Flexible, sustainable electrical systems
Current building codes and standards allow designers to build flexibility into the electrical system.
- Know the codes that define electrical distribution system design.
- Understand flexibility in the compliance path for energy codes.
- Learn about other design methods for sustainable designs.
Although the word “sustainable” may invoke different modern-day connotations, the main target remains the same—to maximize resources. Specifically, the electrical systems that engineers design consume resources both during construction and throughout the life of the building. Although a large part of the designs are driven by certain sections of the codes, these codes also contain some flexibility that allows designers to use fewer resources.
As an example, NFPA 70: National Electrical Code (NEC) Article 220 Part III “Feeder and Service Load Calculations” gives straightforward guidelines on how to calculate feeders and service sizes for commercial and residential projects. For engineers focused on commercial projects, Part IV “Optional Feeder and Service Calculations” is sometimes overlooked. This optional method gives the designer a different approach to calculating both the feeder/service to individual dwelling units and the feeder/service for multifamily dwellings.
For example, consider a condominium with 40 identical dwelling units. Each dwelling unit is 1500 sq ft and includes a kitchen with an electric range. Each unit also has laundry facilities, with an electric dryer, as well as an electric HVAC split system and other general use appliances. Table 1 summarizes how the calculations will differ when using the Part III or Part IV method.
For single dwelling units, the Part III method permits the general lighting load, along with the small appliance and the laundry circuit loads to be taken at the general demand of 35% after the first 3,000 W (NEC 220.52), but only allows for a relatively large demand factor for large appliances like dryers and cooking equipment per NEC Tables 220.54 and 220.55, respectively. Additionally, for other connected appliances, it will allow a demand of 75% only if there are four or more appliances in the apartment unit. HVAC loads are usually taken at the largest coincidental load in the unit, in this case the compressor with the fan coil motor. Using Part III, our load for this unit is 135 amp, which consequently will make us select a 150 amp overcurrent protective device and the corresponding conductor.
In contrast, the Part IV method allows us to combine the general lighting, the small equipment, the laundry, the connected appliances, dryer, and electric range into one sum and demand the first 10,000 W at 100% and the remaining load at 40%. When combined with the HVAC load, which is calculated similarly to the Part III method, the result is a new calculated demand of 108 amp. Although we could select a 110 amp protection size, most of us will use 125 amp for ease of mind. Either way, the NEC has provided flexible guidelines that allow engineers to save clients in conductor costs, especially when considering the entire 40 units. It’s necessary to clarify that Part IV Article 220.82(A) states that this method of calculation is only applicable to “a connected load served by a single 120/240 V or 208Y/120 V set of 3-wire service or feeder conductors with an ampacity of 100 or greater.”
Furthermore, in the same code sections, the NEC gives guidelines for the service calculation for multifamily dwellings. Continuing with the same 40-unit multifamily dwelling example, Table 2 illustrates the differences between both calculations.
When considering the entire 40-unit multifamily dwelling, Part III will allow the application of an additional 25% demand factor to anything over 120 kW of the sum of the general lighting, small equipment, and laundry circuit loads. Additionally, we can apply demand factors to the electric dryer and electric range loads per NEC Tables 220.54 and 220.55. After all factors are taken into account, Part III calculations leave us with almost 1700 amp in calculated demand for which would generally be a 2000 amp service with 6 sets of 500 kc-mil copper (Cu) conductors.
In contrast, Part IV allows us to apply a demand factor per NEC Table 220.84, in this case 28%, to the entire load. This results in a calculated demand of 1047 amp in load, for which would generally be a 1200 amp service with 3 sets of 600 kc-mil (Cu) conductors. This calculation result gives us the opportunity to save resources on the service entrance equipment.
Remaining within the realm of dwelling units, NEC Article 220.61 allows the reduction in size of the neutral conductor feeding the unit, considering the maximum unbalanced load. Table 3 shows that the neutral conductor could be downsized to 76 amps, with the phase conductors being sized to 108 amp. Note that such reduction is not allowed for 4-wire, wye-connected, 3-phase systems, which is reiterated in by Article 310.15(B)(7)(4). Therefore, depending on the distribution system chosen for a project, a reduction of the neutral conductor may not be allowed.
Further along in the Part IV calculations, the NEC also allows us to use optional methods for schools in Article 220.86 and new restaurants in Article 220.88. Because NEC Table 220.86 applies demand factors to the connected load on a per area basis, this option becomes relevant for schools that demand a high power density (kVA/sq ft), which usually is the case for small schools that lack central plants and opt for remote terminal units (RTUs) and electric heat throughout the building. Tables 4 and 5 show the impact such method can have. In this case, it’s a 20% reduction in the service size. The optional method for new restaurants is similarly prescriptive per NEC Table 220.88.
Therefore, we can see how the NEC does allow for flexibility in load calculations for certain types of buildings. This flexibility can help our clients save resources on the front end of the building’s lifecycle.