I sent an email off on the weekend to Pertronix to ask what sort of current I should expect the coil to draw, and if I needed to install a relay. The instructions don’t mention a relay at all.
I received a response from Marvin in the Technical Dept.:
Normally it will draw around 7-10 amps. A relay is a great way to not overload the switch or OEM wiring.
I already know that the Electric Choke can draw up to 10A initially, and will settle down to less than 1A when hot. Simple maths tells me that I need to design for 20A, plus 50% safety factor gives me 30A. I bought a 30A relay for the Ignition circuit
I decided to take the power from the 12V feed to the Ignition Switch rather than run another wire, as the switch doesn’t need a huge amount of current any more. I removed the spade terminal from the connector and replaced it with a pigtail I made using a new connector.
To splice the pigtail in without cutting the wire I opened up the clamp around the insulation and cut it away. I stripped the end of the pigtail and split the wire to run through either side of the clamp, and then squeezed it all back together. I used some soldering flux to clean it all and then soldered the union.
The actual finish is neater than how it appears in the pictures.
I installed the relay using the same screw as the Accessory relay, and earthed both relays using a new hole. A little bit of floristry to arrange the wiring and it was done!
Hopefully this is the last time I work on the Ignition Switch wiring loom.