The size of bonding conductors where PME conditions apply according to 544.1.1 could be up to 50mm2, depending on the CSA of the distributor's PEN conductor. Whereas the size of the earthing conductor is related to the CSA  of the line conductor (meter tails) OR by using the adiabatic equation (543.1) So we could quite easily have a situation where the bonding sizes are bigger than the main earth.

Where would you get the information about the sizes of the supply PEN conductor(s)? If there were a ring main in the supply, i suppose the CSA should be doubled?

Are there tables where you can measure the OD of the cable sheath to get the required figure?

You can't just open up the head and take a peek

The size of bonding conductors where PME conditions apply according to 544.1.1 could be up to 50mm2, depending on the CSA of the distributor's PEN conductor. Whereas the size of the earthing conductor is related to the CSA  of the line conductor (meter tails) OR by using the adiabatic equation (543.1) So we could quite easily have a situation where the bonding sizes are bigger than the main earth.

Where would you get the information about the sizes of the supply PEN conductor(s)? If there were a ring main in the supply, i suppose the CSA should be doubled?

Are there tables where you can measure the OD of the cable sheath to get the required figure?

You can't just open up the head and take a peek

So we could quite easily have a situation where the bonding sizes are bigger than the main earth.

Not for PME though - they've thought of that - Earthing conductors, where PME conditions apply (542.3.1), have to meet the requirements of 544.1.1 as well.

   - Andy.

Are there tables where you can measure the OD of the cable sheath to get the required figure?

Most cable manufacturers have data available - e.g. https://www.elandcables.com/media/39042/aluminium-concentric-bs-7870-pvc-cable.pdf (there might be a small variation between manufacturers or even batches, but they should be of the same magnitudes). Some might be copper rather than aluminium centre core though, so still some guesswork to be done perhaps.

If there were a ring main in the supply, i suppose the CSA should be doubled?

Rings are common for HV, but not sure they're used much at LV in DNO land. Some large urban areas apparently mesh LV, but I've still never heard of a single LV supply having more than one incoming cable (some are looped, one in and one out to next door, but that's not a ring either). Even if it was a ring in the street, it's only the c.s.a. of the 'spur' cable into the property you need worry about for table 54.8.

   - Andy.