This article was posted 04/25/2012 and is most likely outdated.

Mike Holt - 2011 Changes to the NEC - 110.24
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2011 Changes to the NEC - 110.24
Based on - NEC - 2011 Edition

2011 Changes to the NEC – 110.24

The following is an instructional page from our 2011 Changes to the NEC Textbook/DVD Package complete with graphics and video where applicable. As part of our on-going effort to provide free resources to help the industry, we will be sending this content as part of a series of newsletters. Each newsletter will feature pages taken directly from our textbooks. This can be a great training resource for your organization!

There are some important features in this text which help to highlight the changes that you should be aware of:

  • Each Code section which contains a change includes a summary of the change, followed by a paraphrase of the NEC text affected by the change. Any specific change is denoted by underlined text and in the corresponding chapter color
  • Graphics with green borders and 2011 CC icons next to the heading are graphics that contain a 2011 change; graphics without a green border or icon are graphics that support the concept being discussed, but nothing in the graphic was affected by a 2011 Code change.
2011 Changes to the NEC Part 1

110.24 Available Fault Current

A new section requires some equipment to be marked with the available fault current and requires updating of that marking if modifications of the electrical system occur.


110.24 Available Fault Current.

(A) Field Marking. Service equipment in other than dwelling units must be legibly field-marked with the maximum available fault current, including the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved. Figure 110–13

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Figure 110-13 (Click on image to enlarge)

(B) Modifications. When modifications to the electrical installation affect the maximum available fault current at the service, the maximum available fault current must be recalculated to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) must be adjusted to reflect the new level of maximum available fault current.

Exception: Field markings aren’t required for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.

ANALYSIS: All equipment must have an interrupting rating or short-circuit current rating that’s equal to or greater than the available fault current [110.9 and 110.10]. As premises wiring systems age, utilities may change transformers in an effort to become more efficient, or to increase capacity. When this occurs, the available fault current increases, many times resulting in noncompliant (and dangerous) wiring systems. This NEC change is intended to alert Code users to the fact that when utilities change transformers (or when emergency or standby systems are installed), the ratings of equipment must be reevaluated.

Opponents of this NEC change argue that oftentimes the ratings of equipment are based on a “worst case” basis. While this is suitable for designing a system, it isn’t suitable for performing the calculations required to establish the proper personal protective equipment (PPE) necessary to work on the equipment. When artificially high values of fault current are used for equipment ratings, a lower PPE rating is often the result of the calculations.

 

2011 NEC Changes DVD Package
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Don't let the scale of the code changes intimidate you, this package will get you up to speed on the most essential 2011 NEC changes quickly. The book is well-organized, easy to follow, and the full-color illustrations bring the material to life. The DVDs bring together a group of experts from the field to discuss the changes and how they apply in the real-world.

This program includes the following items:

  • Changes to the NEC 2011 Textbook
  • Changes to the NEC 2011 DVD 1 & 2 includes Articles 90 - 810

Product Code: 11CCDVD
Price: $198.00

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Comments
  • Thank you for bringing this code requirement to our attention and spreading the word that this is a requirement that needs to be met. I agree with the statement that the fault currents are typically based on the worst case fault current available. This method of calculating fault currents does tend to lower the PPE requirements as the protective devices clear the faults faster when the fault currents are higher.

    However, I disagree with the Exception. "Exception: Field markings aren’t required for industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment." In my personal experience I almost always find molded case circuit breakers rated less than the available fault current and non fused disconnect switches rated at 10 kA SCI and under rated MCC buckets installed in factories supervised by qualified personnel. The plant maintenance electricians and contractors install these underrated devices as they are the only replacement devices available on the shelves and they need to get a critical piece of equipment back into production or they are the lowest cost device available and help get the job for the contractor. I also see project specifications where the available fault current, SCI, has not been calculated and the equipment has to be replaced when the fault current calculations are finally run as part of a future arc flash study. The Arc Flash study and fault current calculations should be run on the proposed distribution system before the specifications are written. But that would mean we are doing the project engineering up front and our long time lead items are not on order. The installation of under rated equipment is a common problem within our industry. The eventual cost to replace the underrated devices is much higher than the initial cost to install the properly rated equipment. When the under rated device fails the replacement costs and lost production costs far outweigh the cost of doing ti right in the beginning. I would prefer to have another sticker on the panel with the calculated fault current listed along with the arc flash ratings. With the information readily available the plant supervisory personnel, contractors estimators and installing electricians will look at the breaker ratings and the sticker ratings and verify that the new breaker is properly applied.

    Doug Decherd  April 26 2012, 9:31 am EDT
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