ECN Forum Forums General Topics of the Trade General Discussion Area Do All Electrical Engineers Understand the NEC?

Joe,
The issues that you have addressed should be well known by most qualified electricians,the guys that get the work done.Yes I agree the good engineer should be familiar with them as well(your example of raceway fill was excellent) but his job is to use science and physical principles/equations to predict the results and then test to see if they are achieved.
Chris

It would seem that in order to come to a solution, you will need to provide some more information about the loads, conductors and how they are served. It is possible that the solution is not to pull a parallel neutral, but to mitigate the harmonic problem at the load, change the distribution groupings or determine a malfuntioning load. A parallel neutral is a band-aid solution and shouldn't be entered lightly.

oh, and please don't beat up the engineers. Don't be surprised that the next time you look in Section 16050 of your specification, that it doesn't say that you have to bring donuts any time the engineer shows up.

[This message has been edited by Ron (edited 02-12-2003).]

Under engineering s ....upervision

So then these words should be ignored or deleted?

I am not beating up on engineers.

I only asked a simple question, and it seemed to rattle many cages. If you look throughout the NEC there are many rules that require an engineer. See Article 505 for one example.


Here are the rules where engineering supervision is required:

(2) Supervised Installations. For supervised installations, branch-circuit conductor sizing shall be permitted to be determined by qualified persons under engineering supervision.

Supervised installations are defined as those portions of a facility where all of the following conditions are met:

(1) Conditions of design and installation are provided under engineering supervision.

(3) Supervised Installations.

For supervised installations, feeder conductor sizing shall be permitted to be determined by qualified persons under engineering supervision. Supervised installations are defined as those portions of a facility where all of the following conditions are met:

(1) Conditions of design and installation are provided under engineering supervision.

(1) Conditions of maintenance and engineering supervision ensure that only qualified persons monitor and service the system.

(3) The conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors within recognized time vs. current limits for all short-circuit and ground-fault conditions.

(4) Conductors shall be considered to be protected if calculations, made under engineering supervision, determine that the system overcurrent devices will protect the conductors from overload conditions.

(A) Location and Type of Protection.

Feeder and branch-circuit conductors shall have overcurrent protection in each ungrounded conductor located at the point where the conductor receives its supply or at an alternative location in the circuit when designed under engineering supervision that includes but is not limited to considering the appropriate fault studies and time–current coordination analysis of the protective devices and the conductor damage curves.

The overcurrent protection shall be permitted to be provided by either 240.100(A)(1) or (A)(2).

Exception: In industrial and commercial premises under engineering supervision, it shall be permissible to size the ampacity of the neutral conductor to not less than 20 percent of the ampacity of the phase conductor.

Exception No. 4: Under engineering supervision, grounded neutral conductors in sizes 2 AWG and larger shall be permitted to be run in parallel for existing installations.

(1) Tables or Engineering Supervision. Ampacities for conductors shall be permitted to be determined by tables or under engineering supervision, as provided in 310.15(B) and (C).

(C) Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general formula:

(B) Ampacities of Conductors Rated 2001 to 35,000 Volts. Ampacities for solid dielectric-insulated conductors shall be permitted to be determined by tables or under engineering supervision, as provided in 310.60(C) and (D).

(D) Engineering Supervision. Under engineering supervision, conductor ampacities shall be permitted to be calculated by means of the following general formula:

Exception: Where electric enclosures covered by Part IV of this article are part of an industrial wiring system operating under conditions of maintenance and supervision that ensure only qualified persons monitor and supervise the system, they shall be permitted to be designed and installed in accordance with appropriate engineering practice.

If required by the authority having jurisdiction, design documentation shall be provided.

Manholes, vaults, and their means of access shall be designed under qualified engineering supervision and shall withstand all loads likely to be imposed on the structures.

Exception No. 1: Where the setting specified in Table 430.52 is not sufficient for the starting current of the motor, the setting of an instantaneous trip circuit breaker shall be permitted to be increased but shall in no case exceed 1300 percent of the motor full-load current for other than Design E motors or Design B energy efficient motors and no more than 1700 percent of full-load motor current for Design E motors or Design B energy efficient motors.

