The Comprehensive Guide to Cables: Understanding, Choosing and Using Them Correctly
Choose the wrong cable, and you’re left with a tangled mess: devices don’t work, connections are unstable, and, in the worst-case scenario, it can even pose a safety risk. For beginners, apprentices or DIY enthusiasts, it’s not always easy to keep track of everything. This guide explains what matters when choosing cables, and helps you make the right decision.
How a cable is constructed
A cable consists of several layers, each with a specific function. The conductor [1] is at the heart of the cable: this is where the current flows, usually through copper. It is surrounded by an insulating, protective layer [2], usually made of plastic, which prevents short circuits or electric shocks. Not every cable has shielding [3]. This protects against electromagnetic interference, which is particularly important for data cables. The outer jacket is called the sheath [4], and protects against mechanical damage, moisture, or external substances.
Solid or stranded conductors?
Solid conductors consist of a single, solid copper wire and are stable and break-resistant. This makes them ideal for fixed installations, for example, within walls. Stranded conductors (strands), on the other hand, are made up of many thin copper wires, and are therefore considerably more flexible. This makes them better suited to applications involving movement: for example, in extension leads or within appliances and machinery.
Types of sheathing and materials
The sheathing makes a big difference: PVC is inexpensive and flexible, making it perfect for indoor use, such as in the home or office. Rubber, on the other hand, is more robust, heat-resistant, and remains flexible even in cold conditions; perfect for the workshop and outdoor use. PUR sheathing, made of polyurethane, is oil- and abrasion-resistant, making it the most durable choice for industry and machinery.
In addition, a sequence of letters is printed on every cable, for example, ‘H03VV-F’. Each letter stands for a specific property. H means that the cable complies with a standard that is uniform across Europe, and may therefore be used anywhere in Europe. The number that follows indicates the voltage: 03 stands for 300/300 volts, 05 for 300/500 volts, and 07 for 450/750 volts. The higher the number, the higher the voltage the cable is rated for. The first letter describes the inner conductor insulation, the second the outer sheath; here, V stands for a PVC sheath, R for standard rubber and N for a special, heavy-duty rubber. The final letter – F for fine-stranded, or U for rigid – indicates whether the conductors are flexible or rigid. An H03VV-F cable is therefore: compliant with European standards, designed for low voltage, PVC-insulated and sheathed, and flexible. This type of cable is well suited to light domestic appliances.
Meaning of cable marking
| Character sequence | Symbol | Meaning | Explanation |
|---|---|---|---|
| 1st character | H | Harmonised cable | Complies with European standards (HAR system) |
| 2nd character | 03 | Rated voltage 300/300 V | For low voltages and light-duty applications |
| 2nd character | 05 | Rated voltage 300/500 V | For medium-duty applications |
| 2nd character | 07 | Rated voltage 450/750 V | For higher voltages and installations |
| 3rd character | V | PVC core insulation | PVC insulation on individual conductors |
| 3rd character | R | Rubber sheath | Rubber core insulation |
| 4th character | V | PVC sheath | Outer sheath made of PVC |
| 4th character | R | Rubber sheath | Outer sheath made of rubber |
| 4th character | N | Special rubber | Heavy-duty rubber sheath (e.g. neoprene) |
| 5th character | U | Solid conductor | Solid copper conductor (Class 1) |
| 5th character | R | Multi-strand conductor | Rigid conductor consisting of several strands (Class 2) |
| 5th character | K | Fine-strand conductor | Flexible conductor (Class 5), e.g. for control cabinets or installation conduits |
| 5th character | F | Flexible cable | Indicates a flexible cable for applications involving movement |
Beispiele:
- H03VV-F: Harmonised cable · 300/300 V · PVC insulation · PVC sheath · flexible connection cable
- H05VV-F: Harmonised cable · 300/500 V · PVC insulation · PVC sheath · flexible connection cable
- H07RN-F: Harmonised cable · 450/750 V · rubber insulation · special rubber sheath · flexible connection cable
- H07V-U: Harmonised cable · 450/750 V · PVC insulation · solid conductor
- H07V-K: Harmonised cable · 450/750 V · PVC insulation · stranded conductor
What the cable colours mean
The colours of the wires in the cable follow a fixed standard. Each wire performs a different function in the circuit. Brown, grey, or black is the live wire, also known as the phase conductor; this is the wire that actually carries voltage and can be dangerous if touched. Blue is the neutral wire, which completes the circuit back to the source. Green-yellow is the protective conductor, also known as the earth: it does not normally carry any current, but safely diverts the current in the event of a fault. If you mix up these three wires when connecting them, you risk a malfunction, at best, and an electric shock or fire, at worst. But please note: in older installations (prior to 2003), or in other countries, these colours may differ.
| Cable colour | Designation | Function | Does it normally carry current? | Safety note |
|---|---|---|---|---|
| Brown | Phase (L) / Outer conductor | Carries the voltage from the mains to the load | ✅ Yes | Contact may be life-threatening |
| Black | Phase (L) / Outer conductor | Same as brown; often used for additional phases | ✅ Yes | Live |
| Grey | Phase (L) / Outer conductor | Often the third phase in three-phase systems | ✅ Yes | Live |
| Blue | Neutral conductor (N) | Closes the circuit and returns the current to the source | ⚠️ Yes, when in operation | May carry voltage under certain circumstances |
| Green-yellow | Protective conductor (PE) / Earth | Safely diverts fault currents to earth | ❌ Normally no | Must never be used as a live conductor |
An overview of the main types of cable
- Installation cables are designed for permanent installation in walls, ceilings or conduit. The best-known example is the NYM-J: this can be found in almost every domestic installation, for example behind sockets or light switches.
