For single generator configurations, this separation is often implemented by utilizing multiple output breakers within the genset connection box assembly. Historically, this was implemented with multiple breakers in a common connection box. With the NEC’s increased attention to separation of circuits, the most stringent implementation includes isolating these breakers in separate generator connection boxes or complete barrier separation. This has been a Generac standard process since 2009.
The issue becomes more complicated when specifying a parallel generation solution which offers increased flexibility such as Generac’s Modular Power System (MPS). The most common concept is to use a separate vertical section as discussed in NEC700(B)(5)a. This is definitely a valid solution, but potentially inefficient for simple applications with limited feeder circuits. For example, users with an application that has one emergency circuit, one fire pump and one optional standby circuit will find the three-section switchboard configuration is costly and not very space efficient.
In these situations, there are three options to be considered. It’s important to note that these options focusing on the separation of circuits could also be modified to insert additional point of entry disconnect devices, if the disconnects on the outdoor housed generator sets where not meeting application or local requirements.
One approach is to utilize a connection box that feeds multiple disconnects each in their own enclosure per NEC 700(B)(5)a. The connection box provides a single point of power termination creating a common bus. The cabling to the various disconnects are feeder taps. This solution provides separation of circuits, creates a convenient outdoor tap point for future growth, and provides a convenient point for load bank testing. The disadvantage is the capital and installation costs of the junction box and associated cable terminations.
This option applies to applications that have a large optional standby circuit. The optional standby transfer switch could be sized for full generator capacity and could functionally replace the need for the junction box. In this configuration, the optional standby transfer switch forms the common point of connection for the generator bus. The fire pump and emergency circuits could be tapped directly from the generator paralleling switches. This configuration maintains the benefit of multi-generator redundancy for all the load circuits but may require the use of crimp cable connections to accommodate the additional cabling terminations at the paralleling switches.
This option is a variation of Option 2; however, it employs a second output breaker configuration. In this configuration, the emergency and fire pump circuits have enhanced separation from short circuit events by being connected directly to a specific alternator instead of the common bus. This enhances isolation but means that the dedicated generator must be operational in order to support its respective dedicated circuit.
Though all of the above diagrams show a single optional standby transfer switch, all of the configurations could use an optional standby distribution panel feeding multiple switches. For applications like healthcare with multiple emergency circuits, the benefits of using the default approach of a multi-section distribution panel tends to offer the best solution.
These options illustrate the flexibility of the Generac’s MPS approach in meeting various application and NEC code constraints while at the same time providing customers with enhanced reliability, scalability, and flexibility.
If you have questions concerning the separation of circuits for MPS, contact your authorized Generac Industrial Power distributor or dealer.