The National Electrical Code Understanding Wire And Terminal Temperature Ratings Part one

Note to the Reader: These articles are based on the latest Revision of the National Electrical Code (NEC), the National Fire Protection Associations (NFPA), Publication 70, 2008 Revision. My reason for writing these articles is to try to make the Electrician’s Bible comprehensible even to the new do-it-yourself electrician.

The NEC has much to say about temperature ratings as it applies to wire insulation, device terminals, and equipment ratings. Understanding these rules and knowing how to use them is the key to a successful electrical project. There is much confusion over these rules and their applications. More projects fail their inspections because of temperature rule violations than for violations of any other single Code Article. Electrical inspectors know this and look closely for these violations. Taken individually, these rules are easy to understand, the confusion come in when the electrician has to use them together when selecting the appropriate conductors to use for a given projects and in deciding what temperature rating he or she has to assign to them. I hope that after reading this article, you will know how to do just that.

The Importance of the Wire Temperature Rating

You cannot use conductors in a manner that causes its operating temperature to exceed the temperature rating of its insulation as given in NEC Table 310.13(A). The table provides you with everything you will ever need to know about the insulation on the most commonly used electric wire. This table provides you with the following information:

• The conductor’s insulations trade name.
• The conductor’s insulation type letters.
• The insulation’s maximum safe operating temperature
• The types of locations where the wire may be used, its applications
• The type of material the insulation is made of
• The insulation thickness in inches and millimeters, and
• The type of outer covering over the conductors, if any.

Of all the insulation types described in this table, the ones you will encounter the most are TYPES TW, THW, and THHN, which have a Maximum Operating Temperature of 140º F (60º C), 167º F (75º C), and 194º F (90º C) respectively.

Factors Affecting Conductor Operating Temperature

A vast amount of explanatory information accompanies many of the articles and tables in the NEC called "Fine Print Notes." The purpose of the FPNs is to assist the electrician in understanding the rules. In Article 310.10, the FPN explains how the operating temperature of a conductor is determined by explaining the factors that must be considered. The following factors affect a conductor’s operating temperature:

• The ambient temperature of the environment in which the conductor is installed. The ambient temperature may vary from one point to another along the conductor, so you must use the highest temperature in your calculations.
• The heat generated by the current flowing through the conductor is a factor. His includes fundamental and harmonic currents.
• The heat generated by current through adjacent current carrying conductors in a cable or other raceway must be considered. How the other conductors interfere with heat dissipation, must also be taken into account.
• The rate at which the heat is transferred to the ambient medium is the final consideration.

The happy news is that the NEC tables and the FPNs affixed to the bottom of the tables eliminate the need for the electrician to make these calculations.

The Insulation Temperature Rating and Conductor Ampacity

The insulation temperature rating directly effects the ampacity of a given size conductor, the higher the temperature rating the higher the ampacity rating. The ampacity ratings for an AWG 1/0 Copper conductor with Type TW (60º C), TWN (75º C), and THHN (90º C) insulation are 125, 150, and 175 Amperes respectively. The key idea here is, that, in many cases, you can use a smaller conductor to carry the same current load by going to higher temperature rated insulation.

In part two of this article, you will learn how to use NEC Table 310.13(A) and Table 310.16 to select the proper size and type conductors for wiring project.

The NEC is the Electrician’s Bible, and you do need to study it religiously. Now is the time to add a copy of the latest Revision to your library of reference books if you do not already have a copy. The NEC is an expensive book, but it is a "must have" for anyone doing electrical work, professionally or as a do-it-yourselfer.