management companies or energy sales companies prohibit installing machines above a certain rating (for example, no more than 32A for apartments with old wiring).

If you ignore these factors, the consequences can be serious:

⚠️ Attention! A 40A circuit breaker installed on an aluminum cable with a cross-section of 4 mm² (maximum 28A) will lead to overheating of the wiring already at a load of 6–7 kW. The risk of short circuit and fire increases by 3–5 times.

Another typical situation: the apartment has a 25A automatic circuit breaker, but the allocated power under the contract is only 5 kW (22.7A). In this case, when you turn on the refrigerator, washing machine and electric kettle at the same time, the machine will operate, although the wiring can withstand a large load. The solution is not to increase the nominal value, but to contact the power supply for an increase in the allocated power.

How to find out the allocated power for an apartment

Before calculating the rating of the input machine, you need to find out how many kilowatts are allocated to your apartment. This information is provided in:

• 📄 Agreement with the energy supply organization (usually in the "Specifications" or "Maximum Power" section).
• 🏠 Technical conditions (TU) for connecting electricity (if the house is new or there has been a major overhaul of the network).
• 🔧 Meter passport or circuit breaker (sometimes the maximum power is indicated there).
• 📞 Certificate from the management company (can be requested in the office or through the State Services portal).

Standard allocated power values for apartments:

House type	Distributed power, kW	Typical rating of an input circuit breaker, A
Old stock (Khrushchev, Stalin)	3–5	16–25
Panel houses (1980–2000)	5–7	25–32
New houses (after 2010)	7–10	32–40
Luxury houses/apartments with electric stoves	10–15	40–63
Apartments with 3-phase input	15–25	25–40 (per phase)

If the allocated power is unknown, it can be calculated at the nominal value of the input machine, which is already in the shield. For example, if the input is a 32A machine, then:

Maximum power (kW) = Machine rating (A) × Voltage (V) × cosφ

For a single-phase network (cosφ = 1): 32A × 220V = 7.04 kW.

Calculation of the power rating of the input circuit breaker

The basic formula for calculating the machine's nominal value:

Inom = Pmax / (U × cosφ), where:

• IN — rated current of the machine, A;
• Pmax — maximum load power, W;
• U — mains voltage (220V for single-phase, 380V for three-phase);
• cosφ — power factor (for household appliances it is taken 0.95–1).

Calculation example for an apartment with allocated power 7 kW (7000 W) and single-phase network:

Inom = 7000 / (220 × 1) ≈ 31.8A

The closest standard machine denomination is 32A. However, this value must be adjusted taking into account:

- Cross-section and material of the input cable (must withstand a current of at least 32A)

- Starting currents of devices (for example, a refrigerator or air conditioner can produce a short-term load 2–3 times higher than the rated load)

- Temperature conditions (in a hot room the machine can operate at lower currents)

- Reserve for the future (if you plan to add powerful devices) -->

Amendments to the calculation

1. Cable cross-section: if the input cable is copper with a cross-section of 6 mm², it can withstand up to 42A (according to the PUE table). A 32A machine is safe in this case. If the cable is aluminum of the same cross-section (maximum 30A), then a 32A circuit breaker is already risky - it’s better to install 25A.

2. Starting currents: When the refrigerator compressor or washing machine motor is turned on, the current may briefly increase by 3–5 times. Class machine C (the most common) is triggered when the nominal value is exceeded by 5–10 times. Therefore, for powerful devices it is better to choose an automatic machine with a reserve or use a class D (withstands inrush currents up to 20 times the nominal value).

3. Temperature: in a panel with poor ventilation, the machine may overheat and operate earlier. In this case, the denomination is taken 10–15% lower than the calculated value.

Table for selecting an incoming machine according to cable cross-section

The nominal value of the input machine must be less than or equal to maximum permissible current for the cable. Below is a table for copper and aluminum cables according to PUE 7.1.34:

Cable cross-section, mm²	Max. current for copper, A	Max. current for aluminum, A	Recommended automatic, A
2.5	21–25	16–19	16 (for aluminum), 20 (for copper)
4	28–32	23–28	25 (for aluminum), 32 (for copper)
6	36–42	30–36	32 (for aluminum), 40 (for copper)
10	50–55	39–46	40 (for aluminum), 50 (for copper)

Critical error: if the cable cross-section is 4 mm² (copper), and the machine is set to 50A, then at a current of 40A the cable will begin to melt, but the machine will not work - this is a direct road to a fire.

Example: in an apartment there is an aluminum input cable with a cross-section of 6 mm². According to the table, its maximum current is 30–36A. This means that the input machine must be no more than 32A, even if the allocated power allows 40A.

Single-phase or three-phase input: what to choose

Most apartments use single-phase input (220V), but in some cases it is advisable to connect three-phase (380V):

• ⚡ Power above 10 kW: if the apartment has an electric stove, sauna or powerful equipment (for example, machines in a workshop).
• 🔄 Load distribution: a three-phase network allows you to evenly distribute devices across phases, reducing the load on each.
• 🏗️ New houses: many developers initially lay out a three-phase input to be able to connect powerful consumers.

