Which wire to choose for the garage: calculation of cross-section by power and length

Introduction: Why garage wire gauge is critical to safety

Wiring in the garage is not just a matter of comfort, but the basis of fire safety. Incorrectly selected cable section can lead to overheating, melting of insulation and even fire, especially if welding machines, compressors or heaters are used in the garage. Unlike a home network, garage wiring often operates under extreme conditions: temperature changes, high humidity, mechanical stress.

Many car owners mistakenly believe that it is enough to take a “thicker” one and there will be no problems. In fact excess section leads to unjustified expenses, and insufficient - to accidents. In this article we will look at how to calculate the optimal cross-section taking into account equipment capacity, line length and conductor material, and also what PUE standards (Electrical Installation Rules) must be observed when installing in a garage.

1. Basic criteria for choosing a wire cross-section for a garage

Before you run to the store to buy a cable, determine three key parameters:

• 🔌 Total power all electrical appliances (in kW). This includes lighting, sockets, welding machine, compressor, heater, etc.
• 📏 Line length from the distribution panel to the garage (in meters). The longer the cable, the greater the voltage loss.
• 🔋 Conductor material: copper or aluminum. Copper wires are more expensive, but more reliable and allow a smaller cross-section for the same load.

For example, if in the garage there is welding inverter 5 kW and a pair of 100 W lamps, the total power will be ~5.2 kW. For such a load with a line length of 20 meters, a copper cable with a cross section of 4 mm², and aluminum is already 6 mm². Why is this so? More on this in the next section.

2. Calculation of power cross-section: formulas and tables

For an approximate calculation, use the formula:

I = P / (U × cosφ)

Where:

• I - current strength (A)
• P — total power (W)
• U — voltage (220 V for single-phase network, 380 V for three-phase)
• cosφ — power factor (for household appliances ~0.95, for welding machines ~0.7)

The resulting current is compared with the data from the table PUE 1.3.4–1.3.11 (permissible current loads on cables). For example, for copper wire 2.5 mm² maximum current is 27 A (hidden wiring), and for 4 mm² - already 38 A.

Section, mm² (copper)	Max. current, A (open wiring)	Max. current, A (hidden wiring)	Max. power, kW (220 V)
1,5	23	19	4,2
2,5	30	27	5,9
4	41	38	8,3
6	50	46	10,1
10	80	70	15,4

⚠️ Attention: If used in a garage three-phase equipment (for example, machine tools), the calculation is carried out according to a different scheme. The voltage is taken as 380 V, and the current is divided by √3 (~1.73). In such cases, it is better to consult an electrician.

3. The influence of line length on wire cross-section

The longer the cable from the panel to the garage, the more voltage loss. According to PUE, they should not exceed 5% for lighting and 10% for power lines. For calculation use the formula:

ΔU = (I × ρ × L) / S

Where:

• ΔU — voltage loss (V)
• I - current (A)
• ρ - resistivity (0.0175 for copper, 0.028 for aluminum)
• L — line length (m)
• S — cross-section (mm²)

Example: for copper cable 2.5 mm² 30 m long at a current of 20 A, the loss will be ~4.2 V (1.9%). This is acceptable. But if the length is 50 m, the losses will increase to ~7 V (3.2%), and at 100 m - to ~14 V (6.4%), which is already exceeds the norm for lighting. In this case, you need to increase the cross section to 4 mm².

What happens if you ignore voltage losses?

With significant losses (more than 10%), the equipment may operate unstably: dim lighting, overheating of compressor engines, malfunctions of welding machines. In the worst case scenario, equipment failure or fire due to cable overheating.

4. Copper vs aluminum: which is better for the garage?

The debate about which conductor is better has not subsided for decades. Let's look at the pros and cons of each:

• ✅ Copper wire:
• 🔹 High conductivity (bears loads better)
• 🔹 Resistant to oxidation and corrosion
• 🔹 Lasts longer (service life up to 50 years)
• 🔹 You can use a smaller section with the same load
• ❌ 30–50% more expensive than aluminum
• ✅ Aluminum wire:
• 🔹 Cheaper than copper 2–3 times
• 🔹 Lightweight (easy to mount over long distances)
• ❌ Oxidizes in air (contact at joints worsens)
• ❌ Fragile (can break when bent)
• ❌ Requires a larger section for the same load

⚠️ Attention: According to PUE 7.1.34, in residential and domestic premises (and the garage is equivalent to them) prohibited use aluminum wires with a cross-section less than 16 mm². For garages, this means that aluminum can only be used for the input cable (from the pole to the panel), and it is better to make the internal wiring using copper.

