Doing the Circuit

Maintenance Q & A — April 2003
Maintenance Q & A — April 2003
By Capt. Ken Kreisler

Doing the Circuit
Series versus parallel circuits, outboard flooding, and restoring the shine to stainless steel.
 More of this Feature
• Circuits
• Flooding Outboard, and more
• PMY Tries... Marinco Cable Ties

 Related Resources
• Maintenance Q&A Index

What is the difference between series and parallel circuits, and how do they differ in A.C. and D.C. applications onboard a boat? H.L., via e-mail
A basic circuit consists of a power source—a battery, shore power, or genset—a switch, and a device such as a motor or lightbulb. There are two kinds of currents that can run along a circuit: A.C., or alternating current, and D.C., or direct current. Unlike D.C., which always flows in one direction, A.C. reverses its direction at regular intervals called cycles. The number of cycles per second is known as the frequency of the current. In the United States, we operate on a 60-cycle, or 60-Hertz (Hz), frequency. As long as the electricity can pass from one side of the source to the other, through the circuit out from the positive and back to the negative terminal, the circuit will be complete and the device will operate.

In the simple circuit in the first diagram, closing the switch allows the electricity to complete its path and light up the bulbs. Opening up the switch prevents the flow of electricity from returning to the source, so the bulbs go off. In its most basic form, what we have is a series circuit, as there is only one path for the current to follow.

If you add another device or two to this series circuit and flip that switch off, everything in the circuit will go off. Moreover, if one device or wire in the circuit goes bad, the entire circuit will fail because there will be no complete path from one pole of the battery to the other. In some ways, this is not a good idea onboard, especially for multiple A.C. applications like a lamp and entertainment center circuit. If the lamp shorts, the entire circuit will go out.

The parallel circuit, shown in the middle diagram, provides three paths through which electricity can flow. Should one be faulty—a break in a wire, for instance—the other two will not be affected, and there will still be a clear path for the electricity to flow from and to the power source.

In the circuit shown here, one switch controls three devices. Open the switch, and all three shut off. But if you install three switches, as illustrated by the third diagram, you can shut one or two off without affecting the others.

The question of whether to use A.C. or D.C. applications aboard comes down to a practical solution. For example, a boat that needs 1,800 watts of electricity to run its A.C. circuits would require 14-gauge wire conductors. (Gauge is a measure of thickness where the larger the number, the thinner the wire.) For the same application in 24-volt D.C., a two-gauge wire conductor would be needed—larger than a shore-power cord. Some A.C. circuits aboard your boat would include lamps, galley appliances, and entertainment items, while D.C. would cover bilge pumps and electronics, for example.

As with all things electric aboard your vessel, unless you are qualified to do so, let a marine electrician have a look at your system.

Next page > Flooding Outboard, and more > Page 1, 2, 3

This article originally appeared in the March 2003 issue of Power & Motoryacht magazine.

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