As the title states, I am building a control box and need to run 2 elements at the same time off of 1 PID. Each element will have its own SSR and contactor. I did see one of PJ's schematics that did something similar, but with 120V elements. My question is, is it better to run Line 1 through the SSR for one element and Line 2 through the SSR for the other element, to keep it more balanced?
2, 240V elements, 1 PID controller
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WI_Wino
Well-Known Member
I don't think so. With electricity, when the switch is open it's open and no current flows. There isn't "pressure" on it like a hydraulic hose (at residential levels anyway).
240v uses both L1 and L2 at the same time so there is no concept of load balancing with the two elements.
I kind of figured, but wasn't sure, something else was going on because one side of the element was connected directly to the line, and the other through the SSR. Is there some convention that states to do it one way or the other?
Thanks for the replies!
Both of the lines in a 240V system are identical, except for being 180˚ out of phase with each other. Current flows in a loop, so all you have to do is break any conductor in the loop to stop current flow. So, the SSR in a 240V circuit can be on either hot line. If there are circuit branches, then there can be many loops, and things can quickly get more complicated.
For safety reasons, power disconnects must open up all hot lines. So, you need 2 pole switches/contactors for 240V circuits, but only single pole switches (on the hot line) for 120V circuits. It would be possible to turn a 120V circuit on and off by switching the neutral line, but that would violate the safety principle, and would be a code violation.
Brew on
Not splitting hairs here, but in the states I believe there is only 120˚ between L1 and L2. Not that it matters in this conversation.
Nope, in 240V systems, the two hot lines are 180˚ out of phase. In 3-phase systems, the phases are 120˚ off from the others. Each of the 120V hot lines is taken from one end of a center taped transformer winding, and there is a 240V difference between them. The neutral is taken from the center tap, and is grounded at/near the service entry (but nowhere else.)
Brew on
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rudylyon57
Well-Known Member
Why not just run parallel control voltage from your PID to each SSR so they "fire" on and off together? I assume the PID can handle the added control current...maybe not? Split L1 and L2 to each SSR in a parallel manner also. They will be "balanced" since both elements will see the same voltage.
I am going to run parallel control voltage to each SSR, but my question was, should I run line 1 through both SSR'S or run line 1 to the first, and line 2 to the second. Are you saying run line 1 and line 2 to each ssr? If so, I think that will go boom.
<<<SNIP>>>
After talking to a licensed electrician that works for me, you are indeed correct, the transformer creates the 180 degree phase shift.
rudylyon57
Well-Known Member
I am suggesting you run L1 to SSR terminal 1, load to SSR terminal 2 and load back to L2 for each SSR/heater. It does not matter if you run L1 or L2 to the SSR, as long as you have a load on the other side of the SSR that is connected to the opposite line.
I think we are on the same page, I guess I could have avoided all of this confusion if I just would have drew it out and posted it
