Ecmweb 6347 Commercial Building Lighting Loads Pr
Ecmweb 6347 Commercial Building Lighting Loads Pr
Ecmweb 6347 Commercial Building Lighting Loads Pr
Ecmweb 6347 Commercial Building Lighting Loads Pr
Ecmweb 6347 Commercial Building Lighting Loads Pr

Lightening Up on Lighting Loads

May 20, 2014
A change in the 2014 NEC could be a sustainability windfall for the education facilities industry and potential driver for the largest upgrade of commercial building electrical systems in U.S. history.

Electrical professionals have long observed the “underloading” of customer-owned transformers, which, in part, is brought about by National Electrical Code (NEC) Chapter 2 design demand rules. This practice can be traced back to more than 50 years ago — when the economy was growing close to 10% compared to the 1% we see today. This was also a time when customers could visit the local electric utility store to get an incandescent light bulb replaced for free. The widely understood condition of underloading is best summarized in Fig. 1, which shows the average loading of low-voltage dry-type transformers by building type across the United States.

The blending of art and lighting are moving into mainstream lighting designs (Vladimir Voronin/iStock collection/Thinkstock).

The divergence between design demand and measured demand reported in a 1999 study by the Cadmus Group for the Northeast Energy Efficiency Partnership has only grown with the addition of more sophisticated motor controls for HVAC equipment and the lower power requirements of LED-based lighting systems. But a new Exception in the branch circuit load calculation requirements in the 2014 Edition of the NEC (Sec. 220.12) provides inspiration for the re-imagination of lighting supply circuit design (see Branch Circuit Load Calculations).

Fig. 1. Metered load factors for low-voltage, dry-type transformers by building type (1999 study by the Cadmus Group for the Northeast Energy Efficiency Partnership).

As a fire safety document developed by the National Fire Protection Association (NFPA), the principal focus of the NEC is not energy conservation. The concern for fire hazard associated with overloaded transformers, switchgear, and wiring within the building premises takes precedence over all other concerns. Proposals presented for decades by many industry experts to revise the Chapter 2 and Chapter 4 NEC demand calculations on the basis of “too much material waste and heat losses” were routinely rejected, until a University of Michigan-led consortia of colleges and universities presented sufficient data to an NEC Technical Committee that too much energy brought into a building was the root cause of unnecessary flash hazard.

Lighting load was an obvious place to start because lighting presents continuous load on the order of 30% to 40% of a typical general commercial or school building. Lighting load is also on the green agenda of other non-electrical standards developers, such as American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) and the International Code Council (ICC) — both groups are promulgating limits on lighting power densities as summarized in Table 1 (Click here to see Table 1).

A careful look at Art. 220 and Annex D of the 2014 NEC reveals that many industries, such as agriculture, homebuilding, and restaurant, have succeeded over the years in getting exceptions to the Chapter 2 design rules. The largest occupancy class of them all (large commercial facilities) has not been permitted a usable method to narrow the disparity between design demand and measured load — until now. Exceptions provided in Sec. 220.87, Sec. 430.26, and Sec. 645.25 offer some relief from oversizing but not at the speed of energy code promulgation. The proposal presented to the NEC Technical Committee in this latest Code cycle, which does provide immediate relief, found its inspiration in previous proposals from the past four NEC cycles submitted by the American Chemistry Council, the State of Wisconsin, and at two electrical equipment manufacturers.

Monitoring the load

The requirement for a lighting load monitoring method is noteworthy. Although many new buildings are required to have energy management systems as part of their HVAC budget, the Exception in Sec. 220.12 does not require a full-scale energy management system — only a way to monitor the lighting load in a fashion that provides information so that the owner or manager knows about the total lighting load of the building (and not other types of loads that may emerge farther out in the life-cycle of the building). Units should correlate with the units of the adopted energy code. A combination of the appropriate current-sensing relays (or other transducer) and software can deliver a number in terms of amperes or watts in a new energy management platform. A monitoring point could also be built from scratch for about $2,500 in most buildings — where the cost of pipe and wire between the measured point and the monitoring point is not too complicated. Some transformers even have an integrated metering option, which can significantly reduce this cost.

Energy efficiency programs continue to drive innovation in the lighting design arena (alice-photo/iStock collection/Thinkstock).

