Observing and complying with standards and VDE regulations is of major importance in electrical installation - in everyday practice, however, this is often not easy to abide by.

A much discussed point, for example, is the so-called "multi-wire clamping". Is it permitted to connect two conductors at one and the same clamping point? In general, it is certainly not sensible, since two conductors at one clamping point make clear identification in the switch cabinet itself and in the corresponding switch diagrams more difficult. Nevertheless, extra connections, operating or control elements often have to be installed during maintenance work, or a forgotten connection added. The space for an extra modular terminal block is missing and this raises the question as to whether a multi-conductor connection is permitted in this case.

The answer to this question depends of course not on how precarious the particular situation is, but on the relevant standards for modular terminal blocks and clamped connections.

In EN 60 999 (Safety requirements for screw-type clamping points and screwless-type clamping points for electrical copper conductors), for example, a difference is made between clamping points with screw-type clamping and screwless-type clamping points, as the title suggests.

In section 7.7, special attention is drawn to the fact that each conductor must be clamped individually in the case of screwless-type clamping points. In EN 60 947-7-1 too, (modular terminal blocks for copper conductors) it is written that a multi-conductor connection is only permitted in the case of screw terminal blocks. This, however, is only permitted on condition that the manufacturer states type, size and number of conductors that can be connected simultaneously.

EN 60 947-7-1

4.3.4
Rated cross-section
"The rated cross-section of a terminal block is the value of the conductor cross-section that can be connected, as stated by the manufacturer, and which certain thermal, mechanical and electrical requirements refer to.
The rated cross-section must be selected from the standard cross-sections given in table 1."

4.3.5
Rated connection capacity (of a modular terminal block)
"The rated connection capacity of a modular terminal block is a range and/or number of rated cross-sections for which the modular terminal block is intended.
For modular terminal blocks with a rated cross-section of between 0.2mm² and 35mm², the smallest range given in table 2 applies. The conductors can be rigid (single or multiple wire) or flexible. The manufacturer must state the kind of conductors, the largest and smallest cross-sections, and if applicable, the number of conductors that can be connected simultaneously in each terminal block. He must also state all the work required to prepare the conductor end."

EN 60 999 / VDE 0609

7.7
"Screwless clamping points for the connection of two or more conductors must be designed and constructed in such a way that 

• each conductor is clamped individually 
• the conductors can be connected or disconnected simultaneously. 

It must be possible to reliably clamp any number of conductors up to the predetermined maximum."

7.11
"Clamping points must be designed and constructed in such a way that they reliably clamp the conductor between metal surfaces."

In brief:

• Multi-wire clamping is allowed for screw terminal blocks (provided such information is given by the manufacturer) 
• Multi-wire clamping is not allowed for screwless clamping points (e.g. spring cage terminal blocks)

To meet the requirements of practice, an efficient way had to be found to make it quick and easy to make reliable double connections.

The solution is the TWIN terminal block.
In the screw terminal block series, the TWIN terminal block is a further powerful variant, and as explained above, it is an absoute must for a spring cage terminal block series.

On the switch cabinet side, it has two independent conductor connections and can be bridged in the center of the terminal block even with aligned feed-through terminal blocks. As far as the characteristics of conductors used are concerned, there are no restrictions for the TWIN modular terminal block.
Different cross-sections with different structures can be clamped without risk. The TWIN modular terminal block is the alternative to the 2-position feed-through terminal block if a multi-wire clamping has to be avoided or is not possible due to the modular terminal block technology.

The TWIN modular terminal block variant provides the much needed third clamping point to guarantee reliable clamping for "2+1" conductors.

The TWIN terminal block is not only convincing in the area of potential branching. If switch cabinet extensions are forseeable or possible, the use of TWIN modular teminal blocks is also a good solution.

Spring cage terminal blocks
A switch cabinet equipped with spring cage terminal blocks allows no multi-wire clamping. Due to the tolerance of the cross-sections, 100% contacting of both conductors with the current bar is not guaranteed.

QUiX quick connect terminal blocks
For the same reason, even QUiX quick connect terminal blocks can only accept one conductor per clamping point. 

Screw terminal blocks
The use of screw terminal blockss allows multi-wire clamping in accordance with the information provided by the manufacturer. There are, however, some restrictions: A clamping point can only be occupied with 2 conductors if they both have the identical cross-section. Moreover, both must be either rigid or flexible. A mixture is not possible because the difference in structures does not allow even contacting with the current bar. For space reasons, clear labeling of a clamping point with 2 conductors is very difficult.

TWIN terminal blocks
Perfect potential branching - professional and in line with standards.
By means of the third clamping point, various cross-sections with different conductor structures can be combined. The TWIN modular terminal blocks do not need any more space on the mounting rail than the corresponding feed-through terminal block (same design width). All of the TWIN variants have these benefits as well as sufficient labeling options for inputs and outputs of the terminal blocks. The TWIN terminal blocks are available in the proven screw and spring cage technology and now even in the time-saving QuiX technology.

Conclusion: Multi-wire clamping desirable?
No, because clear labeling is very difficult.

Allowed?
For screw terminals only.

Prohibited?
Not allowed for spring cage and QUiX quick connect terminal blocks.

The safe alternative:

If simple potential branching is not sufficient, ...
... e.g. due to conductor loops, potential multiplication is commonly encountered in practice which means that several connections are routed to one terminal block.

In this event, the so-called double terminal blocks (UDK or ZFDK) are used. The double terminal blocks available in screw or spring cage technology can accept four conductors each on each separate terminal point and connect them all via the same current bar. The use of such double terminal blocks obviously saves a great deal of time and money because it is no longer necessary to bring individual feed-through terminal blocks to the same potential by first inserting bridges.

The double terminal block: Four terminal points with one potential

Possible connections for 2 conductors of the same cross-section and same structure in single-wire to fine strand conductor design: