I have a toroidal transformer with mains input and 24V output, which I want to mount in an IP65 ABS plastic case. The transformer comes with two rubber and one metal disc for mounting, along with a metal bolt to connect them.

As this project is intended for outdoor use, I do not want to compromise the seal of the case by drilling a bolt hole into it. Is there a recommended solution to secure the transformer in such cases?

On a related note, where should I connect my earth lead? Given that there is no metal case, the only place I can see is the metal disc/bolt for mounting the transformer.

Edit: Here is a schematic of the general arrangement mounting schematic

  • \$\begingroup\$ A correctly sized bolt with an appropriate set of washers will be spray proof. If you're paranoid you could put a gasket around it. If trying to meet a standard, you should mention that. And the transformer may have a ground connection on the core somewhere, which would be best given the circumstances. \$\endgroup\$
    – user39962
    Feb 28, 2015 at 7:18
  • \$\begingroup\$ How do you intend to get the wires through the case? \$\endgroup\$
    – Dave Tweed
    Feb 28, 2015 at 16:17
  • \$\begingroup\$ @DaveTweed As per the added schematic, the wiring will be at the bottom, with grommets, where I imagine it would be pretty hard for rain to get in. However, a great big bolt hole on the back facing the weather doesn't seem as safe. \$\endgroup\$
    – DaveP
    Mar 1, 2015 at 10:17
  • \$\begingroup\$ @SeanBoddy There is no particular standard, just the ill-defined target of being "weatherproof". I guess this is a lesser standard than IP65 :-) \$\endgroup\$
    – DaveP
    Mar 1, 2015 at 10:18

2 Answers 2


The following is with regards to how waterproof enclosures are installed in industrial environments, where IP56 is the usual minimum standard (if not IP66 or IP68.)

With regards to waterproof grommets for wires - the correct name for these is a 'cable gland' and they are available from IP56 up to IP68 ratings. The metal versions last better than the plastic versions.

Pay attention to the minimum and maximum wire sizes. Cable glands achieve waterproofing by compressing a rubber washer to squeeze around the cable, and this sealing is not effective if the cable is too small for a good fit.

This example is good for cables with outer diameter 2mm - 5mm. It costs about $2.00 in small quantities.

enter image description here

With regards to mounting the plastic case, itself - well designed outdoor enclosures have the mounting screw holes outside the waterproof sealed compartment, so that the screw holes do not allow water to enter.

B&R's Polynova PS is an example of such a box. The box also comes with internal screw holes that you can use to attach things to the inside, again, without compromising the weatherproofing of the box. Usually, you would screw a piece of sheet metal to the back of the box (the 'mounting pan') and then attach other things to the mounting pan. DIN rail is also popular.

enter image description here Image from B&R Polynova PS brochure

Cheaper enclosures may not be so luxurious. The Clipsal 56 series is an example of a box intended to have screws put directly through the back of the box, in the waterproofed area. In this case, it is necessary to use small plugs to ensure waterproofing of the screw holes.

The plugs also provide double insulation between live parts in the box (maybe 240 VAC!) and the screws, which prevents live voltages from appearing on the metallic surface you have mounted the box on.

One supposes that if you are drilling your own holes and don't care about electrical insulation, you could just put a big bolt through the back and use an appropriate rubber washer to provide waterproofing.

enter image description here Image from installation instructions for Clipsal 56CVI switched socket outlet assembly

Edit: Found a sample junction box WISKA 1010 lying around at work, which is an example of a 'deluxe' outdoor enclosure rated to IP66. I don't have any prices for these, but I don't imagine they are cheap.

Outside of box enter image description here

Inside of box - note numerous mounting posts on the bottom of the box, for screwing down a mounting plate. enter image description here

Detail of mounting hole - right at the bottom of the hole. No path for water to enter the waterproof part of the box. enter image description here

Detail of lid gasket - goes around the screw hole. enter image description here

  • \$\begingroup\$ Thanks for the great answer. Do these cable glands only work if the cable cross section is circular? \$\endgroup\$
    – DaveP
    Mar 5, 2015 at 11:53
  • 1
    \$\begingroup\$ @DaveP: You can also get glands for flat cables, i.e. elfa.se/elfa3~tr_en/elfa/init.do?item=55-190-40&toc=19922 . However if the cross-section is concave i.e. 'figure 8' cable, it is probably impossible to gland it. If you want to use such cable I would suggest using a round gland, which will still provide mechanical strain relief, then applying silicone sealant or epoxy to make the assembly waterproof. \$\endgroup\$ Mar 6, 2015 at 2:44

Does your chosen enclosure have any internal mounting posts? I'd be surprised if it didn't.
If so, then make a mounting plate for your transformer.
Mount the transformer on the plate, then mount the plate to the enclosure's internal mounting posts.


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