On Inverters, Converters, and Transfer Switches – Questions From the Road (3)

Welcome to Questions From the Road!

Hello there! Here, I answer (or take a crack at answering) real questions from real RVers, just like you. You might find your question here! If not, please send me an email!

Will I lose circuits going from 50A to 30A?

“Is is logical to think, without opening up a panel, that when you connect a 4-prong, 50-amp RV plug into a 30-amp RV plug adapter (if a camp ground doesn’t offer 50 amp hookup) that I would lose half of my 120volt circuits?”

Is it logical? Sure, you’d be forgiven for thinking that! But thankfully, no, that’s not how things work!

With the right adapter, you can plug your RV into most standard outlets. As I discussed in A Surprising Difference Between 50A and 30A Power and Can You Plug Your RV Into a Dryer Outlet, you can plug a 50A RV into a 30A hookup, a 20A hookup, or even a small 15A household outlet. A 30-amp adapter will “fork” the single-phase 30A power into two faux “legs,” although both are in the same electrical phase. So both 120V legs in your RV should receive power.

Fair warning, though: Don’t MacGyver your own adapter! Many 30A RVs have been damaged by plugging into a 240V outlet.

All circuits will remain operational when you use an adapter. However, you will be limited in how much total power you can draw.

  • A 50A hookup provides two 50-amp legs at 120-volts (240-volts total) for a total of 12,000 watts of power.
  • A 30A hookup provides a single 30-amp leg at 120-volts for a total of 3,600 watts of power.

All circuits will still work; you just can’t use as much power as you’re used to!

In real-life terms, this normally means you can only run one large AC appliance at one time when plugged into 30A power. You generally can’t run two air conditioners, or an air conditioner plus a microwave, for instance.

Does an inverter only feed a limited number of circuits?

“When a pusher coach has a 2,000-watt inverter, the inverter more than likely will go to a separate panel, where it will only feed 120VAC to a limited number of circuits?”

I see what you’re thinking, Bill. 2,000 watts is not much power compared to a standard 50A hookup (12,000 watts), so how could an inverter possibly supply full power to all the circuits?

The trick is that an inverter may be installed in different ways. In a whole-house installation, an inverter probably won’t be able to supply full power to all circuits – but it can still operate any single circuit. In Bill’s case, the 2,000-watt inverter is probably just supplying a limited number of circuits in a pass-through capacity.

In my deep dive on inverters, I talked about how most OEM inverters are hardwired separately from the converter/distribution panel. They are wired to a transfer/relay switch (or have one built-in).

In other words, you’re giving your RV a choice: It can either run off of shore power, or off the inverter (or off the generator, in some cases). But not both. 

Some transfer switches can be flipped manually; most will automatically detect where power is coming from and switch to that source. Think of the transfer switch like an electrical traffic controller.

So an onboard inverter will still deliver power to all your circuits – and not just the 120VAC ones! Because it’s pumping power to your converter, it can run your 12V circuits as well.

(If you connected the inverter to the AC circuits after the converter, using a separate subpanel and relay, you would lose this capability).

But again, the catch is total power will be much, much less compared to shore power. If your inverter is rated for 2,000 watts peak, then it can probably only handle about 1,500-1,600 watts continuously. That’s not enough juice to run most air conditioners (at least not without a soft-start capacitor).

A 2,000-watt inverter is good for powering your outlets and maybe a microwave, not for fully self-contained dry camping. You generally need at least 3,000 watts peak/2,000 watts continuous for that, and that’s just the bare minimum!

Are 12V circuits powered directly by the battery or through the converter?

“Does a diesel pusher have a converter for DC versus running directly off the batteries? The onboard inverter is a Magnum ME2012. Is it also a charger or does it have converter capabilities?”

Lot to work through here, Bill!

1) I don’t quite agree with the assumption behind the first question. An RV converter, technically, transforms AC electricity into DC electricity. That’s how engineers look at it.

Now, RV users look at a “converter” a little differently, because most RVs are built with an all-in-one RV converter + AC/DC power distribution panel (WFCO is a major manufacturer).

(Some RV manufacturers prefer a separate deck-mount converter plus a separate distribution panel, but most combine the two out of cost and efficiency.)

So technically, a “converter” doesn’t have anything to do with 12V DC electriicty from the battery. It’s already DC electricity; it doesn’t need to be converted!

Now, most 12V circuits are wired “to the converter,” but what we’re really saying is that the 12V circuits are wired to a fused DC distribution panel, which is powered by the battery. The 12V circuits don’t go through any fancy gadgetry like a DC-to-DC converter. If you strip away all the excess, they’re still just simple fused circuits.

Now, are the 12V circuits powered by the converter, or by the house battery?

Answer: Yes!

At the risk of oversimplifying the answer:

  • If you’re plugged into shore power, the converter will directly power your 12V circuits AND charge your battery. (A battery cannot be charged and discharged at the same time.)
  • If you’re NOT plugged into shore power, the 12V circuits operate directly off your battery, and the converter doesn’t really do anything.

2) The Magnum ME2012 is an inverter/charger, which basically means it can invert DC to AC electricity and convert AC to DC electricity. So it goes both ways. Pretty cool, huh?

It’s not quite an all-in-one solution. The Magnum doesn’t have a circuit breaker power distribution panel, for instance. Most inverter/chargers are designed to be added onto existing RV electrical systems, which already have power distribution panels.

Don’t get too confused by the converter vs. charger lingo. A charger is a converter, but is a converter isn’t necessarily a charger. Clear as mud?

It’s kind of a semantic thing. Basically, people use the term “charger” to refer to an microprocess-controlled electrical device (usually a “converter”) designed specifically to charge batteries. The ME2012, for instance, has a robust 3-stage charging algorithm to recharge your RV house batteries. 

So yes, your Magnum inverter is also a charger. 

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RV technician & design engineer by day, intrepid blogger by night (and sometimes weekends). My website is about how RVs work, and sometimes why they don't.

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