Branch Circuit Lighting Load Calculations

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meby's picture
Joined: 2012-04-17

What do you think of the change to the 2014 NEC that provides an exception in Sec. 220.12?

Electrical professionals have long observed the “underloading” of customer-owned transformers. In fact, application of NEC rules for sizing services, feeders, and branch circuits has resulted in unused capacity in almost all occupancy classes for decades. Recognizing that aggressive energy codes are driving energy consumption lower — and that larger than necessary transformers create larger than necessary flash hazard — this revision permits designers to reduce transformer kVA ratings and all related components of the power delivery system.

An exclusive EC&M special report (to be published in the May 2014 issue), written by industry experts Michael A. Anthony, P.E., University of Michigan; Philip Ling, P.E., Power Quality Institute; and Jose Meijer, Peter Basso Associates, discusses the possible sustainability windfall that may be in store for the education facilities industry as a result of this change, not to mention the fact that it may prompt the largest upgrade of commercial building electrical systems in U.S. history.

We’d like to hear your feedback. What’s your take on this important change?

ecmjacomen's picture
Joined: 2013-10-08

I’d like to make a few comments regarding the report published in the May 24 issue.

Two situations must be considered.: 1): the service is supplied in low voltage, or 2): the service is supplied in medium voltage.
In a medium to large size commercial building, the major portion of the lighting is fluorescent and is fed at 277V.
In the first case, the service is supplied from the electric utility at 480Y/277V. Then, no step-down lighting transformer is necessary.
The savings from the application of the Exception to Sec. 220.12 will come basically from the reduction in wires, raceways and related equipment sizes, but not from the transformer.

In the second case, the medium-voltage power transformer pertains to the owner of the facility, and in this case, there will be a reduction in its size when the Exception to Sec. 220.12 is applicable. However, according to the description and graphics in the report, this doesn’t seem to be the case.

On the other hand, the cost comparisons are based on using the requirements of IEEC (0.90 W /sq FT.). The NEC doesn’t establish a limit, but it is worth to remember (as the report also noted) that the NEC is a safety code, and as such, in designing branch-circuits, good sense will call of more conservative values, taking into account future growth.

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