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This topic is my favorite. You can go to the hardware store and find a 1 HP pump that is 4 amps, or a 1 HP pump that is 10 Amps. Seriously? How many amps is a 1 HP pump? It would seem marketing driven horsepower is no longer just for the automotive industry. What’s even better? You put that “10 Amp” pump on the bench and it pulls 8.1 amps. Your mom might call that lying — engineers call it a “safety margin.” Sorry, Mom.
It should be mentioned that float switch manufacturers don’t get to play this “horsepower-current-marketing-shell game”. It would put customers in danger. If we state something is good to ½ Horsepower at 120V, that takes a certain gauge of wire, and a switch tested to that rating. To get to 1 HP requires thicker wire (smaller gauge), and a heavier duty switch. That leaves you dealing with the pump horsepower amp dilemma.
So how many amps is a 1 HP pump, really? The current most commonly used for testing is from UL 508 Table SB4.1, which more than aligns closely with NEC 430.248. They use the same values — for 1 HP at 120V, that’s 16 amps. For ½ HP, it’s 7.2 amps. Here is the data for 120V pumps from 0-2 HP.
If you peek at UL 508, they’re the same numbers.
But wait a minute, the float switches Sump Alarm produces are rated for 10 AMP or ½ HP. How does that work, and which one do I follow? We put a Honeywell microswitch into our floats in part because Honeywell has amazing testing data. What they are telling us is that they have tested the microswitch up to ½ HP. If they say it is good for 100,000 cycles at ½ HP, then that is the standard that it was tested to. Naturally, you would ask yourself, “Did they test it with the marketing driven 2 amp ½ HP pump, or did they test it with a 6-amp ½ HP pump?” Well, if you check the table above it means that they tested it at the 7.2 Amps – so neither 😊.
Most of the world (except for our nation’s plumbers) are afraid of saying something wrong. That switch was tested with a 7.2 Amp inductive load, meaning it has an inrush current that spikes every time it starts up, and (following on the example above) they started it a quarter of a million times.
Inrush current is another place where we need to stay grounded with facts. I remember as an entry engineer asking an older engineer how long a motors inrush current last. He said, “a couple of seconds”. I also see statements online that the inrush current can be “5 to 6 times” the normal running current. As broad statements, those extremes may be true for some applications, but frankly I’ve yet to see it in pumps working in water – and I’ve yet to see it at “locked rotor” either. We’ve locked rotors. We’ve started pumps in water. We’ve shut the valves on the discharge and changed the vertical pumping height. I’d still like to try pumping maple syrup. It’s almost maddening that I cant get inrush much over 2 times the full load amperage of the pump, or an inrush current that lasts more than a fraction of a second. We have seen what pump in rush currents look like on an oscilloscope. Wanna see it? here it is:
So, what’s the end game of all of this? HP is a useful gauge of which pump is bigger will have higher head, better pressure, and more volume in any selection from the same manufacturer. I believe Zoeller’s 2 HP pump is bigger than Zoellers’ 1 HP pump. But I can’t tell you without looking at the curves and amp data if Zoellers 1 HP pump is bigger than Liberty’s 1 HP.
As far as the float switch is concerned, take the amps (unfortunately from the nameplate) compare them to NEC Table 430.248, and that gives you the standardized full-load current — which you can use to determine the horsepower your float switch should be rated for. Based on NEC 430.28, that 1 HP, 4 Amp pump is less than ¼ HP.
That is the real engineering and math behind this.