Why E-Bike Controller Phase Cables Melt and How to Fix It

E-bike controller phase cables melt primarily due to excessive heat caused by too much electrical current flowing through them, a condition known as overload. This often happens because of a poor or loose connection, which creates resistance and heat, or an undersized wire gauge that can't handle the power demands of your motor. This blog post will dive deep into these causes, show you how to diagnose the problem safely, and provide step-by-step solutions to fix and prevent it from happening again.

Root Causes of Phase Cable Meltdown

The melting of e-bike controller phase cables is a symptom of underlying electrical stress, often stemming from a confluence of factors that push the system beyond its designed limits.

Overloading

The most fundamental reason for wire melting is the electrical system drawing more current than the wires or connected components are designed to safely handle. This phenomenon, often termed "overloading," generates significant heat due to increased electrical resistance.

Demanding riding conditions are a frequent contributor to excessive current draw. Riding up hills that are too steep for the e-bike's power, or navigating challenging terrains such as deep sand, thick mud, or tall grass, forces the motor to work much harder. 

This increased effort translates directly into higher current demands from the battery and controller, which then flow through the phase wires, generating heat. 

Similarly, carrying excessively heavy cargo, towing trailers, or having a rider who exceeds the vehicle's specified weight limit significantly increases the mechanical load on the motor. This sustained high load necessitates a continuous high current draw, stressing the electrical components.

Mismatched components or modifications can also lead to overloading. For e-bikes that have been modified or custom-built, an improperly selected gear ratio between the motor and the wheel can be a major culprit.

If the gear ratio is too high (i.e., the motor is geared for too much speed and not enough torque), the motor will constantly struggle and draw excessive power, leading to wire melting and motor overheating. 

While controllers have internal current limits, if the controller's phase current limit is set too high for the motor or the wiring, or if the actual usage consistently pushes the system beyond its design parameters, the phase wires will experience dangerously high currents.

Recommended Reading: E-Bike Motor Overload: Symptoms, Causes and Prevention Guide

Component Overheating

The e-bike's key electrical components, particularly the controller and motor, generate heat during operation, and excessive heat can lead to cable melting.

The controller houses sensitive power electronics, most notably MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), which regulate power flow. These components naturally generate heat when handling high current, especially during periods of sustained full throttle or long, arduous climbs.

Most modern controllers incorporate a built-in thermal protection system. If the internal temperature rises too high, the controller will temporarily cut off or reduce power – a safety mechanism known as thermal shutdown. While this protects the controller in the short term, repeated or extreme overheating can permanently damage internal components like MOSFETs or capacitors.

Water ingress is a significant threat to controllers; moisture can seep into the Printed Circuit Board (PCB), creating unintended conductive paths or "water bridges" between components.

This can cause MOSFETs to turn "on" unintentionally, leading to uncontrolled motor activation or a "shoot-through" condition where both high and low side MOSFETs conduct simultaneously, resulting in rapid power discharge, dangerous surges, and severe overheating. 

Signs of controller overheating include the casing becoming extremely hot to the touch, emitting a burning smell, causing motor stuttering or power loss after heavy use, or displaying error codes.

Similar to the controller, an overloaded motor can also overheat, leading to excessively hot coil windings. This internal motor heat can transfer to the phase wires connected to it, causing their insulation or connectors to melt.

Many motors also have their own thermal protection mechanisms that can cause power cutouts during intensive rides. A lingering burnt smell from the motor often indicates that the stator windings inside have been severely overheated.

The melting of phase cables is not an isolated event but a direct and often predictable consequence of the electrical system struggling to cope with power demands. An increased mechanical load, such as climbing a steep hill or carrying a heavy rider, directly leads to a higher electrical current demand from the motor and controller.

As this current flows through the wires and connectors, the inherent electrical resistance, even if small, becomes a significant heat source. This transforms these components into miniature heating elements. This process generates excessive heat, which in turn causes wire and connector insulation to melt.

This melting can then further increase resistance or even lead to short circuits, perpetuating a vicious cycle of damage.

Poor Connections & Undersized Wires

The quality and specification of the wiring and connectors play a crucial role in preventing overheating.

Inadequate wire gauge is a common issue. The thickness, or "gauge," of an electrical wire directly dictates its current-carrying capacity (ampacity). Wires that are too thin (undersized) for the actual current demands of an e-bike system will experience high resistance and consequently heat up excessively.

