Read a water pump's amps from its nameplate, not from its horsepower. Why two pumps of the same horsepower draw different currents, starting current, daily electricity use and choosing the pump controller model.
"How many amps does a 1 hp pump draw?" has no fixed answer. Two pumps of the same horsepower can draw different currents. The right figure is printed on the pump's own metal nameplate, and that is the number you need to choose the pump controller, the miniature circuit breaker and the cable.
How horsepower and amps are related
One horsepower is 746 watts. That figure is the motor's output power, the work it does at the shaft. To deliver that power the motor takes more electricity from the mains, because part of it turns into heat.
On a 220 V single-phase supply, every 1000 watts of input power is about 4.5 amps of current. The motor's actual current is higher than that, because the motor's power factor is less than one. Losses and power factor vary from one pump to another, which is why horsepower only gives you an estimate.
For any electrical decision, go by the amps on the pump nameplate, not the horsepower.
Where to read the pump's current
- The pump nameplate: the motor's running current in amps is printed on the nameplate.
- The controller display: on HYDROSET, while the pump is running, press the Down key to read the pump's actual current on the display.
- Your phone: the pump controller settings app also shows how much current the motor draws.
If the nameplate has worn off, the figure on the controller display during normal running is the best substitute.
Starting current
At the moment it starts, a motor draws several times its normal running current. In the manual's specifications, the maximum starting current is 10 amps for PM821-8 and 30 amps for PM821-16. The miniature circuit breaker is chosen for that moment; a breaker smaller than needed trips at start-up.
Every time the pump starts, it draws this large current once. That is why a pump that keeps switching on and off wears out its switch and capacitor sooner.
How much electricity a water pump uses
Electricity use is power multiplied by running hours. An example: if a pump takes one kilowatt and runs for three hours in total over a day, it uses 3 kWh a day and about 90 kWh a month. Your pump's actual running hours depend on the building's consumption.
Things that push consumption up for no reason:
- A pump that runs nonstop; usually because of a leak or a set pressure beyond what the pump can deliver.
- Low pre-charge in the pressure vessel; the pump starts for every small draw. See pressure vessel pre-charge.
- A stop pressure (F1) higher than the building needs.
The HYDROSET controller itself uses less than 5 watts.
Choosing the controller model by current
All three HYDROSET models share the same core and differ only in maximum pump current. The model depends only on the pump nameplate current:
Always choose a model rated at or above the nameplate current. If the pump current exceeds the model's rating, the unit reports an over-current fault (E1) and stops the pump.
The circuit breaker table and model selection by number of floors are in the pump controller selection guide.
