Why is my immersion heater wired with 4mm cable?!

In our 1970s house, they mostly used as little copper as possible in the original wiring. Cables pulled very tight, lights wired in singles, etc.
However, the immersion heater circuit is wired with 4mm T+E. I was replacing it with 2.5mm, and just thought I'd check this is OK? Searching, the consensus on here seems to be that even 1.5mm is OK- ish, so what were the original installers thinking?
It's clipped direct (actually buried in plaster on blockwork), and run through a stud-partition wall. It's not grouped or bundled with anything else.
Doesn't really matter - just wonder if there's a reason to be paranoid that I've overlooked?!
Cheers, David.
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1970s is too late for imperial cable, which can look like like 4mm.
4mm FTE and 2.5mm FTE both have 1.5mm CPC (earth). The difference is in Current Carrying Capacity.
Unless the cable is run through insulation or grouped with other cables or subject to high temperature there is no need to use 4mm cable for a single 3kW immersion element. Some even wire short immersion runs in 1.5mm, but most prefer 2.5mm at least when DIYing.
It may have been done purely so it could be converted to a 32A radial at a future date if the immersion was dispensed with, alternatively someone perhaps planned a DIY twin 3kW element shower in pumped store, no boiler. It may have been done purely because someone had some left over from a previous job - simple as that. Old school sparks if doing a favour for someone did use somewhat uprated cable so as to provide "future flexibility" and of course a bigger cable runs cooler and lasts longer (although 3kW run even 5hrs a day is not going to stress even 2.5mm much).
So you can use a 2.5mm cable providing the upstream circuit protective device is 20A or lower. Alternatively you can buy 4mm FTE cut-length from TLC online, it is not uncommon but due to lower usage can be more expensive.
Check the terminals on the immersion element & that a safety stat has been fitted (they can be changed if the slide-out type). Usually neutral is a bit charred, Screwfix do 2.5mm Butyl cable in 1m cut lengths (note pre-cut) quite cheaply if you want to over-engineer or the cable runs in a warm location (by the tank).
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