Electricity 101 for Boaters

Since the majority of failures in boating have to do with something electrical, we thought it might be a good idea to post a series of articles on electricity and trouble shooting. Electricity 101 will be the first in that series.

Understanding the terminology of electricity is the first step to starting to understand electrical theory. I personally like to compare electricity to the more straight forward discipline of simple plumbing. I am assuming that each of you has, at some time, used a water hose. That is the simple plumbing to which I am referring.

When you turn on the spigot connected to a water hose, the amount of pressure that you have at the end of the hose depends on how much you open the spigot. This pressure can be measured in pounds per square inch. With the spigot opened fully you will have a constant pressure at the end of the hose. The pressure can vary if you reduce the length or diameter of the hose or add a nozzle which restricts flow at the end therefore increasing pressure.

Electricity works the same way. Think of voltage as equivalent to pressure in the water hose and the hose itself as the wire that carries the electricity. Water flowing through the hose has resistance from the inside hose surface itself as well as at the nozzle. This same resistance occurs as current flows through an electric wire. However, in electricity the resistance is measure by a unit called the Ohm. With greater pressure, water flow increases through the nozzle and resistance is greater. The same is true with electricity, with greater† voltage (pressure) the flow of electrical current increases though the wire and resistance (Ohms) also increase. On the other hand, if pressure drops, water flow decreases. The same happens if voltage drops, current slows down causing a "brownout".

Resistance is an important part of the electrical equation. A smaller hole in the nozzle of a water hose increases resistance. In pipes, friction on the walls makes resistance. In wires there is resistance from the wire itself and, if you add a light bulb with a fine tungsten filament, even more resistance is produced. You might think of the light bulb as the nozzle. The larger the pipe the less resistance is created, the same holds with wire; the larger the wire, the less the resistance.

Water flow is measured in gallons per minute. Electrical current flow is measured by the Ampere or Amp.

So far we have explored in the plumbing analogy the following:



Pressure =


Resistance =


Water flow rate =

Ampere (Current)

How do these all interact? This brings in something called Ohm's Law. Let's say you have turned the spigot on only half way, producing some resistance, with a hose attached to fill a pot with water. As you fill the pot with a constant pressure from the spigot you have constant flow rate through the hose. The same is true with electricity; with a constant voltage you have a constant flow of Amps through the wire. If you open the spigot all the way and double the pressure you are also doubling the flow rate. Electricity works the same way, if we double the voltage we double the current in Amps.

Remember the resistance part? Water resistance flowing through a hose is the same as resistance through a wire. This resistance is measured in Ohms. The amount of this resistance can be measured by pressure and flow rate or voltage and amps. One Ohm of resistance lets one Amp of current flow when the pressure is one volt. Two volts would give you two amps of current, Six volts would give you 6 amps, etc. You can calculate the amount of Ohms, Volts or Amps with the following equation.


From this equation we can calculate any of the three variables if we know any two. For example, if we applied 12 volts and were able to measure that the 12 volts was producing 2 amps, what is our resistance in Ohms?

12 Volts/2 Amps = 6 Ohms

What's a Watt?

Back to our plumbing analogy...the amount of power of the water flow can be controlled. Obviously, the power of a stream of water can be increase by adding a nozzle. (Ever try to wash leaves off the driveway without a nozzle or putting your thumb over the end of the hose?) Power comes from the volume and speed of the water jet. This is similar to power (Watts) of electricity. Volts times Amps give Watts.

As an example of why this is important, try to follow the following example. Remember Volts X Amps = Watts

Each of the lights below are 12 watts. As such they should give about the same amount of light. What we want to know is how many Amps are used under various voltage situations. To measure Amps we will use the formula but the illustration will show an Ampere Meter.

6 Volts

6 Volts X 2 Amps = 12 Watts

12 Watts

12 Volts

12 Volts X 1 Amp = 12 Watts

12 Watts

120 Volts

120 X .1 = 12 Watts

12 Watts

If you study the figure above more carefully you will notice that to produce the same amount of light at lower voltages more current (Amps) must flow to the bulbs tungsten element. In order to have more current flow through the 12 Volt or 6 Volt filament, the filament in the 12 Volt bulb must have a lower resistance than the filament in the 120 Volt light. Consequently the filament in the 6 Volt bulb must have a lower resistance than the 12 Volt bulb so that double the current will flow through its filament even though only half the pressure or voltage is there to make the current.

