Length? Long enough to reach from the 125VDC battery distribution panel to the relay. You will have to look at your project's physical layout drawing and estimate the wire length.
Size? - If it is a power wire to deliver power to the relay, a #12 or #14 AWG size wire is probably more than large enough to carry the load without appreciable voltage drop. If it is just an input signal, like breaker position indication, the current draw is so small that a #18 awg is large enough but it might not have the physical strength to withstand the wire pulling stresses during installation.
Your question leaves a lot of assumptions to be made or questions unanswered. What are you worried about? Voltage drop to the relay input? Voltage drop during tripping? How much wire or cable to order for the new substation?
rcwilson,
First of all thanks for your response. What I am worried about is the distances wires have to cover in high voltage yards to reach from control center to equipment in the yard. Sometimes these distances can be in upwards of 1000ft and my question is at what point do you go with a size higher than let's say #14 or #12. For example if you run a #12 wire from the breaker to the control center for tripping, closing and auxiliary contacts then at what distance would it become unreliable, where something like a #10 would be required. I am not running a single conductor from the yard to the control house to be worried about pulling strength, it would be more like an 8 to 12 conductor cable.
For circuits delivering significant current such as trip/close circuits, you'll need to do voltage drop calculations. Select the wire based on the minimum design control power voltage and required operating voltage + a safety factor. I don't think there's a rule-of-thumb.
For signal circuits where operating time is critical, you should evaluate the affect of capacitance of the wiring.
Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.
