What Happens When a Pump Runs Dry
An irrigation pump running without water in the casing is a pump damaging itself. Centrifugal pumps rely on the fluid they're moving to lubricate the shaft seal and cool the motor. When the suction side loses prime — because a well dropped below the pump intake, a strainer clogged, or a foot valve failed — the pump continues spinning, but heat builds up fast. The shaft seal overheats first, then the impeller begins to cavitate against air rather than water, then the bearing surfaces start to fail.
A submersible pump in a well that runs dry for 20 minutes can sustain seal damage that costs $800 to repair. A surface pump running dry for an hour can destroy an impeller assembly that costs $3,000 to replace. For a large turbine pump on a center pivot, the damage from a prolonged dry run can total $12,000 or more. The frustrating part: this happens most often at night, when nobody is watching, and the pump runs dry for hours before anyone finds out.
The Current Signature of a Dry-Running Pump
Here's the physical fact that makes current-based dry-run detection work: a centrifugal pump moving water draws significantly more current than the same pump spinning in air. Water is dense — moving it requires work, and work draws amps. A pump running dry is essentially unloaded. It spins fast and free, drawing only enough current to overcome mechanical friction and windage losses.
On a typical 10 HP irrigation pump, normal operating current under load might be 28-32 amps at 240VAC. The same pump running dry drops to 14-18 amps. That's a 40-50% reduction that shows up clearly on a current clamp. If you know your pump's normal operating current range, you can set a lower threshold alert: if current drops below that threshold while the pump is commanded to run, something is wrong.
The same signature works for submersible well pumps, though the numbers are different. A 5 HP submersible running at normal output draws around 22-26 amps. Dry-running or cavitating, that same unit drops to 11-14 amps. The ratio holds across motor sizes — expect roughly 40-55% current reduction when a centrifugal pump loses prime.
What You Need: Hardware and Wiring
The hardware required for current-based dry-run detection is a current transformer (CT) clamp — a split-core sensor that clamps around one leg of the motor feed wire without cutting or splicing. The CT outputs a 0-5V or 4-20mA analog signal proportional to the current flowing through the conductor. FarmHQ's analog input channel reads this signal and converts it to a current reading displayed in the dashboard.
CT clamps for this application typically cost $40-80 for the sensor itself, plus $20-30 for a rail-mount signal conditioner if needed for impedance matching. A complete installation with enclosure mounting and conduit work runs $200-350 at most pump panels. The commonly used CT models in FarmHQ installations are the AccuCT 100A split-core (for pumps up to 20 HP at 240V) and the CR Magnetics CR5310-150 for larger 3-phase installations.
Installation is straightforward: open the panel, clamp the CT around one phase conductor between the motor contactor and the motor, run the signal wire to FarmHQ's analog input terminal, and configure the current threshold in the dashboard. The pump does not need to be shut down to install a split-core CT — the clamp snaps over the insulated wire without contact. Confirm the installation with a power measurement at startup: the dashboard should show current within 10% of what a clamp meter reads at the same conductor.
Setting the Dry-Run Threshold
The threshold you set needs to sit between normal dry-run current (too low, pump is in trouble) and normal operating current (expected range when the pump is moving water). A simple approach: let the pump run for 5 minutes under normal operating conditions immediately after priming and record the dashboard current reading. That's your baseline operating current. Set the low-current alert threshold at 60% of that baseline.
Add a time delay before the alert triggers — most FarmHQ installations use a 3-minute hold before a low-current condition fires an alert. This prevents false alerts during the pump startup transient, when current varies as the pump pressurizes the system from a cold start. After 3 minutes at steady state, if current is still below threshold, something has changed and you want to know about it.
For well pumps where the water table fluctuates seasonally, revisit the threshold twice a year — in late spring before peak irrigation demand and in late summer when aquifer levels are lowest. A pump operating at the bottom of its design curve during low water table conditions draws less current than the same pump at peak head conditions, and you don't want summer drawdown to trigger false dry-run alerts.
Combining Current Monitoring With Pressure Sensing
Current monitoring catches dry-run conditions. Pressure monitoring catches a different failure mode: a pump running at full current but with no pressure on the discharge side (typically caused by a check valve failure, a broken discharge line, or an open valve that's bypassing flow back to the source). Current stays high — the pump is moving water — but pressure at the system header is zero and nothing is irrigating.
Using both inputs together gives you a more complete picture. Normal operation: current high, pressure normal. Dry run: current low, pressure low. Discharge failure: current high, pressure low. Blocked discharge (closed valve): current high, pressure very high. Each combination points to a different fault. FarmHQ's dashboard shows both readings in real time and supports alert rules that combine both channels — for example, alerting if pressure drops below threshold AND current is above threshold (which indicates a downstream failure rather than a prime loss).
What This Costs vs. What It Prevents
The total hardware cost for a current-based dry-run protection setup — CT clamp, signal conditioner, and installation materials — is typically $250-350. Adding a pressure transducer for combined protection brings the total to $400-500. An afternoon of installation labor.
Against that cost: a submersible pump replacement runs $2,000-8,000 depending on depth and motor size. A turbine pump rebuild is $5,000-15,000. And that's before accounting for crop loss from the irrigation event that didn't happen because the pump failed overnight and nobody found out until morning.
From a pure cost perspective, the current monitoring setup pays for itself the first time it catches a dry-run event before the pump is damaged. FarmHQ installations with current monitoring have, on average, reported 2.3 dry-run alert events per season per site — almost all of them caught within the 3-minute hold period before any mechanical damage was documented. That represents a lot of avoided repair costs across a fleet of pump stations.
Installing on an Existing FarmHQ Deployment
If you have FarmHQ hardware already installed and want to add current monitoring, the analog input channel on the module accepts a 0-5V or 4-20mA signal from a CT clamp with a signal conditioner. The installation requires opening the pump panel to mount the CT and run a signal wire to the FarmHQ module — typically 30-60 minutes of work. The new analog channel then appears in the dashboard as a sensor input, and you configure the threshold and alert behavior through the standard alert settings interface.
Questions about which CT model to use for your pump size or how to configure the threshold for a variable-frequency drive application? Email support@farmhq.org with your pump motor nameplate data (HP rating, FLA current, voltage) and we'll give you specific recommendations.