Navigating NEC Article 358: A Pro’s Guide to EMT Installation & Support Rules

NEC Article 358 provides the essential framework for the safe and compliant installation of Electrical Metallic Tubing (EMT), one of the most common raceways used in commercial and industrial settings. For any professional journeyman electrician or master electrician, a deep understanding of this article is not optional—it’s fundamental to every project. This guide focuses on the critical rules within NEC Article 358, covering everything from permitted uses and support spacing requirements to conductor fill and bending limitations. Mastering these regulations, particularly the rules for EMT conduit support found in NEC 358.30, ensures your raceway installation is not only efficient but also meets the strict standards of the National Electrical Code. This knowledge is a cornerstone of professional electrician training and is vital for passing certification exams and delivering safe, reliable work in the field.

What is NEC Article 358?

NEC Article 358 is the section of the National Electrical Code that specifically governs electrical metallic tubing (EMT). It details the material’s applications, limitations, and the exact methods required for a compliant installation. This includes specifications for the tubing itself, the fittings used to connect it, and the supports needed to secure it. While EMT is known for being lightweight and easy to work with, Article 358 ensures it is not misused in ways that could compromise safety or the integrity of the electrical system. Adherence to these rules is mandatory for any raceway installation using EMT, and inspectors will verify compliance against these standards.

Uses Permitted and Not Permitted (NEC 358.10 & 358.12)

Understanding where you can and cannot install EMT is the first step in planning a compliant project. The nec code book draws clear lines based on environmental conditions and the risk of physical damage.

H3: Uses Permitted for EMT Conduit

According to NEC 358.10, EMT is permitted for use in a wide range of applications, both exposed and concealed. Its versatility is a key reason for its popularity. Key permitted uses include:

• Exposed and Concealed Work: It can be run on the surface of walls or hidden within them, above ceilings, or under floors.
• Corrosive Environments: EMT can be installed in locations subject to corrosion, including in concrete or in direct contact with the earth, provided it is supplied with supplementary corrosion protection that is approved for the specific condition.
• Wet Locations: EMT is permitted in wet locations when all supports, bolts, straps, screws, and associated couplings and connectors are made of corrosion-resistant materials or are protected by a corrosion-resistant coating. All fittings must be listed for use in wet locations.

H3: Uses Not Permitted for EMT Conduit

NEC 358.12 outlines specific situations where EMT installation is prohibited to prevent failure and ensure safety. These restrictions include:

• Where it would be subject to severe physical damage.
• For the support of luminaires or other equipment (with an exception for conduit bodies meeting specific criteria).
• In or under cinder concrete or cinder fill where subject to permanent moisture, unless protected on all sides by a layer of noncinder concrete at least 2 inches thick.
• In any hazardous (classified) location except as permitted by other articles in the NEC.

Critical Installation Rules: Securing and Supporting EMT (NEC 358.30)

Properly supporting emt conduit is one of the most critical aspects of a professional installation and a frequent focus of electrical inspections. The rules for EMT conduit support are detailed in NEC 358.30.

H3: Support Spacing and Securing Requirements

The general requirements are straightforward but demand precision. Failing to have the conduit securely fastened can lead to failed inspections and costly rework.

• Standard Support Spacing: EMT must be securely fastened in place at least every 10 feet.
• Support Near Terminations: A support is required within 3 feet of every outlet box, junction box, device box, cabinet, conduit body, or any other tubing termination.
• Unbroken Lengths: One exception to the 3-foot rule exists for unbroken lengths of EMT. If you are fishing the conduit through access holes in framing members in finished buildings, securing it within 3 feet of a termination is not required. However, the standard 10-foot spacing rule still applies where possible.

Bending, Sizing, and Conductor Fill

Beyond support, a compliant raceway installation involves correct bending techniques and adherence to conductor fill limits. These skills are essential for both the journeyman electrician and master electrician.

