Simplified Short-Circuit Calculations

Thomas Mason, PE

Course Outline

The course contains front-matter which superficially discusses the content, the intended audience and internet locations for follow-up pursuit of the topic.

The core content is made up of two .pdf documents.  The first is a complete copy of the look-up tables for determining available short-circuit current on a 480V system.  There are 15 tables for 15 common transformers, 8 columns on each table for 8 common distribution feeders and 10 entries in each row, for 10 reasonable feeder lengths.  Interpolation is discussed and demonstrated in the second document.

The second .pdf document is a guide for use of the tables.  It is a summary of the guides provided with the IEEE spreadsheet program (IEEE.com) and the guides provided with commercial system modeling software and courses in short-circuit calculation.

The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course the student will be able to:

• Show the National Electric Code requirement for short-circuit calculations;
• Show the OSHA requirement for j/sq-cm arc flash labels based upon short circuit amps;
• Know where to get a free arc-flash label calculator;
• Report the effect of utility available short circuit amps on plant short circuit amps;
• Report the reason to be very leery of utility short circuit amp numbers;
• Report the effect of transformer size kVA on plant short circuit amps;
• Report the effect of transformer %Z impedance on short circuit amps;
• Perform manual calculations for transformer terminal short circuit amps;
• Perform manual calculations for transformer-plus-feeder short circuit amps;
• Do short circuit amp s table-lookup for 15 transformer sizes, 8 feeder sizes and 10 feeder sizes;
• Understand the problems of using the IEEE 1584 arc flash spreadsheet;
• Apply reasonableness tests to short circuit amps results;
• Know three well-respected vendors of power system modeling software;
• Explain the limitations of using transformer impedance magnitude and cable impedance magnitude to determine short circuit amps;
• Recognize the assumption of lossless switchboards with no protective function;
• Know the 10 kA limit on unfused disconnects and motor control centers;
• Recognize the cost penalty of a unit substation;
• Recognize the conventions of system graphic representation in commercial modeling software;
• Know the reasonable approximation for motor current contribution based upon HP; and
• Recognize the limitations of the look-up tables provided.

Intended Audience

This course is intended for practicing design engineers who must select electric power equipment early in the construction project.  It is intended for Architects and design supervisors who perform a reasonableness-test on finished designs.  It is intended for Construction Managers and Field Inspectors who wish to avoid unpleasant surprises during start-up.  It is intended for Contractors who wish to avoid delays by use of intelligent requests-for-information.

Benefit to Attendees

The primary benefit to the audience is that the method presented makes it easy to comply with the National Electrical Code and Occupational Safety and Health Act.

The secondary benefit is that the guidelines included help everyone on the project do their jobs better, with more confidence.

Course Introduction

The National Electrical Code requires “field marking to warn qualified persons of potential arc flash hazards.”  (2008 NEC 110.16)  Note that it does not require the design engineer, with the system one-line diagram and software tools to compute the arc-flash magnitude.  It requires the installer, with a screwdriver and pliers to apply a label.

The almost-universal interpretation is for the designer to include a specification for the Contractor to perform and submit the arc-flash calculations, prepare the labels and attach them.  When it happens, the Contractor hires an outside engineer to do the work.  Usually, it doesn’t happen.

This course offers a simplified first step towards the arc-flash calculations.  Several vendors offer no-charge on-line computers for arc-flash, IF THE AVAILABLE SHORT-CIRCUIT CURRENT IS KNOWN.  (us.ferrazshawmut.com/arcflash)

There is an apparent ambiguity in that the NEC does not require numerical values.  OSHA does.  (CFR 1910.132)  Regardless of this fine point, the specification to the Contractor is to do the calculations and provide the detailed arc-flash label with numbers.

Course Content

In this lesson, you are required to download and study the following course contents in PDF format:

Course Summary

This course offered a simplified method of calculating available short-circuit current on a commercial or industrial power distribution system.  This calculation is required by the National Electric Code (2008 NEC 110.9, Interrupting Rating).  It is also required by OSHA in order to prepare a detailed Arc-Flash warning label with j/sq-cm and personal protective equipment level.

The course was a set of look-up tables prepared with commercial electrical system modeling software and accompanied by coaching on how to use the tables correctly.

A multiple-choice quiz was included to provide confidence in the learning achieved.

Related Links and References

For additional technical information related to this subject, please visit the following websites or web pages:

2008 NEC 110.9, Interrupting Rating  -  http://www.nfpa.com

IEEE 1584 Arc Flash  -  ieee.com  -  The spreadsheet used to be available from multiple sources on the internet, but they have disappeared.  IEEE offers it for \$29.  GM has a derived version of the spreadsheet that they give to contractors for free.  It includes special GM electrical equipment data.  There are ~10 interacting pages of the spreadsheet to produce a single label.

Detailed arc-flash calculator, if you know the SCA  -  http://www.us.ferrazshawmut.com/arcflash

Power Modeling Software
EasyPower, ESA  -  http://www.EasyPower.com
http://www.skm.com
http://www.etap.com

Publishing software that fought me every step of the way on this course
http://www.Microsoft.com

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.