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Polygon
I'm not an electrician but I understand you need to down rate the current as they can't dissipate heat as well. The derating factor can be about half depending on the cable material and insulation depth. For something like an immersion heater I'd get it checked by an electrian with the tables.
Or you could search for "APPENDIX 1: Correction Factors for Sizing of Cable Conductors" which might be right and hope your house doesn't burn down.