Electrical Power Management 101

by Wendy on May 19, 2013

Provided by the Country Coach Technical support department –

Have you never tripped a breaker on shore or generator power? Do you believe it possible to run both roof air conditioners together on your older Intrigue while on generator power? Do you keep your coach plugged into a fifteen amp outlet while in storage knowing that all will be well and your batteries charged upon your return? If you answered yes to all the above then read no further, you clearly have a good understanding of the power management principles we shall be covering below. If however some or all of the above doesn’t quite make sense or perhaps you have some time to burn then please read on…

Voltage, Current, Power, Load & Battery Capacity,
Let’s start with a brief explanation for each of the above before we thoroughly confuse you with the remainder of the article. The first and easiest to explain is our old favorite, voltage (measured in volts). Volts are essentially a measurement of pressure, electrical pressure in this case. Tightly hold your thumb over the end of a garden hose and turn on the faucet. The pressure you feel against your thumb from the water is called voltage in the world of electricity.

Now fill a bucket with water using the hose. Once the bucket is full, turn off the hose and review what you have done in this step. You have before you a bucket of water and the question you have to answer now is simple. At what rate or speed did the water come out of the hose? Exciting isn’t it…If you didn’t monitor the time as you filled the bucket then you have no way of knowing how fast it was coming out of the hose. If the bucket holds five gallons and it took one minute to fill then the flow rate was five gallons per minute or 5gpm. If the bucket only took thirty seconds to fill then the flow rate must have been ten gallons per minute (10gpm) because the same amount of water filled the bucket in half the time. As you can see, water flow is a function of both quantity and time, and the same is true of current flow in the world of electricity which is measured in amps (short for amperes).

In simple terms we can think of current as the speed at which electricity is flowing through a wire but give some thought to the above and you will see that speed and flow are two completely different entities.

Power is a measurement of work done against time. If you burn off one hundred calories (or joules if you’re from Europe) to climb a hill in five minutes then you have used a certain amount of power. To run up this hill and burn off the same one hundred calories in just one minute will take five times as much power. I’ve forgotten much of what I learned in physics over the years so forgive me if I don’t calculate the actual numbers here but the power in question can be expressed in a variety of units including the very familiar watts (W) and horsepower (hp). It is interesting that we measure the power from an engine in horsepower yet we use a metric unit for power when looking at electricity. Remember this when you next shop for a microwave and tell the sales assistant you are looking for a model with at least 1.34hp (for which the metric equivalent is 1000W).

When used in the context of electricity, power is generally always expressed in watts and is quite simply a product of the aforementioned volts and amps. In other words, power = volts x amps or W = V x A. We shall return to this formula when we look at inverters a little later in this article.

Load is simply a generic term for any device or appliance that is consuming electricity. The roof air conditioners, refrigerator, washer/dryer, water heater and lights etc are all examples of electrical loads.

Last but not least is battery capacity which is measured in amp hours or Ah. (Note that amp/hours or A/h is incorrect). This is perhaps one of those measurements that are often difficult to understand because of their simplicity. Example: you turn on a light for four hours and the light consumes a constant one amp from the battery until it is turned off. You have used 1A x 4h = 4Ah from the batteries. Another example… Two fresh 8D wet cell batteries will have an advertised capacity of 220Ah each, or 440Ah total for the battery bank. How long can you operate this one light from the battery bank before they become fully discharged? Well, 440Ah means the battery bank can theoretically supply 440 amps for 1 hour, or 1 amp for 440 hours. The answer is a theoretical 440 hours in this example. The usable amp hours are much less in reality however due to limitations of the chemistry and construction of the battery. To complicate matters further, as you draw current from the battery faster, the total amp hours it produces before the battery does dead will sharply decrease.