Remember that resistance, in OhmS, is also easy to calculate. It is the ratio of voltage to current:

Resistance (Ohm) =


Why are we concerned with the current flow rate or Amperes? Well...when connected to shore power or using a generator to generate 120 volts we might not be too concerned. However, when we are at anchor and operating off our batteries, we only have so many Amps we can use before your batteries die.

Using these formulas let's try an example as shown below.

The diagram represents a 12 Volt battery powering a light bulb. We are measuring voltage with a volt meter, amperes with an amp meter and Ohms with an Ohm meter. (Actually, you can purchase a multimeter which has the ability to measure all of these) As shown, we have 12 Volts, 3 Amps and 4 Ohms. By using the formulas below we can find each of these measurements by only knowing two.

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12 Volt

= 3 Ampere (Amps)

4 Ohm

4 Ohm X 3 Ampere

= 12 Volt

12 Volt

= 4 Ohm

3 Amps

As the current flows through the resistor (light bulb) the resistor gets hot. How hot you ask, how about hot enough to generate 36 Watts, the heat of a small light bulb.

12 Volt X 3 Amps = 36 Watt

Basic Electrical Theory for Boaters

Multimeter - The Invaluable Tool

meter.jpg (13688 bytes)Many of the problems that you will encounter on your vessel are electrical problems. It is because of this that the multimeter is an invaluable tool. With it you can do a lot of troubleshooting and track down potential problems without calling an electrician.

I prefer a digital multimeter that gives a more accurate digital readout over the less expensive analog models that have a needle that moves over a set of printed numbers on the dial. You can get a good, dependable multimeter at most marine supply stores or Radio Shack for well under $100.00.

Understanding AC verses DC

Multimeters can be used to measure voltage (pressure), amperage (flow rate), and Ohms (resistance). These measurements can be made on AC or DC systems. Before using a multimeter, make sure you understand the basics of electricity as outlined in the aforementioned article.

AC, or alternating current, is the same current that you find in your home that is used to turn on your lamps, television, radio, etc. This is usually 120 volts and can be dangerous. This same AC current comes to your boat via your shore power cord, an extension cord, etc. You might be simply operating a sander with an extension cord from the dock or, on larger vessels, operating all the appliances and comforts of home via your shore power cord when docked and your generator when underway.

DC, or direct current, is that current supplied by your batteries. This is what starts your engine(s), runs your electronics, your VHF radio, your DC lighting system, running lights, etc. This is generally a 12-volt system, except on larger boats where you may find a 32-volt system.

An important factor when using a multimeter is to make sure that you are using the correct scale depending on whether you are measuring AC or DC. (This should be clearly marked on the face of your multimeter.) If you measure AC current while on a DC scale you could destroy your meter and, worse yet, get a severe shock. If you donít have at least a working knowledge of the basics donít attempt to measure or test AC systems.

AC and DC systems on boats often share the same electrical panel. Make sure you know which is which and never work on the panel with the AC power still active from the dock.

Using the same water analogy described in Electricity 101, lets review what we can test for with our multimeter.

Measuring pressure (voltage). Water pressure is measured by inserting a gauge in the line and comparing the difference to the pressure in the line to the surrounding atmospheric pressure. Because atmospheric pressure is all around us, by leaving one side of the gauge open, the difference can be measured. Voltage is measured in much the same way except the reference point is not atmospheric pressure but ground. The meter cannot measure the difference without being connected to the ground reference point. Thus, we must attach our meter to both the positive and negative (ground) sides of the circuit.

Measuring flow (amperage). To measure water flow, the rate of flow could be measured by inserting a paddle wheel into a pipe and seeing how fast it spins as the water flows through. Your multimeter, using the amperage scale, is the electrical equivalent of the paddle wheel and measures the flow through the circuit.

Measuring resistance (ohms). As water flows (amperage) through a pipe at a regulated pressure (voltage) and this rate of flow is measured, the resistance can be deduced. The same theory is used to measure electrical resistance. The multimeter, when used to measure ohms, uses an internal battery to supply current (amperage) at a carefully regulated pressure (voltage) to the circuit being tested. Using ohm's law the resistance can be calculated.

Using The Meter

As with any piece of equipment, you should carefully read the manufacturer's instructions prior to use. Various multimeters have basically the same features but the features may be selected in different ways. Obviously, the more you pay the more features you will get. The following methods will be based on a fairly inexpensive generic multimeter.

