Secondary consumer unit from SWA cable

[pc1966]

Others have already covered the main points. However, on the specifics of feeding the SWA cable you don't need or indeed want a 30mA RCD/RCBO at the supply point as it is just asking for spurious trips, and due to the earthed armour you are already protected from shock on a cable penetrating accident (e.g. nailing a buried section).

But how you protect the new sub-main against faults or overload depends on the characteristics of your incoming supply. If it is of adequately low Ze (fault impedance) then a fused-switch is you best choice and you do get metal bodied ones that the SWA cable can be directly glanded on to.

If you cannot meet an adequately low end of sub-main Zs to achieve fast enough disconnection times (5s for sub-main on TN earth, or 1s on TT earth) then you would need a RCD, but that could be a 100mA or 300mA delay type so you achieve selectivity with downstream RCD/RCBO in the garage CU and don't have everything going off on a single fault on a final circuit.

Finally if going with a switched-fuse you might want to consider your supply fuse (in the DNO cut-out) both for the above points about the total load you can safely draw, and also so you can chose the sub-main fuse to be selective with that so a major fault such on the garage circuit will not take out the home's supply and need a call to the DNO to replace the sealed fuse in their equipment. For BS88 series fuses that is usually achieved at a 1.6:1 ratio, so if your main supply is 100A then a 63A sub-main fuse should be totally selective with it, if your supply is 80A then a 50A sub-main, etc.

You could use a MCB to feed the sub-main but generally they have much poorer selectivity with downstream faults, and going for a high magnetic trip curve to help mitigate that normally results in a harder to meed end of cable Zs, compared to a similar rating of fuse.

 

[tvrulesme]

May seem like a minor point, but please realise the difference between an EV charge point, and an EV charger.

An EVCP presents the supply "13A", 16A 32A etc, and uses the charger within the vehicle, so the charge is limited by the lower of the EVCP or vehicle itself (eg a vehicle with a 5kW charger internally could only charge at ~20A when on a 32A EVCP or 16A when on a 16A EVCP)

An EV charger is usually something like 50kW or 75kW plus and provides DC directly to the vehicle which bypasses the charger within the vehicle.

Thank you Julie. Shows my naivety. I thought the name was interchangeable. I do indeed mean an EVCP. The ones I have been looking at are 7.4kw 32 amps

 

[Julie.]

Thank you Julie. Shows my naivety. I thought the name was interchangeable. I do indeed mean an EVCP. The ones I have been looking at are 7.4kw 32 amps

Most cars still have onboard chargers around 7kW, but PHEVs are usually around 3.3kW

There are newer vehicles with 11kW chargers onboard.

 

[timhoward]

Make sure any plan fully considers the EVCP as this as it can affect several factors of this design.

I have a mini electric on order which won't arrive until Jan 2023 so I have a bit of time. if it has to be 13amp would be a shame but so be it

You certainly don't want a 13amp EVCP. (In my view these should really be banned!)
I'd be fairly certain there will be a way to fully utilise a 7Kw EVCP but you'll need your sparks help to arrive at this. There are solutions available for some charge points to wirelessly connect a current monitor.