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As buildings and mixed-use facilities continue to grow in scale and complexity, the demand for reliable, high-capacity electrical distribution systems grows too. With more modern constructions, electrical engineers must design systems that deliver power to multiple distribution points and handle higher energy demands – all while working within the limited space allocated for these electrical systems. 

When power requirements increase, the secondary side of a transformer often needs to supply several different circuits. Running multiple sets of conduits can get the job done, but it creates crowded layouts, leads to longer installation times, and can generate maintenance challenges over the lifetime of the system.  

That’s where today’s topic –the pad mounted bussed secondary enclosure (also known as a Tapbox) – comes in. A tapbox helps streamline power distribution from the utility transformer, allowing engineers to design systems that are neater, safer, and easier to maintain. The following sections outline what a tapbox is, when and why it’s used, and how EPI ensures project success by working closely with clients to select the right tapbox for each application.

What is a Bussed Secondary Tapbox? 

A bussed secondary tapbox is an outdoor electrical enclosure that provides a centralized point for distributing power from a transformer’s secondary (output) side to multiple downstream circuits. Tapboxes are typically pad-mounted and are built to NEMA 3R standards, providing reliable protection against rain, sleet, and other environmental exposure. 

Depending on project requirements, they can handle ampacities from 600 to 10,000 Amps. Inside, copper buss bars carry electrical current from the transformer and distribute it to several outgoing circuits. These conductors create a low-resistance path for electricity, improving efficiency and ensuring reliable performance under high-load conditions. This enclosed design keeps all secondary connections organized and protected, reducing clutter and maintaining safe operating conditions. 

Typical Applications of Bussed Secondary Tapboxes

Tapboxes are used in projects with high energy demand, such as large malls, office buildings, and multi-tenant complexes where a single transformer must serve multiple service disconnects or distribution panels. These environments often involve multiple outgoing circuits and limited space near the transformer – conditions that make traditional wiring methods difficult to manage.

A tapbox is preferred over a wireway or direct conduit connections when organization, accessibility, and safety are priorities. In setups with several conductors leaving the transformer, using individual conduits can quickly cause congestion and complicate maintenance. 

Engineers also favor tapboxes in projects anticipating future expansion or modification. Their enclosed bus design allows additional connections to be added safely and efficiently, supporting long-term flexibility.

Key Engineering and Design Factors to Consider

Selecting the right tapbox requires careful consideration of several design factors. Engineers must account for ampacity, number of termination points, and sufficient space for conductor placement when specifying the enclosure best suited for their project.

Ampacity directly influences the size of the copper buss bars, as higher current ratings require thicker conductors to carry the load without overheating. Additionally, the number of lug spaces needed determines the required length of the bars. Together, these factors – along with the training space needed for cable entry or exit – define the overall size of the enclosure. Careful consideration of these specifications ensures that the tapbox can be used safely, easily maintained, and comply with local utility codes.

Because every project presents unique challenges – whether its limited space near a transformer, unconventional conductor routing, or utility specifications – custom enclosure design and manufacturing are essential. A well-engineered tapbox addresses these conditions by optimizing enclosure dimensions, buss spacing, and training space to meet each project’s requirements.

This is where EPI’s engineering collaboration adds real value. By working closely with clients from the design stage through fabrication, EPI ensures every tapbox fits its application, meets code requirements, and installs smoothly in the field. The result is a system that performs reliably and supports long-term operational success.  

Bussed secondary tapboxes are essential for efficient, safe, and organized power distribution in modern facilities. Designing the right tapbox requires careful attention to design, installation, and performance goals. Through collaborationand custom engineering, EPI helps engineer solutions that perform dependably and meet project requirements. 

By combining technical expertise with a hands-on approach, EPI ensures every tapbox supports what matters most: powering the future.