Well, before you panic, ask do you really need to ?

Is the cable showing signs of thermal distress?, do you actually intend to run all the circuits in the annex flat out for long periods ?

Assuming it is not buried in insulation, that  cable will be OK to run at about 50 A (about 12kW) all day, rather more on a cold day, and a lot more again for short periods of a few minutes.

What size is the fuse at the house end, and has it ever blown ?

You may  suffer a bit from voltage drop issues, flickering lights etc  if you do load it like that, , but this is not really a dangerous issue, more of an irritation.

Then if you decide there really is a problem, you could reduce the fuse rating at the house end to match or be less than the cable rating, cutting supply if there is excess demand,  or you could run a new supply along side, or remove some of the heavier loads from the annex.

Mike.

It MIGHT be OK.

More detail needed, in particular, what size fuse or circuit breaker is installed at the supply end ? Under favorable circumstances 6mm SWA MIGHT be acceptable for 63 amps, and is almost certainly acceptable if the load is short term or intermittent.

My buddy, an electrical contractor, built his own house. 3500sq feet, raised 4 children. A temporary supply was brought in from the old farmhouse about 50m away when the family moved in. The intake was a neat arrangement with two 24 way consumer units all supplied by a 2.5mm2 twin with earth cable run from the farmhouse up a ditch and into 30A open choc-bloc connectors before dividing into each consumer unit. I guess that was the situation for about 15 years before the DNO cable was connected. By that time the kids were grown and almost ready to fly the nest!

John Boy said:

So my question is how do I protect the cable in this situation?

Simple answer - for protection from overload - the same as for any other circuit - make sure the rating of the overcurrent protective device that supplies it isn't greater than the rating of the cable (as installed). Other methods are also possible (such as taking into account the nature of the connected loads, or downstream protective devices).

   - Andy.