The Exception leaves leeway for the designer to determine where total building lighting load should be monitored. Figure 2 (click here to see Figure 2) presents two places where the lighting load might be measured — at one or more dedicated feeders to lighting load only or at one or more lighting panels supplied by local lighting-only transformers.

In addition, the Exception does not specify which lighting calculation method to apply (“whole building” versus “space by space”). Because energy codes are moving quickly, this will require some coordination among energy conservation and electrical code enforcement authorities. Whatever small differences in power density may exist between them is likely to remain insignificant when compared to the safety and energy conservation gains realized when the Exception is applied.

Smaller transformers

Now we can re-imagine more lighting systems supplied through a network of fewer, smaller, 480V delta or 480V/277V delta-wye lighting circuits. That 800A dedicated substation lighting feeder and its corresponding distribution panel can now be designed around a 3-wire (i.e., without a neutral) feed that supplies smaller transformers positioned in more compact electrical closets.

This also creates a cascade of follow-on design possibilities. Because the delta winding in the smaller local transformers effectively traps ground fault current that originates from energy at the service, incident energy at these smaller transformers is reduced. This results in a big difference in electrician safety when he or she is up on a ladder changing a 277V fluorescent ballast and installing 277V LED power supplies.

Sample commercial calculation

To illustrate how to apply the new NEC Exception at a typical 50,000-sq-ft office building (most of the square footage on a large university campus can be classified as commercial/administrative space), we will use the 2012 IECC.

NEC Table 220.12 requires a load of 3.5VA per square foot for office building lighting. For our 50,000-sq-ft building, this results in a total calculated lighting load of 175,000VA. Under the 2012 IECC, the same building at a maximum installed general lighting density of 0.90VA per square foot yields a total load of 45,000VA. The resulting difference of 130,000VA between the two codes is significant no matter how you distribute it (156A @ 480V or 361A @ 208V).

Fig. 3. On the left, an electrical room is sized without taking advantage of the Exception in Sec. 220.12 of the NEC. On the right is an electrical room sized using the 220.12 Exception. The 80-sq-ft room can be reduced to 52 sq ft by taking the Exception into consideration.

This change will also reduce the footprint of the electrical closet from 80 sq ft to 52 sq ft (Fig. 3) and the size of HVAC cooling system equipment (fan coil units, air ducts, etc.). Apply that same type of savings to the millions of square feet of office space being built across the country every year, and the gains in safety and economy are obvious and disruptive.

Sample school calculation

To illustrate application of the new NEC Exception at a school building, we will use the 2010 ASHRAE 90.1 Standard.

Table 2. For the 2011 NEC column, no Exceptions were provided to design the lighting supply circuits to correlate with energy codes. All lighting was assumed to require 3VA per sq ft. For the 2014 NEC column, the Exception in Sec. 220.12 permits the lighting supply circuits to be designed according to an energy code like the IECC, which required no greater than 0.87VA per sq ft.

NEC Table 220.12 limits the allowable general lighting load for a school or classroom building to be no less than 3VA per square foot. Under the whole building method, the standard allows 0.99VA per square foot. As shown in Table 2, there would be a reduction of 201kVA in building load just for the lighting loads. This would allow for a reduction in substation transformer size from 1,500kVA to 1,000kVA (or even down to 750kVA with a forced air rating of 1,000kVA). The smaller transformer will also allow for a reduced secondary distribution (from 2,500A to 1,600A), and quite possibly a reduced size secondary switchgear section depending on the breaker requirement.

Anticipated restrictions offset by huge efficiency gains

Many Authorities Having Jurisdiction (AHJs) may interpret the Sec. 220.12 Exception to mean that no other 277V load — such as heat-tracing or special equipment — is permitted to be supplied by the dedicated lighting transformer. However, this is a relatively minor restriction in light of the enormous safety and cost benefit projections to the building’s operating economy overall — and is easily remedied with step-up or step-down transformers from non-lighting supply circuits.

Approximately 30% to 40% of electrical load in a school building is due to lighting systems (BRPH/iStock collection/Thinkstock).

