The live red cable running from the starter relay up to the ignition barrel...what is the amp rating of that cable please? 
Thanks.

What's the go, do you want a siren, police lights and cb, or something like that?

I measured 100 Amps on mine with a stall test. Dunno how relevant that is though.

Not likely to be more than 33amp @ 12 volt rating. The K loom is load-distributed i.e. not all of the power goes through the ignition switch - see the load shed relay, for example, which provides power to the lights, indicators and horn. When rewiring I've only ever used a 25amp cable on that run and never had any trouble.

The 1984 handbook wiring diagram shows a 2.5mm red wire from the starter relay COM terminal 30, which goes to a loom junction and there splits and goes to various loads. One of these loads is a 1.0mm red wire to the ignition switch. This might be the wire you're on about.

If you're after adding the 'police sirens' etc (  ) i.e. extra loads on this red wire, then its load capacity/max current can be calculated in theory, but it depends on de-rating factors such as ambient temperature. I'd suggest that the aged condition of the end terminations/ignition switch contacts etc. is a factor too, especially if you're getting near the limit. It is therefore not usually practical to do anything better than a good guesstimate, as Dai has done, and then play safe - you want wires not heating elements!

Best thing is to describe what you've got in mind and you'll get good advice on here. Safest route for add-ons is usually spangly new wire from the battery, a fuse and a relay.

chris846 wrote:The 1984 handbook wiring diagram shows a 2.5mm red wire from the starter relay COM terminal 30, which goes to a loom junction and there splits and goes to various loads. One of these loads is a 1.0mm red wire to the ignition switch. This might be the wire you're on about.

So... it appears I really, really believe in overkill. 1.0mm.sq PVC (which is what the original is) has a current rating of 8.75amps @ 12 volts. Makes sense. Trace the green wire back down through the loom and essentially it's only providing power for the load distribution relay coils. And a little bit else.

Next one I do only gets a 16.0amp cable for that feed

In truth, there's also the mechanical strength aspect to consider. I could run an 11amp cable to the ignition switch which would be more than adequate power-wise, but I'd prefer the increased stength of a 16amp cable in an area that gets a lot of movement.

That’s great info from everyone...thanks!
I am repairing damage to the cable from when my K75S was recently stolen. I am not adding anything else to the load regarding electrics. I just want to run a new cable rather than repair the old one.