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I am assuming your heaters are mains voltage 110 or 230VAC etc?
My 4 x 600W heaters are 220VAC (around 2.5A each) and are simply wired in parallel and then connected to the load side of the 25A solid state relay (10A load)
I have also earthed the heat bed for safety and used high temperature silicone mains wire due to the high temps.

Hope this helps.