New supply to my garage

This is not a diy job, a few cable calculations to be made, method of cable installation, distance, RCD protection needed, main earth, and bonding arrangement & size. You would be much better ( and safer) to engage the services of a registered electrician as the sort of advice you are seeking is not possible without a site visit.

You need to employ the services of a qualified & registered electrician who would know what was required, the work you are describing falls under part p and requires a full installation certificate with building control notification. As a side note, if you need to ask these types of questions then you are clearly not qualified or skilled to do this type of work and therefore should not attempt it.

Hi there, You will need to run a 6mm SWA cable direct from you fuse-board. It will need to be RCD protected so instead of an MCB I recommend using an RCBO. The SWA cable must be glanded off correctly and into a new fuse-board in your garage and from there you can run a lighting circuit, socket circuit, etc.

You require 2.5mm ring circuit for your appliances, with a 32A circuit breaker. You also require 6mm2 armoured cable from your consumer unit to your garage. This will also allow you to have a light circuit.

In this instance you will require the services of an electrician to design the circuit for you. The circuit will have to be designed, installed and tested to BS7671:2018, as well as notifying building control under Part P of the building regulations. If you are not capable of doing this then by attempting to do so puts the safety of yourself and other users of the installation at risk.

That would be difficult to answer without knowing the distance that the garage is from the CU. For example, If you want an 8kW supply to the garage, and it is 10m from the CU then a 4mm 3 core SWA will suffice, but if it is 30m or 40m from the CU, you would need a 10m 3 core SWA. That SWA must also be RCD protected and comes under building regs (part P) so the local council will need to be informed beforehand and they will test after installation. Alternatively, employed a registered domestic installer

I would say disconnect the 2.5mm from the 40amp ASAP, it’s afire waiting to happen. You should not have a 2.5mm connected to a MCB bigger than 20amps. Regarding the new sub-mains the earth fault loop impedance needs to be measure at the main board. You can then calculate the resistance that will be required for the new cable to achieve the correct disconnect time. As the cable is external I would recommend you need to take external influence into consideration, like installing a SWA cable. Within the garage you will also need a consumer unit to create separate supplies and diversity. The entire installation should be completed by a competent person that can certify his/her work.

I would recommend installing a 4.00mm 3 core armoured cable from the main consumer unit fused at 40amp,run to your 2 way R. C. D consumer unit in the garage. Fit a 6amp MCB to feed your lights and a 32amp MCB for the ring circuit feeding your appliances

I would run 6mm SWA coming from the consumer unit in the house going to the garage consumer unit RCD protected with 1 32a MCB for the ring main circuit and a 6a MCB for the lighting circuit and a 40a MCB in the consumer unit in the house for the SWA