The current rating of wire to the motor isn’t a huge issue, as the lengths are so short. Any voltage drop, and consequent heat dissipation, in the pickup wire is tiny in comparison to that dissipated from the motor itself.
Looking at short-circuit faults, the most likely place for this to take place is on the track where a locomotive inadvertently bridges a gap between two rails at different polarity. This particular short circuit current does not pass through the motor pickup wire. While track shorts can be prevented by appropriate signalling and interlocking, and I’ve done it, it remains the most likely place for it to happen.
Short circuits within the loco are rather less likely, and can be designed and constructed out, which means that, in the limit, the connection between pickup and motor can be the same size as the motor winding wire. Practically, it needs to be a bit larger than that to allow for movement during the fitting of the chassis to the loco body. Stranded, insulated, wire will give more satisfactory performance than single-strand wire.
- This reply was modified 1 year, 8 months ago by Nick Ridgway.