The electrofusion control units are meant for the welding of thermoplastic pipes (e.g. made of PE-HD, PE80, PE100 or PP) when used with electrofusion fittings (that have an input voltage of less than 48 V).
The range of fitting dimensions for which an electrofusion control unit can be used depends essentially on the power consumption of the used fittings.
Since the power consumption of the fittings is different for different fitting manufacturers, it is not possible to provide a general rule which covers all the possible fitting dimensions.