**By Steve Froese, F276276**

The electrical system is an important part of the RV infrastructure, yet many owners don’t understand how it works or how to even begin to troubleshoot problems that may arise in their home on wheels. This is understandable, since the electron flow that creates the power can’t be seen (with the obvious exception of sparking and arcing). For many people, not having the ability to visually follow an electrical fault makes problem solving difficult. While this article will not cover troubleshooting specifically, it will discuss the basics of electricity and provide an example for readers who aren’t familiar with the terms or need a refresher.

The most important electrical troubleshooting tool to carry in your RV is a multimeter, also referred to as a volt-ohmmeter (VOM). This easy-to-use instrument allows for the measurement of resistance, volts, current (I recommend an AC/DC “amp-clamp”-style VOM), and other functions. Other tools that are important to have include a plug-in circuit tester and a non-contact voltage tester.

To properly use the above-mentioned tools and troubleshoot electrical issues, some definitions and two simple formulas help.

First the definitions:

**Current:** Describes the quantity of electrons passing through a point in a circuit at a given instant in time. Current is measured in amperes.

**Voltage:** Describes the potential difference in electrical charge between two points in an electrical circuit. Also known as electric potential, voltage is measured in volts.

**Resistance:** Describes the capacity of a circuit element to resist or impede the flow of electrons in the circuit and is measured in ohms.

**Watt:** Describes the power produced by a current of one ampere across a potential difference of one volt.

Now for the formulas. Note that variation exists in the letters used. The first formula is one you may be familiar with — Ohm’s law. This law simply states that the voltage in a circuit (V) is equal to the current (I) times the resistance (R). This equation is commonly shown as **V=IR**. Ohm’s Law can be rearranged as R=V/I or I=V/R, depending on the variable we are solving for.

The second useful equation is Watt’s law, which states that watts (W) equals volts (V) times current (I). Again, there may be some variation in the letters, but the equation in its basic form is expressed as **W=VI**. By comparing the two equations, we can see that both utilize volts (V) and current (I). Therefore, we can combine both equations when we are dealing with volts, amps, resistance, or watts.

For instance, let’s assume that when the water heater in our RV is using the electric energy source (120 volts AC), it is not producing hot water, but when it operates on propane, the hot water is fine. We can use Ohm’s and Watt’s laws to troubleshoot. For this example, I will employ some assumed yet common values to demonstrate the use of the equations.

The most common possible causes of no hot water while on 120 volts AC are loss of electrical power, a failed element or control board, or loose wiring. We will assume for this example that we have verified the voltage at the water heater to be 120 volts AC. It is not easy to know whether the control board or AC wiring is good, so I would check the element next, especially since they are prone to fail over time. Also, many electric elements will fail quickly if they are energized when no water is in the hot water tank.

**CAUTION: Please exercise extreme caution when working with RV electrical systems. De-energize the power whenever possible, and when it’s not possible to remove the power (such as when taking voltage measurements), take extreme caution to avoid electric shock. If you are not comfortable working with electricity, seek professional assistance. **

We have confirmed the voltage (V) is 120 volts AC as measured by a multimeter directly at the element terminals (it is very important to verify voltage directly at the component first), and let’s assume we know from our water heater owners manual that the power output of the element is 1,500 watts. We can rearrange Watt’s law to calculate the current at the element as follows: I=W/V = 1500/120 = 12.5 amps.

Now we can use Ohm’s law to calculate the expected resistance of the electric element. We know the voltage is 120 volts AC, and we have calculated the current to be 12.5 amps. So, we can rearrange Ohm’s law as R=V/I = 120/12.5 = 9.6 ohms. This tells us that the resistance of the element should be 9.6 ohms. ALWAYS disconnect electrical power to an electrical component before measuring resistance. In our example, we can de-energize the water heater by removing AC power to the RV or turning off the water heater circuit breaker. Next, we remove one wire from the water heater element. Doing so ensures we have isolated the element to allow accurate measurements. The heating element is a large resistor, which is what causes it to get hot. We expect the element to have a resistance of approximately 9.6 ohms, based on our calculations. Using a multimeter to measure resistance across the two water heater element terminals, we discover there is an open circuit (infinite resistance reading on the meter). This immediately tells us that the element has broken and is not allowing any current flow. Replacing the element should solve the issue.

Note that the above example did not require any disassembly or advanced circuit troubleshooting of any kind, just an awareness of how to use Ohm’s and Watt’s laws, as well as how to use a multimeter. To reiterate, this article was not meant to introduce electrical troubleshooting tips to the reader, but rather to introduce the topics of how to calculate watts, current, voltage, and resistance by using, combining, and rearranging two important electrical laws. It is important to point out that two variables must already be known to calculate the others. In an RV, we will generally know the value of V (either 120 volts AC or 12 volts DC nominal), and the other known variable will depend on the component and troubleshooting being undertaken. Some variables may be printed on the component itself; indicated in the owners manual; or obtained elsewhere, such as a component specification.