Designing for electrical system flexibility

Current building codes and standards allow designers to build flexibility into the electrical system.

09/29/2013


A positive consequence of today’s building codes is typically a built-in degree of flexibility in the electrical system. In the U.S., these codes have stringent requirements for how electrical loads are calculated, and they require that certain demand factors are used to help delineate connected load (the sum of the building load: lighting, HVAC, and appliances that are physically connected to the building’s electrical system) vs. demand load (the load that a building will likely ever experience at any one time). As any electrical designer will tell you, using these allowed demand factors assures that the electrical system is sized to meet the NFPA 70: National Electrical Code criteria. 

For example, consider the NEC requirements for determining the electrical load of the receptacles in a building. Per NEC 220.14, most receptacles must be calculated at 180 VA (1.5 A at 120 V) throughout a building. This load is then summed for every receptacle throughout the building. Per NEC 220.44, the maximum demand factor that can be applied to nondwelling receptacle loads is 100% for loads of up to, and including, 10 kVA, and 50% for loads of more than 10 kVA. Next consider a 100,000-sq-ft office building with a total connected receptacle load of 200 kVA. This code dictates that the demand load must be at least equal to 105 kVA. Now, consider how many receptacles in a typical building do not even have appliances, such as computers, radios, or TVs, plugged into them, and for those that do, how often the equipment is used. Also, there may be a refrigerator plugged in that cycles on for only a few min every hr. That load is considered constant but runs only intermittently. As a result of these factors, the actual receptacle load should always be less than the value allowed by code. 

Another example is air conditioning loads. When designed, these systems are sized assuming a worst-case scenario of having a maximum number of people in a space, with all the equipment running simultaneously, on the hottest day of the year to determine the heat in a space that the air conditioning must handle. Realistically, most days won’t come close to this hottest day of the year. And even on the year’s hottest day, when will every room be at maximum occupancy with all the equipment running at the same time? These air conditioning loads will nearly always run at a reduced rate compared to their capacities. However, the electrical system is designed to accommodate their full capacity per code. Similar stories might exist for other building loads, such as lighting, kitchens, laundries, etc. 

As a result of these differences between code allowed diversity and the actual loads likely to be encountered when a building is in use, a degree of flexibility is inherently built into the design. 

Building in flexibility

There are other ways to design an electrical system to build in flexibility. These concepts may increase the overall cost of the system but can have huge returns if they prevent the need for new electrical costs down the road. Planning and forethought improves electrical design flexibility.

Plan now for future loads in panels: Is there space available for future loads? Filling the electrical panel today prevents the addition of loads in the future unless new panels are added. Having full panels, which leads to the need to add panels for any future work, will cause more disruption and cost to a facility during future renovations and additions. 

Understand possible future loads: If the panel does have expansion capacity, consider what the future load is likely to be. Will it be a small 20-A breaker for more receptacles or lights, or will it need to accommodate a large new motor? How much power would this equipment draw? Make sure the panel (and upstream equipment) has enough capacity—ampacity as well as physical space for a breaker—to accommodate this. 

Electrical room locations: Early in a project, work with the architect to include as many localized electrical rooms as possible. If successful, typically those rooms can be a little smaller so that the overall area needed doesn’t change significantly. It’s much easier to add new loads or otherwise modify the electrical system later when there is a nearby electrical room compared to running several hundred ft away when one isn’t close to an addition or renovation. 

Electrical room sizes: Determining the sizes of electrical rooms also requires coordination with the architect. Try to design electrical rooms so that there is future space for another panel or two to be added in the room. Later, if another panel becomes necessary due to new loads, it’s a lot easier to accommodate if it can fit into an existing electrical room vs. asking for more space and possibly disrupting important building functions. 

Considering these concepts during design can lead to more flexible building electrical systems for many years to come. For example, if the panel is already designed, ensuring it has space for future breakers is a minimal expense. On bid day, this is an item that may not have any additional cost. Also, by asking for a room that is just 2 ft longer during design (for example, 10 ft by 10 ft instead of 10 ft by 8 ft), you may be able to accommodate two more panels later and save the need for another electrical room altogether (more than offsetting the initial investment).


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Anonymous , 12/15/13 08:10 PM:

Verry useful.
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