that it would be an unusual scenario.

Of course, but it stands as just one example of range of possibilities - we've had decades of repairs and alterations to underground metallic pipework done using plastic so there must be numerous other examples of relatively similar situations out there.

I guess my hope would be that the regulations describe what in principle what would produce a satisfactory installation, rather than a specific policy that only mostly works in practice or merely satisfies the 80/20 rule or somesuch.

We could perhaps try some other numbers too - the outbuilding might not be 165m away (enough to get 2 Ohms out of a 1.5mm conductor at 12.1mΩ/m) - around here we're probably more modest and often have the garage just a footpath width away from the house - so say a 5m submain if the main CU happened to be close - so 0.06Ω might be more realistic. Also 50A feels a bit high to me - if we're looking for something that might prove satisfactory in the long term for a conductor in a cable shared with other possibly loaded conductors. The rating for a 1.5mm² in a cable with three loaded cores, method C, is 17.5A - so I could argue that twice that - say 35A might well be unsatisfactory in the long term. So V=IR then gives us just 2.1V - which doesn't feel quite so implausible.

    - Andy.

 that it would be an unusual scenario.

Of course, but it stands as just one example of range of possibilities - we've had decades of repairs and alterations to underground metallic pipework done using plastic so there must be numerous other examples of relatively similar situations out there.

I guess my hope would be that the regulations describe what in principle what would produce a satisfactory installation, rather than a specific policy that only mostly works in practice or merely satisfies the 80/20 rule or somesuch.

We could perhaps try some other numbers too - the outbuilding might not be 165m away (enough to get 2 Ohms out of a 1.5mm conductor at 12.1mΩ/m) - around here we're probably more modest and often have the garage just a footpath width away from the house - so say a 5m submain if the main CU happened to be close - so 0.06Ω might be more realistic. Also 50A feels a bit high to me - if we're looking for something that might prove satisfactory in the long term for a conductor in a cable shared with other possibly loaded conductors. The rating for a 1.5mm² in a cable with three loaded cores, method C, is 17.5A - so I could argue that twice that - say 35A might well be unsatisfactory in the long term. So V=IR then gives us just 2.1V - which doesn't feel quite so implausible.

    - Andy.