Back in the July 2000 issue ofEngineered SystemsI devoted this column to the National Electrical Code (NEC) and, among other things, mentioned the difference between the intent of the code as a minimum set of standards for safety and its frequent use as a design manual. The fact that this distinction continues to be lost on owners and engineers was brought home to me by a recent project experience, and prompted the decision to editorialize a bit this month.

In reviewing plans for a mission-critical facility, I recommended a change to the design of the grounding system to bring it in line with design practices intended to protect critical, expensive equipment from extensive damage under ground fault conditions. After reviewing the recommendation, the owner, contractor, and engineer (the latter were one and the same, as it was a design-build project) informed me that the design would stand because the recommended changes were not required by the NEC. This may only mean that I failed to make a convincing argument for the need, or that the owner was reluctant to spend additional money to make the change, but invoking compliance with the code as an argument against bettering a design always bothers me.

Not A Design Manual

The NEC, with occasional modifications, has been adopted as law throughout the United States. Its purpose, as stated in Article 90, is the protection of persons and property from the hazards associated with electricity, and it places a strong emphasis on preventing electrocution and fires of electrical origin. Under paragraph 90.1(b) it states, "Compliance [with the Code] and proper maintenance will result in an installation essentially free from hazard, but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use." In paragraph 90.1(c), it states, "This Code is not intended as a design specification nor an instruction manual for untrained persons." Given these words, it is understandable that I once heard a very smart person say, "If the only defense you can offer for your design is to say that it meets the code, you might as well say that if you did any worse, it would be illegal."

For a simple commercial facility with a short life expectancy and no design criteria other than having an adequate supply of power at each item of electrical equipment, a design meeting only the minimum requirements in the code may be acceptable and appropriate. At least there is a reasonable assurance that it won't kill anyone or burn the building down. In facilities where reliability, durability, maintainability, or life-cycle cost are design criteria, the requirements of the code must be viewed as minimums that should be exceeded in areas of the design. Let me provide an example from my short list of personal pet peeves.

Consider Maintenance

The NEC requires minimum clearance in front of electrical equipment adequate to permit it to be safely examined, serviced, and maintained while energized. The required distance depends on operating voltage and is never less than 36 in. Where rear access is required to work on de-energized parts, a minimum of 30 in. is required, regardless of voltage. It is common practice to install switchboards having the feeder cables leaving the switchboard connected in the rear with 30 in. of clearance behind the equipment. When challenged, the designer will state that the main switch can be opened, de-energizing the switchboard, to work on those connections.

Technically, this is correct, and rarely considered a code violation. However, the requirement to de-energize the switchboard precludes a number of common maintenance activities, such as infrared photography and measurement of feeder currents using a clamp-on ammeter. It's hard for me to see how this can be good design, even if it meets the code.

"In facilities where reliability, durability, maintainability, or life-cycle cost are design criteria, the requirements of the code must be viewed as minimums that should be exceeded in areas of the design."

It is incumbent on us as engineers to be aware of the operation and maintenance requirements of the facilities we design, and the way real people go about performing those tasks in the real world. In this example, I can guarantee that at some point in the life of the facility, personnel will access the switchboard from the rear while it is energized even though they should know they aren't supposed to. Should an accident occur, the employer will be cited by OSHA for performing energized work without proper clearance, but doesn't it make more sense to consider the real world up front and design in adequate clearance?

When you run into an electrical issue requiring a design decision, don't ask the electrical engineer, "What does the code require?" but rather ask, "What should we do here to provide a good design?" The answer to the second question must satisfy, but may go beyond, the first. ES