Wow, that's a textbook example of why proper grounding and bonding are critical, and it sounds like you caught a potentially dangerous situation just in time. The unbonded transformer XO is the smoking gun, and your instinct to rewire the room is absolutely the right call. Here's a breakdown of what likely happened, and a plan for rewiring the pool equipment room to bring it up to code and ensure safety

What likely happened:
Unbonded Transformer XO

This is the root of the problem. The XO terminal on a transformer provides the reference point for the neutral/grounded conductor. Without it being properly bonded (connected to the grounding electrode system), the potential of the neutral can float to dangerous levels relative to ground.

Faulty Conductors & Backfeed

The deteriorated conductors in the conduit were causing a current path through the water. This is enough to create the backfeed you were seeing.

Voltage on the Neutral

When you broke the circuit and got shocked, this indicates that the unbonded neutral was carrying voltage relative to ground. This voltage could be coming from an imbalance in the loads connected to the transformer, or from a fault somewhere else in the system.

Pump Failure

The voltage fluctuations and unstable neutral caused by the unbonded transformer likely damaged the pumps, or prevented them from operating correctly.

Corrosion

Pool equipment is highly susceptible to corrosion due to the chemicals and moisture present. Using wrong material for the environment will cause failures like this one.

Rewiring Plan


Disconnect Power

Shut off the main breaker feeding the transformer and the pool equipment room. Verify with a meter that the power is off on all circuits. Lockout/Tagout procedures are essential.2.

Inspect and Document

Before you start ripping things apart, thoroughly document the existing wiring. Take pictures, make notes on wire sizes, conduit runs, and how everything is connected (or was connected). This will be invaluable for troubleshooting.3.

Transformer Bonding (Priority #1)



Verify the Transformer

Confirm the transformer's KVA rating, primary and secondary voltages, and any other relevant specifications.

Bonding Conductor

Install a proper-sized bonding conductor (typically copper) from the transformer's XO terminal to the main grounding electrode system (GES) of the building. The size of this conductor is determined by the transformer's KVA rating (refer to NEC 250.30).

Grounding Electrode System

Ensure the GES is adequate and in good condition. This typically includes a grounding electrode conductor to a ground rod, water pipe, or other approved grounding electrode.

Clean and Tight Connections

Use appropriate connectors and ensure all connections are clean, tight, and corrosion-resistant. Apply anti-oxidant compound to aluminum connections.4.

Conduit and Wiring



Replace Everything

Given the condition of the existing wiring, it's best to replace all conduit, conductors, and junction boxes in the pool equipment room.

Conduit Type

Use rigid PVC conduit (Schedule 80 for direct burial) or stainless steel for above ground runs, as these are resistant to corrosion from pool chemicals. Do not use EMT or other ferrous metal conduit in this environment.

Conductor Type

Use THHN/THWN-2 conductors rated for wet locations.

Wire Sizing

Size the conductors appropriately for the load of the pumps and any other equipment, considering voltage drop. Use the appropriate ampacity tables in the NEC.

Separate Equipment Grounding Conductor (EGC)

Run a dedicated equipment grounding conductor (EGC) in each circuit to provide a low-impedance path for fault current back to the source. Do not rely on the conduit as the sole EGC. Size the EGC according to NEC Table 250.122, based on the overcurrent protection device size.
5.

GFCI Protection



Ground Fault Circuit Interrupters

All 120V receptacles and circuits serving pool equipment must be GFCI-protected. Install GFCI breakers in the panel or GFCI receptacles where needed.6.

Bonding of Metal Parts



Pool Bonding Grid

The NEC requires a bonding grid that interconnects all metallic parts within the pool area, including the pool structure itself, metal light fixtures, ladders, handrails, pump housings, and any other metallic components within 5 feet of the pool. This bonding grid helps to equalize the potential and minimize the risk of shock. Use solid copper bonding conductors, sized according to NEC Article 680.

Bonding Jumpers

Use bonding jumpers to connect metal conduits, enclosures, and equipment.7.

Overcurrent Protection



Circuit Breakers

Install appropriately sized circuit breakers to protect each circuit. Use HACR-rated breakers for motor loads.

Motor Protection

Consider adding separate motor overloads for the pumps to provide better protection against overheating and damage.8.

Disconnect Switches



Service Disconnect

Install a readily accessible service disconnect switch for the pool equipment room.

Equipment Disconnects

Consider installing individual disconnect switches for each pump to allow for safe servicing.9.

Wiring Practices



Waterproof Connections

Use waterproof connectors and fittings in all junction boxes and enclosures to prevent moisture from entering.

Proper Terminations

Torque all terminations to the manufacturer's specifications. Loose connections are a common cause of problems.

Labeling

Clearly label all circuits, breakers, and equipment.

Conduit Support

Properly support all conduit runs to prevent stress and strain on the wiring.10.

Inspection and Testing



Professional Inspection

Before energizing the system, have a qualified electrical inspector review the work to ensure it complies with all applicable codes and regulations.

Testing

Perform the following tests:

Ground Resistance Test

Measure the resistance of the grounding electrode system to ensure it is within acceptable limits (typically 25 ohms or less).

Continuity Testing

Verify the continuity of all grounding conductors.

Insulation Resistance Test (Megger)

Test the insulation resistance of all conductors to ensure there are no shorts or grounds.

Voltage Drop Test

Measure the voltage drop under load to ensure it is within acceptable limits.

GFCI Testing

Verify that all GFCI devices are functioning correctly.

Polarity Testing

Confirm correct polarity of all receptacles and circuits.

Safety Considerations



De-energize Completely

Never work on electrical equipment without first de-energizing the circuit and verifying it is dead with a meter.

Lockout/Tagout

Use proper Lockout/Tagout procedures to prevent accidental re-energization of the circuit.

Personal Protective Equipment (PPE)

Wear appropriate PPE, including safety glasses, gloves, and insulated tools.

Qualified Electrician

This type of work should only be performed by a qualified and licensed electrician.

Code References (NEC)



Article 250

Grounding and Bonding

Article 680

Swimming Pools, Fountains, and Similar Installations

Article 430

Motors, Motor Circuits, and Controllers*

Article 300

Wiring Methods

In summary: The unbonded transformer XO created a dangerous and unstable electrical system. Rewiring the pool equipment room with a focus on proper grounding, bonding, GFCI protection, and corrosion-resistant materials is the only way to ensure the safety and reliability of the equipment. Do not take shortcuts. Hire a qualified electrician who understands pool wiring and NEC requirements. Your diligence in spotting this and your commitment to fixing it right could very well prevent a serious accident. Flag for review