Is there a 30 to 50-amp transformer?

Why can’t you use a transformer to boost a 30-amp outlet up to 50-amps?

Can a transformer boost 30-amps up to 50-amps for an RV?

Hey Mike,

Someone told me you said it’s not possible to use a transformer to boost a 30-amp outlet to 50 amps for an RV. Why won’t that work? - Fast Eddie

Hey Eddie,

Here’s the original question you’re referring to that was asked by an RV technician in 2022: “I have a question from a customer. Do they make a step up transformer to convert 30-amps from a pedestal to a 50-amp RV?”

Great question! I answered this seemingly simple inquiry from someone else a while back. The bottom line is that transformers can indeed boost amperage or voltage, but they can’t boost both amps and volts at the same time. The relationship between voltage and current flow in a transformer is called an inverse ratio. It’s all about the watts!

How it works…

You can get a rather heavy and expensive 120-volt to 240-volt step-up transformer that will convert the voltage. But by boosting the voltage by a factor of 2:1, the 30-amps of current at 120-Volts from the pedestal would drop by a factor of 1:2 to only 15-amps at the RV. It’s all about the watts, and there’s no way to make more watts out of thin air.

Here’s the numbers…

You can only get 3,600 Watts from a 30-Amp outlet because 30 Amps x 120 Volts = 3,600 Watts. But a 50-Amp pedestal outlet can provide up to 12,000-Watts of power because there are 2 legs of 120 Volts at 50-Amps of current per leg. Rennet that Amps x Volts = Watts, so 50 Amps x 120 Volts = 6,000 Watts per leg, and 6,000 Watts x 2 legs = 12,000 Watts of total power from a 50-Amp pedestal outlet…

You want more power, but…

Transformers behave like transmissions in a car or truck where RPM is traded for torque. So, for example in first gear if the engine RPM is stepped down to one third by a 1:3 gear ratio, its torque is tripled by the 3:1 inverse ratio.

That’s why in first gear your car will have a lot more pulling power (think amps) but less speed (think voltage). You’re not making more horsepower, in low gear the transmission is trading speed for torque. The opposite happens on overdrive where the driveshaft RPM is increased with respect to the engine speed with loss of torque at the inverse ratio.

Let’s transform it…

The same thing occurs with a step-up transformer. If you use it to double the voltage from 120 to 240 Volts AC (a 2:1 ratio) the amperes of current will be halved (a 1:2 inverse ratio). But because Volts x Amps = Watts, you still have the same watts going into the transformer that are coming out. So 30 Amps at 120 Volts = 3,600 Watts, and 15 Amps at 240 Volts = 3,600 Watts. “Watts is watts”, as I like to say!

Yes, I have a 120-volt to 240-volt step up transformer under my desk…

But my test transformer weighs 30 lbs and is just 1.5kVA (1,500 watts), so it's less than half the size of what would be needed for a 30-amp pedestal connection. And even then a 3.6kVA transformer could only provide 15 Amps of current at 240-Volts, and never be able to supply the 50-Amps per leg (100-Amps total at 120 Volts) needed to fully power an RV with a 50-Amp shore power cord. That would require a 12kVA step-up transformer powered by a 100-amp, 120-volt circuit breaker. Yikes! 😁

Final analyses

You can’t use a transformer to boost wattage, it can only boost or cut voltage with an inverse change in the available amperage.

”There ain’t no such thing as a free lunch.” Robert A. Heinlein

Let’s play safe out there…. Mike Sokol