There are two relationships to keep in mind when dealing with power management. The first is the relationship between voltage and current. In most cases, the voltage will drop as current increases and vice versa as power is consumed from a source. If you have a digital or analogue gauge package for your AC power, you might observe that the voltage from the generator drops down significantly when it is heavily loaded. The alternator on your engine is another good example. When the chassis or house batteries are low on charge, this alternator will produce a lot of current to charge them quickly. During this time when the alternator is putting out a lot of current, it is perfectly normal for the battery voltage to be low in the region of 13 volts. The message here is to never look at volts or amps in isolation of each other when trying to diagnose a problem. They are eternally interrelated so be wary of the technician that says your alternator is bad after having measured low battery voltage using only a voltmeter.  This of course never happens…

The second relationship we need to consider is the one that exists between power and load. Simply stated, as available power from the source and the load become evenly matched, the relationship between voltage and current will become more extreme. Let’s use shore power for an example, and let’s assume that the power lines that supply power to your shore hookup via a big transformer have the ability to carry a lowly one megawatt. Remember the W = V x A formula from above? If you plug a million watts and 240 volts into this formula you will see that the power line in this example can supply over 4000 amps to your coach. Your coach can only use 50 amps of course due to limitations in the sizing of the shore cord and transfer switch but even at maximum capacity, the 50 amps you might consume in your coach won’t make much of a dent in the 4000 amps that are available so don’t expect the voltage to drop much as you pile on the current.

Your generator is a different story however. Let’s say you have a 12.5KW generator and you have it fully loaded with air conditioning and lights etc inside the coach. Plugging 12,500 watts and 240 volts into our favorite formula shows that the generator can only produce a maximum of 53 amps. This will be rapidly consumed as you make the best use of your 50 amp service so expect to see a much sharper drop in voltage as current increases while running on generator power.

The engine alternator is of course another power source however at about 1900-2400 watts depending on the coach model, it pales into insignificance when compared to the 12,500 watts from generator power and the 1,000,000 watts that is used in the above example for the power line.

50A Service, 30A, 20A/15A. What is the Difference?
50A service has two power ‘legs’ and the voltage between each leg and ground or neutral is 120V. Additionally, each leg can supply 50A for a combined total of 100A.

The voltage from leg to leg is 240V which is necessary for some coaches that have a 240V electric barbeque or range installed. It is very important to keep your shore cord well maintained when connected to 50A service because a loose neutral wire can easily damage much of the 120V equipment in your coach including the inverter, microwave and entertainment system. The Surge Guard that is installed in current motorhomes protects against this scenario but owners of older coaches will want to periodically check the connections inside the shore cord plug that connects to their coach to ensure the wires are tight and secure. The plug that connects to the power pole should have no cracks or evidence of heat damage and the contacts should be straight and clean. Always use the breaker to disconnect power before inserting or removing this plug. If there is no breaker on the power pole then use the main breaker in the coach’s distribution power to protect your coach as you connect or disconnect from shore power.

30A service only has one power leg available so the maximum voltage in this case will be 120V and the maximum current is 30A unlike the above ‘50A’ service that actually supplies 100A. The good news here is that there is no risk of damage to coach equipment from an ‘open neutral’ while on 30A service so if you run into problems with your transfer switch or Surge Guard, consider using only 30A service until you get it repaired to avoid the risk of damage. Keep an eye on any adaptor that you may be using to connect your 50A shore cord to 30A service. These adapters are prone to loose or corroded connections and any evidence of heat damage around the contacts is a sure indication that replacement is necessary.

20A or 15A service is simply a regular electrical outlet that we use everyday to power such mission critical equipment as the coffee maker, microwave and hair dryer. You can actually operate your coach from this limited service and it is very advisable to do so while your coach is in storage to prevent permanent damage to your batteries. If you’re not sure how to do this without tripping a breaker then please read on!

Running Loads on 50A Shore Power.
You shouldn’t have too much difficulty meeting your electrical needs while on 50A service but be aware that either leg will trip the breaker if it exceeds the breaker’s rated 50A. Watch out for cycling loads that are typically overlooked and may cause you to exceed 50A if you have been pushing the limit.

