Okay, here's a breakdown of why MC cable touching the sprinkler pipe is a problem, the electrical theory behind it, and why each suggestion has its own implications

The Core Problem: Grounding/Bonding and Dissimilar Metals

The biggest concern here is a combination of grounding/bonding requirements and the potential for corrosion due to dissimilar metals. Grounding/Bonding

Electrical codes (like the NEC in the US) require all metal parts of an electrical system, including conduit and cable armor (like MC), to be effectively grounded or bonded. This creates a low-impedance path for fault current to flow back to the source in the event of a ground fault (a short circuit to a grounded part). This low impedance path allows the overcurrent protection device (circuit breaker or fuse) to quickly trip and clear the fault, minimizing shock hazards and fire risk.

Dissimilar Metals and Corrosion (Galvanic Corrosion)

Sprinkler pipes are often made of ferrous metals like steel (sometimes galvanized), while MC cable armor is often aluminum. When dissimilar metals are in contact and an electrolyte (like moisture) is present, you create a galvanic cell. One metal acts as an anode and corrodes, while the other acts as a cathode and is protected. In this case, the

aluminum in the MC cable armor will likely corrode over time. This corrosion can weaken the MC armor, potentially compromising its ability to provide a reliable ground fault path. Also, the corrosion can eventually cause arcing as the pipe is no longer bonded and a fault can not return. Why Just "Not Touching" is a Good Rule of Thumb

Simply put, it's a preventative measure. By keeping your electrical installations separate from other metallic systems, you minimize the risk of creating these galvanic couples and ensure that the grounding and bonding system remains effective. It's easier to avoid the problem than to try and perfectly mitigate it every time contact occurs.

Why the "Ferrous Metals" Comment?

The superintendent likely mentioned ferrous metals because the steel pipe is more prone to causing galvanic corrosion with aluminum than some other materials would be.

Evaluating the Solutions Re-pull the MC (Superintendent's suggestion)

This is the best solution from a code compliance and long-term reliability standpoint. It completely eliminates the problem. It might be the most labor-intensive but it is the right thing to do.

Mastic Pad (Foreman's suggestion)

This could be an acceptable solution if done properly, but it needs to be carefully considered:

Proper Material

The mastic pad must be a material that is specifically designed as a dielectric barrier and a corrosion inhibitor for use with dissimilar metals. Just any mastic pad won't do. It needs to completely isolate the metals and prevent moisture intrusion.

Complete Coverage

The pad needs to completely cover the contact point and extend beyond it to ensure that no moisture can bridge the gap.

Long-Term Reliability

You need to consider how well the mastic pad will hold up over time. Will it dry out, crack, or lose its effectiveness?

Code Compliance

While a mastic pad might address the corrosion concern, it may not fully satisfy the grounding and bonding requirements. The electrical inspector may not accept this solution, as the MC cable is still technically in contact with a metal object that is required to be bonded.

Doing Nothing (Your original situation)

This is the worst option. It creates a potential for corrosion, compromises the integrity of the MC cable's grounding path, and could violate electrical codes.

In Summary

While the "don't touch other trades' stuff" rule might seem arbitrary, it's rooted in sound electrical principles related to grounding, bonding, and the prevention of corrosion. Re-pulling the MC is the preferred solution. If a mastic pad is used, ensure it's the correct type, properly installed, and approved by the inspector. Flag for review