Input cable to the apartment: calculation of cross-section and correct choice

Planning the power supply of your own home is a fundamental stage on which not only the comfort, but also the safety of residents depends. Errors at the conductor selection stage can lead to regular breakouts of the circuit breakers, overheating of the insulation and even a fire. That is why the question of what input cable cross-section necessary for your apartment, requires detailed consideration and accurate calculations.

A modern apartment is full of powerful appliances: electric stoves, ovens, air conditioners, boilers and underfloor heating systems consume significant currents. Old standards often do not take into account such a volume of equipment, so you cannot rely on the “neighbors’ experience” or standard solutions from the 80s. You will need updated materials and an understanding of the physical processes that occur in a conductor under load.

In this article we will analyze all aspects of the choice: from the material of the cores to methods for calculating the total power. You'll find out why copper supplanted aluminum in modern construction, and how to properly coordinate your desires with the capabilities of the home network. A competent approach will avoid costly alterations in the future.

Conductor material: copper vs aluminum

The first thing a person faces when choosing a cable is the material dilemma. Historically, aluminum was widely used during Soviet times due to its low cost. However, modern safety standards dictate different rules. Copper cable has better conductivity, less tendency to oxidize and, what is critically important, can withstand repeated bending without breaking the wire.

Aluminum tends to “flow” under pressure, which leads to weakening of contacts at joints and subsequent heating. In addition, with an equal cross-section, aluminum passes less current than copper. If you live in an old house where the riser is made of aluminum, switching to copper inside the apartment is mandatory, but requires compliance with the rules for connecting dissimilar metals.

⚠️ Attention: Direct twisting of copper and aluminum wires is strictly prohibited due to electrochemical corrosion. Use special terminal blocks or adapter sleeves.

When choosing between VVGng-LS (copper) and AVVG (aluminum) for the entrance line to the apartment, priority is always given to the first option, if the budget allows. The difference in price when entering a standard apartment is small, but the margin of safety and security is disproportionately higher. Reliable electrical wiring is an investment in a restful sleep.

Calculation of cross-section for power and current

To determine what is needed cable section, it is not enough just to look at the rating of the input machine. It is necessary to sum up the power of all electrical appliances that can operate simultaneously. Of course, the probability of turning on all devices simultaneously is small, so simultaneity factor, usually 0.7–0.8 for residential premises.

The calculation is carried out using the formula for the dependence of power on current and voltage. For a single-phase network (220V), the formula looks simple, but requires accuracy in calculations. The resulting current is rounded up, and the nearest standard cross-section is selected using the PUE (Electrical Installation Rules) tables. Always leave a reserve of 15-20% for future expansion of your fleet.

• 🔌 Determine the total power of all consumers (W).
• ⚡ Calculate the current: I = P / (U × cos φ), where cos φ for household appliances ≈ 0.95.
• 📏 Select a section from the table with a safety margin.
• 🛡️ Match the selected circuit breaker with the cable cross-section.

It is important to understand that the cable heats up not only from the flowing current, but also from the laying conditions. If the input cable is hidden in a wall under plaster or in a conduit with other cables, its ability to dissipate heat is reduced. In such cases rated current conductor must be higher than the calculated one.

Single-phase or three-phase network

The type of network directly affects the choice of the cross-section of the input cable. Most apartment buildings use a single-phase 220V network, where two wires (phase and neutral) or three (phase, neutral, ground) are used. In this case, the entire load falls on one phase, and the cross-section must be sufficient to pass the full current of the apartment.

A three-phase 380V network is more often found in private homes or luxury new buildings with high-power electric stoves. Here the load is distributed between three phases, which allows the use of a smaller cable to transmit the same power. However, three-phase input requires a more complex distribution board and uniform distribution of consumers across phases.

With three-phase input, it is critical to maintain load symmetry. If you “hang” a powerful oven and air conditioner on one phase, and only light on the others, you will experience phase imbalance. This will lead to a voltage drop in the loaded phase and an increase in current in the neutral conductor, which is dangerous for the equipment.

Network type	Voltage	Application	Cable selection feature
Single phase	220 V	Standard apartments	2 or 3 cores, cross-section based on the sum of all currents
Three-phase	380 V	Cottages, large apartments	4 or 5 cores, smaller cross-section for the same power
With grounding	220/380 V	Modern standard	A grounding conductor (PE) is required.