For example, while 12 AWG copper wire is typically rated for approximately 20-30 Amps, 10 AWG is recommended for higher current applications around 30 Amps. It is crucial to use pure copper wire, often tinned for corrosion resistance, as aluminum has significantly lower ampacity for the same gauge and is less suitable for e-bike applications.

Faulty or low-quality connectors are another frequent point of failure. Many standard bullet connectors, commonly found on e-bikes, are not designed to handle the sustained high amperage and heat generated by powerful e-bike systems.

Their plastic sheathing (shrouds) are particularly prone to melting and deforming under load. Poor-quality connectors, improper installation techniques (e.g., cold solder joints, weak crimps), or simply using connectors not rated for the system's voltage and current can cause them to overheat, melt, and compromise the entire electrical system.

Loose connections and corrosion also introduce significant resistance. Connections that are loose, whether due to vibration, poor initial installation, or wear, introduce increased electrical resistance into the circuit.

This elevated resistance directly generates heat, which can lead to melting. Under heavy loads, loose connections can also cause significant voltage drop, potentially triggering the battery's Low Voltage Cutoff (LVC) shutdown, or even producing dangerous sparks and crackling sounds.

Corrosion, often a result of moisture exposure, forms an insulating layer on metal contacts. This dramatically reduces electrical conductivity and increases resistance, leading to overheating and eventual connection failure.

Finally, friction-induced damage can compromise wire integrity. Wires, especially those not properly secured, can rub against each other, the bike frame, or other metal structures due to constant vibration during riding.

This friction can slowly abrade the wire insulation, eventually exposing the bare copper conductors and leading to short circuits. Additionally, simply bunching up excess wire length can trap heat due to the wires' close proximity, causing insulation to melt and wires to fuse together.

Short Circuits

A short circuit is a direct and immediate cause of melting. It occurs when electricity travels an unintended, low-resistance path, leading to an uncontrolled surge of current. If any of the phase wiring shorts out, this massive, uncontrolled current flow leads to immediate and extreme overheating, causing wires and connectors to melt rapidly.

Shorts can be internal or external. Internal shorts can occur within the motor if severe overloading causes the motor's internal coil windings to overheat to the point of insulation breakdown.

Within the controller, malfunctions such as software glitches corrupting signals, or more commonly, water ingress, can cause MOSFETs to short-circuit (fail "on"). This allows power to flow to the motor uncontrollably and can lead to dangerous surges and overheating within the controller.

External shorts typically result from damage to the wire insulation. This can happen due to friction from wires rubbing , wires being pinched by the frame or other components , or simply worn-out insulation. When the bare copper conductors are exposed, they can short against each other or against the metal frame (ground).

A short circuit in the phase wiring is particularly dangerous because it initiates a rapid and severe chain reaction. The initial overheating and melting of insulation can lead to a direct short, which then overloads and destroys the controller's power stage (MOSFETs).

Once the MOSFETs fail in a shorted state, the entire battery current can be dumped directly into the motor windings, causing them to burn out. This uncontrolled power surge can turn a localized problem into a system-wide destruction.

This cascading failure mode underscores the extreme urgency of addressing any signs of melting or potential shorting to prevent total system destruction and ensure safety.

Recommended Reading: Fixing Short Circuits in E-Bike Wiring: A Comprehensive Guide

Recognizing the Warning Signs of a Meltdown

Recognizing the warning signs of a phase cable meltdown is crucial for timely intervention and preventing further damage.

The most undeniable indicator is the physical evidence: visible melting, charring, or deformation of the wire insulation or connectors, particularly around the phase wires. This physical damage is often accompanied by a strong, acrid odor, frequently described as burnt plastic, burnt electronics, or burnt lacquer.

This smell is a tell-tale sign of overheating components and can originate from the controller or the motor's internal windings. If the bike was recently ridden, carefully and cautiously touching the controller casing or the motor hub (if accessible) to check for excessive heat is also a clear warning sign.

While minor warmth is normal, extreme heat indicates the component is under heavy strain or failing to dissipate heat effectively. Some controllers are even designed to automatically shut down if they overheat as a protective measure, causing the bike to cut out until it cools.

Performance issues are another common symptom. The e-bike may experience sudden power loss, abruptly cutting out either intermittently or completely, as if it has lost all power. This often indicates the controller is failing to consistently deliver power from the battery to the motor.

