Voltage Drop

This “TIPS” page is a basic discussion of voltage drop. It is intended to emphasize the importance of controlling voltage drop. “TIPS” pages 23A & 23B (soon to be posted) will discuss the more complex technical issues with respect to testing and correcting voltage drop. For the purpose of this page, this discussion will be limited to DC (Direct Current) Systems. Currently, the predominantly used system found in recreational boating for onboard equipment is the 12 Volt DC System.

Basic: Voltage drop is inherent in electrical circuits. What is important is to keep this “drop” within acceptable levels.

Equipment requiring electrical power must have sufficient power to function properly. Manufacturers are careful to include the electrical requirements for their products in the Owner’s Manual provided with the equipment at the time of purchase. This manual will normally include (but not limited to) installation, operation and maintenance sections. Information such as “nominal” voltage requirements, a wiring size/length-of-run charts, safety precautions and other useful information are included in a good manual. (Please pay attention to the safety notices.) It is in the best interest of both the end user and the manufacturer to understand and to adhere to instructions. Read all sections prior to beginning any installation. Should you have any question after reading the instructions, you should call the Technical Services Department or consult a knowledgeable marine technician to answer these questions. By checking out the installation requirements for a given product, you can determine if this product can be successfully installed and thereby prevent unnecessary surprises.

When we speak of voltage drop, we are referring to the difference in the voltage reading taken at the source of power for a particular circuit and the voltage reading taken at the product (equipment) when this equipment is operating (under load). The difference between these readings is the “voltage drop” occurring in that circuit under the load imposed on that circuit at the time the test is being conducted.

A Voltage Drop Example: 13.6 Volts is the industry standard for checking bilge pump ratings in GPM (Gallons Per Minute). A Rule Model 10 bilge pump is rated @ 2000 GPM 13.6 Volts Open Head (no hose attached). The amp load at this voltage is 12.0 amps. This same pump will deliver 1740 GPM @ 12 Volts with an amp load of 8.4. As you can see, there is a significant reduction in the GPM delivery.

Most of the onboard equipment with the exception of lights and radios will have an electric motor as a major component of the equipment. Lower voltage (electrical power) as shown in the above paragraph equates to reduced motor speed (less work). In the example above, the wattage (volts time amps) is 163.2 and 100.8 watts respectively.

Below is an example of a simple DC (Direct Current) circuit. To really keep it simple there is no fuse or switch shown in this circuit. A circuit begins at the power source, is directed to the equipment served and goes back to the power source. The distance used for determining the wire size for the amp load requirement consists of measuring the entire length of the circuit. NOTE: In actuality, current flows from negative to positive. For many years (forever) for instructional purposes it has been taught as shown below. In reality circuits are more complex and can have a number of variables. Some of the SUBJECTS to think about are addressed in “TIPS 23A & 23B” listed below.


Some Of The Subject To Be Covered In “TIPS 23A & 23B”. Power Source (Battery) supported/unsupported. Design requirements (3% and 10%) for voltage drop. Load variables and the associated effect on voltage drop. Deterioration of electrical circuits. Proper installation of electrical circuits. General and Pinpoint Testing.