If you are planning a transition from a single-phase to a three-phase network, first obtain technical specifications from the energy sales organization. Unauthorized switching can lead to an accident and fines.

Standard cross-sectional values for apartments

In the practice of electrical installation, certain standards have been formed that are used in most standard projects. For apartments with an area of up to 60-70 square meters, where the use of powerful electric heating systems is not expected, copper cable with a cross-section of 10 mm². This value ensures reliable operation with a 50A input circuit breaker.

For small studios or one-room apartments with a limited range of equipment, a section is often sufficient 6 mm². This cable can withstand current up to 40-50A depending on installation conditions, which covers the needs of most families. However, if you are planning to install a instantaneous water heater or electric boiler, this will not be enough.

It is worth mentioning about apartments with electric stoves. In houses with gas stoves, the allocated power is usually 3-5 kW, while with electric stoves it is 7-10 kW and above. Accordingly, the cross-section of the input cable in houses with electric stoves is initially designed to be large. When replacing wiring, be guided by the design power of the house.

⚠️ Attention: Never install a circuit breaker with a rating higher than the cable cross-section allows. A 6 mm² cable will not withstand a 63A machine under prolonged load.

Influence of route length and laying method

The length of the input cable plays a role when it comes to significant distances, for example, when entering a private house from a pole. In apartment conditions, the length of the route is usually short (up to 20-30 meters), and the voltage drop can be neglected. However, if the switchboard is located far away, you need to check loss of voltageso that at the entrance to the apartment it does not fall below acceptable standards.

The installation method also influences the choice. A cable laid open in the air cools better than one buried in a wall or laid in a tight bundle with other cables. For hidden wiring in walls, it is recommended to choose a cross-section one step larger than the calculated one if there are doubts about heat transfer.

Use cables that are marked ng-LS (non-flammable, low smoke emission) for installation inside residential premises. In the event of a fire, such insulation will not support combustion and release fewer toxic substances, which is critical for the safety of people in the confined space of the apartment.

What does the VVGng-LS marking mean?

VVG - vinyl-vinyl-naked (PVC insulation and shell, without external armor), ng - does not propagate combustion, LS (Low Smoke) - low smoke and gas emissions. This is the optimal choice for residential premises.

Protective automation and coordination

The selected cable cross-section must be protected by a circuit breaker. The main task of the machine is to turn off the power before the cable heats up to a dangerous temperature. The relationship here is direct: the larger the cross-section, the higher the rating of the machine, but always with an eye to the “weak link” of the chain.

A common mistake is installing a powerful machine "for growth". If you have a 6 mm² cable (maximum 50A), and the machine is 63A, then at a current of 60A the cable will overheat and melt, and the machine will not even click. This situation is fraught with fire. The machine protects the cable, not the device!

• 🔒 For a cross-section of 6 mm² (copper), set the machine to no more than C40 (maximum C50).
• 🔒 For a cross-section of 10 mm² (copper), we will allow an automatic machine C50 or C63.
• 🔒 For a cross section of 16 mm² (copper), you can use C63 or C80.

Also do not forget about residual current devices (RCD) or differential automata. They protect a person from electric shock during a leak, but do not protect from overcurrent. Therefore, the combination “Automatic + RCD” or “Difavtomat” is mandatory for the introductory group.

Frequently asked questions (FAQ)

Is it possible to use an aluminum cable to enter an apartment?

Technically it is possible if the cross-section allows (aluminum is taken 1-2 steps thicker than copper), but modern standards and common sense recommend only copper. Aluminum is fragile, oxidizes and requires special reinforcement.

Is a 3x6 mm² cable enough for an apartment with an electric stove?

For most apartments with a standard electric stove (up to 7-8 kW), a cross-section of 3x6 mm² of copper will be sufficient if the input circuit breaker does not exceed 40A. For powerful hobs and ovens, it is better to use 3x10 mm².

What to do if the cable gets hot?

Either the connection point (poor contact) or the cable itself heats up due to overload. In the first case, you need to tighten or replace the terminal, in the second, reduce the load or change the cable to a thicker one.

Is grounding required in the input cable?

Yes, definitely. The third conductor (PE) is necessary for the operation of modern automation (RCD/DIFs) and the safety of metal housings of devices. Without grounding, operating modern equipment is dangerous.