Although it does not matter which leads from your multimeter you use to test AC current, when testing DC, it is imperative that you use the positive (red) lead on the positive (+) side and the negative or ground lead (black) on the negative (-) or ground side of DC circuits. It is a good idea to get into the habit of using the positive and negative leads consistently even on AC current.

To measure voltage you simply need to touch the positive (red) lead to the positive side of a circuit and the negative (black) lead to ground or the negative side of the circuit. For instance you could put the positive lead on the positive (+) side of your battery and the negative lead on the negative (-) side of your battery to measure the voltage in the battery. A new fully charged battery should read approximately 12.5 volts.

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Your meter will have several voltage choices to choose from. For instance mine has, on the ACV(volts) side: 750 and 200 and on the DCV side: 200m, 2000m, 20, 200, 1000. Unless your meter has a feature that automatically selects the correct voltage, always start at the highest voltage selection available first. Why you ask? Suppose you have a situation where you have both 120v AC and 240v AC circuits. If you select the 750v option first, you wonít blow out your meter if you mistakenly touch the 240v circuit. Once the voltage is confirmed in the read out you can select the lower range to get a more accurate reading.

What other situations might prompt you to measure voltage? Letís say you have an aerator pump in your live well that connects to, and is powered by, your battery. It worked fine last time you went fishing however today you put in the bait, turn on the switch and nothing happens. Do you have enough voltage in your battery to run the pump? You check this and find that you do. Next you use the meter to check the positive (red) and negative (black) wires going into the switch. HmmmÖno voltage. The solution must be that there is a loose or broken wire between the battery and the switch.

Amperage (current) can be measured (both AC and DC) by connecting the meter in series with the appliance you are measuring. On board you may have many appliances such as water pumps, fans, stereos, radios, electronics, etc. All these items draw current from your battery. These appliances may draw from .5 to 6 or 7 amps. Your battery, however, only has so much current that can be drawn before its voltage drops to a point that it will not run anything.

Most appliances will have ratings on them that tell the voltage required and the amps or current that they use. However, lets say you bought a 12-volt lamp at a marine yard sale that has no rating listed. You want to find out how many amps the lamp will draw. By connecting the meter in series with the lamps wiring and your battery you can measure the amperage that the lamp will use.

batlamp.jpg (16954 bytes)

You can measure resistance in Ohms using your multimeter. Make sure that you only measure resistance on circuits that are free from a power source or you will blow your meter. The meter itself supplies the power source from its internal battery. This battery will discharge over time and may need to be replaced. Why would you want to measure resistance? Basically when you are measuring resistance you are measuring to see if you have a complete circuit. When you place the meter in a circuit that has been disconnected from its power source you can tell if the circuit is complete. You can start by checking the circuit in the meter itself. Turn the selector switch to the area labeled OHM. Depending on your meter you may get a display of the number 1 or the infinity symbol. This indicates that there is a break in the circuit. Now touch the ends of the positive and negative leads together. You reading should be zero or very close to zero. SoÖwhat you ask?

Letís take an example, say you have two light bulbs rolling around in a drawer. You remember that one was burned out and you just threw it in the drawer in order to make sure you got the correct replacement. You got the replacement and threw them both in the drawer thinking you would change them out later. Now you donít know which one is the bad one and which is good. They are both of the frosted variety so you canít physically see the element to see if it is broken in one bulb. You donít want to do the trial and error test because you just bought this cool multimeter. How do you identify the good bulb? You can use the multimeter to measure the resistance to find which one has an open circuit. This would be the bad one.

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Good bulb
bulb2.jpg (6259 bytes)
Bad bulb

Turn the meter to the OHM position. Put the one lead on the bottom part of one of the bulbs. This is the part that touches the contact in the middle of the socket. Then, put the other lead on the metal side of the bulb. This is the part that screws into the socket. Your multimeter changes from 1 to 0 or perhaps 0.01 which mine did. This means you have a closed circuit and you have just tested the good bulb.

Other uses of measuring resistance might be to check a pump that has stopped working. You have voltage to the pump but it does not run. You can test its integrity by measuring resistance. If you place the meter across the circuit (remember to turn off the power) by placing the positive (red) lead on the positive (red) wire going into the pump and the negative (black) lead on the negative (black) or ground wire going into the pump you will be measuring the resistance. If the meter reads 1 or the infinity symbol you know that there is a short in the pump wiring.


Courtesy of:

- Genesis Marine