These devices prevent breakers from tripping by monitoring power usage and adjusting electrical loads.
By Mark Quasius, F333630
Today’s motorhomes contain more power-hungry accessories than coaches once did. Thirty-amp systems often can’t supply enough electricity to meet all the needs that owners of RVs demand these days. Even 50-amp services can be challenged when everything in an all-electric coach is switched on.
Many RV parks have stepped up to meet the demand by providing electrical systems that can handle modern RVs, but there may be times when only a 30-amp power supply is available to run a motorhome’s 50-amp electrical system. If your power consumption exceeds what is available, you’ll spend lots of time trekking outside, resetting the circuit breaker on the campground pedestal, and switching off unnecessary loads.
An energy management system (EMS) automatically handles such circuit switching. It’s important to note that we are not talking about surge suppressors, even though some models are also advertised as energy management systems. Surge suppressors protect an RV from low voltage or surges, but they do not manage power consumption. A true EMS, on the other hand, monitors power usage and makes adjustments to the loads to reduce nuisance tripping of the pedestal breaker. Many motorhome manufacturers incorporate an EMS into their premium units. Precision Circuits Inc. and Xantrex are two companies that produce such systems.
Power Consumption Basics
Before we go further, an overview of RV power consumption may be helpful. A 30-amp shore power supply utilizes a three-wire connector that plugs into a 30-amp RV-specific receptacle on the campground pedestal. The service has a single-pole, 30-amp, 120-volt feed that provides 3,600 watts of power.
A 50-amp supply uses a four-wire connector that provides two poles — each delivering 50 amps — producing a total of 12,000 watts of power. So, a 50-amp service provides more than three times the power of a 30-amp supply.
In an RV with a 50-amp electrical system, ideally the electrical loads should be evenly distributed between the two poles of the breaker panel. When connecting a 30-amp outlet to a 50-amp shore power cord, a dog-bone adapter converts the 30-amp plug to a 50-amp receptacle. The adapter then supplies power to both poles.
On a large inverter-charger (typically between 2,000 and 3,000 watts), the inverter’s AC outputs are connected to a subpanel; it supplies certain circuits with AC power while you are driving or dry camping, eliminating the need to run the generator. The inverter uses battery power to invert the DC battery voltage into 120 volts AC and supply power to the subpanel. Also, the inverter’s AC input connections are supplied from the main breaker panel so that pass-through AC power can be fed to the subpanel whenever shore power or generator power is present; that pass-through AC power also drives the battery charge circuitry in the inverter-charger.
A basic energy management system monitors power usage via a current loop transformer on the breaker panel’s neutral line. The EMS also tests the two hot poles to determine whether the power supply is 50 amps; the EMS does this by checking for the presence of 240 volts across line 1 and line 2. If this is not detected, the EMS will assume a power supply of 30 amps, although the remote display panel could be set manually to 20 amps or 15 amps if that is all that is available.
Once the current draw exceeds the incoming power, the EMS will reduce power consumption by switching off certain circuits until power usage drops below the threshold. This load shedding is done according to a predetermined order set for each particular coach’s needs. Once the extra load is removed, power is restored as circuits that were previously shed begin to come back on line.
Here’s an example. An RV is running two 11-amp air conditioners and is using another 4 amps for battery charging to meet 12-volt power needs. That’s a total of 26 amps. Now, someone turns on the microwave oven, which requires another 10 amps. That brings the total load to 36 amps, which is too much for a 30-amp pedestal breaker. To prevent tripping the breaker, the EMS sheds whatever circuit is first on the list — most likely the rear air conditioner. That reduces the power demand to 25 amps. Once the microwave is off, the shed circuit is restored automatically without any operator intervention. And, let’s say the electric water heater element was switched on before the microwave oven but was shed as soon as the second air conditioner kicked in. Once the second air conditioner is finished running, the power to the water heater will be restored as well.
Advanced energy management systems take this process to an even higher level. Remote displays show detailed information about voltage and amperage draws. The systems also monitor generator power as well as shore power and manage the AC loads accordingly. Perhaps the biggest advancement is the feature called inverter assist, which was introduced by Precision Circuits in 2006.
Inverter assist facilitates communication between the EMS and an inverter. The feature increases the total usable power by adding the inverter’s output to the available shore power. When power demand exceeds supply, the EMS instructs the inverter to use battery power to operate accessories that are connected to the inverter’s output via the AC breaker subpanel. In the above example, switching on the inverter would not cause the rear air conditioner circuit to be shed. Instead, the EMS would trigger the inverter to use the batteries to power the microwave, rather than use any pass-through shore power. If the additional loads were too large for the inverter to handle, the EMS would begin to shed loads.