Instead of spinning smoothly, the motor may jerk, vibrate, stutter, or shudder when attempting to start or during operation. In more severe cases, the motor might not turn at all, sometimes accompanied by a low humming noise.

This usually points to the controller failing to sequence the motor phases correctly, or a problem with one of the motor's internal windings. Riders often describe this as the bike "cogging". 

Furthermore, erratic throttle or assist behavior can manifest as unpredictable acceleration, a delayed response from the throttle, unexpected power surges, or jerky pedal-assist. Such erratic behavior is a hallmark of controller trouble, indicating it is not processing input smoothly.

Unusual noises or display errors can also signal trouble. A failing controller might produce odd buzzing or clicking sounds, especially when the motor is under load. Many e-bikes are designed to flash specific error codes on the display when the controller or another component has an issue. The LCD itself might flicker, reset, or show incorrect readings (e.g., inaccurate speed) due to power delivery problems.

A burning smell, motor stuttering, and an overheated controller together offer a clearer diagnostic signal than any symptom alone. While single issues like stuttering could point to various causes, a combination of symptoms helps pinpoint the problem faster and more accurately—leading to quicker, more effective repairs.

Recommended Reading: Troubleshooting Guide for Electric Bike Screens

Pinpointing E-bike Phase Cable Meltdown Problem

Accurately diagnosing the cause of melted phase cables requires a systematic approach, combining visual inspection with precise electrical tests.

Initial Visual and Olfactory Inspection (Safety First!)

Before attempting any inspection or testing, safety must be prioritized. Always ensure the e-bike is powered off and the battery is completely disconnected. This non-negotiable step prevents accidental shocks or further damage. If any components were hot, allow them to cool down completely before handling or beginning work to prevent burns and ensure component stability.

Begin with a thorough visual scan. Systematically examine all external wires and connectors from end to end. Look for any obvious signs of damage: frayed or pinched wires, melted insulation, visible scorch marks, or corroded connectors.

Pay close attention to the connections where the phase wires meet the controller and the motor. Ensure all plug connections (if applicable) are firmly seated and that no pins are bent or burnt.

The "sniff test" is also valuable: carefully sniff around the motor casing, vents, and controller. A distinct burnt odor, reminiscent of burnt lacquer or wire varnish, is a strong corroborating sign of internal overheating, even if no visible damage is immediately apparent.

Using Multimeter

A digital multimeter is an indispensable tool for accurately diagnosing e-bike wiring issues, allowing for precise testing of continuity, resistance, and shorts.

The Continuity Test for Phase Wires (Motor Side) verifies the integrity of the motor's internal windings and their connections to the phase wires, checking for any broken (open) circuits.

To perform this, set the multimeter to the continuity setting (often indicated by a speaker icon) or the lowest resistance (Ohms) range. Touch one probe to one phase wire (e.g., Yellow) and the other probe to a second phase wire (e.g., Green).

A beep should be heard (for continuity) or a very low resistance reading (typically less than 1 Ohm) should be observed. This test must be repeated for all three possible combinations: Yellow-Green, Green-Blue, and Blue-Yellow.

All three pairs should yield similar low resistance readings. If one pair shows infinite resistance (indicated as "OL" or "Open Line" on the multimeter), it means that winding or its connection is broken, indicating an open circuit.

The Short-to-Ground/Hub Test (Motor Side) is critical for checking internal short circuits within the motor windings to the motor's metal casing or axle. Such a short can cause severe damage to the controller if left unaddressed.

For this test, keep the multimeter on the resistance (Ohms) setting, but select a high range (e.g., 200k Ohms or M Ohms). Touch one probe to any one of the phase wires (e.g., Yellow).

Touch the other probe to a clean, unpainted metal part of the motor axle or casing, ensuring good metal-to-metal contact. The multimeter should display an "OL" (Open Line) or infinite resistance reading. 

Any low resistance reading (e.g., a few hundred Ohms or less) indicates a short circuit, which is a serious fault that can damage the controller. This test must be repeated for all three phase wires.

The Controller Phase Output Test (Optional, with caution) helps determine if the controller is properly outputting voltage to the motor's phases. Disconnect the motor completely from the controller.

Connect the controller to its power source (the e-bike battery). Turn on the e-bike system. Set the multimeter to DC voltage (e.g., 20V range). Carefully connect the positive probe to one phase wire output from the controller and the negative probe to another phase wire output.

