Power Distribution and Management

Designing Submetering under Title 24: A Question of Space Versus Cost

As discussed in the previous section, the recently updated Section 130.5(b) of the California Energy Commission’s Title 24 efficiency standard adds new requirements to designs for new electrical services rated over 50 kVA. Understanding that “what can’t be measured can’t be managed” these requirements intend to provide greater visibility to energy usage within a facility. “Disaggregation of electrical circuits” is intended to make it relatively easy to measure energy consumed by lighting, HVAC-related motors and elevator systems – and, in multitenant settings, by building area, such as lobbies and tenant-controlled spaces.

At a minimum, the design of the electrical system must allow measurement of loads by specific types and these requirements increase as the size of the service increases. Specific load/building-area types that must be addressed are outlined in table 130.5(b).

This is only a wiring requirement in the 2013 version of Title 24, and providing meters is optional. Best explained in the Title 24 Compliance Manual, “The intent is to have a single feeder or breaker with each type of load (such as lighting) on it, such that a meter could be placed on the feeder to report energy use by that load type.” This arrangement of circuits provides for future submetering, even if that’s not part of the owners current plans.

Title24_CTABannerTo help illustrate the allowed wiring, the Compliance Manual says this: “This section of the Standard requires buildings to be wired in a manner that separates loads by types into independent feeders and risers through the building.” It is clear that the intent of the allowed wiring is to separate the circuit types after the service meter to allow simple measurement. The code describes distribution methods allowed to accomplish this that can be scalable to buildings of larger size:

  • Provide “separate switchboards, motor control centers, or panelboards to which are connected only the required load or group of loads.” The physical layout should ensure adequate space inside or adjacent to each panelboard for the future addition of individual meters.
  • For additional loads of a particular type, it further allows “SUBPANELS OF THE ABOVE to which are connected ONLY the required load or group of loads AND for which the subpanel load can be INDEPENDENTLY measured in aggregate.”

Exception to the distribution system design requirement of Section 130.5(b)

There is one exception to strictly segregating circuits to allow measurement at a single point. This is if “a complete metering and measurement system IS PROVIDED that at a minimum measures and reports the loads called for in Table 130.5(b).” This type of system can calculate the total energy use of a particular load type from multiple measurement points, using additive or subtractive methods. Under this plan, the panelboards must incorporate permanent branch-circuit current transformers that communicate to permanent metering and measuring systems provided at time of construction.

Although metering equipment has a cost, an advantage of this approach is that it allows load types to be consolidated and mixed within a smaller number of panelboards. Such systems can save additional space if contained within the panelboard. Jobsite conditions can favor this approach.

Essentially, the decision boils down to an evaluation of available space versus a project’s budget. Disaggregated, multi-panel designs mean more electrical equipment, which also means more space dedicated to non-revenue infrastructure. The consolidated design requires fewer panels, but those panels become more expensive due to the required addition of current transformers at each circuit and the associated metering devices. However, it makes it much more likely that the building owner will actually get the information they need to help cut energy use and lower the monthly utility bill.

Many consulting engineers can have an “a-ha moment” when they tally up the cost of a mixed-load design. If the end user attaches a premium to space savings – or has a true interest in using the metering data – projects can justify the upfront cost of mixed-load panels with permanent metering included. This becomes especially true in larger jobs requiring 10 or more mixed-load panels, where economies of scale and other savings can help the approach make economic sense.

For more help, check out Schneider Electric’s Professional Engineer Portal for additional information and for access to experts you can call on for advice on how these new requirements might affect plans for your next project in California or other jurisdictions that might be following Title 24 guidelines.

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