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The error is telling you that you’re providing a string value when the parameter is not a string. I’m guessing your parameter takes a number but you’ll have to confirm that yourself.
There are custom nodes for dealing with circuits (MEPover and OpenMEP are usually good options to start with) or you can always use Python and the Revit API to do exactly what you’d like.
Create Method (Connector, ElectricalSystemType)
Create Method (Document, IList(ElementId), ElectricalSystemType)