Heat bed wires get hot (12 V connected to RAMPS 1.4 shield)
The PCB heatbed Mk2B has a reported resistance between 1.0-1.2&nbsp;&ohm;. The current that is drawn from the power supply equals about 12/(1.0 or 1.2) = 10-12&nbsp;A. Note that this amount of current requires cables that can carry that load, too thin cables heat up. 

Note that you have wired the bed correctly for 12&nbsp;V (see image below), the wires might have a too-small cross-section, try thicker wires. Personally I use AWG 12-14 silicone wires depending on the bed resistance.



AWG 12-14 size silicone, multiple threads (these are the best for the bed because they are more flexible), should be able to handle the load fine. Note that cables also have resistance; if there are heat spots in the cable or the connection, this indicates that the cable may be broken or the connection has too much resistance.