I’m setting up a backup system for my home’s sump pump. Rather than deal with the installation/maintenance of a separate backup pump I’ve decided to go with an inverter & 12v battery setup. My sump pump claims 9A draw, so I purchased a 2000W inverter & a battery. What I didn’t realize is that the inverter is only for providing constant power, but does not switch between power sources. The inverter has a standard AC outlet on it for plugging in a load. What I think I’m looking for is a relay that switches between two 120V power sources and sends the power to a single load. I’ve tried searching, but I feel like I might be missing the right terms to narrow down to what I’m looking for. Any tips on finding a relay like this? Does such a thing exist?
Figure 1. Automatic transfer circuit.
Figure 1 shows one way of doing the job.
- RLY1 needs to be a contactor (heavy-duty relay).
- Coil voltage = 120 V or 240 V AC, depending on your supply.
- Contact rating 10 A inductive or motor start rated at the AC voltage you are switching.
- Contact arrangement: 2-pole, changeover.
When mains power is lost (as shown in the schematic) power will be supplied from the inverter. When power is restored the relay will energise and power the pump from the mains.
Normal safety precautions apply. Install in box that requires tools or key to open. Gland all cables to prevent chaffing. Add a fuse to cut supply to the coil in the event of a solenoid fault. It may prevent a fire.