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Jack & Danielle Mayer

30 Amphere Inverter in 50 Amphere RV

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Design Considerations for RV Genset and Inverter Installations 

Most modern, large 5'ers have 50 amp shore power (a 50 amp RV actually has 2 - 50 amp lines, each feeding one side of the loadcenter, for a total of 100 available amps). This section assumes the following issue, which is fairly common: If you have a 50 amp RV, how do you safely integrate an inverter, or inverter/charger that has an internal transfer switch rated at 30 amps? This design is specifically done to address the issue of an inverter/charger with a 30-amp transfer capability used in an RV with 50-amp service. The purpose of the design is to circumvent the limitations of the 30-amp transfer switch, and to avoid using a sub panel for the inverter-fed circuits. The best design is always one that isolates the inverter circuits to a sub panel, but that is not always practical in a retrofit implementation.
 
Given a choice, I would always purchase an inverter/charger that has a 50 amp transfer switch. This would allow you to avoid the complexity of TS2 in this design and allow you to place the inverter directly in-line. Many manufacturers of high-powered inverter/chargers now offer a model with a 50 amp transfer switch. So if you are buying new, do yourself a favor and get one - it will simplify the installation.

An alternative to an Inverter/Charger
rvinverterinstallation.jpg

Inverter with a Separate Battery Charger
 
The simplest system is shown in the conceptual design drawing to the left. The major difference in this design is that you use an inverter, instead of an inverter/charger, and a separate battery charger.
 
The separate battery charger is required because you can't run 50 amps through the inverter pass thru relay - the original problem. This stops you from having  120 volt available to the battery charger, since the circuits inside the inverter for passthru and battery charging both operate on the same 120-volt input.
 
Inverter/Charger with an External Transfer Switch
 
In the design below, we use an inverter/charger, but do not use the transfer function, protecting against overcurent.

An alternative, using inverter/charger
rvinverterinstallation2.jpg

This design utilizes the battery charger in an inverter/charger but does not use the power passthru function. The input AC is fed directly from TS1, avoiding the RV loadcenter. This input AC to the inverter is ONLY used for battery charging, and is never passed on to the RV loadcenter. The reason the input is taken off of TS1 directly is to prevent a backfeed situation from occuring if TS2 fails. (If power was obtained from the loadcenter and you were inverting then you would be in a circular input loop.)
 
In this scenario, the battery charger is supplying the battery bank, and power is passing through the passthru relay, but it is "stopped" by the TS2 transfer relay, which is set to "prefer" the other input. This way no 50 amp load is ever placed on the 30 amp inverter transfer relay. Power only passes to the loadcenter from the inverter if there is NO power available from TS1 (no shore or genset). In that case, if the inverter is in invert mode, power will pass from the inverter to the RV loadcenter. 
 
The Preferred Design - All Charging Sources Integrated

RV electrical design for inverter/solar/genset
rvelectricaldesign2.jpg

This drawing provides an overview of the RV electrical system, and identifies major components used to support battery charging via: solar energy, the existing converter, and a new, high-powered battery charger contained in the inverter. It is a more detailed view of the design directly above.

 

Electrical input sources include a genset (either a portable, an RV mounted or a truck mounted), and shore power sources. Optionally, two main shore power sources are shown, controlled by a separate 50-amp transfer switch (TSO). These are intended to provide for a shore connection at the front of the 5er, and at the rear of the 5er. The existing converter is shown connected to an external power source (other than the RV) for optional use. This should never be plugged into the RV, but only to an external source via an extension cord.

Assumptions:

 

  1. The RV is wired for 50-amp shore power. This is actually 2-50 amp lines, for a total of 100 amps available at the loadcenter, on 2 legs. All shore power is assumed to be using 6ga wire, except where noted.
  2. All the transfer switches are 50 amps.
  3. The inverter is an inverter/charger with a high output battery charger that replaces the RV converter for normal use.  The inverter has a pass thru power capability controlled by a 30-amp transfer relay.
When inverting, the loadcenter is fully energized. It is up to the user to provide manual load management. In other words, don’t turn on the air conditioner, electric hot water heater, or other large loads. Turn the breakers off, if you are prone to forget.

A 400-amp catastrophe fuse is used to protect against an inverter short. It is placed either directly on the battery positive (if not placed in a fuse holder), or as close to the battery as possible, if a fuse holder is used. See the Truck Electrical Center page for additional details and sources for the fuse and other components.

 

The shunt is a 500-amp shunt. It must be placed “downstream” of all loads to get an accurate measure of amps/amp hours. Place it between the distribution hub and the battery negative.

