In a two-pump, battery-backed sump system, how do people avoid excessive power draw when the battery charger and primary pump want to run at the same time?

I'm imagining a post-hurricane scenario: The sump basin continues to refill with water when the grid power is finally restored. At that point, the primary pump and the battery-charger will both want to run at the same time.

I assumed people would solve this by using some kind of intelligent power switch, that only gives power to the battery charger when the primary pump isn't running. But I haven't found anything like that on the market, so I assume that's not a typical solution.

So how do people avoid this problem? Is it as simple as providing as 20A circuit instead of 15A?

(My question is similar but a little different from this one)

  • Most sumps and battery chargers should be okay coming on at same time. If you add a fridge or something else on the same circuit might have trouble.
    – crip659
    May 4, 2021 at 15:55
  • 1
    There are many ways to do this but a 20 amp circuit needs to have the max load below 2400w. A 15 amp circuit max below 1800 watts. I have run MWBC’s to a duplex to provide separate circuits prior to the GFCI mandate (my state did exempt GFCI for this case) each pump system or charger are usually made for 15 amp circuits so having 2 with only 1 extra wire gives more bang for the buck or 3600w max vs 1800 or 2400 with a 15 or 20 amp circuit. If your state requires the GFCI’s on the sump I would not use a multiwire branch circuit but a 20 amp or separate circuits.
    – Ed Beal
    May 4, 2021 at 16:12
  • Can we see the specifications sticker on your battery charger? Unlike newer chargers for cars and eBikes, most chargers for systems like yours work slowly and draw trivial amounts of power.
    – jay613
    May 5, 2021 at 20:35

2 Answers 2


By sizing the loads appropriately

Every load has a nameplate or published draw. You are to use the numbers on the various nameplates to assure that you are not overloading the circuit, or (more literally to Code), to assure you provide large enough circuit(s) for your loads.

You can get any ampacity of battery charger that you want.

You seem to be looking for an electronic solution; that's grossly over-complicated compared to simply sizing circuits (or appliances) correctly in the first place. But it is permitted in Code, mainly used for "load shedding" to allow an automatic transfer switch with a generator that is too small to pick up all provisioned loads. You couldn't use that gear here because it is monitoring for sagging voltage or AC frequency from the generator bogging, and that won't occur on a utility-supplied circuit wired with proper wiring. Unless the wiring is extremely long.

The DC pump is not powered by AC, it is powered by battery. The AC battery charger which refills the battery, does not need to be large enough to carry the full power of the pump, and it does not have a startup surge. So it might be 2-3 amps, leaving 12-13 amps for the AC pump.

  • Thanks but I think you've misunderstood my question. I'm not worried about both pumps operating simultaneously. I'm worried about the backup pump's BATTERY CHARGER drawing significant current at the same time as the primary pump is operating. I.e., what you'd get when grid power was restored after a hurricane. May 5, 2021 at 17:08
  • @ChristianConvey Edited. Oh, well, that's a matter of sizing the battery charger and circuit. a) they don't all need to be on the same circuit. b) you can size anything you like. May 5, 2021 at 19:55

Here is how IMO most people would solve this problem:

  • Install a larger circuit. Sure, 20A.
  • Install a second circuit
  • Run an extension cord to the charger from an existing second circuit
  • Buy a smaller charger.
  • Replace the backup pump with a water-powered one that requires no electric power (Random link provided for illustration, not a recommendation).

Nobody would install logic-controlled switch gear to handle this situation in a residence. There are plenty of arguments against this, but IMO the biggest one is that such gear would increase system complexity thus reducing the reliability that a backup pump is intended to provide. If done wrong it could create a single point of failure for both pumps.

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