You're right, finding the correct NEC guidance for low-voltage DC systems that are not explicitly part of a PV system can be challenging. Your concern about using MC cable is valid, and while the PV section provides a clear example, it's not the only way to justify your position. Here's a breakdown of why MC cable is a reasonable and often required choice, and how to argue for it

Why MC Cable is a Good Choice for 48VDC Power Cabling
Safety

48VDC, while low voltage, can still deliver significant current and pose a fire hazard if improperly installed. MC cable provides physical protection for the conductors, reducing the risk of damage, shorts, and subsequent fires. The metal sheath acts as a grounding conductor (when installed correctly), further enhancing safety.

Physical Protection

Running cables through a cable chase five floors presents numerous opportunities for physical damage. MC cable's robust construction is far superior to other wiring methods in resisting abrasion, impact, and other hazards encountered in a commercial building.

Compliance with General Wiring Methods

While Article 720 covers low-voltage circuits, it doesn't specify every acceptable wiring method. It often refers back to Chapter 3, "Wiring Methods and Materials." You can argue that using a wiring method listed in Chapter 3, like MC cable, is acceptable for a 48VDC power system.

Building Codes and Local Amendments

Local building codes may have additional requirements for wiring in specific applications, including data centers or areas with critical equipment. Check local amendments to the NEC as these can significantly influence the permissibility of wiring methods.

How to Justify MC Cable in a Non-PV Application1. Focus on Safety and General Wiring Practices

Instead of solely relying on the PV section (690), start with the fundamental principles of safe wiring practices. Refer to NEC Article 300, "General Requirements for Wiring Methods and Materials." This article emphasizes the need for wiring methods to be suitable for the environment and protected from physical damage. Emphasize the importance of minimizing fire risks and preventing accidental shorts. Use the PV section as an example of best practice for DC wiring within a building, even if it's not explicitly mandated in your specific application. "If the NEC requires MC for PV systems inside a building for safety, it makes sense to follow the same principle for our 48VDC system, especially given the long run and potential for physical damage." Mention the grounding benefits of MC cable and how it reduces the risk of ground faults.2.

Refer to Relevant Sections in Chapter 3

Specifically, point to Article 330, "Metal-Clad Cable: Type MC." This section outlines the permitted uses of MC cable, which includes power, control, and signal circuits. Check the allowed uses and restrictions in 330.10 and 330.12 for Type MC cable3.

Consult with a Qualified Electrical Engineer or Inspector

If you encounter significant resistance, the best approach is to consult a licensed Electrical Engineer familiar with NEC and local codes. They can provide a professional opinion and stamp a design that specifies MC cable, lending credibility to your argument. Engage the local electrical inspector early in the process. Their interpretation of the code is what ultimately matters. Showing them your proposed design and reasoning will help ensure compliance and prevent costly changes later.4.

Document Your Reasoning

Create a document outlining your justification for using MC cable. Reference the specific NEC articles you're using to support your decision. This document will be helpful when discussing the project with others and can be used as a record of your decision-making process.

Alternative Arguments (if needed)



Consider Intermediate Metal Conduit (IMC) or Rigid Metal Conduit (RMC)

These are other metal raceway options that provide excellent physical protection. While they require more labor and fittings, they are unquestionably compliant with NEC and offer a similar level of safety to MC cable.*

Use a Flexible Metal Conduit (FMC) with appropriate conductors

If you need flexibility in certain sections, FMC can be used in conjunction with THHN/THWN conductors. Ensure the FMC is properly grounded.

In summary, while the NEC doesn't have a single, explicitly clear statement mandating MC cable for all low-voltage DC power systems, you can build a strong case by focusing on safety, general wiring practices, and Chapter 3 of the NEC. Emphasize the physical protection provided by MC cable, its grounding capabilities, and the overall reduction in risk compared to other wiring methods. If you encounter pushback, consulting with an Electrical Engineer or the local electrical inspector is highly recommended. Flag for review