Arduino IO long distance travel I would like to use Arduino to control locks around a building which is 5 storeys high and on every floor there is 2 door. 

I'm concerned whether the wires is able to travel all the way from the ground floor to the 5th floor to provide enough power to the relay located at the door. 

The lock will be a 12V magnetic lock and it will be unlocked thru a SSR relay which will be controlled by the arduino
Yes, losses in wires are proportional to the length, so an excessively long wire may not be able to deliver enough power. Many factors come into play here:


higher current consumed by the relay increases the losses
smaller wire diameter increses the losses
the voltage required by your relay defines how much you can afford to lose


For example, a pair of AWG23 wires 30m long will have a total resistance of 4 Ohm. If your relay consumes 100 mA, you will lose 0.4V in the wires. If Arduino delivers 5V, and the relay requires a mininum of 4.5V to work, you're just good enough: you're delivering 4.6V to the relay after the losses.