How to mount a toroidal transformer in a waterproof plastic case
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.



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&amp;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.


Image from B&amp;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.


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


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


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


Detail of lid gasket - goes around the screw hole.