Voltage drop is the decrease in electrical potential (i.e., voltage) that occurs as current flows through a conductor, connection, switch, fuse, circuit breaker, or other component in a circuit. They all haves some resistance.

According to Ohm’s law: Voltage Drop = Current × Resistance

This means voltage drop increases as either current or resistance increases. In a marine electrical system, excessive resistance can be caused by conductors that are too small, long conductor runs, loose or poor quality connections, and corrosion. Excessive resistance also produces heat, making poor electrical connections both a performance problem and a potential safety concern.

Here is a simple example for a 12 VDC nominal system.

Suppose a piece of equipment draws 10 amps and the total resistance of the circuit, including the positive and negative conductors and all connections, is 0.06 ohm.

Voltage Drop = 10 A × 0.06 Ω = 0.6 V

If the source voltage is 12.0 volts, the equipment will receive approximately:

12.0 V − 0.6 V = 11.4 V

A 0.6 volt drop in a 12 VDC nominal system is a 5% voltage drop. That might not sound like much, but it exceeds a 3% voltage drop limit. For comparison, 3% of 12 volts is only 0.36 volt, while 10% is 1.2 volts. In low voltage DC systems, even a fraction of a volt can matter.

Excessive voltage drop can cause lights to dim, motors to operate poorly, electronics to malfunction, and other equipment to receive less voltage than intended.