Okay, I understand the situation. You have a 200A feed-through panel and want to add twin lugs to feed two subpanels, but the bolt holes behind the existing lugs are too small for readily available twin lugs. You're also out of breaker spaces, so an add-a-lug breaker isn't an option. Here's a breakdown of potential solutions and things to consider

Understanding the Problem & Safety First
Panelboard Listings

It's crucial to maintain the panelboard's listing. Modifying a panelboard by drilling new holes, altering its busbar, or using components not specifically designed and listed for it can void the UL (or equivalent) listing. This can have serious implications for safety and insurance.

Ampacity and Wire Size

Make sure your conductors are sized correctly for the ampacity of the subpanels you are feeding.

Torque Requirements

Any new lugs you install must be torqued to the manufacturer's specifications. Incorrect torque can lead to loose connections, arcing, and potential fire hazards.

Qualified Electrician

This type of work is best left to a qualified and licensed electrician. They have the experience and knowledge to ensure the job is done safely and in compliance with electrical codes. If you're unsure, hire one.

Potential Solutions (with varying feasibility and risk)


Manufacturer-Specific Lugs

This is always the best option.

Contact the Panel Manufacturer

The ideal solution is to contact the manufacturer of your panelboard (e.g., Square D, Eaton, Siemens, GE). Provide them with the model number of your panel. They may offer specific twin lug kits or replacement lugs that are UL-listed and designed for your panel's busbar and bolt size. This ensures safety and compliance. They may also provide information on allowable tap rules in your area, as well as if your existing conductors for the pass-through panelboard are correctly sized.2.

Terminal Blocks (Distribution Blocks) with Fuses

This is a safer approach but requires space in or near the panel.

Install a Separate Enclosure

Mount a small junction box (UL-listed) adjacent to your panel.

Feed to Terminal Blocks

Run the existing conductors from the panel's existing lugs to appropriately sized terminal blocks within the junction box.

Feed Subpanels from Terminal Blocks

Then, run conductors from the terminal blocks to your two subpanels. This is where the fuses come in. Add properly sized fuses inline after each terminal block so you can protect the wire going to the subpanel.

Advantages

Avoids modification of the panelboard itself. Provides a clean and organized way to distribute power. Allows for easier future modifications. Is a safer route that could prevent future issues.

Considerations

Requires additional space. Adds complexity and cost. Terminal blocks must be rated for the appropriate voltage and current.3.

Transformer Stud Lugs with Adapters (Least Recommended)

This is a more risky proposition.

The Idea

Use transformer stud lugs (which are designed to clamp onto a flat busbar) along with an adapter or custom-made piece to fit the existing bolt pattern on your panelboard.

Significant Risks



Listing Issues

Extremely likely to void the panelboard's UL listing.

Contact Area

Ensuring good electrical contact between the stud lug and the busbar can be challenging. Poor contact can lead to overheating and failure.

Mechanical Stability

You need to be absolutely certain that the adapter provides a secure and reliable mechanical connection. Vibration or movement could loosen the connection.

Not a Standard Practice

This is not a common or recommended practice, and most electricians would advise against it.

Why it's not ideal

Finding the right stud lug and creating a reliable adapter that can handle the 200A current safely and reliably is very difficult.

Important Considerations



Local Electrical Codes

Always comply with your local electrical codes. They may have specific requirements for tapping into existing feeders or modifying panelboards.

Load Calculations

Ensure that the combined load of the two subpanels does not exceed the ampacity of the main 200A feeder.

In summary, the best approach is to contact the panel manufacturer for compatible twin lugs. If that's not possible, consider using terminal blocks (with fuses), in a separate enclosure. Avoid modifying the panelboard itself unless you are absolutely certain you can do so safely and in accordance with code. A qualified electrician should be involved in the decision-making process and the installation. Flag for review