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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)

Christian Convey
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2 Answers2

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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.

Harper - Reinstate Monica
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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.

jay613
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