If you run the star delta connection on a 12kw element, your element will run at ~3.6Kw (you will have LOW watt density) I run the delta configuration.
First, I think you should consult an electrician familiar with three phase...
To get 2 elements without powering the 3rd element one has to split the connection, as I mentioned. A competent electrician would take one of the connection bars off and run an extra wire, one from each contactor to each separated lead... when they do this, they essentially have a dual element and a single element that happen to share a single wire(they can run 5 wires and have them totally separate.) They will normally use two 3-pole contactors, but they could use a 2-pole and a 3-pole(I wouldn't)
top one is 5 wires to the element, bottom is 4 wires to the element.
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