Energy savings in electrical distribution systems
Consider these energy efficiency options for retrofitting existing and designing new buildings’ electrical distribution systems.
- Recall the codes and standards that pertain to energy-efficient design.
- Identify some energy-efficiency upgrades that should be considered during design as well as measures owners can take for energy efficiency.
Engineers and building owners often focus on payback periods and return on investment as economic decision-making thresholds for energy efficiency investments. These energy efficiency improvements typically revolve around energy-efficient lighting (i.e., LEDs), using variable frequency drives (VFDs), or using National Electrical Manufacturers Association (NEMA) Premium motors, for example. However, there are other parameters that are more accurate and effective in making investment decisions for building systems, specifically electrical distribution systems.
The codes and standards associated with energy efficiency establish the minimum energy efficiency requirements needed for the design of new buildings and renovations to existing buildings. The codes, however, are geared toward the efficiency of mechanical and lighting systems. Not a lot of information is provided within these codes to establish energy efficiency measures for the design of the power distribution systems, just the systems that the power distribution systems serve. These codes and standards include the following:
- ASHRAE Standard 90.1
- International Energy Conservation Code (IECC)
- State energy codes
- Local stretch codes (city or county).
The adoption of energy codes and the energy code used in each state varies greatly.
ASHRAE 90.1 includes a chapter on power (Chapter 8). Although the standard includes the requirement for transformers to meet the Energy Policy Act (EPAct) of 2005, it does not discuss other aspects of the power distribution system.
The standard also establishes that the voltage drop shall not exceed 2% for feeders and 3% for branch circuits (Chapter 8.4.1). Although ASHRAE 90.1 is not more stringent than NFPA 70: National Electrical Code (NEC) voltage-drop recommendations outlined in the fine point notes (FPNs) section included in Article 210.19, ASHRAE 90.1 does establish voltage drop as a requirement for meeting the standard. An NEC FPN recommends a maximum voltage drop of 5%, with the feeders to not exceed 3%; because this is an FPN, it is not a code requirement. However, accounting for voltage drop is important in designing an energy-efficient power distribution system. Voltage drop can cause overheating and shorter lifespan of equipment due to undervoltage conditions for inductive loads. The excess heat will require additional HVAC requirements for cooling spaces. Additionally, voltage drop causes inefficiency in lighting loads by causing losses in the cables rather than the power being fully applied to the lights. The increase in cable size will permit higher output of lights without applying additional power from the source.
The remainder of Chapter 8 discusses submittals (shop drawings) and controls for receptacles. The remainder of the standard primarily pertains to mechanical equipment and building measures to achieve energy efficiency. However, it does address some aspects of electrical design including lighting (Chapter 9) and motors (Chapter 10). This article does not discuss lighting or motors, except in regard to VFDs.
The automatic control of receptacles was included in ASHRAE 90.1-2010 and has been a point of contention in the standard. Although automatic receptacle control provides additional energy efficiency to plug loads, which has been difficult item to reduce energy consumption in commercial and residential applications, it also requires educating owners on the proper application and the benefits.
Similar to ASHRAE 90.1, the energy efficiency measures outlined in the IECC pertain mostly to building measures and mechanical equipment. The code discusses lighting requirements for power density and controls (Chapter 4), but unlike ASHRAE, they do not establish electrical distribution system energy efficiency design minimums. The Massachusetts Stretch Energy Code, for example, is based on the 2009 IECC, however, in order to meet the requirements the designer is required to exceed the baseline IECC standards by 20% for commercials (medium or large) buildings or providing incremental improvements of the baseline IECC standards for (medium) buildings by choosing two of the three options available in the code, which include improve HVAC efficiency, on-site power generation, or further reduction of lighting power density.
Because they are more stringent than the IECC, using ASHRAE 90.1 or a stretch energy code as a basis for design is a good starting point in any project and should be considered as a standard practice for engineering design firms.
Copper versus aluminum
Copper and aluminum are the most commonly used materials for conductors, busing in distribution equipment, and windings in transformers. There is a common misconception that because copper is more conductive than aluminum, copper distribution equipment will be more energy-efficient than aluminum. That is not the case. There are other factors to take into account including conductor size, equipment size, cost, and weight of the equipment and conductors.
Depending upon the alloy of aluminum used for the conductors or bus, the conductivity of aluminum is approximately 56% to 61% that of copper. Although the difference in conductivity is significant, this will not significantly affect the overall efficiency of the distribution equipment because the panelboards, switchboards, and transformers because, regardless of the material used, the equipment is still required to meet NEMA and UL standards for temperature rise, which would affect the efficiency of the equipment.
Similarly, although the conductors will be larger, the efficiency of the cables will not be affected. The increase in size of the conductors will require larger conduit and potentially additional feeders, depending on the rating of the conductors. As an example, per Table 310.16 of the NEC, if 380 amp is required, then a 500 MCM cable is required, the equivalent aluminum conductor is a 750 MCM cable. This is a 50% increase in the cross-sectional area of the conductors.
The cost of the materials is dependent upon the market. However, it is typical to see the following cost savings when using aluminum: 30% to 50% for dry-type transformers, 20% for substation dry-type transformers, 25% for liquid-filled pad-mounted transformers, $1,000 per vertical section for a 1,000-amp switchboard, and $1,500 per vertical section for 3,000/4,000-amp switchboards.
Additionally, in regard to aluminum cable, the voltage drop is a larger factor to be considered because it is less conductive. On average, the equivalent aluminum conductor will reduce the length a cable can be run by approximately 40% to still meet the ASHRAE-recommended 3% voltage drop.
Although the busing or windings within the equipment will increase, there typically is not a significant increase, if an increase at all, in the overall size of equipment enclosure when using aluminum versus copper. Large equipment, such as pad-mounted oil-filled transformers and switchboard/switchgear, may increase based upon the current or kVA rating of the equipment. However, panelboards and dry-type transformers will not require an increase in the enclosure size when choosing between these materials.
The more significant difference is the reduction in weight, even though the busing/windings of the equipment have increased in size. As an example, a 1,000-amp busway will be approximately 22% larger for aluminum; the copper bus will be approximately 50% heavier. This will dramatically increase as the bus ratings increase. For instance, for a 4,000-amp busway, the size increase to aluminum over copper is approximately 27%; the weight increase is approximately 73%.
Although there isn't a significant energy efficiency advantage using copper versus aluminum, the material selected for the project application should always be evaluated during design.
As noted above, the major differences between the metals are in regard to the weight and cost of the materials, although there are many other factors that should be taken into account when choosing a current-carrying material, which includes environment. Manufacturers have useful information as well as many white papers on this topic that can be reviewed for additional information. Although the efficiency difference between copper and aluminum transformers is not significantly above 15 kVA, there is enough difference that dictates copper should be used when energy efficiency is the main goal of the project. However, when cost is accounted, the initial cost savings for aluminum often outweighs the loss in energy efficiency.