Let’s say it is a hot day and you have all three roof air conditioners running. Two of these at least will be powered from one leg and they consume about 15A each for a total of 30A. If you also have the coffeemaker running and are watching a movie then the current can quickly add up an additional 15A so now you are using a total of 45A on one leg. You are still below the 50A limit at which the breaker will trip so life is good right? Probably not… Cycling loads such as electric water heaters and refrigerators are often overlooked and these can each add an additional 10A on top of the 45A you are already squeezing out of the system. At 55A, the breaker will naturally trip.

The solution? Pay attention to the current on both legs and try to balance your loads across them so you don’t have a situation where you are drawing say 45A on one leg, and 10A on the other. If you have three roof air conditioners and you only have the front two running then try turning off the front unit and run the bedroom unit instead to keep your coach cool. This can help balance the loads by taking 15A away from the leg that is running at 45A, and transferring it to the other leg that is only running at 10A. If you have an all electric coach with two inverter/chargers, you can also use these to help balance the loads when needs dictate. Simply turn of the inverter/charger that is operating on the leg you wish to reduce current on and leave the remaining inverter/charger to charge the batteries on its own. Modified Sine Wave (MSW) inverter/charters such as the Heart/Xantrex Freedom series and the Trace RV series will consume about 20A – 25A of AC when heavily charging your batteries. The true sine wave inverter/chargers such as the Xantrex Prosine and RS2000 are much more efficient during charging consuming as little as 14A for an identical charge current to the batteries.

Running Loads on 30A Shore Power.
First the good news, you don’t have to worry about balancing your loads across both legs because there is only one with this service. Now the bad news, you only have 30A to work with as opposed to the luxury of 100A above. Not a problem you say? You want to run both your roof air conditioners on a very hot day to cool things down a little? Well yes, it is actually possible on most coaches with propane refrigerators and water heaters. The first thing you should realize is that each air conditioner will consume about 14A – 15A and both together will eat up just about all the 30A available to you. Start by setting your refrigerator and water heater to propane only (turn off the breaker labeled ‘water heater’ in the distribution panel). Next, turn off any other 120V loads that you might be running and then… turn off the battery charger at the remote panel. You should now be able to comfortably run both roof air conditioners without tripping a breaker.

The eagle eyed reader will notice that the batteries are not being charged during all of this which may be perfectly acceptable for the hour or more it takes to cool your coach down if you have been away from it for awhile. If you wish to get even more adventurous, you can reduce the fan speed on both roof air conditioners to their lowest setting which may free up the five or more amps you need to keep the batteries charged at a low setting and watch TV. If you were not aware that the charge rate from the inverter/charger can be adjusted to use less AC current while providing a slower charge rate then please read on…

Running Loads on 20A/15A Shore Power.
Believe it or not, by taking similar steps to operate two roof air conditioners on 30A, you can actually run a single roof air conditioner on 20A or even 15A service. Fortunately you shouldn’t encounter many occasions where you will need to do this but if you do, make sure you are using a beefy extension cord to hook your coach up to the outlet and uncoil it fully if it is on a reel to avoid a meltdown.

The main reason why you might want to power your coach from such a limited supply is to keep the batteries charged during storage. Even when disconnected, batteries will self discharge in as little as three weeks depending on their health and construction. Batteries that are left in the discharged state for more than eight weeks or so can become permanently damaged. Solar panels are a great solution to this problem because although they have a very limited output, it is just enough to keep your batteries topped off and fully charged even during extended periods of non use. The other solution is to first adjust your inverter/charger to a lower charge rate, turn off all other electrical loads such as water heaters and refrigerators etc then connect your coach to an outlet using a beefy extension cord. The easiest way to ensure all other loads are disabled is to turn off all the breakers in your distribution panel except for the main breaker and one of the inverter/charger breakers. Next, turn off the breakers for the inverter circuits which will either be mounted on the inverter itself, or in small load center in close proximity to it. Check the documentation for your inverter/charger to determine how to reduce the charge rate. This feature has various names depending on the make and model of the inverter/charger but look for instructions on setting shore power, power sharing or simply charge rate. For the first two, just select a number that is lower than your available service which in this case would be 5A, 10A or 15A. The 5A setting may actually prevent your charger from working at all if there are any other loads enabled so it is best avoided. Revert this setting back to 30A when taking your coach out of storage. If you have an option to reduce the charge rate, then simply lower it down to about 10A or 10% whichever is applicable.

