How Many Amps Should a One-Half Horsepower Septic System Pump Pull?

Motors draw electric current to convert electrical energy into rotating mechanical energy. The amount of current they draw depends on how much work they do. Two reasons for knowing how much current a sump pump draws include properly sizing the circuit breaker for the sump pump circuit and troubleshooting a sump pump circuit breaker that trips repeatedly. The amount of current a sump pump draws is measured in amps.

Motor Startup Current

A motor's starting current is quite high compared to when it has reached full speed. An electric motor during start up overcomes inertia, begins turning and establishes the magnetic fields around its windings. These all contribute to a very high starting current even for small motors.

The start-up current is sometimes called the inrush current or locked-rotor current. During start up, the current rises 6 to 8 times higher than the motors full-load or running current. As the motor begins turning and the revolutions per minute increase, the current falls until the motor reaches running speed.

Full Load Current

The full-load current of a motor is the current it draws while operating at its rated speed and driving a load at its rated horsepower. At its rated speed, the motor is no longer working to overcome the inertia of the load it is turning, and the magnetic fields that cause the motor to turn give it some resistance to the flow of electricity.

As the load on the motor increases, so does the current it draws. When the current rises, the motor's temperature also rises, which is fine for a few moments or even a minute or so and provides reserve capacity. If the temperature goes too high and remains too high, the motor's thermal protection device stops the motor.

National Electric Code

The National Electric Code governs the installation and use of motors in homes and businesses. Every motor ships with an attached nameplate that gives information about the motor, including its operating voltage, full-load current draw, horsepower and other information.

The NEC also provides for instances where the motor nameplate cannot be read or is inaccessible. For example, if the motor's horsepower and voltage is known, a table provided by the NEC allows you to look up the full-load current for the motor. The values are not necessarily absolute but give a good estimate of the motor's full-load current to size circuit breakers.

Sump Pump Current

A sump pump motor drives the pump to eject water from a sump pit. The load on the pump is the water column in the pipe leading to the outside and the water it draws into the pump housing and pushes out through the pipe.

According to National Electric Code Table T430-148, the full-load current of a 1/2 horsepower motor operating at 115 volts is 9.8 amperes. The maximum breaker size for a sump pump motor of this size is 250 percent of the full-load current or 24.5 amps. You can't go higher, so you must use the next lower size circuit breaker, which is 20 amperes, and the circuit wiring must be a minimum of 14-gauge wire.

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