- Power cords/connection cables link devices to the mains supply. They are fine-stranded and flexible.
- Network cables, such as a patch cable from the router to the PC, transmit data within a home network or office. The category (CAT) determines the transmission speed and bandwidth.
- Coaxial cables are used for TV and aerial signals. A metal braided shield around the inner conductor shields the signal and prevents interference.
- Control and data cables are found in industry and automation technology. They transmit control signals between devices, for example, in machines or control cabinets.
The right cable for your project
How to choose the right cable?
- In the home, a lightweight connection cable is sufficient for light, portable devices. An H03VV-F with 2× 0.75 mm² , for example, is the thin, flexible cable used in a table lamp. An H03 or H05 cable is also suitable for sockets on an extension lead. For sockets in or on the wall, however, you need an NYM-J installation cable with at least 3× 2.5 mm².
- In offices and for IT projects, it is worth using network cables of at least Cat6a standard – this achieves data transfer rates of up to 10 Gbit/s, and is more than sufficient for streaming, work and gaming. Standard connection cables, suitable for the device’s power requirements, are sufficient for connecting devices.
- In workshops and industrial settings, robust rubber cables such as the H07RN-F are suitable. The ‘R’ stands for a rubber sheath that withstands oil, heat, and mechanical stress.
- Outdoors, UV-resistant and waterproof cables are essential. Standard PVC quickly becomes brittle when exposed to sunlight, and moisture can quickly become a hazard. For outdoor lights or garden sockets, you therefore need special outdoor cables, such as an H07RN-F rubber cable or a specially marked NYY-J underground cable, which can even be laid directly in the ground.
A comparison of common cable types
| Cable type | Voltage | Insulation / Sheath | Conductor | Typical use |
|---|---|---|---|---|
| H03VV-F | 300/300 V | PVC / PVC | Flexible | Light household appliances, lamps, chargers |
| H05VV-F | 300/500 V | PVC / PVC | Flexible | Household appliances, extension leads for indoor use |
| H05RR-F | 300/500 V | Rubber / Rubber | Flexible | Workshops, portable equipment |
| H07RN-F | 450/750 V | Rubber / Special rubber | Flexible | Construction sites, gardens, industry, outdoor use |
| H07V-U | 450/750 V | PVC insulation | Rigid | Permanent building installations, laying in conduits |
| H07V-K | 450/750 V | PVC insulation | Fine-stranded | Control cabinets, wiring requiring high flexibility |
Common mistakes and how to avoid them
Selecting a cable cross-section that is too small
On the one hand, the cross-section depends on the current load: cables that are too thin overheat under high loads. As a rule of thumb: 1.5 mm² for lighting and less heavily loaded sockets, 2.5 mm² for heavily loaded sockets (e.g. tumble dryers), and at least 4 mm² for electric cookers and more powerful machines in a hobby workshop or garage. For very high-power appliances, even larger cross-sections may be necessary.
On the other hand, the distance to the fuse box is also important: the longer the cable, the greater the voltage drop, and the larger the cross-section should be. In practice, DIN 18015 generally recommends a maximum voltage drop of 3% from the meter to the appliance; at 230 V, this amounts to 6.9 V.
An example: a 20 m long cable with a cross-section of 2.5 mm² at 16 A produces a voltage drop of around 4.5 V (approx. 2.0 per cent), which is still within the acceptable range. With a cross-section of just 1.5 mm², the voltage drop would already be ~7.5 V (approx. 3.3%), which narrowly exceeds the 3% limit. At a length of 30 m, it would rise to ~11.2 V (approx. 4.9%). In this case, 4 mm² would be the correct choice.
Incorrect wire connection
Always check the colours, and if in doubt, measure the voltage. A multimeter is recommended for this purpose, as it allows you to reliably check the voltage, continuity, and correct wiring.
No residual current device (RCD)
A residual current device (RCD) constantly monitors the current flow in the circuit and automatically cuts off the entire circuit in an emergency. It is mandatory for outdoor and damp-room installations, and can save lives.
Choosing the right cable safely
Whether for domestic wiring, a home network, a workshop or the garden – choosing the right cable always depends on the location, the load, and the environmental conditions. If you understand the key differences in construction, cross-section, materials and markings, you can avoid making the wrong purchase, operate equipment safely, and extend the service life of the installation.
Images: Adobe Stock, reichelt elektronik
