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.
ABYC E-11 Requirements
ABYC E-11, AC and DC Electrical Systems on Boats establishes two maximum voltage drop categories for DC conductor sizing:
- 3% maximum voltage drop: Panelboard main feeders, bilge blowers, electronic equipment, navigation lights, and other circuits where voltage drop should be kept to a minimum.
- 10% maximum voltage drop: Lighting other than navigation lights and other circuits where voltage drop is not critical.
When using voltage drop tables, circuit length is the length of conductor from the positive source of power connection to the device and back to the negative source of power connection. In other words, both the positive supply conductor and the negative return conductor are included in the total circuit length.
Conductor sizing cannot be based on voltage drop alone. ABYC E-11 requires the conductor to meet both its required ampacity and the applicable voltage drop limit. If the ampacity requirements and voltage drop tables indicate different conductor sizes, the larger conductor size must be used.
Controlling voltage drop begins with correct conductor sizing and continues with good installation and maintenance practices. Properly sized conductors, properly made terminations, and clean, tight connections all help reduce unwanted resistance. Voltage drop testing under load provides another valuable tool for confirming that DC equipment is receiving the voltage it needs to operate reliably and safely aboard a boat.
In Part 2, we will look at how to measure voltage drop and how to use this simple troubleshooting method to pinpoint high resistance areas in a circuit that can create performance problems and potential safety issues.
For additional information, refer to ABYC E-11, AC and DC Electrical Systems on Boats or contact ABYC Standards and Compliance Department at [email protected].