Holding to Electrical Specs

Consulting-Specifying Engineer.  October 1998

Engineers should maintain their positions on electrical system design, whether for code issues, good design practice- or the owner’s best interest

"Why on earth did you design it that way?"

This is a plaint often heard from the contractor, construction manager, or owner when documents are issued for construction. More often than not, the statement is liberally littered with modifying expletives. The engineer so challenged,must first decide if the question has a legitimate basis. If so, is it a National Electrical Code (NEC) requirement, good practice, or merely designer preference?

The key is to examine code requirements, how the engineer can maintain a position, and the potential impacts to both the engineer and the project if they fold on their position.

Engineers are charged with a mandate to "safeguard life, health and property, and promote the public welfare," as well as meet project requirements. In addition, there are instances in which they are called upon to certify that drawings to which they have affixed their seals meet all requirements of the NEC and the authority having jurisdiction (AHJ). To meet these weighty obligations engineers must consider seriously their mandate and- on a more practical level- interpret applicable NEC sections for each project and assure that their design meets all requirements.

By stamping and signing design drawings, engineers are signifying that they believe the documents meet project and code. When these documents are sent to contractors for construction bidding, the questions begin. The most common topics of discussion- from contractors, owners and others, comprise the following:

Wire Sizing

  • Ampacity. There have been numerous articles written on the subject of conductor sizing to meet the NEC requirements as described in Article 310 (see sidebar, "NEC Conductor Sizing,") These articles agree

The Cast of Characters

Like it or not, there are differing viewpoints on a construction project, and as long as we live in a free market society, there always will be. Participants each have their own agendas and issues of paramount importance. A description of the cast of character every engineer encounters (and often embodies) follows. While these stereotypes are certainly unfair to many, at the same time there is an underlying truth to the generalizations.

The Owner. The owner wants the best project for either the lowest first cost or life-cycle cost. Meeting code is not always a top priority, particularly when code requirements conflict with project requirements-and when the cost of compliance increases the budget.

The Construction Manager. Typically hired by the owner, the CM’s views, for the most part, parallel those to the owner.

The AHJ. The authority having jurisdiction (AHJ) wants to project that is safe and meets their interpretation of the various codes. They are usually not interested at all in whether their interpretations impact the cost of the project.

The Contractor. The builder wants to complete the project and make a profit; they’re not necessarily interested in whether the project meets Code, but only that it passes inspection. Also, the contractor be interested in changes to the design after the contract is signed that mean greater profit for the contractor. So, all opportunities to reduce cost or find design errors can increase the contractor’s profit margin-and his influence with the owner.

The engineer must consider all of these viewpoints, and make value judgments in light of code, cost, constructability, safety and consumer satisfaction (as well as their own profit picture). They must advise the owner on the future operation of the project. Any changes proposed by the contractor must be beneficial to the owner.

that a number of issues must be addressed to properly size conductors, including ambient temperature, termination temperature, number of current-carrying conductors in a raceway, character of the load and type of conductor insulation. While now included in NEC appendix B- which is not a code mandate- the Neher-Macgrath calculations for sizing of conductors installed in underground duct banks should also be considered to properly size conductors in a partially insulated environment.

The diligent engineer applies each requirement in the appropriate manner and arrives at a conductor size suitable for the load, the environment and the overcurrent protection. Yet, the controversy often begins when a member of the construction team compares the conductor sizes on the contract documents to the uncorrected, 75-degree column in table 310-16 in the NEC and concludes that the engineer has sized the conductors incorrectly. The owner or contractor then takes the position that the conductors are too large and, to economize, suggests that the sizes be reduced to match the referenced column.

  • Voltage Drop. Ampacities are just one factor in proper conductor sizing. Just as important, though often neglected, is voltage drop. NEC Articles 210-19 (a) (FPN no. 4) and 215-2(b) (FPN no.2)- which are not mandatory rules- recommend sizing both feeders and branch circuits to prevent a voltage drop exceeding 3 percent at the farthest outlet, where the maximum total voltage drop of the feeders and branch circuits does not exceed 5 percent

When the feeder and branch circuit conductors for long circuits are sized on this basis, the conductor sizes are increased even more than that required for ampacity. Perhaps because it is not mandatory, contractors and owners sometimes forget this basic fact. It should not be overlooked, however, because performance and the operating life of utilization equipment can be adversely affected.

