Our Odyssey

AC Electrical System

This is arguably the most complex system on the coach, and took a great deal of engineering to get right. Be forewarned: the following narrative is lengthy and arcane. If AC power fascinates you (or if you're converting a bus, or maybe building a yacht), read on.

When we bought the coach, the existing conversion was all-electric, and all-whizbang, by which I mean pretty much nothing on the coach worked unless you were either plugged into shore power, or running the enormous 16.5KW on-board generator.

For example, the stove was a 240-volt Gaggeneau cooktop, which meant you had to run the genset just to boil water, even if you had, say, 30-amp shore power available.

Our taste runs more to boondocking, so we started over with pretty much a blank sheet of paper. The one thing we decided to keep was the generator. It's about twice as big as we need, but the economics were just not there to trade it for an 8-10KW unit, which would still have been about as large physically.

Given that we already had excess 24-volt alternator capacity from the main engine, and that we wanted to be able to run electronic equipment of any type from our battery plant, we quickly settled on the Xantrex/Trace SW4024 Inverter/Charger to be the heart of our 120-volt power system:

This is a high-dollar item, with lots of features that are designed for off-grid living (the fixed-dwelling term for "boondocking"), and we wanted to maximize our investment by making use of as many of these features as made sense. Note that we bought the standard SW4024, which is designed (and listed) for residential usage. There is also an SW4024-MC2 model, which is listed for RV and marine use. We bought the residential one, because they are more widely discounted and/or surplussed and we got a much better price. Click here to read more about this issue.

The combination of the massive generator and the complex SW4024, while a powerful team, presented a number of challenges:

  1. The SW4024 is a 120-volt 3-wire device, while the generator and shore cord are 240/120-volt 4-wire systems.
  2. The SW4024 has a 60-amp internal transfer switch for grid/generator, but it is only 120-volt single-phase.
  3. The SW4024 does not provide a ground-to-neutral bonding relay, while the generator had the ground and neutral bonded internally.
  4. The SW4024 provides a generator start/stop signal if the batteries discharge below a set point, but this system is defeated if grid power is hot.
  5. The generator can deliver 70 amps at 240 volts, but most transfer switches made for the rv/conversion/yacht market top out at 50 amps.

After fiddling around with numerous designs involving off-the-shelf components, none of which could address all of these issues, we finally concluded that either (a) we had to give up some of the automated features of the SW4024, or (b) we had to convert to an entirely 120-volt, 3-wire system, or (c) we had to custom-build a transfer switching system to integrate these components.

Option (a) was unacceptable, as some of the off-grid features of the SW4024 were exactly the reason we selected it, and we considered them essential to full-timing. Option (b) would have meant downsizing the generator (or leaving more than half of its power output unusable), or rewiring it for 120-only and going to 140-amp main breakers (with all that entails, including very large mains). That left option (c), which is what we did.

The resulting custom-built Automatic Transfer Switch with Energy Management is detailed here, along with drawings and notes. For those unwilling to delve that far into the gory details of the design, this device does the following:

  1. Provides automatic connection to either shore power or generator power when either is available. If both are available, the generator is given preference.
  2. Provides a ground-to-neutral bond for the coach that is automatically broken when shore power is available and connected. (If shore power is available but not connected, the shore neutral is also disconnected from the system.)
  3. Passes a single-phase, 120-volt feed from both shore and generator, as the generator first comes on line, to the SW4024 so that the SW4024 can make its own timed selection between them using its internal transfer switch. This allows the SW4024 to sense that the generator has properly started in auto-start mode, and allows it time to synchronize its wave form.
  4. Delays connection of loads (other than the SW4024) to the generator by a selectable time period to allow the generator to stabilize after starting.
  5. Drops grid power upon low-battery signal from the SW4024, thus allowing the SW4024 to start the generator (see issue #4 in the previous list). This is useful if shore power is available, but not large enough to keep the batteries from running down.
  6. Connects non-critical loads (everything other than the SW4024 and its connected loads) to shore power only if both sides of the shore power are separately fed (50a/240v service, vs. 30a/120v service or less). A signal is also sent, when shore power is available but not 240v service, to an Energy Management System at the main load center.
The Automatic Transfer Switch:

As it turns out, there is no actual 240-volt equipment aboard Odyssey, except the transfer switch itself. This allowed us to simplify at least part of the design, as we don't need any 240-volt main panelboards. We also never need worry about finding 240-volt power, and everything on board can, if the need arises, be connected to the inverter. Here's a "single-line diagram" of the AC power layout.

There is a small 240v/120v load center connected directly to the generator output. The feed to the transfer switch comes from this panel. Also connected to this panel ("Panel G") are the electric cooling fans for the generator itself, and a 50-amp "buddy plug" so we can actually power something outside the coach, such as another coach, or perhaps a small third-world country.

Everything else in the coach is connected to one of three 120-volt "panels" fed from the inverter and/or the transfer switch. For space and wiring reasons, the three panels are really three busses in one enclosure.

Panel "I" is the "Inverter" panel, fed from the output of the SW4024 (through a 60-amp main breaker/disconnect). This panel is hot all the time, and all our critical loads are connected here, along with one roof air conditioner.

Panel "N" is the "Normal Power" panel. This panel is hot only when the generator is running, or when 50amp/240volt shore power is available. This is where electric heaters, air conditioners, and other auxiliary devices are normally powered.

Panel "S" is the "Switchable" or "Shared load" panel. This panel is fed by a small transfer switch which, in turn, is fed from panels "I" and "N." This is our Energy Management System (EMS).

A panelboard schedule detailing what's connected to which panel is here. There is also a floorplan showing AC outlet locations (differentiated by panel) here. Some of the outlets are downstairs, shown here. Note that three roof air conditioners are fed from three different panels. A multi-way selector switch allows us to select which air conditioner(s) will be fed from the inverter. Back to main technical information page