Kettle 3kW heaters 2 x 3 kW so looking at 9 kW so around 40 amp, so bang it into my software.
View attachment 273197
So it would in theory need 10 mm² to ensure the volt drop was within the limits for the lights in the shed. Would I use 10 mm² no I would not, I would use a 32 amp MCB not 40 amp, and I would use 6 mm² cable, as in the main LED lights can stand a bit of volt drop, but rules say lighting allowed 3% volt drop, so "Hypothetically speaking" it needs 10 mm².
In fact since I don't want to pay the LABC their fee, either I would use a scheme member electrician to do it for me, or if in England would use a 13 amp FCU for the supply and cross my fingers it does not rupture the fuse. At 13 amp we get
View attachment 273202
So 2.5 mm² cable, and use a 13 amp fuse, remember to turn off heater before using kettle, since a caravan seems to work with a 16 amp supply with 2.5 mm² cable at 25 meters long, I would say 2.5² cable is ample, and the heater in the caravan could be set seem to remember starting at 750 watt and highest setting was 3 kW, and the central heating even with snow on the ground was ample on the 750 watt setting.