Okay, this is a frustrating situation! Let's break down the problem and explore some troubleshooting steps. Since you've already addressed the obvious electrical components, we need to think a bit outside the box.

Understanding the Problem



Random Tripping

The key here is "random." If it tripped predictably (e.g., every time the heaters turned on), it would point to a simple overload. The random nature suggests an intermittent fault.

Past Performance

The fact that this didn't happen for 8 years is crucial. Something has changed.

Potential Causes and Troubleshooting Steps


Voltage Drop (Likely Culprit): Given the distances and the fact that it's multiple heaters on one circuit, voltage drop is a prime suspect. Theory

As the distance from the power source increases, voltage decreases due to wire resistance. When the heaters all try to draw power at the same time, the voltage at the end of the line (Pasture C) might drop significantly. This causes the heaters to draw more current to compensate and maintain their heat output, potentially exceeding the breaker's capacity.

Testing



Measure Voltage Under Load

This is critical. Use a multimeter to measure the voltage at the heater's junction box in Pastures A, B, and C while the heaters are actively heating. Do this with all three waterers connected and see what the voltage is. Compare it to the voltage at the breaker panel. A significant drop (more than 5-10%) indicates a voltage drop problem. Do the same at Pasture D for comparison (it is less likely to be having this issue, due to being on its own breaker.)

Solution (If Voltage Drop is the Issue)



Increase Wire Gauge

This is the most effective solution. Larger wire provides less resistance, reducing voltage drop. However, you need to ensure the new wire fits in the conduit and the junction boxes. You might be able to pull larger gauge wire through the existing conduit, but it will be difficult and may require special lubricants.

Split the Circuit

Divide the Pasture A, B, and C waterers onto separate circuits. This would require running new wiring from the barn to each pasture. This is a significant undertaking.

Lower Wattage Heaters

Replace the existing heating elements with lower wattage versions. This will reduce the current draw and potentially alleviate the voltage drop problem. However, it may not provide sufficient heat in extremely cold weather.

Dedicated Transformer

Install a small step-down transformer closer to the pastures to boost the voltage. This is a more complex solution.2.

Loose Connections (Despite the Electrician's Check)



Theory

Even though the electrician checked the connections, they could still be loose or corroded. A loose connection creates resistance, which can cause heat and intermittent tripping.

Testing



Carefully Inspect ALL Connections

Go back and meticulously check every connection in the circuit: At the breaker panel. In the junction boxes at each waterer. Inside the waterer itself (where the heater connects).

Look for

Discoloration, corrosion, loose screws, or signs of arcing (burnt wires).

Tighten EVERYTHING

Use a screwdriver to ensure all screws are tight. Be careful not to overtighten and strip the screws.

Use Electrical Contact Cleaner

Clean the connections with a good quality electrical contact cleaner to remove any corrosion or contaminants.

Solution



Replace Suspect Connections

If you find any damaged or corroded wires or connectors, replace them immediately.

Use Anti-Oxidant Paste

When re-making connections, apply a small amount of anti-oxidant paste (like Noalox) to prevent future corrosion.3.

Ground Fault (Even Without GFCI)



Theory

A ground fault occurs when current leaks from a hot wire to a grounded surface (e.g., the metal casing of the waterer). Even without a GFCI breaker, a significant ground fault can trip a standard breaker.

Testing



Megger Test (Insulation Resistance Test)

This is the best way to check for ground faults. A megger (insulation resistance tester) applies a high-voltage DC current to the wiring and measures the resistance between the wires and ground. Low resistance indicates a ground fault. This test should be performed by a qualified electrician.



Continuity Test (With Power OFF)

Disconnect the wires at the breaker panel and the waterer. Use a multimeter to check for continuity between the hot wire and the ground wire, and the neutral wire and the ground wire. There should be NO continuity. If there is, you have a ground fault.

Solution



Locate and Repair the Fault

Once you've identified the ground fault, you need to find the source and repair it. This might involve replacing a damaged wire, a faulty heating element, or a cracked insulator.4.

Water Ingress (Water Getting into Electrical Components)



Theory

Water can seep into the junction boxes, conduit, or the waterer itself, causing a short circuit or ground fault.

Testing



Carefully Inspect for Water

Check all junction boxes and the waterer itself for signs of water. Look for rust, corrosion, or water stains.

Ensure Proper Sealing

Make sure all junction boxes are properly sealed and that the conduit connections are watertight.

Solution



Seal All Entry Points

Use silicone caulk or electrical sealant to seal any potential water entry points.

Replace Damaged Components

If any electrical components have been damaged by water, replace them.5.

Rodent Damage



Theory

Rodents can chew on the insulation of the underground wires, causing short circuits or ground faults.

Testing (Difficult Without Digging)



Visual Inspection (Limited)

Carefully inspect any exposed wiring for signs of rodent damage.

Megger Test (As Mentioned Above)

A megger test can help detect rodent damage, even if you can't see it.

Solution



Replace Damaged Wiring

If you suspect rodent damage, you'll need to replace the damaged section of wiring. This might involve digging up the underground conduit.

Rodent Control

Implement rodent control measures to prevent future damage.6.

Other Appliances on the Same Circuit



Theory

Despite what the electrician says, there might be other appliances or devices connected to the same circuit that are drawing power intermittently and contributing to the overload.

Testing



Turn Everything OFF

Turn off every breaker in your barn and house, except the breaker for the waterers. Then, systematically turn on each breaker one at a time and monitor the current draw using a clamp meter at the breaker panel. See if any other circuits show a significant current draw when the waterer circuit is active.

Unplug Everything

As a test, unplug everything in the barn and surrounding area to see if the tripping stops.

Specific Questions for Your Situation



What is the wattage of each heating element? This is critical for calculating the total current draw.

What gauge wire is currently used for the underground runs? You mentioned 12 gauge going into the box, but what about the long runs from the barn?

What is the voltage at the breaker panel when the waterers are NOT running? This will give you a baseline voltage to compare against when the heaters are on.

Were the thermostats replaced with the exact same specifications? Could there be some variance causing the heater to stay on longer?

Recommendations


Voltage Drop Testing

Start by measuring the voltage at the heaters under load. This is the most likely cause, given the distances.2.

Meticulous Connection Check

Go back and carefully check all connections.3.

Megger Test

If voltage drop and connection issues are ruled out, consider having a qualified electrician perform a megger test to check for ground faults.4.

Document Everything: Keep a detailed log of when the breaker trips, the weather conditions, and any other relevant information. This can help you identify patterns and narrow down the problem.This is a process of elimination. Start with the simplest and most likely causes and work your way towards the more complex ones. Good luck! This is definitely solvable. Flag for review