You've stumbled upon a common point of confusion and misapplication of the "skin effect." Let's break it down

Understanding Skin Effect
What it is

The skin effect is a phenomenon where, at high frequencies, alternating current (AC) tends to flow primarily on the surface (the "skin") of a conductor, rather than being distributed uniformly throughout its cross-sectional area. The higher the frequency, the shallower the skin depth.

Why it happens

AC current induces eddy currents within the conductor. These eddy currents oppose the flow of current in the center of the conductor and reinforce it at the surface.

Key factor: Frequency

The skin effect becomes significant at frequencies of kilohertz (kHz) and megahertz (MHz) and above. It's negligible at typical power frequencies (50/60 Hz) and lower.

CAN Bus and Skin Effect - The Likely Reality

Your TRANE technician's statement that a wire nut splice disrupts skin currents in a CAN bus cable is highly unlikely to be correct in the context of a typical HVAC system's CAN bus. Here's why: CAN Bus Frequency

CAN bus in HVAC systems generally operates at relatively low frequencies (typically 125 kHz to 1 MHz, but often much lower). While 1 MHz might seem "high," the signal voltages are usually low enough that the skin effect impact is practically negligible, especially in short wire runs. The skin depth at these frequencies, even for copper wire, is still significant enough that the current distribution across the conductor's cross-section isn't severely limited.

Signal Integrity, not Skin Effect

The issue with splicing CAN bus cables isn't usually skin effect. It's far more likely related to

signal integrity, which encompasses several factors: Impedance Matching

CAN bus cables are often designed to have a specific characteristic impedance (typically 120 ohms). A poorly made splice (like a twisted wire nut) can create an impedance mismatch, causing signal reflections. These reflections can distort the data signals, leading to communication errors.

Signal Attenuation

A bad splice can increase the resistance of the connection, causing the signal to weaken (attenuate) over the cable length.

Noise Susceptibility

Loose or poorly insulated splices can make the CAN bus cable more susceptible to electromagnetic interference (EMI) or radio frequency interference (RFI), which can corrupt the data.

The Problem with Wire Nuts for Data

Wire nuts are generally designed for joining solid-core electrical wiring, where the contact area is relatively large and stable. When used on stranded data cables (as CAN bus cables often are), wire nuts can: Not provide a reliable, low-resistance connection. Damage the conductors due to the twisting action. Be prone to loosening over time due to vibration or thermal cycling.

Why the Tech Didn't Suggest an Alternative

The tech might not have suggested an alternative because: Company Policy

TRANE might have a policy against splicing CAN bus cables in the field at all.

Lack of Knowledge

Sadly, not all technicians are deeply familiar with CAN bus principles. The "skin effect" explanation might have been a misunderstanding or a way to avoid further explanation.

"Best Practice"

The safest and most reliable approach is often to replace the entire cable run.

Better Alternatives to Wire Nuts for CAN Bus

If splicing is unavoidable, use these methods:1.

Proper Crimp Connectors

Use crimp connectors specifically designed for data cables. These connectors maintain a more consistent impedance and provide a reliable connection. Be sure to use the correct crimping tool!2.

Insulation Displacement Connectors (IDCs)

Similar to those used in telephone or network cabling. These connectors pierce the insulation to make contact with the conductor. However, ensure they're rated for the voltage and current levels of the CAN bus.3.

Terminal Blocks

Use screw-down terminal blocks. Strip the wires, insert them into the terminal, and tighten the screw. This provides a more secure and organized connection. Make sure the terminal block is suitable for the wire gauge.4.

Soldering and Heat Shrink

While more involved, soldering the wires together and covering the joint with heat shrink tubing provides a very reliable connection. Be careful not to overheat the wires and damage the insulation.*5.

CAN Bus Specific Connectors

Look for connectors specifically designed for CAN bus applications. These will often provide proper shielding and impedance matching.

Recommendations


Consult TRANE's Documentation

The most important thing is to check TRANE's official documentation or consult with their engineering support. They will provide the definitive guidance on proper wiring and splicing procedures for their CAN bus systems.2.

Prioritize Replacement

If possible, replace the entire cable run instead of splicing. This is the most reliable solution.3.

Use Appropriate Connectors

If splicing is necessary, use one of the alternatives mentioned above, making sure the chosen method is rated for the voltage, current, and environmental conditions of the application.4.

Test the Connection

After any splice, thoroughly test the CAN bus communication to ensure there are no errors.

In conclusion, the skin effect is almost certainly not the reason why you shouldn't use wire nuts on a CAN bus cable in your HVAC system. Signal integrity and reliable connections are the real concerns. Follow TRANE's guidelines and use appropriate splicing methods to ensure proper system operation. Flag for review