 

Use appropriate size welding cable for the DC inverter runs. Consult the inverter installation instructions. Do not use less than 2/0. I prefer to use 4/0 in most situations if the inverter is 2000 watts or more. The inverter should not be more than 10 feet from the battery (cable run).

Note 1

 

Optionally, I show two main shore power cables. When using an external generator (either portable or truck mounted) it is often convenient to have a shore power cable at the front of the rig. You simply use another 50-amp transfer switch – that way you can’t have both “live” at once, or energize the other plug. This is obviously optional, but when wiring the transfer switches and deciding where to break into the main shore power cord you might consider leaving enough slack in the line to accommodate a future transfer switch if you decide not to do this right away.

 

Note 2

 

The line designated by Note 2 is the 120-volt AC input to the inverter. The inverter normally would pass this thru the transfer relay and on to the AC loadcenter. The issue is that the transfer switch is rated for 30-amps and the potential load on a single leg of the shore power line is 50 amps. The transfer switch built into the inverter can be overloaded. The purpose of TS1 is to circumvent this issue and still allow use of the high-powered battery charger in the inverter/charger. Thus, AC power needs to be supplied to the battery charger, but the power passing through the inverter must be stopped from reaching the loadcenter.  The inverter will pass power to TS2 when shore power is available, but since TS2 is already receiving shore power on the preferred input, the power from the inverter is blocked. Thus the relay in the inverter can never be overloaded, since no load is ever placed on it (when there is shore power available).  However, the battery charger is energized, and can charge the battery bank (assuming the inverter control is set to charge). Power is taken directly off the TS1 transfer switch output lugs. The line to the inverter from TS1 is a single 120-volt line, so only one of the hot terminals is tapped (along with ground and neutral). Since the only load on the AC lines into and out from the inverter are from the 1) battery charger and 2) the inverter, while inverting and passing power up to the loadcenter, these lines do not have to be sized the same as the main shore power lines. Depending on the size of the inverter, and the max "surge" of the inverter, you could use 12 ga. I recommend 10 ga., to cover any additional surge capacity various inverters might have. Each inverter will be different.

 

When wiring the input from the inverter to TS2 you need to jumper the hot lines. The inverter is only passing one hot line through, and you want to energize both of the lines going to the loadcenter. It is permissible to jumper between the two hot lines on  the input side of the TS2 inverter line to support this.

 

Note 3

 

Distribution hubs are used for DC power connections. The existing house DC wires that feed the DC loadcenter are not shown in the drawing, but they should be moved to the distribution hubs. Typically, a wire goes from the converter directly to the battery, and another from the converter to the DC loadcenter. If you are leaving the converter in place you can remove the converter-to-loadcenter wire, and splice a new wire into the line that goes from the battery charger output of the converter to the battery. (Your converter outputs may be different, but you get the idea...)
 
The solar input and conventional converter inputs attach directly to the distribution hubs. You should attach all DC power input/outputs here. Nothing should attach directly to the battery except some of the instrumentation and monitoring lines, and possibly the DC catastrophe fuse (if not in a holder). If you have additional DC loads you are adding, you may want to add a small DC fuse center, which would also attach to the distribution hubs. I usually add one to support fusing for the solar lines, and some of the instrumentation lines which otherwise require inline fusing (which is not as neat, and not centrally located).

Instrumentation

 

In the diagram, the dotted lines denote instrumentation lines. These are not shown in detail – there are multiple connection points and lines for each instrument. Follow the instructions.

 

Sometimes the solar controller will have a remote display, and sometimes the entire controller will mount where the display can be seen – it depends on the controller you use. If you have a choice, acquire the remote monitor for the solar controller. It will make the wire run for the solar line shorter. The solar line should be as short as possible to minimize voltage drop. I prefer to use 6 ga for the drop from the roof to the battery bank. Sometimes that means you have to trim the wire where it goes into the terminals on the solar controller in order to make it fit. That is OK. On the roof, I interconnect the solar panels with a minimum of 10 ga.

 

If the inverter monitor panel has a running amp hour capability (also called cumulative amp hour) then you can eliminate the Trimetric amp hour meter, since it would be redundant. If not, you really need to know your accumulated amp hours (either positive or negative), since that is the best measure of the state-of-charge of your battery bank. You can buy a Trimetric meter, with shunt, for under $200 at www.solarseller.com. If you are using a Heart inverter, the Link 1000 or Link 2000 monitors contain an amp hour meter.