I don't see why you cannot split the ring and add further length, but if that requires > 13 A (how much more?) that leaves very little for the rest of the sockets.

Perhaps the architects should have thought of the air-con in the first place?

I need 15 amps. Is this allowable in any reasonably achievable way please? In terms of the architects, the less said the better. The house has no ventilation, huge windows and needs an absolute tonne of AC installing because of poor material and design choices that have created an incredibly unusable house. It was 27 degrees in a bedroom the other day with no heating in use and an outdoor air temperature of 15 degrees!

Could you please specify which ring circuit you are looking to supply from? Are you referring to those servicing the kitchen area or the upper floor etc ? 

could be possible provided that, under the intended conditions of use, the load current in any part of the ring is unlikely to exceed for long periods the current carrying capacity of the cable as installed, assuming the cable has a installed rating of 20 amperes. 

Luckily the house has quite a few rings, it could be the rear top floor ring, or the middle floor rear ring. Both rings are just bedrooms and a study on the middle. 

Or as Chris suggested, split the ring over two 20 Amp radials. But again would need to check current carrying capacity of the cables as installed.

Don't tell anybody, but a 13 A BS 1362 fuse will run at 15 A all day long.

As others have said, start-up currents are a different matter.

But the 13A fuse will run very hot, and will rapidly cook whatever it's installed in.

Indeed, I observed today within a commercial kitchen environment, the sockets utilised to power 13 Amp food trolleys. Upon replacement of the distribution board that supplies these outlets, a subsequent inspection revealed numerous instances of heat-induced damage to the sockets, which were identified during R1+R2 testing. 

Granted it will dissipate about 33% more power, but will the compressor be running non-stop even on a hot day?

I am aware of 433.1.

I see no reason why the ACU should not be connected by a 16 A MCB in a suitable enclosure, not elegant, but compliant!

Granted it will dissipate about 33% more power, but will the compressor be running non-stop even on a hot day?

I am aware of 433.1.

I see no reason why the ACU should not be connected by a 16 A MCB in a suitable enclosure, not elegant, but compliant!