Installing an RV inverter and automatic transfer switch

Inverter, inverter/charger and transfer switch

• An inverter turns 12V DC battery power into 120V AC.
• An inverter/charger adds a battery charger and usually a built-in transfer relay. When shore power or a generator is present, it passes that power through and charges the battery. When it's not, it inverts.
• An automatic transfer switch (ATS) selects between two AC sources (for example, shore/generator and inverter) so only one feeds the RV at a time. Many RVs already have a transfer switch for the generator.

Danger: 120V AC can kill. Inverters can produce 120V even when the RV is unplugged, and incorrect wiring can back-feed shore power or create shock hazards on the RV body. Have a licensed electrician do or inspect all 120V AC wiring, including the transfer switch and panel connections. Follow the inverter and ATS manuals exactly. RVs sold in Canada are generally built to the CSA Z240 RV standard, and modifications should keep the rig compliant.

Step 1: Choose the inverter

Size it from your planning work (Planning an RV solar system for Alberta: energy audit, panels, batteries and controller): continuous watts for loads that run together, plus surge for motors and compressors. Choose a pure sine wave model for electronics and appliances. For most RVs, an inverter/charger gives the cleanest integration.

Step 2: Decide what it will power

Options, from simplest to most involved:

1. Plug-in only: loads plug directly into outlets on the inverter. No AC wiring changes.
2. Selected outlets: the inverter feeds a few outlets or a small sub-panel through a transfer switch.
3. Whole RV: the inverter/charger or ATS feeds the main panel.

With options 2 and 3, prevent the inverter from powering big loads it can't handle: the converter/charger (which would try to charge the battery from the inverter, wasting energy in a loop), an electric water heater element, air conditioners and electric heaters. A common approach is to move loads that should be inverter-powered to a separate sub-panel.

Step 3: Pick the location

• As close to the battery as practical, to keep DC cables short.
• Dry, ventilated, and with the clearances the manual specifies.
• Not in the same sealed compartment as flooded lead-acid batteries.
• Accessible for the remote panel, reset buttons and fans.

Step 4: DC side: battery, fuse and cables

1. Turn off all chargers and disconnect the battery negative.
2. Install a main fuse close to the battery positive. Many inverter manuals specify a Class T or ANL fuse of a particular size. Lithium banks can deliver very high fault currents, which is why Class T fuses (with a high interrupt rating) are often recommended.
3. Add a battery disconnect switch if the manual calls for one.
4. Run positive and negative cables of the gauge the manual specifies for your cable length. Keep the positive and negative cables together.
5. Crimp lugs with the correct tool, add heat-shrink, and torque connections to spec.
6. Connect the inverter's chassis/ground lug to the RV's chassis ground as the manual describes.
7. Reconnect the battery negative last. Expect a small spark as the inverter's capacitors charge. Some manuals describe a pre-charge step.

Step 5: AC side (licensed electrician)

The electrician will:

• Wire the ATS so shore power or a generator takes priority, and the inverter feeds only the intended circuits.
• Make sure the inverter's neutral-to-ground bonding is handled correctly. Many inverters bond neutral to ground while inverting and remove the bond when passing shore power through. Wrong bonding can create shock hazards or trip GFCIs.
• Size breakers and wiring for the inverter's AC output and pass-through current.
• Test GFCI protection and polarity.

Step 6: Test

1. With shore power unplugged, turn the inverter on and confirm that the intended outlets work and others don't.
2. Plug into shore power and confirm the transfer happens and the inverter/charger switches to charging (if equipped).
3. Use a clamp meter to check DC current under load, and feel connections for heat after a sustained load.
4. Set the charger profile for your battery chemistry.

When to call a pro

Any time 120V AC wiring, the main panel, or the RV's original transfer switch is involved, hire a licensed electrician. If your RV is under warranty, check with the manufacturer or dealer first.

Related guides

• RV solar and battery fusing, breakers, grounding and safety
• RV outlets not working on shore power: GFCI, breakers and pedestal checks
• RV battery not charging: how to troubleshoot the converter and 12V system