Running Loads on Generator Power
Running loads on generator power is pretty much identical to what has previously been discussed for 30A or 50A service depending on the make, model and most importantly, size of your generator expressed in kilowatts or kW. There are however some important points to take into consideration. To begin with, the generator is a very limited power source so expect to see the voltage reading drop as the current reaches its limit. If you are nudging the current limit on one or both legs then don’t be too surprised if the generator shuts down when a cycling load such as the refrigerator or water heater comes on, or a surge load such as that required to start the motors in your roof air conditioners kicks in. Motors such as these typically require up to five times their normal running current to get them moving.

Larger generators above 8kW are typically configured for 240V across two legs similar to the 50A service discussed above. While keeping the loads balanced isn’t critical on shore power, it is important on generator power to avoid damage in some cases. Loads that are not balanced well between the two legs will also cause an imbalance in the voltage which may result in the Surge Guard disconnecting power from the coach due to over/under voltage protection in extreme cases.

Running Loads on Inverter Power.
Have you ever wondered why the lights dim in your coach when you fire up the microwave on inverter power? If you recall from our discussion on power above, watts = volts x amps. This can also be expressed as amps = watts/volts.

Let’s say the microwave is a 1200W model. How many amps will it consume while on shore power? The answer is 1200W/120V or 10A. This 10A current won’t present much of a challenge while on 50A service but how many amps will be consumed from the batteries to run the same load? Remembering that the batteries are 12V, not 120V, the answer is 1200W/12W or 100A. Factor in some efficiency losses especially for true sine wave inverters and that microwave could easily consume up to 120A from the batteries while it is running. This current draw will vary somewhat depending on the actual battery voltage but it will significantly discharge your batteries in a very short time frame. Many of you more than likely run the generator when you wish to use the microwave but don’t forget that any heat producing appliance will present a huge load on your batteries. Examples of such appliances are the aforementioned microwave, toasters, coffeemakers, curling irons and worst of all, that super heavy duty atomic powered 1500W hair dryer!

You might find it interesting that the most popular inverters these days seem to be the 2000W and 3000W models yet to use all the current they can provide would discharge the batteries in a very short time-frame. We occasionally receive requests to add extra batteries to the battery bank but we caution against this because not only are you adding a significant amount of weight to the coach, the charge time required for these extra batteries is often prohibitive. Consider that your inverter can easily discharge two AGM batteries in less than an hour if you really wanted to but to recharge them fully from generator power can take up to six hours depending on coach model and year. You can add four batteries to the bank to increase the duration for which the inverter can supply a heavy load but how do you feel about running the generator for eighteen hours to recharge them?

Conservation of Battery Power during Dry-Camping
There are many tips and tricks to conserve your battery power when shore power is not an option. The simplest of course is to keep the various loads such as lights and the entertainment system turned off when not in use. Other savings are not so obvious however and a good example would be the inverter that can consume a constant six amps from the batteries even when all the loads on that circuit are turned off. The simple advice here is to keep the inverter turned off when you have no need for 120V power.

Automatic generator start systems are a popular addition if not already installed and are a great solution to keep your batteries charged while dry-camping. They are not recommend for use while your coach is in storage however because it is not advisable to have the generator running when nobody is around to keep an eye on it. If you do need to store your coach for extended periods then consider installing solar panels to keep your batteries charged and ready to go when you are.

That’s it for this article and hopefully we were able to help you with any questions you might have had. If not, or should you need further clarification on this article or others then send us an email to tech1@countrycoach.com.

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