You've identified the core problem: a 120/240V RV receptacle needs 240V between the hot legs, and you only have 208V available between your hot legs on a 120/208V 3-phase system. An autotransformer is the right idea, but you need to be very careful about how you implement it, especially regarding the neutral connection.Here's the correct approach and why

Understanding the Issue:
120/240V RV Receptacle

This expects two 120V legs that are 180 degrees out of phase, providing 240V between them.

120/208V 3-Phase

You have three 120V legs that are 120 degrees out of phase. The voltage between any two of these legs is 208V.

The Problem

Wiring two of your 120V legs directly to the RV receptacle won't work. The RV will see 208V instead of 240V, which can damage appliances designed for 240V.

The Solution: Using an Autotransformer1. Choose the Correct Autotransformer



Step-Up

You need an autotransformer specifically designed to step up 208V to 240V. Look for models marketed for this specific purpose. These are sometimes labeled as "buck-boost transformers" and can be wired as autotransformers.

VA Rating

The autotransformer must have a VA (Volt-Ampere) rating that can handle the 50A load of the RV receptacle at 240V. That's 50A 240V = 12,000 VA (12kVA). It's always best to slightly oversize, so a 15kVA autotransformer would be a good choice.

Neutral Connection



Crucially, look for an autotransformer specifically designed for single-phase, 3-wire (2 hots + neutral) operation. Many autotransformers are intended for control circuits and don't have the appropriate neutral provisions for supplying power to a full load. Some single-phase autotransformers have taps for various voltages; these can often be configured to boost 208V to 240V. The wiring diagram is critical.2. Wiring (Assuming you find an appropriate autotransformer with a neutral connection)



Input (from Panel)



Hot 1

Connect one of your 120V hot legs from the panel to the appropriate input terminal on the autotransformer (typically labeled "H1" or "208V In").

Hot 2

Connect another of your 120V hot legs from the panel to the appropriate input terminal on the autotransformer (typically labeled "H2" or "208V In").

Neutral

Connect the neutral wire from the panel

DIRECTLY to the neutral terminal of the RV receptacle. Do NOT run the neutral through the autotransformer. This is absolutely critical for safety. Ground

Connect the ground wire from the panel to the ground terminal of the autotransformer AND the ground terminal of the RV receptacle. Make sure the autotransformer chassis is also grounded.

Output (to RV Receptacle)



Hot 1

Connect the corresponding output terminal of the autotransformer (typically labeled "X1" or "240V Out") to one of the hot terminals on the RV receptacle.

Hot 2

Connect the other output terminal of the autotransformer (typically labeled "X2" or "240V Out") to the other hot terminal on the RV receptacle.

Neutral

(As mentioned above, connected directly from the panel to the RV receptacle's neutral).

Ground

(As mentioned above, connected from the panel to BOTH the autotransformer and RV receptacle's ground terminals).

Important Safety Considerations



Licensed Electrician

This work must be done by a qualified and licensed electrician. Incorrect wiring can lead to fire, electrocution, and equipment damage.

Permits

Obtain all necessary permits from your local electrical inspector before starting the work.

Overcurrent Protection

Ensure the branch circuit breaker in your panel is properly sized for the 50A RV receptacle.

Grounding

Proper grounding is absolutely critical for safety. Do not skip or compromise on the grounding connections.

Autotransformer Enclosure

The autotransformer must be installed in an appropriate enclosure, suitable for the environment (indoor/outdoor).

Inspect the RV

Verify the RV is actually designed to use 240V. Some RVs only use 120V.

Why the Neutral Goes Straight Through

The neutral wire provides a return path for unbalanced 120V loads within the RV. The autotransformer is only boosting the voltage between the two hot legs; it does not affect the 120V voltage between either hot leg and neutral. If you were to run the neutral through the autotransformer, it would disrupt the voltage relationships and potentially damage equipment.

In summary

1. Find a single-phase autotransformer specifically designed to step up 208V to 240V with a neutral connection and sufficient VA rating (15kVA or higher is recommended).
2. Connect the two 120V hot legs from your panel to the appropriate input terminals of the autotransformer.3. Connect the 240V output of the autotransformer to the two hot terminals of the RV receptacle.
4.

Crucially, connect the neutral wire from the panel directly* to the neutral terminal of the RV receptacle. Do not run it through the autotransformer.5. Connect the ground wire from the panel to both the autotransformer chassis and the ground terminal of the RV receptacle.
6. Have a licensed electrician perform the installation and obtain all necessary permits.This is a complex electrical installation, and safety is paramount. If you are not a qualified electrician, do not attempt this yourself. Flag for review