We are trying to keep our discussions in some kind of order so that they make sense. When there is so much to talk about, I get excited and want to throw all sorts of new ideas at you. Then I realize that would discourage some and I have to hold myself back. I want to spend more time discussing Ohm’s law because it is extremely important to understand this law when we talk about electrical systems.

However, we are going to take a short break this week to introduce yet one more vocabulary term: Power. What is “power”? Wikipedia, the online encyclopedia, defines power as “the amount of work done or energy transferred per unit of time”. The important word here is “work”. There are many forms of work and the one we want to consider here is the electrical form. Most electrical work shows up as heat, and you can generally tell if electrical work is being done if there is heat present.

(I want to get a little off topic here to talk about “work” a little more. More specifically, about conversion of energy. In physics, there is a “law of conservation of energy”, which tells us that we cannot destroy energy, we can only convert it into another form. For instance, when you are driving your RV down the highway, you are converting the energy in the fuel in the tank into mechanical energy to propel the RV down the road. Something most folks don’t think about is that when you turn on your windshield wipers, or your headlights, you are also using energy. That energy has to come from someplace, and so, when you turn on your accessories, you actually reduce your fuel mileage by a slight amount.

Anyway, when you do these things, you accomplish “work”, and this is done by generating power. Power in an electrical system is a “product”, like the product we all learned in 7^{th} grade math class. It is the product of multiplying volts time amps, and is another form of Ohm’s law as shown below, with “P” representing power:

P=E*I

So, if we know the volts in an electrical system, and the amps, we can multiply them together and find the amount of power being used in the electrical system. A simple example would be if you have a 12v system running a lamp with 1 amp of current flowing, the lightbulb would produce, or use 12 watts of power. Turn that around a little and you can find what amperage will flow in a circuit if you know the lightbulb wattage rating and the circuit voltage. This can be very useful when working on an electrical system.

## David

Of course, there are some simplified ways to remember all this, but the formula “P = E * I” can be confusing when talking about “Power,” “Volts” and “Amps.” In a way, this discussion is confusing because you’re talking about two different groups of terminology. Add to this the discussion of “Ohms” before, and things can get more confusing.

Power = Work = Watts

Electromotive Force = Pressure = Volts

Current = Flow (of electrons) = Amps

Resistance = Load = Ohms

When working with the DC current within your RV, all the formulae are simple, falling into two easy ‘pie’ charts.

P E

I E and I R

Simply put, P = I * E, I = P/E and E = P/I for the first one

with E = I * R, I = E/R and R = E/I for the second one. With these six basic formulae, you can troubleshoot almost any simple DC circuit with a DMM and a calculator.

Of course, most circuits on your RV aren’t all that simple, but that’s what this series of blogs is all about, isn’t it?

## David

Just so you know, in my previous reply, the P is supposed to be centered above the I E and the E is supposed to be centered above the I R. It appears the site ignores spaces it perceives as extraneous.

## Gene

What should be the AC voltage at an RV site?

There is a site that is to provide 30 amps of AC voltage that is thought to be about 110VAC.

Because of various problems with the AC at the site, I went to Camping World and purchased an AC Voltage Meter the plugs into an AC outlet in an RV. That meter read less than 110VAC. The RV Park then put the rig on a site with 50 amp service.

The meter reads about 117.5 VAC with appliances turned off. When the air conditioner is turned on, the Camping World meter shows about 113VAC. As other AC items are turned on, the Camping World meter shows even less voltage.

What is causing this situation? The RV Park appears to be over 10 years old. Our unit is less than 2 years old.

Sincerely,

Gene

## David

I’m not the creator of this particular blog and don’t know how he will answer this question. However, I will start by telling you that AC power is quite a different worm from DC.

As to your question itself, under normal circumstances AC power at the box should be 110-120VAC. Most AC appliances can operate reasonably well even if voltage drops to around 90VAC. However, such a significant visible drop in supply voltage just by turning on your unit’s air conditioner leads me to believe the park’s supply transformer is either too small for the job or aging to the point of near-failure. Of course, that can also depend on how many RVs are connected and running at the same time. If the park is near capacity for spaces, then the combined load of all those RVs will likely have its supply nearly maxed out.

Another possibility is that the local generation source (power plant, hydroelectric dam, etc.) is running at near capacity itself due to population growth in the area. Eventually things will reach the point that demand will exceed supply; but that’s nothing for you to worry about.

One other thing you have almost no control over is the wiring in your coach itself. Obviously to keep the vehicle as light as possible, they use only heavy enough wire to carry the expected load in the vehicle. An air conditioner is a heavy load, no matter how you look at it. Unlike a permanent home that has power come in on a very heavy line (wire gauge possibly 0 (which is very thick,)) you have semi-flexible 3-lead wire on a plug that is probably 14 ga. at the thickest. If you’re lucky, maybe it’s 10 ga. (lower number is thicker wire.) The thinner this wire is, the less current can be carried and the more noticeable the air conditioner load will be.

In other words, everything really sounds about normal for you. By moving you to a 50Amp site, they put you on a heavier feed line than you really needed since the RV wiring probably can’t handle much more than one room in your permanent home.

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