Hi Chris, 12 gauge wire should be good for the heater. If the wires on the element are exposed, you could create some type of cover so you can get to them later. I think I would wrap with electrical tape and the put a plastic cover over them too. Be creative :). We're you able to get all your parts?
Hi Chris, You have done well. Either wall wart would be fine.. What ever fits your layout better will work.
The installation of the SSR on the heatsink isn't rocket science. The Thermal Grease is the stuff between the SSR and heatsink. I put a spot in center and smash two together. Basicly this just thermally connects the two pieces of metal so heat flows from the source to the heatsink quickly to help cooling. Once you have this system wired up and you turn it on. Don't toutch the contacts with power, but feel the back of the SSR or the heat sink. It will be warm but shouldnt be too hot to touch. If it is really hot, then think about mounting it outside of an enclosure. Using the 40 amp ssr vs a 25 amp will mean you are using a device capable of more power. This means it will create less heat and should be less problem there. Like I said. Wire it up and see how much heat it generates. More than likely you will be able to put in the 8x8 encloser and be fine.
Grounding. Always ground anything A/C powered. Water and A/C are dangerous. Good grounds are essential. That said. The Controller is only touching the water with it's temp probe and then a DC wire from wall wart is going through it. If there is a ground screw or terminal, then use a power cord with a ground and ground it.
As for the wall wart, no worries once it is converted to DC.
The heater element is another story. This is going to bring 120 volt directly to the element and in immediate proximity to the water. There is no ground connection on the element to tie to. So you can address this in a few ways. The nut portion of the element is steel and attaches to the pump/filter housing you are installing the heater into. If there is a way to connect the ground to this, that would work. If this doesn't prove feasible, then go to the nearest piece of metal that has a bolt on it that you can put the ground wire under. Pump bolt, mount foot what ever. Just imagine if there is a short from either wire going to the element, the electricity needs to find a fast path to ground. Otherwise it energizes the water near it, kills your fish and potentially you. So use the ground wire to carry this to the safe ground of the plug. Then ensure that the element is plugged into a ground fault protected plug.
Little crimp on ends are tidy, but twist the strands of wires together tightly and secure them under the screw and you will be fine.
SSR to heat sink. What ever works. You don't need to crush them together. Just held firmly together. your PC has a heat sink similar to this on the CPU to shed heat. They are held on with springs and work reliably.
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Well done Chris - I've nothing to add to Steve's suggestions - the PC CPU Heat Sink is a very good suggestion.
Send us photo's of your completed box. Hope your fish are soon cured..