Now that I am thinking about this again, my brain is in "work-gear" and ideas started running through my head. There are hundreds of ways you could design one of these systems, from the most basic to over-the-top, the latter of which was often referred to by a friend/former boss as "creeping elegance." It is obviously more ideal to keep things simpler, but depending on one's own situation, there are always answers and options. A common answer to the question "can you make it do THIS?" was "how much money do you want to spend?"
Most of the initial confusion over the whole subject is related to terminology.
PROCESS VARIABLE (PV) is the actual temp value of the material being sensed.
SETPOINT (SP) is the desired temp value we have configured the controller to achieve and maintain.
These two terms are used ON the controller displays and extensively in the user/operator's manual.
Remember that the CONTROLLER itself will have terminals for:
INPUT POWER, which is usually 120VAC for our purposes and runs the thing;
SIGNAL INPUT, which is the SENSOR INPUT, like a TC or RTD, and most controllers have terminals for both and are configurable for various types of either;
An OUTPUT, which is a LITTLE switch power to the BIG switch (SSR) to your heat load. Read that again - your controller output is a little switch, controlling a big switch, which is controlling your heat load. The controller output is going to be solid state or relay contact. Solid State (magic rocks) is like the SSR we switch the heat load with, in that it doesn't physically open or close a circuit, but its composition allows it to "pass power" or not "pass power," depending on whether you've goosed it with the correct input voltage. A relay contact is a physical switch which is caused to physically/mechanically open or close, allowing power to pass or not pass. Outputs have voltage and current ratings - pay attention.
There are also ANALOG controller outputs, which produce a 0% to 100% signal (rough ananlogy - think dimmer switch in the living room), but is overkill for what we're doing, unless you're controlling a gas burner, but the control system/hardware for that would be complicated and expensive.
A Solid State Relay (SSR), the one we will use to switch our LOAD (the "BIG switch" for the heat source), has an input and an output as well. The input is typically a range of DCV from low to something over 24VDC. The SSR output is capable of switching an AC load and it is important to SIZE the SSR based on the number of AMPS you'll be switching, which is mentioned below. Pay attention to terminal markings and voltage and current ratings when ordering and wiring. THIS is the most common things my students mess up on this lab - putting the wires where they don't belong. SSRs get HOT. If yours fails after a couple years, you may have cooked it. A heat sink and proper clearance around it will help.
Someone mentioned making sure the controller output was 24 VDC:
Note that for a Solid State Relay (SSR) the INPUT on the SSR will be DC. The one I have handy requires 3VDC to 32VDC to "turn it on," or to cause the output side to switch the AC to "ON" going to your load (heating coil or outlet FOR the heating coil). The input voltage sent to the SSR has to be within that range. I've managed to "turn on" SSRs with as little as 1.5VDC, but don't count on it. No "special powers" on my part, just sloppy tolerances when manufactured. On the SSR, not me.
If you happen to have a RELAY output controller, it could still work to control an SSR one of four ways:
1) You have a separate DC power supply in your box, which you would switch to your SSR INPUT with the CONTROLLER OUTPUT. This is more money, more wiring, more heat, another fuse...
2) You could substitute a DC cell or battery and appropriate holder for the DC power supply (see the unappealing "option 1," above) and anything 2V through 32V will power the input to the SSR. A pair of AAs or AAAs, a 9V,... THEN, you have to be mindful of the cell or battery not dying on you eventually.
3) IF there's an onboard DC source - on the CONTROLLER - even if only 5V or 10V. Doesn't HAVE to be 24 VDC, but check your SSR to be sure. You could switch that onboard source through the RELAY OUTPUT on the CONTROLLER to the SSR INPUT.
4) Switch 120 VAC to a 120 VAC, Mecury Displacing Relay (MDR), which will not require a heat sink. These are like $200 though, but I had one and I used it. I just looked and these are something like $200 at Newark now.
BEST BET is to pay heed to the comment about the CONTROLLER OUTPUT being a DC output, but options 2 or 3 aren't terrible if you already have a relay-output controller.
Some things I don't normally see in such discussions:
AMPS!!!
P=IxE "Easy as PIE" to remember, and can be rearranged to determine either I or E as long as you know the other two variables.
My LEE pot is 500W( that's "P"). If I divide 500W by 120V (which is "E"), I should draw 4.1666 AMPS ("I," of course).
I would multiply 4.1666A by 1.25 to get a fuse size of "5.2" (I'd round to 5A) for the heating element or outlet for the pot.
I would size the control fusing separately based upon the ratings for the controller and SSR input, which won't be much.
My PC oven (commercial personal-sized pizza oven) is 1450W. If I divide 1450W by 120V, I should draw 12 AMPS.
You can take it from there.
The branch circuit you are plugging into in your garage IS a short-circuit-protected branch circuit, but you could still cook your element, your SSR, your controller, etc. before the circuit-breaker blows.
Wire size and outlet rating:
First, I use a SINGLE RECEPTACLE. Yeah, you probably already have 15A, 69-cent, ivory duplex receptacles and covers left over from a kitchen project from ten years ago, but if you spend a couple bucks on a 20A, single receptacle and cover, it looks more professional, no one will be tempted to use the "spare" receptacle and you have peace of mind. I never thought I'd run a PC oven off mine when I started, and it's three times the electrical load of my puny LEE lead pot. For what little wiring you have in one of these, and since you are putting in a 20A receptacle, use 12 AWG wire for the load circuit. The controls can be smaller wire, but it's nice to know you have a safety factor/expandability on the load.
NOTE that if your pot is 240 VAC, you will have to FUSE AND SWITCH BOTH LEGS! You have TWO HOTS on "residential 240."
TC type is usually configurable within the controller. I personally use J-Type TCs - because I had some really nice J-Type TCs, suitable for submersion in molten bullet alloy and all my K-Types were better suited to "air." Shop around, pay attention to temp ranges and especially the end you stick in the pot. Costs a bit more for a protected hot-junction (where the wires are welded together at the "hot end" of the TC), which might be why RCBS put theirs in in the heater box? You actually CAN use TC "lead wire" (like an extension cord made of the two dissimilar metals for that TC-type) and twist the two wires together at the hot junction and get it to work, although it's not ideal. I watched a team from a large aerospace company spend two weeks in my shop testing an oven, because only THEY could do it "right enough" for safety, implying that us hicks in Podunk might mess it up. The "TCs" they tossed and which I dug out of the trash can after they left were actually done like that. That's not why I don't fly, but I'm glad I don't.
Hope this helps someone.