However, three-phase input also has disadvantages:

• ⚠️ Difficulty of installation: a three-phase meter, automatic circuit breakers for each phase and correct load distribution are required.
• 💰 More expensive connection: Energy sales may charge an additional fee for three-phase input.
• 🔌 Limitations on single-phase devices: If the load is not distributed correctly, one phase may become overloaded.

For three-phase input, the machine's rating is calculated for each phase separately. For example, if the allocated power is 15 kW (380V), then:

Inom = 15000 / (380 × √3) ≈ 22.8A

This means that for each phase an automatic machine is needed 25A (nearest standard denomination).

⚠️ Attention! For three-phase input, be sure to install three-pole circuit breaker — it turns off all three phases simultaneously in case of overload. The use of three single-pole circuit breakers is prohibited by the PUE!

Typical mistakes when choosing and installing an introductory machine

Even experienced electricians sometimes make mistakes when selecting an input machine. Here are the most common:

• 🔌 Ignoring cable size: they install the machine “according to power”, without checking whether the cable can withstand such a current. For example, a 40A circuit breaker for a 4 mm² aluminum cable.
• ⚡ Lack of stock: if the total power of the devices is 6.5 kW, set the machine to 32A (7 kW), not taking into account starting currents or future additions of equipment.
• 🔄 Wrong machine class: for a network with large starting currents (for example, with a pump or compressor), select a class B instead of C or D, due to which the machine is triggered when the devices are turned on.
• 🏠 Independent replacement without approval: in some houses (especially with a common panel), replacing the input circuit breaker requires permission from the management company.
• 🔧 Poor contact: loose terminals on the machine lead to heating and melting of the contacts.

A practical example: in an apartment with an allocated power of 10 kW, a 50A automatic machine was installed, motivating this “for the future.” As a result, when the heater and kettle were turned on (~5 kW in total), the 4 mm² cable began to heat up, and the machine did not work. The reason is excess current for the cable (maximum 28A for aluminum). Solution: replaced the circuit breaker with a 32A one and laid a 6 mm² copper cable.

What happens if you install a machine with a higher rating than the cable allows?

If the permissible current for the cable is exceeded (for example, 40A for 4 mm² aluminum, which can only withstand 28A), the following occurs:

1. The cable begins to heat up and the insulation melts.

2. The risk of short circuit due to exposed wires increases.

3. The circuit breaker does not operate because its rating is higher than the cable overload current.

4. In the worst case, a wiring fire.

Which brand of machine to choose: review of manufacturers

The quality of the circuit breaker directly affects safety. Cheap Chinese machines may not work when overloaded or, conversely, turn off for no reason. Recommended brands:

• 🇩🇪 ABB: reliable, durable, but expensive. Series SH200 - the best choice for an apartment.
• 🇫🇷 Schneider Electric: series Acti9 — good price/quality balance. Suitable for most household networks.
• 🇷🇺 IEK: budget option, but the quality is lower than European analogues. Series KEAZ is an acceptable choice for a temporary solution.
• 🇩🇪 Siemens: series 5SY — premium segment, high accuracy.
• 🇯🇵 Mitsubishi: rarely counterfeited, stable operation under high loads.

The cost of the introductory machine depends on the brand and denomination:

Brand	Denomination, A	Price, rub (2026)
ABB SH200	32	1 200–1 500
Schneider Acti9	32	800–1 100
IEK KEAZ	32	300–500
Siemens 5SY	40	1 500–1 800

Advice: buy machines only in trusted stores (for example, 220pro.ru, ElectroTorg, Leroy Merlin). Counterfeits are outwardly indistinguishable from the originals, but may not work in a critical situation.

FAQ: Frequently asked questions about introductory machines to apartments

❓ Is it possible to install an introductory machine with a higher rating than the allocated power?

No, this is a violation of energy supply regulations. For example, if 5 kW (22.7A) is allocated, and you install a 32A machine, then if the limit is exceeded, the protection at the substation will work and the lights will be turned off in the entire entrance. In addition, the management company may impose a fine for unauthorized changes to the scheme.

❓ Which machine should I install if the apartment has old aluminum wiring?

For aluminum wiring with a cross section of 2.5 mm², the maximum current is 16–19A, so the input circuit breaker should be no more 16A. If the cross-section is 4 mm² - up to 25A. We recommend replacing the wiring with copper or reducing the load.

❓ Do I need to coordinate the replacement of the introductory machine with the management company?

Yes, if the machine is installed in a common panel on the landing. If the shield is located inside the apartment, approval is usually not required, but it is better to check with the Criminal Code. Unauthorized replacement may lead to conflicts during inspections.

❓ Is it possible to install several instead of one introductory machine?

Yes, it's called selective protection. For example, at the input there is a 40A machine, and after it there are two at 25A and 16A for different groups of consumers. The main thing is that the total rating does not exceed the allocated power.

❓ Why does the introductory machine heat up?

Reasons:

• Poor contact in the terminals (needs tightening).
• Exceeding the rated current (replace the machine with a larger one, if the cable allows).
• Fake or defective (check the certificate, replace it with the original).
• Power overload (turn off some devices).