Electrical working clearance

Clearance for electrical equipment are specified in NEC Articles 110-16 and 384. Working clearances are based on voltage level and conditions of installation, and for installations below 600 volts, these clearances range from 3 to 4 feet in front and in back of equipment (where the equipment has rear access). Headroom is also given as a minimum of 6.5 feet to prevent the installation of electrical equipment in crawlspaces.

Article 384 covers dedicated space in the vicinity of electrical equipment for the installation of conduit and limited physical protection of equipment. Generally speaking, this space is equal to the footprint of the equipment extending from the floor on which the equipment is mounted to the next structural floor, or 25 feet, whichever is less. The article also restricts the intrusion of pipes and ducts into this space.

While the passage clearly states that no equipment foreign to the electrical installation shall be installed in the zones, this often becomes a point of contention between engineer and contractor. In the contractor’s view, the shortest distance between two points is a straight line, even if that line is a pipe passing directly over electrical equipment.

While the code seems clear and straightforward, this is a frequent problem at the construction site. Clearance requirements are often violated to the extent that equipment doors cannot be opened, and owners and even AHJs may prefer to look the other way. Many engineering firms have specific coordination stipulations, both in drawings and specifications, to prevent space conflicts between mechanical and electrical systems. In the field, however, there must be a commitment to implement them,

Transformer protection

Transformer overload and short-circuit protection are addressed in Article 450 of the NEC. Depending on voltage levels and conditions of maintenance and supervision, one of two protection schemes can be used: either primary overcurrent protection only, or primary and secondary overcurrent protection. Also, the NEC permits fairly wide latitude in the sizing of overcurrent devices.

As a matter of good practice, however, better protection will be realized with breaker settings or fuse ratings lower than the maximum allowable. Likewise, primary-only protection is a minimum-only requirement, which – unlike primary and secondary protection- does not provide the best protection under all types of fault conditions. Plus, it may introduce nuisance tripping due to magnetizing inrush, cold load pickup and emergency operating conditions.

Using primary and secondary protection, with the primary breaker set between 175 and 200 percent, there is less opportunity for nuisance tripping while offering adequate short-circuit protection for both primary conductors and the transformer. The secondary overcurrent device, set at 125 percent of full load, provides overload protection. In addition, the primary overcurrent device should be examined to ensure that the transformer damage curve (defined by ANSI/IEEE C57.109) is fully protected by the primary device, while allowing the transformer magnetizing inrush point to fall below trip characteristics. That way, energizing the transformer will not cause a nuisance trip.

Unfortunately, contractors routinely question the use of larger overcurrent devices and primary conductors on the primary side of the transformer.

Engineers are accused of "overdesigning" projects, overlooking cost savings from reduced conductor and overurrent device sizing. Yet, in most instances, the larger overcurrent device is the same frame size; so, for example, reducing the trip size from a 200-ampere to a 125-ampere trip on a breaker results in no savings at all. The difference in cost between the conductors and conduit that is perhaps 4 feet long is negligible. In reality, the time wasted in asking the question is probably more valuable than the cost of the materials needed for the more reliable, better protected installation.

Grounding

Conflicts on grounding most often occur between the engineer and the owner’s equipment suppliers. The NEC is very clear. In Article 250-54, it spells out the standard for connecting all grounding conductors to a single grounding electrode.

When electronic systems are installed in a facility, the equipment installation technicians typically request an isolated ground not connected to any other ground in the building, often stating that their equipment will not work without the isolated ground. The conflict between the NEC requirement and the supplier’s desire is easily resolved: Connect the ground for the electronic system as close to the grounding electrode as possible; this meets the NEC requirement and gives electronic systems their solid, earth ground connection.

The real issue for the systems is having a minimum voltage above ground on the signal ground for the system. By connecting the system ground close to the grounding electrode, there is minimum impedance between the system ground and earth ground, reducing the voltage to a low value, no matter what the ground current.

An associated but less common request from system technicians is that the voltage difference between neutral and ground at the utilization equipment be less than 1 volt. The equipment ground installed as noted above will minimize the voltage on the ground; reducing voltage level on the neutral at the utilization equipment is more problematic. Utilization equipment often has a switching power supply, which generates significant harmonic currents that are impressed on to the neutral of the system with a resultant increase in neutral current.