Trip settings above 800 percent for other than Design E motors or Design B energy efficient motors and above 1100 percent for Design E motors or Design B energy efficient motors shall be permitted where the need has been demonstrated by engineering evaluation. In such cases, it shall not be necessary to first apply an instantaneous-trip circuit breaker at 800 percent or 1100 percent.

(3) Evidence acceptable to the authority having jurisdiction such as a manufacturer’s self-evaluation or an owner’s engineering judgment.

A flexible cord shall be permitted for connection between portable lighting equipment or other portable utilization equipment and the fixed portion of their supply circuit.

Flexible cord shall also be permitted for that portion of the circuit where the fixed wiring methods of 501.4(A) cannot provide the necessary degree of movement for fixed and mobile electrical utilization equipment, in an industrial establishment where conditions of maintenance and engineering supervision ensure that only qualified persons install and service the installation, and the flexible cord is protected by location or by a suitable guard from damage.

The length of the flexible cord shall be continuous. Where flexible cords are used, the cords shall be as follows:


(3) Evidence acceptable to the authority having jurisdiction such as a manufacturer's self-evaluation or an owner's engineering judgment

A flexible cord shall be permitted for connection between portable lighting equipment or other portable utilization equipment and the fixed portion of their supply circuit.

Flexible cord shall also be permitted for that portion of the circuit where the fixed wiring methods of 505.15(B) cannot provide the necessary degree of movement for fixed and mobile electrical utilization equipment, in an industrial establishment where conditions of maintenance and engineering supervision ensure that only qualified persons install and service the installation, and the flexible cord is protected by location or by a suitable guard from damage.

The length of the flexible cord shall be continuous. Where flexible cords are used, the cords shall be as follows:

Exception: Sequential delayed automatic connection to the alternate power source to prevent overloading the generator shall be permitted where engineering studies indicate it is necessary.

(3) Engineered Systems. For an electric vehicle supply equipment ventilation system designed by a person qualified to perform such calculations as an integral part of a building’s total ventilation system, the minimum ventilation requirements shall be permitted to be determined per calculations specified in the engineering study.

This annex provides application information for ampacities calculated under engineering supervision.

Most of us seem to be confused here.

Electrical design is based on physical laws.

The NEC is a set of perscriptive rules that approximate the physical laws. These rules will be sufficient under most circumstances. The NEC points out some circumstances where the rules may not be sufficient.

An engineered design will be based on the physical laws and the specific circumstances. An engineered design will always be sufficient.

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With regard to the neutral problem. An engineer can make a decision that the NEC fill/derating rules are too conservative and another neutral can be pulled without derating the condutors.

For course this incurs the rath of the AHJ who thinks the NEC is to be followed. A lot of yelling follows. A few people land in jail and the hospital and then the lawyers go to work.

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I should point out the most engineers prefer to use perscriptive rules that cover their area of practice better than the NEC covers their area of practice.

Engineers do a good job at their work.

I work for a large engineering/construction company. All of our engineers in a supervisory role are registered, and most are Master Electricians. We do engineering and design for many of the major Fortune 500 companies, and many who are not in that category. We all have to know the NEC and how to apply its various rules. Most of the young engineers just out of college don't yet know the NEC, but are getting a few lessons in learning. All that I work with have been given the instructions to get registered (as an engineer) and to pass the Journeymens and Masters exams. I worked with one engineer that told me the Masters exam was harder than the PE. Maybe she was just smart. But she is actually a good engineer, and could probably teach the NEC.
Like some of the other posters here on this board, I feel the NEC could be written in a more understandable down to earth language to make it more readily applicable. However, I just don't see that happening any time soon. Its people like Joe, Mike Holt, Charlie Trout, and a few others that make the NEC understandable. My thanks go to these and others around the country who dedicate themselves to the safe use of electricity.
Just my 2 cents worth.

Thanks for all of your replies!



[This message has been edited by Joe Tedesco (edited 02-13-2003).]