Our Odyssey

Inverter/Charger -- Xantrex Trace SW4024

Inverter/charger selection for a project like ours can be daunting -- there are perhaps a dozen models on the market that could do the job, each with its own strengths and weaknesses. Once upon a time, these dozen or so models were made by three or four major manufacturers, and one of the decision factors would have been the reputation, service, and support of the manufacturer. In the past few years, however, the three largest inverter manufacturers -- StatPower, Heart, and Trace -- have been acquired and consolidated by Xantrex, a Canadian firm. Xantrex has kept most of the product lines unchanged and in their original packaging, meaning there is a lot of overlap in the Xantrex catalog, and even today, choosing the model that is right for the application can be challenging.

We looked seriously at what we were going to be running on the inverter, coming up with a size requirement of around 4KW (the VannerVerter inverter we removed from the coach was a 5KW unit, and we never hit that limit), which would let us run pretty much anything we wanted plus one air conditioner. We also wanted "true sine-wave" output, to be kind to our high-end electronics. Lastly, we wanted 24-volt input, to keep the cable sizes down and to match up with our existing 24-volt system that includes a massive 270-amp alternator. Taking all these factors into consideration, we quickly zeroed in on the top-of-the-line Trace SW4024 inverter/charger.

Sounds simple, right? Well, as it turns out, not quite: there happen to be two subtly different models of the SW4024 (and it's smaller sibling, the SW2512) -- the "standard" SW4024, and the "RV/Marine" version, the SW4024-MC2. Sussing out the real differences between these models proved to be quite difficult indeed. I will spare you all the gory details of what we went through on this issue, and, instead, just present our findings directly:

(Drum roll, please.) There is absolutely no difference whatsoever in the hardware between these two models. The differences between these units are twofold: 1. The SW4024-MC2 has a special software release, the key differences for which I will relate below, and 2. The two units carry very different regulatory approvals (and the associated labels), which I will also discuss.

Software differences: These turned out to be less spectacular than one might imagine. Before I discovered this, I spent a good deal of time trying to find out if the MC2 software can be retrofitted into the standard model (answer: Xantrex will not sell you the software, which comes pre-loaded on ROMs, for this purpose, however, if you could "find" a set of ROMs -- perhaps in a salvage unit -- there is no reason they would not work.) After I found out the real differences (by obtaining a copy of the MC2 manual supplement, a document notably absent from Xantrex's otherwise quite complete on-line document library), I decided it was not worth the trouble. The differences are:

  1. All software associated with "selling" power back to the utility has been removed from the MC2 model.
  2. The battery equalization mode has been defeated in the MC2 model, on the assumption that all modern RVs and yachts are using AGM or gel batteries (an assumption I believe to be erroneous).
  3. If both Generator and Grid (shore) power are available, the MC2 model will default to Generator, while the standard model will default to Grid (even if Grid is smaller than Generator) -- this is not user-selectable.
  4. The MC2 model has a setting on the main menu for Shore Cord Size. On the standard model, one needs to enter the setup menus and change the "AC1 Amps" setting to change shore cord size.

We don't intend (nor is anyone in an RV permitted) to "sell" power back to the utility, so item #1 did not concern us at all. Item #2 was a factor in favor of the standard model -- we will be using AGMs, but defeating this capability means that flooded batteries are no longer a real option. Also, there are times when AGMs may need an equalization charge. Item #3 was a real issue for us: we would much rather have generator priority over shore power -- if we chose the standard model, this would have to be addressed somehow. Lastly, item #4 was a selling point for the MC2 model, but only a nuisance and not a show-stopper if we chose the standard.

Regulatory approvals: This became an issue for us mostly because I am a total code weenie. I am a real believer that most provisions that make it into the electric code do so only after someone (often, far more than one someone) has died, and I am reluctant to overlook code issues even when they "don't apply to me". The specific issue here is that the standard model is UL listed for fixed installations, but not mobile ones, while the MC2 model is UL-listed and USCG-approved for RV and Marine installations, but not for fixed installations. The units are not cross-labeled -- you will not find the approval labels for one application on the other model. This means that, if you need to pass a regulatory inspection, you need the standard model for fixed use, and vice-versa. For example, the standard model will likely not pass a USCG inspection.

The NEC section on mobile installations requires that all components of the electrical system be "listed" and that the listing be specific to the use. Whether or not the standard model's "listing" can be construed to be within the code guidelines for RV use is somewhat of a gray area. It's UL listed for use in standby power systems, but not specifically for mobile use. Then again, a standard electrical junction box is also listed, but not specifically for mobile use, yet these items are clearly intended to be permitted by the code. In any event, the nature of Odyssey's conversion is such that it is not subject to any sort of code inspection or regulatory approval (I'm philosophically opposed to this, but it's the reality). After hemming and hawing about this for quite a while, we decided that use of the standard model would be acceptable, for two reasons: 1. The code clearly allows use of many standard electrical components that are "listed" without specific listing for mobile use and 2. the standard model and MC2 model are physically identical -- any test that one would pass ( e.g. flame spread) the other would pass as well, it being simply a marketing decision on Xantrex's part not to submit both models for testing to each agency.

In the end, for us, it came down to cost: Both models list for $3,495. The MC2 model is widely discounted to around $3,200, and is generally available only from RV and Marine dealers. The standard model, however, is far more widespread, as the SW4024 is sold mostly for residential applications such as off-grid, photovoltaic, or standby power. As such, it is much more heavily discounted, and we purchased ours for around $1,900 -- a savings of about $1,300, or 37%, over the best deal we could get on an MC2. As a bonus, we still have our equalization capability.

Having purchased the SW4024 standard model, our next challenge was to come up with a way to allow the generator to become the priority source, especially in the case where the shore power input is insufficient to keep the batteries from discharging. We also had to find a way to implement a neutral-ground bond switching scheme that would handle all combinations of shore/generator/inverter power correctly -- neither the MC2 model nor the standard model has any internal provisions for this. Return to the AC power page to read more about how we accomplished this.

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