What does a transfer switch do in a van?

A transfer switch selects between two 120V AC sources — typically an inverter (battery) and shore power (campground hookup). When shore power is connected, the transfer switch sends shore power to your 120V outlets and disconnects the inverter. When shore power is disconnected, it switches back to the inverter. This prevents backfeed from the inverter onto the shore power line, which is a safety hazard.

Do I need a transfer switch if I have both shore power and an inverter?

Yes. If your van has both a shore power inlet and an inverter powering the same outlets, you must never run both simultaneously — the two AC sources are not synchronized and connecting them can damage your inverter. A transfer switch ensures only one source is active at a time. The automatic version does this without manual intervention.

Inverter Transfer Switch for a Van: Do You Need One?

If your van has both a shore power inlet and an inverter, a transfer switch is essential — not optional. Here's why and how it works.

Why you can't run shore and inverter together

Your inverter creates 120V AC at 60Hz from battery power. Shore power from a campground is also 120V AC at 60Hz, but not synchronized (same frequency, different phase, different timing). Connecting them to the same outlets simultaneously would cause large currents to flow between sources — potentially damaging or destroying your inverter, blowing breakers, and creating a fire hazard.

A transfer switch prevents this by ensuring only one source connects to your outlets at any time.

Types of transfer switches

Manual transfer switch

A rotary switch or toggle that you physically move between "Inverter" and "Shore." Simple, inexpensive ($30–$80), and requires no automatic logic. The downside: you have to remember to switch it when you plug in or unplug, and if you forget while shore power is connected and the inverter is on, you create the problem above.

Automatic transfer switch (ATS)

Detects the presence of shore power and automatically switches to shore. When shore is disconnected, switches back to inverter. Typical response time: under 100 milliseconds — too fast for most devices to notice.

Recommended: Progressive Industries ATS-30 or ATS-50 (~$80–$120). Designed for RV use, reliable, integrates with standard 30A and 50A shore connections.

Budget option: Reliance Controls 30A ATS (~$60) works but has less surge rating — fine for light builds.

Inverter/charger with built-in transfer switch (Victron MultiPlus, etc.)

The cleanest solution. The MultiPlus, Renogy inverter/charger, and similar combo units include a built-in transfer switch and charger. When shore power connects, the unit:

Switches outlets from inverter output to shore power (under 20ms)
Charges the house battery from shore
When shore disconnects, switches back to inverter automatically

No external ATS needed. The better choice for any build that uses shore power regularly. See inverter vs inverter/charger for the full comparison.

Sizing the transfer switch

Match the ATS to your shore power connection:

30A shore power (NEMA TT-30): ATS rated for 30A at 120V
50A shore power (NEMA 14-50): ATS rated for 50A at 120V (or 30A + 50A adapter for 30A ATS)

The ATS continuous rating should equal or exceed your shore connection.

Wiring diagram

Shore power inlet (TT-30 or 14-50)
         ↓
[Surge protector — optional but recommended]
         ↓
[Automatic Transfer Switch]
         ↓                ↑
[Inverter output] ────────┘
         ↓
[AC distribution panel]
         ↓
[120V outlets, cooktop, etc.]

Add a surge protector on the shore power line

Install a Progressive Industries EMS-PT30X or similar surge protector between the shore inlet and the ATS. Campground power is notoriously dirty — surges, low voltage, and miswiring are common. A $120 surge protector can save a $500 inverter.