Safe method to use either 120 or 240

What I really want to do is get started and I won't have 240 4 wire available for several weeks and I want to get started on my control panel so that I can use a 120v RIMS tube I bought that has a 240v 4500W element in it which yields about 1125W at 120v.
I want to make the control so that I can plug the RIMS tube in and control the temperature at the exit and later when I get 240 available then either rewire the RIMS tube or just jump in with both feet and put a 5500W element in my pot.
Can this be done without wasting a lot of money for the flexibility. In my mind I can do it, but I need some encouragement.

Usually, everyone is switching one leg of 240V to the element. Build it as if you were then just switch the hot of 120V through the SSR. Should work...

You could design a 240 4 wire panel with 4 wire element connector and connect the 120 or 240v element as required.

I’ve looked at several 220V wiring diagrams and they all have line 2 going through a contactor to one side of the element and nowhere else. The neutral goes everywhere. With 120V there is no line 2 so neutral needs to go to the side of the element that line2 goes to.
I’m beginning to get an idea here that if I resign myself to hardwiring the supply cord then when I have the 120V cord installed I can just jumper the neutral to line 2 and everything should work, only slower. Then I just have to remember to remove the jumper when I wire in the 220V cord. I don’t need any extra contactors are anything. Is this right?
On the other hand, instead of a jumper between line 2 and neutral, I could just wire from the contactor feeding the line 2 side of the element to neutral and then if I forget to move that wire when I wire in the 220V cord, there is no smoke, it just won’t heat any faster.
Does this make sense?

You say you won't have 240v available for 'several weeks' - how quickly do you plan to have this panel built and operational? I can tell you it takes most people on here several weeks to source parts, finalize a design, source more parts, build, and test the whole thing. You may be putting a lot of effort into coming up with some hybrid design that's only going to be useful for a week or two, if at all. If your end goal is 240, and it's not years away, why not just build to that and do it right, one time, the first time?

Handsaw said:

I’ve looked at several 220V wiring diagrams and they all have line 2 going through a contactor to one side of the element and nowhere else. The neutral goes everywhere. With 120V there is no line 2 so neutral needs to go to the side of the element that line2 goes to.
I’m beginning to get an idea here that if I resign myself to hardwiring the supply cord then when I have the 120V cord installed I can just jumper the neutral to line 2 and everything should work, only slower. Then I just have to remember to remove the jumper when I wire in the 220V cord. I don’t need any extra contactors are anything. Is this right?
On the other hand, instead of a jumper between line 2 and neutral, I could just wire from the contactor feeding the line 2 side of the element to neutral and then if I forget to move that wire when I wire in the 220V cord, there is no smoke, it just won’t heat any faster.
Does this make sense?

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A 4-wire 240v power cord has 2 hots, 1 neutral, and Gnd. If you design your panel with a 4-wire output connector then you can connect the appropriate 3 wires on the elements power cord to the element (i.e. Hot, Hot, Gnd for 240v OR Hot, Neutral, Gnd for 120v) No sparks, no magic smoke.

I have a 220V 3 wire dryer receptical near where I want my spa panel. Can I just wire in a 3 wire dryer plug and get the ground from a cold water pipe? The whole house is plumbed with copper and there is a jumper around the water filter.

The pipes are grounded but they may not be a ground... there are some pretty specific requirements for what can serve as a ground under NFPA70.

Well, if I disregard meeting code, is it safe to use the cold water for ground? It was the norm for years and years. I think the plumbers were at risk if someone put in a water filter and didn't put a jumper around it. Maybe the plumbers and anyone else who grab hold of the pipe.

I can assure you that they don't take things that used to be perfectly safe and pull them out of code books just for kicks...

You can run a separate ground right back to the source. How much further is that?

I hadn't thought of that. How big does the ground wire need to be? Is it just there to give the GFCI something to measure to?

Handsaw said:

I hadn't thought of that. How big does the ground wire need to be? Is it just there to give the GFCI something to measure to?

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It's there to carry fault current and trip the breaker if there is a short. The equipment ground is sized by the breaker protecting the circuit.