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Plug-In Energy Monitor for Measuring Peak Usage

steve_smith | Posted in General Questions on

This seems like a better venue than most for energy conservation and monitoring questions.  I have a Kill-a-Watt, which is great for many things, but not every use case.  I am looking for a plug in energy monitor that can measure and retain the peak energy use.  I am trying to make sure that I can plug in two different devices into the same circuit without tripping the breaker (or worst case, cause a fire) long term.  I am looking to plug in two water pumps and want to make sure their start up power draw is okay on a 15 amp circuit.

Does anyone know of a good 120V plug in energy monitor that can measure and retain the peak energy use?


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  1. walta100 | | #1

    The peak from a correctly sized load on a circuit will not trip the breaker or risk a fire.

    15 amps is a pretty light circuit for 2 motors. You can use up to 80% of the 15 amps so using more than 12 amps would be the max you should plan on pulling. Most motors are allowed draw 25% over the number on the name plate unless the number on your plate is 4.8 or less you do not want two on one 15-amp circuit.

    The peak could be important if you have to worry about stalling out a generator.


  2. Expert Member
    BILL WICHERS | | #2

    The "80 percent rule" only applies to CONTINUOUS loading on a circuit, which the code defines as being more than 3 hours. Motor inrush (startup current) is in the range of a few seconds at most, so the 80% rule does not apply in this case. Where the 80% rule does apply is if your two pumps will be running together for more than 3 hours at a time, and their combined running load (not startup) will exceed 12 amps, which is 80% of the rated capacity of the 15 amp circuit.

    For the peak current, you want the LRA (Locked Rotor Amperes) number from the motor, or the letter code for the motor which will give you an LRA range based on the running load (RLA) for the motor. Motor start current doesn't generally cause any issues tripping circuit breakers, since circuit breakers don't trip at 15.00001 amps, and they don't trip immediately, either. A typical 15 amp circuit breaker might trip in a few minutes with 30 amps of load, under a few seconds at 100 amps of load, and well under a second for a short circuit which might be a few thousand amps. Circuit breakers are not precision devices, their exact trip points are defined by a time curve and cover a pretty broad range.

    I would not worry about trying to measure the inrush currents here. All you'd need to do that would be a peak reading ammeter though, which is a pretty basic piece of test equipment that can be found for under $100 or so. What I would do is look at the rated load amps, the running amperage for the motors, and see if both could run together on under 15 amps. You don't even need to worry about the 80 percent rule if you don't expect BOTH motors to run for MORE than 3 hours at a time. I wouldn't worry about trying to determine starting current draw, since it is unlikely to really matter, although I would do a few test starts of both together just to be sure.

    If the two "water pumps" are basic things like sump pumps, then they can probably share a circuit without any trouble. You may want to consider putting them on seperate circuits for a bit of redundancy though, to ensure that a fault in one pump won't trip the circuit breaker and make BOTH pumps stop operating. If the run of wiring is particularly long, you may have voltage drop issues too, which could be a problem for the pump motors. If these are motors like sump pumps, I would recommend running a 20 amp circuit and 12 gauge wiring (or 10 gauge for a long run) to provide a little extra margin and keep voltage drop under control.


    1. steve_smith | | #5

      They are a pair of substantial water pumps (think big aquarium), but there was a breaker trip. I didn't think they would trip it on startup, so here I am digging a little bit. Putting them on different circuits does seem to be the safest option.

  3. bfw577 | | #3

    My Emporia smart plugs pick up the surge on my furnace blower. Here is a screenshot below where you can see it spiked to 1800 watts when it started. It draws around 650 watts running.

    The Emporia plugs are really cheap with 4 of them costing $35. I also have their Vue 2 system monitoring my electric panel. The plugs integrate into the same software/ap.

    1. stevedavis | | #4

      +1 for Emporia. I have them throughout the house. They measure in 5 second increments which hopefully is accurate enough to define your peak.

    2. steve_smith | | #6

      This looks like just the ticket. I've order an Emporia smart plug. I'm guessing this is case closed for me.

      Thank you so much!

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