The old owners left an unfinished electrical box that was the to be outlet for an EV car charger and I am thinking of gearing up to finish it( 3 foot run from the panel) however the only thing that is a little confusing to me is what kind of strain relief should I use on both sides as in this setup as there is a tube connecting the electrical box to the main panel.
Thank You
You don’t use any strain relief. Wires are supposed to be able to slide completely freely through conduit.
You terminate the wires inside the junction boxes, of course, and they don’t move around when you’re not working on the wiring. Any strain happens to the device cord, which is designed for movement.
The moment you put any wires in that pipe, you must put a lid on the box. This is mandatory!
The lid can either be a blank (they're like 20 cents), or can have a cutout for a socket aka receptacle.
For instance if you are connecting the EVSE with a cord and plug, you would choose a lid that has a cutout for the socket that you need.
Or, if you are hardwiring an EVSE, you extend conduit from another side knockout to the EVSE itself. At that point the box is only used for splices or a pass-thru, and you would use a blank cover.
The lid, and cover plates if used, are the thing that keeps curious fingers from getting shocked.
Your box is a 4-11/16" square box. (Also called 120mm even in the States). It was a good choice because it has lots of room. For instance you have room to splice #6 wire (65A, or 52A continuously). If you're not using it for an EVSE, it'll also fit two GFCI sockets comfortably. Or one and an EVSE socket. Possibilities are endless. You may have trouble finding a good variety of lids with socket openings for that box, if so, try a real electrical supply house not the box stores.
As far as strain reliefs, let's be clear. Flexible cord coming out of the EVSE must have a plug on it. It cannot go either into a knockout on the box, or into the face of the box and into that tube. That is not how wiring is done.
The wiring that gets used in that tube is called THHN and it is individual wires. You need to use:
The rule letting you re-mark white wires to be hot does not work with individual wires.
The wires just lay in the pipe. There is no need for strain relief unless the wires go 4-5 storeys vertically, we're talking the kind of building Bruce Willis likes to run around barefoot, not a 3-story house.
Note the ground dimple in the lower left. Code says with metal boxes, you must ground to the box FIRST. So put a 10 AWG pigtail on that ground screw, and pigtail it to the cable(s) coming in.
If the cover holds a receptacle, and the cover has flats where the screws attach to this box, then the screw, flats and cover will carry ground to the receptacle. You don't have to wire ground to the recep.
An EV charger is a box with internal electronics that communicates with the car about how much current it is allowed to draw. A hardwired wall unit with appropriate circuit lets the car charge more quickly than with the measly 1.5KW that the portable charger plugged into a standard 15A outlet is capped at.
From the electrical box you would have another conduit into the EV charger or depending on the existing location of that electrical box you might be able to put the conduit directly into the charger box.
No strain relief necessary inside the conduits. But you should put in a bushing on the threaded end of the conduit to avoid the sharp lip of the conduit being able to cut into the insulation.