5. Types of garage cables: which one to choose?

Not every wire is suitable for garage conditions. Optimal options:

• 🔧 VVGng-LS - non-flammable copper cable with low smoke emission. Ideal for hidden and open wiring.
• 🔧 NYM — imported analogue of VVG, with additional rubber insulation. Convenient for installation, but afraid of UV rays (cannot be laid under direct sun).
• 🔧 SIP-4 — self-supporting aluminum cable for input from a pole. Does not require a support cable and is weather resistant.
• 🔧 PVS — flexible stranded wire for carrying and temporary connection. Not suitable for fixed wiring!

For input cable (from pole to garage) more often used SIP-4 16–25 mm² (aluminum) or VVGng 10–16 mm² (copper). For internal wiring — VVGng 2.5–6 mm² (depending on load).

✔ The cross section corresponds to the calculations (check the markings)

✔ Insulation without cracks or damage

✔ Certificate of conformity (GOST or TU)

✔ For the street - a brand with the prefix “ng” (non-flammable) and “LS” (low smoke)

✔ Solid cores, not twisted from thin wires (for stationary wiring) -->

6. Typical mistakes when choosing and installing wiring in a garage

Even experienced car owners sometimes make critical mistakes:

1. Using PVA for fixed wiring. This wire is intended for portable use and not for permanent installation. Its insulation is not designed for long-term loads.
2. Twists instead of terminals. In a garage with its humidity and temperature changes, twists oxidize in 1–2 years. Use terminal blocks or sleeves.
3. Neglect of RCD. Must be installed in garage residual current device (30 mA for sockets, 100 mA for input). This will save you from electric shock if the insulation breaks down.
4. Cable laying without protection. Exposed wiring in the garage must be installed in corrugation, cable channel or metal hose - this will protect against mechanical damage and rodents.

⚠️ Attention: If the garage is metal (for example, made of corrugated sheets), prohibited attach the cable directly to the walls without insulating spacers. Metal can damage insulation and also create stray currents when struck by lightning.

7. FAQ: Frequently asked questions about garage wiring

❓ Can I use a 1.5 mm² cable for sockets in the garage?

No. According to PUE 7.1.45, the minimum cross-section for socket groups is 2.5 mm² (copper). The 1.5 mm² cable is only suitable for lighting with a load of up to 3.5 kW.

❓ Is it necessary to ground the garage?

Yes, definitely. Without grounding, any equipment malfunction (for example, a breakdown on the body of a welding machine) can lead to electric shock. For grounding use yellow-green wire (PE) with a cross-section of at least 6 mm² (copper) or 10 mm² (aluminium).

❓ Which machine should I put into the garage?

The rating of the machine depends on the cross-section of the input cable:

• 🔹 6 mm² (copper) - 32 A
• 🔹 10 mm² (copper) - 50 A
• 🔹 16 mm² (aluminium) - 50 A

Usually enough for a garage 25–40 A. The machine must be type C (for household loads).

❓ Is it possible to stretch a cable from a house to a garage over the air?

Yes, but subject to the rules:

• 🔹 The height above the roadway is at least 6 m, above the pedestrian zone - 3.5 m.
• 🔹 Use SIP or VVGng in a steel pipe.
• 🔹 The distance between supports is no more than 25 m (otherwise an intermediate support is needed).

❓ What to do if the voltage in the garage “drops” when you turn on the welding machine?

This is a sign insufficient cable cross-section or weak transformer at the substation. Solutions:

• 🔹 Increase the cross-section of the input cable (for example, from 6 mm² to 10 mm²).
• 🔹 Install voltage stabilizer for sensitive equipment.
• 🔹 Contact your energy sales office to check the line (the problem may be on their side).