More information is required for an accurate answer.
Such as
KW rating of immersion heater
Length of proposed cable run
Ambient temperature
How will the cable be installed. Eg, trunking, clipped direct etc.?
Is the cable grouped with any other circuits?
Is the cable ran in thermal insulation (how thick)
What type of protected device will be used?
There are many factors to consider to attain an accurate answer.
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