Slowly twist the throttle. Fluctuating voltage readings should be observed as the controller attempts to commutate. If one or more phases show no voltage or erratic readings compared to the others, the controller itself may be faulty.

Extreme caution is paramount during this test to avoid accidentally shorting the controller outputs, which can cause immediate damage.

Manual Motor Rotation Test (Simple Short Test)

A quick and simple way to check for phase wire shorts within the motor without needing a multimeter is the manual motor rotation test. With the motor completely disconnected from the controller, try to spin the motor wheel freely by hand.

Note how easily it turns. Then, touch any two of the three thick phase wires together (e.g., Yellow and Green) and try to turn the wheel again. If the motor's internal windings are healthy, touching two phase wires together should create a noticeable magnetic resistance, making the wheel feel "lumpy" or significantly harder to turn.

If it still turns freely when two wires are shorted, there might be an open circuit in one of those phases. If the wheel always feels lumpy, even when the phase wires are separated, it indicates an internal short within the motor windings.

The Process of Elimination for Troubleshooting

Electrical diagnosis can be complex, as multiple issues can cause similar symptoms. A systematic process of elimination is crucial. Always start with the most basic and easiest-to-check components, typically the battery (ensuring it's charged and functional), then move to the controller, and then individual components like the motor or throttle.

Rule out simpler issues first: for instance, try a different battery if available, or check if the display shows any error codes. Repairing obvious external wiring damage can often resolve symptoms without needing to delve deeper into the motor or controller.

Effective e-bike diagnosis is akin to detective work, requiring a systematic and hierarchical approach. Starting with basic, non-invasive checks like visual inspection, smell, and manual motor rotation allows for quick identification of obvious issues or the elimination of simple problems.

Progressing to precise multimeter tests, such as continuity and short-to-ground checks, enables the accurate identification of electrical faults like open circuits or internal shorts. Each test is designed to either rule out certain problems or confirm others.

For example, a successful continuity test rules out an open circuit in the windings, while a failed short-to-ground test strongly indicates an internal motor short. The manual rotation test provides a quick, tactile confirmation of phase integrity.

Understanding how each test result points to a specific component (motor winding, controller, external wiring) is crucial for efficient, accurate troubleshooting, and ultimately, for implementing the correct repair. The process of elimination emphasizes that symptoms are often interconnected.

A short-to-ground in the motor, for instance, would explain not only melted phase wires but also motor stuttering and controller overheating. This highlights that diagnosis is about connecting the dots between various test results and observed symptoms.

Repairing Damaged Cables & Components

Repairing melted phase cables goes beyond simply replacing damaged parts; it necessitates addressing the root cause to prevent recurrence.

Immediate Action: Safety First!

The single most crucial step before attempting any repair is to identify and correct the root cause of the overheating and melting. Simply replacing damaged parts without addressing the underlying issue will almost certainly lead to a recurrence of the problem. Always power off the e-bike and immediately disconnect the battery.

This is a non-negotiable safety step to prevent electrical shock or further damage. If any components were hot, allow them to cool down completely before handling or beginning work. This prevents burns and ensures component stability.

Addressing the Root Cause (Crucial Step Before Repair)

For unmodified vehicles experiencing overloading, the solution involves adjusting riding habits to respect the e-bike's design limits. This means avoiding overly steep inclines, prolonged rides through deep sand or mud, towing heavy loads, or exceeding the manufacturer's specified rider weight limit.

For modified or custom-built vehicles experiencing overloading, the vehicle will either need to be operated under less demanding conditions or, more effectively, the gear ratio between the motor and the wheel must be changed to a lower, more appropriate ratio. This reduces the strain on the motor and electrical system. Resources such as sprocket diameter calculators can assist with this adjustment.

If the diagnosis indicates that the motor itself was overloaded and its internal coil windings overheated (e.g., burnt smell, visible charring inside), this fundamental motor issue must be addressed. Depending on the severity, this might require professional motor repair or, more commonly, replacement of the motor.

If the controller is determined to be faulty (e.g., blown MOSFETs due to a short, or internal overheating), it will likely need to be repaired or replaced. When replacing a controller, ensure the new unit is fully compatible with the e-bike's display and other connected components, as command protocols can vary between manufacturers.

