Manufacturers know how much voltage each of their connections, switch contacts, or lengths of wire will drop. Connectors are listed as having a voltage drop across them based on each amp that flows through them. Small connectors can cause a voltage drop of 4mVolts per amp, whereas, large connectors can cause a voltage drop of 10mVolt per amp. Each connector is rated to handle a certain amount of amperage based on the physical size of their blades or pins. Vehicle connector blade sizes used today range from .8mm (.032 Inches) to 8.0mm (.315 inches). Their amperage carrying ratings range from 0 to 15A, 25A, and 33A for the most common connectors in use.
A small connector rated to drop 4mVolts per amp would drop .060Volts (60mV) with 15amps flowing (.004V X 15A = .060V, E=IR). A large connector rated to drop 10mVolt per amp would drop .300V (300mV) with 30 amps flowing (.010V X 30A = .300V, E=IR). Voltage drop is always proportional to current flow.