What's the maximum length across a single core wire will a high signal last on a raspberry pi GPIO pin?
I.e if I have a 3 metre signal core wire attached to a relay when the raspberry pi sends the high signal will it reach the wire?
If the current is low the wire resistance won't have any significant influence and you can have a very long wire. Of course, resistance is not all and inductance and capacitance will also rise and will take a toll on the high frequences. Short answer for the relay case: don't worry. The question here is if you are driving the relay directly and in that case if the gpio has anough current available.