As Ohm’s Law states, when current is increased through the constant circuit impedance, voltage increases. Therefore, the neutral has an inherent voltage impressed at the utilization equipment terminals well above the requirement outside of 1 volt over ground. In fact, no equipment manufacturer has been able to demonstrate either: 1) good reason for maintaining the neutral-to-ground voltage less than 1 volt; or 2) any equipment malfunctions attributed to this voltage difference.

Fault Duty

Fault duty on distribution equipment should be a focal point of the engineer’s concentration. Not only is it critical to safety and reliability, but it is an NEC-mandated issue in Article 110-9, which says that equipment intended to break current at fault levels shall have an interrupting rating sufficient for the current that is available at the line terminals.

Contractors, owners and construction managers often seem to think that fault duty of electrical equipment is a myth propagated by engineers to increase the costs of projects. Yet, the fact is that the engineer must meet the NEC requirement by either designing a fully rated system or, as permitted by Underwriters Laboratories (UL), designing a series-rated system. NEC enforces UL requirements in Article 110-3(b), which says that listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

The contractor that has some passing knowledge of fault duty will suggest that the distribution system be series rated in place of a fully rated system. This suggestion becomes difficult when the series connected rating is applied to systems with a mixture of multiple manufacturers or with various overcurrent devices that are not listed as series connected by UL.

As UL plainly states, "These ratings are applicable only when the series connected devices have been investigated by UL in combination with the end-use equipment and the equipment in which these devices are used is marked with the series connected rating." Therefore, if series connected devices are not listed in UL Recognized Component directory, they are not suitable for series-rated application.

Between the NEC and UL requirements, engineers very carefully must meet the specific requirements for series ratings confirming in shop drawings equipment labeled for that specific rating- or maintain that all systems must be full-rated.

Resolving codes and costs

With all of these viewpoints clouding the issues, the engineer has a difficult problem. Because the engineering firm is hired by the owner, either directly or indirectly, their first commitment must be to the client- the owner.

Along with this commitment, however, engineers must also meet the mandate to protect public safety and welfare. This becomes one of the most sensitive issues engineers face, especially when they choose to hold to a design or specification against the owner’s wishes. When this happens, these options are available to engineers:

  • A simple approach: Show the owner where the code does not allow a proposed change. In most cases this is enough
  • A cost-based approach: Connect the code-mandated design with construction cost savings or operational savings,
  • A diplomatic approach: Develop an alternative solution meeting the owner’s requirements and code requirements. Sometimes, this implies more design time and additional construction cost, but it may be the path of least resistance when owners are adamant about a change.

If these approaches don’t work, further options become less pleasant. Where there appears to be no major hazards, engineers may comply with the owner’s desires and go on record with a letter to the owner stating the engineer’s objection to the approach and recording the reasons why.

For more critical issues, engineers can send a letter absolving themselves of any responsibility- and simply not incorporate the requested approach into the contract documents. This position may risk the loss of future work with the same owner, but this may be a better option than violating a public mandate to design safe buildings.

NEC Conductor Sizing

There are four criteria used for conductor sizing to meet the NEC requirements:

1) The overcurrent device must be sized for the sum of the non-continuous load plus 125 percent of the continuous load. The overcurrent device may be rounded up to the next available standard size, as long as the device does not exceed 800 amperes.

2) The conductor must be sized so that termination temperatures do not exceed 60 C for conductors smaller than 100 amperes or #1 American wire gauge (AWG), and 75 C for conductors larger than 100 amperes or #1/0 AWG. This results in using the 60 C column of 310-16 for all conductor sizing of circuits 100 amperes and smaller, and the 75 C column of 310-16 for conductors larger than 100 amperes. Even if insulations are utilized, such as XHHW, Z, TFE, etc. two columns must be used to determine conductor sizes.

3) Conductor size and insulation type must take into consideration conditions of use to prevent insulation damage. Conditions of use include more than three current-carrying conductors in a single raceway and high-temperature ambients. High temperature ambients are-according to the heading of the correction-factor chart for 310-16-any ambient conditions that exceed 30 C or 86 Most electrical rooms and mechanical rooms and mechanical rooms where mechanical rooms where electrical equipment is installed will exceed 86 F under typical operating conditions

Wire sizing and conductor sizing must not only meet code requirements, they must also meet special needs, such as those for partially insulated environments.

4.) The overcurrent device must protect the conductor under all conditions of use.

Copyright © 2002 Cahners Business Information, a division of Reed Elsevier Inc. Reprinted with permission.

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