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National Electrical Code Articles and Information

Where 90 Degree Conductors Fit

by Mark Lamendola

Special thanks to Fred Hartwell for helping me to understand this. We can all thank Fred for Example D3(a) in Annex D of the NEC, which is the reference for this article.

A raceway may run through a high temperature area, while its terminations are in a low temperature area. Many industrial processes result in extensive heat at the ceiling level (where you would run a raceway). But fans, ventilation, and the natural thermal gradient sharply reduce the temperature at the operator's level.

An example would be a set of open thermal pots for melting aluminum. Along the ceiling, the temperature stays at about 140 DegrF. in moderate weather. Worst case, you are going to see 165 DegrF on a really hot day (but the pot operators are working at 80 to 90 DegrF). Further, the pot area is surrounded by air curtains or doors to cordone it off from adjacent areas of the plant.

So you are running a 480V feeder from the service entrance to a lighting transformer located in an area adjacent to the potting area, and your feeders pass through the potting area. There are no terminations in the potting area and you are using THHN, so you can use the 90 Degr column. You know you need 1 AWG to handle the 160A load you calculated. But when you apply the correction factor, you find you need 500 kcmil.

Now you apply the correction factor to your THHN for temperature at the transformer. But the worst case is you are going to see 95 degrees on a really hot day. But since you've applied the correction factor to your THHN for the worst case of the potting ceiling area and sized the conductor accordingly, you can apply the 75 degree column to that same THHN. The 500 kcmil has an ampacity of 357A--so you are actually oversized.

But suppose you had not used the 90 degree column. That means you would have had to run 600 kcmil to accomplish the same thing. Even at 500, you are talking about special tools and some very strenuous work. But using a wire that is 20% larger now makes the job 200% harder. How do you support the raceway needed for so much extra conductor--how many additional holes must you drill? And how do you make bends, given that your bender isn't large enough?

The difference between the cost of the first installation (using the 90 degree column) and the cost of the second installation is huge. It's not just a larger wire size running in the "same 1-inch EMT" you were going to run, anyhow. I haven't even calculated the raceway size, but I would guess you are going from 4-inch raceway to 6-inch.

Even with a smaller load, these numbers can make a huge difference.

Now, perhaps, you can see why we find 90 degree conductors in various NEC tables.

 

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How the NEC is arranged

  1. The first four Chapters of the NEC apply to all installations.
  2. Article 90 precedes Chapter One, and establishes the authority of the NEC.
  3. Article 80 follows the body of the NEC; it exists as Annex H. It provides the requirements for administration.
  4. Chapters 5, 6, and 7 are the "special" chapters, covering special: occupancies, equipment, and conditions (in that order).
  5. Chapter 8 provides the requirements for communications systems.
  6. Chapter 9 provides tables.
  7. The appendices provide mostly reference information.
  8. Appendix D contains examples that every NEC user should study.

Try your NEC moxy:

  • Do you know the difference between bonding and grounding? Hint: Look in the NEC, Article 100.
  • Does the NEC refer to grounding incorrectly in any of its articles? Yes! So be careful to apply the Article 100 definitions. Don't ground where you should bond.
  • When doing motor load calculations, which Article covers hermetic motors? Answer: While Article 440 covers the application of hermetic motors, it does so only by amending Article 430 because hermetic motors are a special case of motors. For motor load calculations, refer to Article 430.
  • Does the NEC provide a voltage drop requirement? Yes! It does so in a special case, which is Article 648 Sensitive Electronic Equipment. But for general applications, it does not provide a requirement; it merely provides a recommendation in a couple of FPNs.
  • Take our Code Quizzes.

Remember other applicable codes, rules, standards, and references:

  • OSHA's electrical worker safety rules.
  • IEEE standards.
  • NETA standards.
  • NFPA standards.
  • International Codes (if applicable to the installation).
  • State Codes (if the state has them).
  • Local ordinances and permit requirements.
  • Local fire codes.
  • Manufacturer requirements or guidelines.
  • Customer security requirements.
  • Industry standards.
  • Your company's own internal standards, practices, and procedures.
  • Engineering drawing notes.

 

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