Repairing/Replacing Damaged Wires & Connectors

When replacing damaged phase wires, it is imperative to use wires of an adequate gauge (thickness) to safely handle the current demands of the system. Thick power wires, typically 10–14 AWG, are recommended for battery-to-controller connections and for the phase wires themselves.

For most standard 36-48V, 250-750W e-bike systems, 12 AWG copper wire is common and can safely carry approximately 20-30 Amps. For higher-power systems or those drawing closer to 30 Amps, 10 AWG is often preferred for battery and motor leads.

Always opt for pure copper wire, often tinned for corrosion resistance, as aluminum wires have significantly lower ampacity for the same gauge and are less suitable for e-bike applications.

Many standard bullet connectors, especially those of lower quality, are notorious for not handling the high amperage and heat generated by e-bike systems effectively. Their plastic shrouds are particularly prone to melting. Consider upgrading to more robust, high-amperage rated connectors.

Table: High-current E-bike Connectors

Crucially, always ensure that the chosen connector is explicitly rated for the specific voltage and current requirements of the e-bike system to prevent future overheating and failures.

Best practices in soldering and crimping

Crimping (Often Preferred for High Current)

Many professionals and engineers prefer crimped connections for high-current applications in e-bikes. Properly executed crimps offer superior mechanical and electrical contact compared to many solder joints, and they prevent the wire strands from stiffening, which can lead to fatigue breaks.

Achieving a good crimp requires using the correct, specialized crimping tools (e.g., ferrule crimpers, open U/double barrel crimp tools) and properly sized connectors. For the strongest connection, intermesh the strands of both butt-ends of the wires before crimping them together.

Soldering (If Crimping Not Possible/Preferred)

If soldering, use a powerful, adjustable-temperature soldering iron (80W+ is recommended for heavy e-bike wires) equipped with a wide, chisel-style tip. A small pencil tip will struggle to transfer enough heat efficiently. Always use leaded 60/40 rosin-core solder. It is imperative to 

NEVER use acid-core plumbing solder, as it will corrode the wires, and avoid lead-free solder which is much harder to work with for these applications. Apply electronics-grade paste or liquid flux liberally to all metal surfaces intended for soldering. This helps the solder flow smoothly and creates a stronger joint.

Before applying solder, create a good mechanical joint by twisting or interleaving the wire strands together. The solder should then flow into and around all the strands (known as "tinning") to create a shiny, well-wetted connection. To prevent accidental shorts, always work on one wire at a time. Fully connect and insulate one wire before stripping or working on the next.

To minimize the risk of future shorting, trim the phase wires to different lengths so that none of the splices or connections overlap each other. This provides an extra layer of safety even if the insulation were to fail.

The Critical Role of Heat Shrink Tubing

After making any connection (soldered or crimped), proper insulation is vital. Adhesive-lined (dual-wall) heat shrink tubing is highly recommended as it creates a durable, waterproof, and mechanically strong seal that is far superior to electrical tape.

Remember to slide the heat shrink tubing onto the wire before making the connection. Ensure the tubing is long enough to completely cover the finished joint and overlap onto the original wire insulation on both sides by at least 5-10 mm. Use a heat gun for safe and even shrinking.

Keeping Your E-Bike Cool & Connected

Preventing phase cable melting is not a singular action but a continuous commitment to comprehensive e-bike care.

Avoid Overloading, Respect Your E-Bike's Limits

Understanding the e-bike's design limits, including its maximum weight capacity, the types of terrain it is designed for, and its sustained power output capabilities, is crucial. Consistently pushing these limits is a primary cause of overheating.

Moderate riding practices are advised; avoid prolonged periods of continuous full-throttle use or sustained climbs at maximum power, especially during hot weather, as these conditions put immense strain on the motor and controller.

Utilizing pedal-assist modes effectively helps reduce the direct electrical strain on the motor and controller, allowing rider power to contribute. After particularly strenuous rides (e.g., long climbs, high-speed runs), allowing the e-bike's motor and controller to cool down is essential. If components feel scorching hot to the touch, it is a clear signal that they are at their thermal limits and need a break.

Optimizing Thermal Management

Both the controller and motor benefit from optimized thermal management.

For controller cooling solutions, passive cooling is the first step. Ensuring the controller has adequate airflow and avoiding enclosing it in tight bags or compartments that trap heat significantly hinders its ability to dissipate warmth.

