You're right to be concerned about supporting a rigid conduit riser above a roof. The NEC (National Electrical Code) doesn't specify an exact height where guy wires must be used for 2" rigid conduit. Instead, it focuses on adequate support and strength to prevent damage from weather, physical abuse, and the weight of the conductors.Your 42" recollection likely comes from common industry practice based on several factors: Wind Load

A 2" rigid conduit extending 42" above a roof presents a considerable surface area for wind to act upon. This creates a significant lever arm, placing stress on the conduit, its connection to the roof, and the enclosure it serves.

Physical Abuse

The higher the riser, the more vulnerable it is to being bumped, hit, or leaned on, especially on commercial roofs where HVAC maintenance or other activities occur.

Conductor Weight

Even though the conductors themselves are relatively lightweight, their weight contributes to the overall stress on the riser.

Material properties

All materials, including steel conduit, have tensile strength that varies based on temperature.

Support Spacing

The NEC (specifically Article 344 for Rigid Metal Conduit) provides tables for support spacing for rigid conduit, but these are for horizontal runs and don't directly address vertical risers.

Here's a breakdown of the key considerations and where you can find guidance


NEC Article 344 (Rigid Metal Conduit)

This is your starting point. While it doesn't provide a specific height limit for risers without additional support, it emphasizes the need for secure and adequate support. Look at the support intervals for horizontal runs as a minimum guideline.2.

Manufacturer's Instructions

Check the specifications of your conduit fittings, supports, and the enclosure it connects to. Manufacturers often provide recommendations for maximum unsupported heights.3.

Engineering Judgment

This is crucial. Consider the local wind load conditions (check local building codes), the type of roof mounting, the likelihood of physical abuse, and the overall stability of the installation. If you're unsure, err on the side of caution and provide additional support.4.

Local Codes and AHJ (Authority Having Jurisdiction)

Your local electrical codes might have more stringent requirements than the NEC. Always check with your local AHJ for specific guidance and interpretations.5.

Acceptable Supports

Consider these support options for risers:

Guy Wires

Use galvanized steel guy wires attached to the conduit near the top and anchored securely to the roof.

Strut Supports

Attach the conduit to a section of strut channel mounted to the roof. This provides rigid support.

Stand-off Brackets

Use sturdy stand-off brackets designed for conduit support and secure them to the roof.*

Unistrut

Use Unistrut bolted to the roof, and a clamp or bracket to secure the pipe to the Unistrut.

In Summary

While there's no absolute "42-inch rule" in the NEC, it's a reasonable guideline based on common sense and best practices.

Assess each installation individually, taking into account the factors mentioned above. When in doubt, provide additional support (guy wires, strut, brackets) to ensure a safe and durable installation that complies with the NEC, manufacturer's instructions, and local codes. Always check with your AHJ for their specific requirements. Flag for review