Inverter assist also allows the EMS to control the inverter’s battery charge rate. The inverter/charger is one of the largest appliances in an RV. Inverter assist initially lowers the battery charge rate for short periods to allow other appliances to operate. It does this in an intelligent manner. If the batteries are low and the charger is running in bulk charge mode, charge reduction will not occur. The EMS will lower the battery charge rate immediately in order to prevent tripping the breaker. The EMS then slowly raises the charge rate in 10 percent increments while monitoring the current to prevent exceeding the breaker’s rated capacity.
Precision Circuits Inc. has been supplying energy management systems to motorhome manufacturers since 2006. Its Power Control System (PCS) consists of a breaker panel that contains an integrated control module with imbedded relays and control logic circuitry. The system can shed up to four AC circuits and three additional DC circuits. The DC circuits can be used to control air-conditioning compressors or may be connected to DC relays that control AC circuits. The system is programmed via software according to each RV’s specific electrical system.
In addition to load shedding, the PCS also can communicate with a Magnum inverter to provide inverter assist. The PCS system automatically identifies the incoming power source and manages the energy demands accordingly; the system will manage energy usage when running on either a generator or shore power to prevent tripping the main breaker on the generator.
The recent trend toward all-electric coaches with electric cooktops, heated tile floors, and residential refrigerators has placed additional demands on electrical systems and can cause even 50-amp pedestal breakers to trip. Fortunately, the PCS is designed to manage a 50-amp shore power feed as well as 30-, 20-, and 15-amp feeds. An LCD remote display panel provides a wealth of information pertaining to electrical usage, such as voltage and current draw on both legs. It displays each controllable circuit with a plain English name as well as that circuit’s status so you can see which loads have been shed, if any. It also checks the power line configuration and warns against any open neutrals or ground conductor failures.
The PCS doesn’t require additional components outside of the breaker panel. Simply replace the original breaker panel with the PCS breaker panel; make the electrical connections inside the panel; connect the inverter and remote display via a Cat 5 cable; and you are ready to go. The system is preprogrammed by Precision Circuits according to a coach’s electrical needs, so there is no tricky setup after installation. In fact, I installed this system in my coach without any difficulty. The PCS does not incorporate an automatic generator start (AGS) control, but an existing AGS module from Magnum or Onan will work just fine with this system.
A recent addition to the product line is the Mini-PCS. The existing breaker panel is retained, and a compact input/output (I/O) module is installed inside the breaker panel. This system does not manage 50-amp shore power but does manage generator feeds. And, it does not offer inverter load support but is a less expensive system capable of managing 30-, 20-, or 15-amp shore power feeds. A compact display panel with LED indicator lamps provides service information and load shed status, as well as the total amperage draw. The I/O module can handle two AC circuits and two DC circuits.
Xantrex, a longtime inverter manufacturer, recently introduced the Freedom Sequence energy management system. At this writing, it had not yet been installed in original equipment manufacturer (OEM) applications. Like the Precision Circuits system, the Freedom Sequence manages energy by shedding loads, and it, too, offers inverter assist, but it is used specifically with Xantrex Freedom SW series inverters.
Unlike the PCS, the Freedom Sequence is a stand-alone module that intercepts AC lines after they leave the breaker panel. The module requires additional space to mount, but it can be placed in a basement compartment as long as the module can access the AC wiring for the circuits that the unit will control. The module can manage four AC circuits and four additional DC circuits, as well as power sources from a generator or 50-amp shore power.
The Freedom Sequence is connected to Xantrex’s Xanbus data network via Cat 5 cabling. The network then connects to the System Control Panel (SCP), the Freedom SW inverter, and an optional Xantrex AGS module to control the generator auto-start functions. The SCP is menu-driven; one single controller display accesses the inverter, the AGS module, and the EMS. While the installation is a bit more involved than the Precision Circuits offering, it does have the advantage of minimizing the number of display panels typically found in a motorhome.
These energy management systems generally are installed as OEM equipment on new motorhomes, but they also can be retrofitted to existing coaches. It isn’t all that hard; as I noted, I’ve done this myself.
Both the Precision Circuits and Xantrex systems work well and are similar in what they do. They differ mainly in how they are incorporated into a motorhome. Your brand choice depends primarily on how your existing electrical system is configured. If you have a Xantrex Freedom SW series inverter, or if you do not want to swap out your breaker panel and you have room to add an additional device, then Xantrex’s Freedom Sequence is a good choice. If you have a Magnum inverter, or you would rather swap out your breaker panel than try to find room for an additional device, the Precision Circuits PCS system may be the best choice. If all you want is load shedding for a 30-amp pedestal, the Mini-PCS would be an inexpensive and easy installation.
In any case, an energy management system offers seamless operation of a motorhome’s electrical accessories and eliminates treks to the campground pedestal or generator to reset tripped breakers.
Precision Circuits Inc.
2538 Wisconsin Ave.
Downers Grove, IL 60515
3700 Gilmore Way
Burnaby, BC V5G 4M1