Electrical Supply to Multiple Flats

So, by trawling through the regs, the on-site guide, and pestering the local supplier to prise out required information, this is what I came up with. I hope it can come in useful for anybody else.

Supply wiring arrangement to three households

The supplier's three-phase cable comes in to the supply head with it's 100A fuse. From there it is broken into three single-phase supplies, with a short length of 25mm² supply tails connecting to each meter, then some 25mm² meter tails connecting to each installation's isolation switch. As the consumer unit is "at a distance" from the supply head, a Residual Current Detector or Earth Leakage Detector Phase&Neutral isolation switch is required.

We are aiming to avoid using long, expensive lengths of 25mm² cables and supply each consumer unit with smaller cables. When using supply cables smaller than 25mm², you are required to use the next size up from the usual cable size to carry the required current.

If the installation required an 80A supply, the Regs tell us that 16mm² cable has a current carrying capacity of 87A when clipped direct. However, we would need to use the next size up as a supply cable, which puts us back to using the 25mm² we are trying to avoid.

If the installation is designed to require a 60A supply, the Regs tell us that 10mm² is rated at 65A when clipped direct. So, we can use the next size up of 16mm², so avoiding the long, expensive lengths of 25mm².

The supply cables then have an installation rating of 65A, as opposed to a manufactured rating of 87A, so they need to be protected by a 60A overcurrent protector. Combined with the requirements due to the consumer unit being at a distance from the supply, this means that each installation needs a 60A P&N RCD or ELD. Note also, that the overcurrent/isolation needs to be "nearby" the supply meter. As a rule of thumb, no more than a couple of feet away.

Earth cable feeding a consumer unit is required to be no smaller than the next size down from the supply cables. Each installation is fed with 16mm² cables, so the earth cable can be 10mm². The bonding cable between the incoming supply and the incoming gas and water pipework has to be at least 10mm² as normal.