Question and answerEXC-QA-187

Can you add extra terminals or conductors to an Ex e junction box?

excodex editorial teamInstallation and inspectionOct 31, 2026

Abstract

Only within the box's certified rating: Ex e terminals of an allowed type, and a total load within the rated maximum dissipated power or the table of terminal numbers, sizes and currents. One conductor per clamping point unless the terminal is certified for more.

Yes, but only within what the junction box is certified for. An increased safety (Ex e) junction box has a rating that limits the heat its terminals and conductors may produce, so that no point inside exceeds its limiting temperature and the temperature class stays valid. Extra terminals must be of a type the certificate allows, fitted as the instructions show, and the total load must stay within the rating. Each clamping point takes one conductor unless the terminal is certified for more.

Short answer

IEC 60079-7 gives every general-purpose Ex e connection or junction box a rating, expressed either as a rated maximum dissipated power or as a table of permissible number and size of conductors and maximum current for each terminal size. The rating is on the certificate, the label or in the instructions.
IEC 60079-14 expects the manufacturer's instructions to state the permissible number of terminals, conductor sizes and maximum currents. If they do not, all terminals are assumed to be loaded at the same time.
Only Ex e terminals of a recognised type, mounted as the manufacturer specifies (rail, end plates, spacing), keep the creepage and clearance distances the certificate relies on.
IEC 60079-7 does not allow a terminal to take more than one conductor per clamping point unless it is designed and assessed for that. IEC 60079-14 also forbids two conductors of different sizes in one terminal unless the manufacturer's documentation permits it or they are first secured in a single compression ferrule or by another method the manufacturer specifies (2007 edition, 11.2.3).
Adding terminals or conductors beyond the rating is a modification; IEC 60079-19 says terminations must not be modified without reference to the manufacturer.

Why the rating exists

In an Ex e box there is nothing that sparks in normal operation, so the remaining risks are hot spots and failing connections. IEC 60079-7 notes that in a box containing only terminals, most of the heat comes from the conductors connected to them, not from the terminals themselves. Every extra loaded conductor adds heat to the same air space. The box was tested with a "worst case" terminal and conductor arrangement to find how much power it can dissipate before the limiting temperature is reached. That figure, or a table derived from it, is the rating.

A draft IECEx decision sheet on terminal ratings (2017) makes the same point from the other side: in large boxes, very few terminals may carry their full rated current, and terminals and cables may need to be de-rated. The rated current printed on the terminal block is not automatically the current allowed in that box.

How to check whether you can add terminals

Maximum dissipated power method. For each terminal, add the resistance at 20 °C of the terminal (from its Ex component certificate) and of its conductors, each conductor assumed to be half the internal diagonal of the box long. Multiply by the square of the maximum current. The sum for all terminals must not exceed the rated maximum dissipated power.

Defined arrangement method. Read the table for your terminal size: it gives the permissible number of terminals and conductors, the conductor size and the maximum current. Stay within it.

Example: A box with an internal diagonal of 0.40 m has a rated maximum dissipated power of 12 W (illustrative). Each conductor counts as 0.20 m. A 2.5 mm² plain copper conductor (class 1, solid) has a maximum resistance of 7.41 mΩ/m at 20 °C (IEC 60228), so 1.48 mΩ per conductor. Take a terminal resistance of 0.5 mΩ from its certificate (illustrative). A feed-through terminal with two conductors: 0.5 + 2 x 1.48 = 3.46 mΩ. At 16 A it dissipates 16² x 0.00346 = 0.89 W. Twelve such terminals give 10.6 W. Adding four more at 16 A would make 14.2 W, above the rating, so the box would lose its temperature class. Adding four at 4 A adds only 0.22 W (total about 10.9 W), which fits, provided there is physical space and the terminals are of an allowed type.

Real values always come from the certificate and instructions of the box and the terminals.

What else limits extra conductors

Space and distances. Terminals must fit on the rail with the specified end plates and partitions, and the wiring must not be crushed against the lid or pressed onto other terminals.
Entries. Each new cable needs a certified Ex e gland in an entry the box allows, with the IP rating kept. Unused cores must be terminated properly; IEC 60079-14 does not allow heat-shrink sleeves to insulate unused cores inside Ex e enclosures.
Torque. All screw connections are tightened to the torque specified by the terminal manufacturer.

Common mistakes

Two conductors under one clamp of a terminal certified for one, or a thin and a thick conductor together.
Using an ordinary industrial terminal because it fits the rail.
Counting only the terminal's rated current and ignoring the box's dissipated power limit.
Adding a terminal block to a box rated by table without checking the table for the new terminal size.

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Standards and further reading

IEC 60079-7:2015 (edition 5.0; consolidated 5.1 with amendment 1:2017 is current), increased safety "e": electrical connections, rating of general-purpose junction boxes (clause 5.7, informative Annex E); publicly available as incorporated by reference: law.resource.org
IEC 60079-14 (current edition 2024), installation: conductor terminations, combinations of terminals and conductors in junction boxes, unused cores; the 2007 text, published as AS/NZS 60079.14:2009 (clauses 9.6.3, 11.2.3 and 11.2.4): archive.org
P. Thurnherr (convenor of IEC maintenance team MT 60079-14), "Electrical installations design, selection and installation of equipment, including initial inspection", presentation summarising IEC 60079-14:2024 (permissible number of terminals, conductor connections, unused cores, torque) at the 2024 IECEx International Hydrogen Conference, Singapore: presentation PDF
IECEx ExTAG draft decision sheet, terminal ratings in general purpose junction boxes (2017): iecex.com
IEC 60079-19, repair (current edition 2025); 2006 edition as IS/IEC 60079-19, Ex e terminations: law.resource.org
IEC 60228:2023 (edition 4.0), conductors of insulated cables: maximum resistance of class 1 solid conductors at 20 °C (Table 3 in the publisher's preview): standards.iteh.ai

This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.