Are you saying that such a load should never be supplied via a CU (OP has called it a fuse box as well)?Opinions may vary, but I was saying that I personally wouldn't be all that happy (i.e. would not deem the situation to be particularly 'acceptable {to me} and satisfactory) to have such a load supplied by a 'standard domestic CU' (and the OP seems to imply that it is the only one in the house), even if I lived in Chatsworth.
If not, what is the maximum you would favour on a 100A CU.
If there were two, would you think differently?
Or a small CU just supplying the heater?
I suppose, in that respect, a small house with a 100A supply and very little else would have more spare capacity than a large house with the same supply.
I just got the impression everyone was poo-pooing the OP's question and thought it was a silly thing to ask or contemplate.