Regularly cleaning any dirt, dust, or debris from around the controller's casing and cooling fins helps maintain optimal airflow. Consider enhancing passive cooling by adding larger or more efficient external heatsinks, especially if the controller routinely runs hot.

Crucially, ensure proper application of high-quality thermal paste between the controller's internal components (like MOSFETs) and its heatsink or casing. Insufficient or poorly applied thermal paste is a common cause of localized overheating.

For high-power e-bikes or those frequently ridden under demanding conditions (e.g., slow speeds with high loads), passive cooling may be insufficient. Integrating a small cooling fan can significantly boost heat dissipation by forcing air over the controller or its heatsink.

When considering a fan, attention should be paid to its size (40mm or larger blowers are effective), airflow direction, and ensuring any modifications maintain or improve waterproofing.

For motor cooling, similar principles apply. The motor can overheat under heavy loads. Monitoring its temperature is important; if the motor hub is scorching hot after a cutout, it is likely a motor thermal issue. Allowing the motor to cool down after hard riding is essential for its longevity and performance.

Superior Cable Management

Proper cable management is vital for preventing physical wear and heat buildup. All cables should be routed with gentle, sweeping bends, avoiding sharp angles or kinks that can stress the wires, degrade performance, or even break internal conductors.

Adequate slack must be ensured in cables to accommodate full handlebar sweep, suspension travel, and any other moving parts; cables should never be pulled taut. Utilizing rubber grommets or dedicated entry ports where cables pass into the bike frame protects the cable jackets from sharp edges and seals out dirt and moisture.

For motor wires, creating a "drip loop" – a short section of the cable that dips downwards before heading up into the fork – ensures that any water running down the cable drips off the loop instead of entering the motor housing.

Securing wires effectively is equally important. Cables should be grouped logically (e.g., all brake lines together, all sensor wires together) and bundled neatly using zip-ties or reusable silicone straps.

However, it is crucial not to over-tighten these ties, as excessive clamping force can pinch the insulation and damage the internal wires, potentially leading to shorts or signal interference. Any bulky connectors or cable junctions should be secured firmly to the frame to prevent them from dangling or snagging.

If there is excess cable length, it can be neatly rolled into a small bundle and secured, possibly inside a piece of old inner tube, to prevent tangles and reduce the risk of friction.

Protecting against friction, vibration, and water ingress is paramount. High-current power cables should be routed to avoid running parallel to sensitive signal wires (e.g., throttle, Hall sensors) for long distances, as this minimizes electromagnetic interference (EMI) that can cause erratic behavior.

 If crossing is unavoidable, it should be done at right angles. All external connectors should be properly sealed, or waterproof connector versions used where possible, to prevent moisture ingress. Water can cause corrosion and short circuits. Applying dielectric grease can also help protect contacts.

Wires should be tethered firmly to the bike frame or the motor's stator (internal to the motor, if accessible) to prevent them from bouncing loose, getting caught in rotating parts, or rubbing against other components due to vibration. This friction is a common cause of insulation wear and shorts.

Regular Maintenance & Inspections

Consistent, proactive maintenance checks are essential. Monthly visual inspections of all cable runs are recommended. Diligently look for any signs of wear, fraying, kinks, corrosion on connectors, or loose connections.

Particular attention should be paid to areas that experience stress or movement, such as near pivots or entry points into the frame. All connectors should be kept clean from dust, dirt, and debris using a non-corrosive cleaner to ensure consistent electrical conductivity and reduce the risk of overheating.

Finally, strict adherence to best practices for battery charging and storage is important. This includes using the correct charger, avoiding overcharging, and never charging lithium-ion batteries below 0°C (32°F). Proper battery maintenance indirectly protects the controller and wiring by ensuring a stable power supply and preventing battery-induced electrical stress.

FAQs

How do I know if my phase wires are the problem?

The most obvious signs are melted or discolored wire insulation, a burning smell, or a complete loss of motor power. You might also notice your motor stuttering or making unusual noises.

Can I just wrap the melted wire with electrical tape?

No, this is a temporary and unsafe fix. The underlying issue (excessive heat) will likely return, potentially causing a short circuit or even a fire. You must properly repair or replace the damaged wire and connectors.

What's the most common cause of phase cable melting?

The most frequent culprit is a poor connection, typically at the bullet connectors between the controller and the motor. A loose or corroded connection creates high resistance, which generates enough heat to melt the plastic insulation.