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Why do appliances recommendations differ to what a cable is rated for.
In theory you could run two 13 amp devices on a double socket that’s on a 2.5mm spur.
But a single item pulling 16amps is recommended to have 4mm and a 19 amp on 6mm.
IIRC 2.5mm is rated at 21 amps ( before any derating factors).
I usually spec 4mm for radials.
I’ve no insulation so I assume my de rating factor is low, although it’s a fairly long run.
More reason to follow the regs (voltage drop)
Is that based on distance of run?
I see a handy tlc calculator for run. Thanks.
2 items plugged into 2 sockets are very unlikely to be drawing max amps at the same time or if they do not for very long
A 16 or 19 amp connected to a spur WILL draw peak amps and likely for a reasonable period of time jence they spec up.
i often wonder about diversification but as you say is the Time as well as the total draw . So 32 amp fuse doesn’t blow but cable is getting slowly hotter with a big load. If it’s the 30 mins to heat the dishwasher water and washing machine drawing 6kw then no bother but two 3kw fan heaters on for a few hours might be more problematic.
Voltage drop is a problem for motors and the like I assume.
It's called diversity. Some circuits have an expected load of 100% like car chargers and other circuits are less for example lighting is 66%
https://the-regs.co.uk/blog/?p=223
FYI
Cable calcs aren't just worked out on maximum load but also the disconnection time of the protective device under fault current. The earth cable in twin and earth is smaller than the live and neutral cores so this needs to be taken into account. As well as thermal effects etc
There's a reason that software exists for working this out as it is a time consuming thing to carry out cable calcs properly.
In theory you could run two 13 amp devices on a double socket that’s on a 2.5mm spur
in theory you could, in practice it just doesn’t happen. Which is fortunate, because a double socket is not designed nor tested to sustain such a load. Double sockets are tested and certified for a 20A load. A few are rated at 26A (good old MK).
Cable ratings another story. A single 2.5mm^2 clipped to a wall in open air is rated for 27A. In conduit, buried in a wall is rated for 20A.
Rings are another matter, that’s another play at de-rating a circuit based on a lot of assumptions, mostly safe.
Why do appliances recommendations differ to what a cable is rated for
Because they're not designed based on specifics
Voltage drop is a problem for motors and the like I assume
Voltage drop is a problem for any circuit but the allowances change:
Lighting 3%
Other 5%
Motor inrush can exceed those %