Many electrical renovation possibilities will be pushed to an economic tipping point. Where a single reason is insufficient for a total upgrade of a building’s electrical system, we now have several reasons — taken together — that can define a new package of “green” electrical upgrades than can be financed, safely built, and economically run. Benefits to the electrical industry include:

• Annual no-load losses (assuming ½% of rated kVA at 1.0 power factor at $0.10/kWhr) for medium-voltage transformers will be on the order of $43,800 per 10,000kVA of avoided transformation. (Losses on smaller, low-voltage transformers are on the order of 4% of nameplate — so the smaller the better.)

• A 1,000-ft 400A legacy feeder (copper wire) that costs about $7,500 could now be built as a 300A feeder (aluminum) for about $2,500. On campuses where the buildings are in close proximity, medium-voltage services can be replaced altogether with low-voltage services from feeders between buildings — a huge recovery of enterprise space or just to give electricians more space that’s safer to work in.

• Owners and operators of district energy systems (such as hospitals and universities) may see a reduction in reactive load as the effect of smaller transformers rolls out over time. One or two percent higher power factor on a 100MVA grid could save hundreds of thousands of dollars per year, depending upon the local electric utility tariffs.

• The ratings of on-site generators for emergency and optional standby power will see downward pressure.

• If reconfiguration of electrical services is going to happen, then this would be the time to install smart metering, exterior breakout circuits for mobile standby power hookups, and provisions for interactive, alternative energy switchgear.

This exceedingly simple Exception has the potential of being the leading edge of the largest reconfiguration of electrical power systems in U.S. history. Its underlying philosophy — getting building power systems “right-sized” — is currently being conveyed into a project established by the NFPA research foundation — “Evaluation of Feeder and Branch Circuit Loading.” More information about that study is available via the NFPA website (http://bit.ly/1lxdUXZ).

It will be extremely interesting to see how quickly the electrical design community acclimates itself to this Code change and takes advantage of its possibilities. Some designers will want to know what the practical effect of running closer to rated capacity might mean for distribution gear life cycles. There will also be offsetting I2R losses associated with running closer to capacity. This change might also provide the impetus for innovation in product life cycles and result in building power systems that are safer, smaller, lower-cost, and longer lasting for all industries.

For more discussion on this topic, visit EC&MTalk online forum at http://bit.ly/1lwt4a3 to share your feedback.              

Author’s note: The authors would like to thank Larry Spielvogel, P.E., consulting engineer, Bala Cynwyd, Pa., for his technical review of this article.                                                       

Anthony is a senior manager of National Infrastructure Standards Strategy for the University of Michigan Plant Operations and leads the U.S. education facilities industry in assertive technical standards advocacy. He can be reached at [email protected].

Ling is a licensed professional engineer (electrical) and LEED accredited professional with the Power Quality Institute in Brampton, Ontario, Canada, who is active in the fields of energy efficiency and power quality. He is a member of the Advisory Committee of the Association for the Advancement of Sustainability in Higher Education. He can be reached at [email protected].

Meijer, a principal with Peter Basso Associates, has 24 years of electrical engineering design and project management experience. He holds a degree in electrical engineering from the University of Detroit-Mercy and is a LEED Accredited Professional. He can be reached at
[email protected].

SIDEBAR: Branch Circuit Load Calculations

Branch circuit loads shall be calculated as noted in Secs. 220.12, 220.14, and 220.16 of the 2014 NEC.

“220.12 Lighting Load for Specified Occupancies. A unit load of not less than that specified in Table 220.12 for occupancies specified therein shall constitute the minimum lighting load. The floor area for each floor shall be calculated from the outside dimensions of the building, dwelling unit, or other area involved. For dwelling units, the calculated floor area shall not include open porches, garages, or unused or unfinished spaces not adaptable for future use.

“Informational Note: The unit values herein are based on minimum load conditions and 100% power factor and may not provide sufficient capacity for the installation contemplated.

“Exception: Where the building is designed and constructed to comply with an energy code adopted by the local authority, the lighting load shall be permitted to be calculated at the values specified in the energy code where the following conditions are met:

(1) A power monitoring system is installed that will provide continuous information regarding the total general lighting load of the building.

(2) The power monitoring system will be set with alarm values to alert the building owner or manager if the lighting load exceeds the values set by the energy code.

(3) The demand factors specified in 220.42 are not applied to the general lighting load.

About the Author

Michael Anthony, P.E. | Senior Manager of National Infrastructure Standards

About the Author

Philip Ling, P.E. | Electrical Engineer

About the Author

Jose Meijer | Principal

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