Cross-section reduction refers to the deliberate downsizing of the conductor cross-section in a circuit. It is permissible if current-carrying capacity, voltage drop and protection against overload and short circuit still comply with the standards of the DIN VDE 0100 series. In control cabinets, the 3-metre rule according to DIN VDE 0100-430 applies: a reduced section may be at most 3 metres long and must be installed so that short circuits and earth faults are minimised. The insulating parts that separate and cover inside such assemblies are made by GOBA as customer-specific insulating parts to drawing. This page explains the rules, shows the fuse rating table per cross-section and the limit values for voltage drop.
A smaller cross-section saves copper and space, but carries three risks
A thinner conductor needs less copper or aluminium, which lowers material costs, noticeably so on long cable runs. Smaller cross-sections are easier to install, particularly in tight spaces and crowded cable ducts. And a cross-section sized for the actual load avoids overdimensioning without any loss of safety.
Against this stand three risks: overheating if the conductor loading is calculated incorrectly, a voltage drop that reduces the performance of the consumers, and in the worst case a cable fire if the fusing does not match the reduced cross-section. The rules in the following sections keep these three risks in check. Exact cross-sectional geometries from insulating material, from 5 mm cut width with ±0.1 mm tolerance, are produced at GOBA by stamping and forming.
How to Calculate the Correct Cross-section Reduction
Factors for Calculating the Conductor Cross-section
Several factors play a role when selecting the optimal cross-section:
- Current-carrying capacity of the cable (depending on material and installation method)
- Cable length and the associated voltage drop
- Fuse rating for protection against overload or short circuit
- Ambient temperature, since higher temperatures reduce the permissible current
- Installation method, for example in cable ducts, on plaster or under plaster
Formulas and Methods for Cross-section Calculation
Calculation of the conductor cross-section is based on the voltage drop formula. The relevant quantities are: voltage drop in volts (deltaU), current in amperes (I), cable length in meters (L), specific resistance of the conductor material (rho) and cross-section in mm^2 (A). For copper, the specific resistance is 0.0178 Ohm mm^2/m, for aluminum 0.0282 Ohm mm^2/m.
deltaU = (I x L x rho) / A
A = pi x (d / 2)^2
For domestic installations, DIN VDE 0100-520 specifies a maximum voltage drop of 3 % for lighting and 5 % for other consumers.
Which Standards and Regulations Apply to Cross-section Reduction?
In Germany, various VDE and DIN standards govern the correct dimensioning of electrical cables:
- DIN VDE 0100-430: Protection against overload and short circuit
- DIN VDE 0100-520: Selection and erection of electrical equipment
- Supplement 2 to DIN VDE 0100: Detailed cross-section tables for various installation methods
The standards specify how cable cross-sections must be dimensioned depending on voltage, load and cable length in order to ensure safety and efficiency.
Reducing the cross-section in the control cabinet: the 3-metre rule
In control cabinet and panel building, cross-section reduction is everyday practice, for example when transitioning from a heavily fused supply to thinner wiring to individual devices. DIN VDE 0100-430 allows the protective device for short-circuit protection to be positioned downstream if all of the following conditions are met:
- The section with the reduced cross-section is at most 3 metres long.
- The section is installed so that short circuits and earth faults are minimised, for example in a closed duct or with reinforced insulation (installation according to DIN VDE 0100-520, section 521.13).
- There are no branches and no plug-in devices in the section.
- A protective device at the end of the section protects the reduced cross-section against overload.
For machine control systems, DIN EN 60204-1 additionally applies, which places comparable requirements on conductor protection. Overload protection may never be omitted: it must either be located upstream of the reduction and also protect the thinner conductor, or be positioned directly at the end of the reduced section.
How to Select the Correct Conductor Cross-section for Domestic Installations
Typical Cross-sections for Various Applications
The following standard values apply depending on the application:
| Application | Typical cross-section |
|---|---|
| Socket circuit (16 A) | 1.5 mm² or 2.5 mm² |
| Lighting circuit (10 A) | 1.5 mm² |
| Electric cooker or instantaneous water heater | 4 to 6 mm² |
| Main supply cable for distribution board | 10 to 16 mm² |
NYM-J cables are the standard conductors for domestic installations. When installed in insulated walls or cable bundles, however, potential overheating must be considered.
Which Fuse Ratings Apply to Reduced Cross-sections?
The choice of fuse rating depends directly on the conductor cross-section. Guide values for copper conductors with typical domestic installation methods:
| Cross-section (copper) | Maximum fuse rating |
|---|---|
| 1.5 mm² | 16 A |
| 2.5 mm² | 20 A |
| 4.0 mm² | 25 A |
| 6.0 mm² | 32 A |
| 10 mm² | 40 A |
These are guide values, the decisive factor is always the current-carrying capacity according to installation method, grouping and ambient temperature as per the tables of DIN VDE 0298-4.
For long cables (over 15 m), a cross-section increase may be necessary to prevent voltage losses.
How Does Cable Length Affect Cross-section Reduction?
Voltage Drop Calculation for Longer Cables
Excessive voltage drop leads to performance losses and can damage equipment. DIN VDE 0100-520 defines the following limit values:
| Application | Max. voltage drop |
|---|---|
| Lighting circuits | 3 % |
| Socket circuits | 5 % |
For longer runs, an increase in cable cross-section may be required.
Example: A 16 A circuit with 1.5 mm^2 copper conductor may be a maximum of 18 m long in order not to exceed the permissible voltage drop.
GOBA conclusion: the 3 metre rule carries the reduction in the control cabinet
Inside a control cabinet, DIN VDE 0100-430 permits a reduced cross-section without its own fuse when four conditions come together: the section stays under 3 metres, it is installed so that short circuits and earth faults are excluded, it carries no branch and no plug connector, and the overload protection sits at its end. If one condition is missing, the section needs its own fuse. Calculate the voltage drop alongside: a 16 A circuit with 1.5 mm² copper may run around 18 m before the 3 percent limit for lighting is exceeded. The permissible current-carrying capacity itself comes from the tables in DIN VDE 0298-4, by installation method, grouping and ambient temperature. The insulating parts that separate and cover inside such assemblies we produce as customer-specific geometries to drawing.
