The answer to your question is it depends, and here's a breakdown of the factors you need to consider

Key Considerations:
Listing and Labeling

Most importantly, the enclosure (handhole) is likely

Listed and Labeled by a testing agency like UL or ETL. Modifying the enclosure by adding knockouts generally voids that listing. This means the manufacturer's certification that the enclosure meets safety and performance standards is no longer valid. NEC 110.3(B)

The NEC requires that listed and labeled equipment be installed and used in accordance with the instructions included in the listing or labeling. Modifying the enclosure without manufacturer approval violates this requirement.

Manufacturer Approval

If you contact the enclosure manufacturer and they specifically authorize the modification in writing, including providing the proper installation method (e.g., type of knockout, torque specifications), it might be permissible. They are likely to require you to maintain the enclosure's original integrity, corrosion resistance, and mechanical strength. This is the best way to proceed if modifying the enclosure is necessary.

Structural Integrity

Adding knockouts can weaken the enclosure, especially if the steel is relatively thin or if the knockouts are placed close to corners or edges. The enclosure must maintain its structural integrity to withstand environmental factors (weight, pressure, frost, etc.).

Corrosion Resistance

Cutting through the factory finish exposes the base metal, making it susceptible to corrosion. You'll need to ensure the cut edges are properly treated with a corrosion-resistant coating and sealed to prevent water ingress.

Accessibility and Working Space

Consider whether adding knockouts will obstruct access to conductors or splices within the handhole. Adequate working space is required by the NEC.

Authority Having Jurisdiction (AHJ)

Ultimately, the AHJ (inspector) has the final say. Even if you get manufacturer approval, the inspector may still reject the modification if they believe it compromises safety or code compliance. Consult with them beforehand!

Regarding Meeting NEC 314.28 (U-Pull)



Deflection Requirements

NEC 314.28 deals with the minimum size of pull boxes, junction boxes, and handhole enclosures, and it's very important for ensuring proper bending radii and clearance for conductors. The U-Pull calculation is specifically about the minimum distance required for a cable to enter and exit the box without exceeding its bending radius.

Location Matters

If you're adding knockouts to meet 314.28 U-pull requirements, the placement of those knockouts is critical. They must be positioned to allow for the proper bending radius of the cables, and the entry/exit point of the cable must align to facilitate the required bend.

The Existing Opening

The 9" x 14" opening at the bottom might be useful, but it depends on its location relative to the existing and proposed knockout locations. It might also be usable as is to bring in cables if it meets required protection of cable for the environment (like direct burial)

General Advice


Contact the Manufacturer First

This is always the first step. Get their written approval and specifications.2.

Consult with the AHJ

Discuss your proposed modification with the electrical inspector before you make any changes.3.

Consider Alternatives

Before modifying the enclosure, explore other solutions, such as:

Using a Larger Enclosure

This is usually the simplest and safest option.

Re-routing Cables

See if you can adjust the cable routing to use existing knockouts effectively.

Using a different wiring method

Consider if using a different type of cable or conduit fitting might reduce the U-pull requirements.4.

Document Everything

Keep detailed records of all communication with the manufacturer and the AHJ, as well as any modifications made.

In summary, modifying a listed and labeled handhole enclosure in the field is generally discouraged and can void the listing. You must* obtain written approval from the manufacturer and acceptance from the AHJ before making any changes. Prioritize the safety and integrity of the enclosure above all else.

Always consult with a qualified electrical engineer or a licensed electrician for specific advice on your situation. They can assess the requirements of NEC 314.28, evaluate the enclosure, and ensure that any modifications are done safely and in compliance with all applicable codes. Flag for review