H3: Rules for Bending EMT (NEC 358.24 & 358.26)

When bending EMT, the goal is to change direction without compromising the raceway. NEC 358.24 requires that bends be made so that the tubing is not damaged and its internal diameter is not effectively reduced. For a more detailed walkthrough, see our guide on how to bend EMT conduit. Furthermore, NEC 358.26 limits the total number of bends in a single run:

• A maximum of 360 degrees of bends (equivalent to four 90-degree bends) is allowed in one continuous run between pull points (e.g., box to box, or box to conduit body). This prevents damage to wire insulation during pulling.

H3: Conductor Fill Calculations

NEC Article 358 does not contain its own fill table; instead, it directs users to NEC Chapter 9 for all conductor fill rules. Calculating the correct fill is crucial to prevent conductor overheating. While a digital conduit fill calculator can speed up the process, every electrician should know how to do it manually. For a deep dive into this topic, explore our guides on Conduit Fill Calculations: An NEC Guide and using NEC Chapter 9 Table 1.

Here is a simplified step-by-step process for determining correct EMT conductor fill:

1. Identify the size of your EMT conduit and find its total internal cross-sectional area (in square inches) from NEC Chapter 9, Table 4.
2. Determine the allowable fill percentage from NEC Chapter 9, Table 1. For three or more conductors, this is typically 40%.
3. Identify the type and gauge of all conductors you plan to install in the raceway.
4. Find the approximate area for each conductor type from NEC Chapter 9, Table 5.
5. Sum the areas of all individual conductors to get a total conductor area.
6. Verify that the total conductor area does not exceed the allowable fill area (Total EMT Area x Allowable Fill Percentage).

This level of detail is exactly what journeyman and master electricians need to master. Master conduit installation techniques with our practical, state-approved online electrical courses.

Key Considerations for Fittings, Couplings, and Grounding

The integrity of an EMT system relies heavily on its components. Using the correct fittings and ensuring a solid ground path are non-negotiable safety requirements.

H3: Using Listed Fittings, Couplings, and Connectors

Every component in an EMT system must be made for the job. NEC 358.6 requires that all tubing, couplings and connectors, and other fittings be listed. Using unlisted or improper fittings, such as plumbing parts, is a serious violation. When comparing different raceway types and their components, understanding the specific requirements for each is key, as outlined in our guide to conduits, wireways, and gutters.

H3: EMT as an Equipment Grounding Conductor (EGC)

Under NEC 250.118, EMT is permitted for use as an equipment grounding conductor (EGC). However, this is only true if the entire raceway system is installed with listed fittings that are made up tight. A loose setscrew on a connector or a poorly seated coupling can break the ground path, creating a serious shock hazard. This makes it critical to ensure every connection point is mechanically secure to maintain electrical continuity for the life of the installation.

Primary Sources & Authoritative References

The information in this article is based on the standards set forth by the National Fire Protection Association (NFPA). For direct reference, consult the official code book.

• NFPA 70®, National Electrical Code® (Source: nfpa.org)

Frequently Asked Questions (FAQ)

What is the maximum support spacing for EMT conduit under NEC Article 358?

According to NEC 358.30, EMT must be securely supported at least every 10 feet. Additionally, a support is required within 3 feet of every termination point, such as a box, cabinet, or conduit body.

Can you use EMT in wet locations?

Yes, EMT is permitted in wet locations, provided that all fittings (couplings, connectors, straps, etc.) are listed for wet location use and made of corrosion-resistant material. Proper installation of these listed fittings is critical.

How many bends are allowed in an EMT raceway installation?

NEC 358.26 limits the total degrees of bends in a single, continuous run of EMT to 360 degrees between pull points. This is equivalent to four 90-degree bends and is designed to prevent damaging conductor insulation during pulling.

Does NEC Article 358 specify conductor fill limits for electrical metallic tubing?

No, NEC Article 358 itself does not list conductor fill limits. It directs the user to NEC Chapter 9, which contains all the necessary tables and rules for calculating the maximum number of conductors allowed in any type of raceway, including electrical metallic tubing.