wiring on heat exchanger?

Sounds like two different heater cans and the element powers could be different which means the wire sizes would need to be appropriate. The way I would do it is to use a multi-meter to check the element resistances and positions, work out the kW power at 240Vac and check the wire sizes. It sounds like the new can has three elements and that two could be linked together in parallel to make one element, but without seeing it I don't know. If this is a spare part from Gainsborough sent as a direct replacement then they should be able to tell you how to wire it, if not then I personally wouldn't swap it as elements powers are possible different and the wire routing in the shower needs to be changed.
Good luck

Paul M
May 2009