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Rule 4-004, Comparable But Different

Posted By Leslie Stoch, Sunday, July 01, 2012
Updated: Friday, September 07, 2012

You probably noticed a number of important changes from 2009 to the 2012 edition of the Canadian Electrical Code. Section 4 – Conductors has taken the lion’s share of the changes. Rule 4-004 has received a good deal of attention. This article discusses and compares some of the similarities and differences between the 2012 version of Rule 4-004 – Ampacity of Wires and Cables and its 2009 predecessor.

Beginning with underground conductors, the 2009 Canadian Electrical Code rule for calculating the allowable ampacities for underground conductors, direct buried or in raceways went right to the point (minimum conductor size #1/0 AWG and allowable ampacities calculated in accordance with the IEEE 835 standard). A "see Appendix B” note led us to an interpretation of the standard — the diagrams and ampacity tables in Appendices B and D. The 2009 rule offered no clues as to what to do about wire sizes smaller than #1/0 AWG or conductor arrangements different from the Appendix B diagrams.

The new Rule 4-004 addresses these earlier limitationsas follows:

Rule 4-004 refers us directly to the conductor configuration diagrams in Appendix B and the allowable ampacity tables in Appendix D;

It tells us what to do if we decide to arrange underground conductors in configurations different from those shown in Diagrams B1 to B4 — allowable ampacities are to be based on the IEEE 835 standard calculation method; and

It tells us what to do for conductor sizes smaller that #1/0 AWG — use Tables 2 or 4 or the IEEE 835 standard calculation method.

Rule 4-004 of the 2009 CEC specified allowable ampacities for single-conductor cables in free air.For cable spacing at least the diameter of the larger adjacent cable, we could use Tables 1 or 3 to determine allowable ampacities for copper and aluminum conductors. For up to 4 single-conductor cables in contact with each other, Tables 1 and 3 ampacities needed to be corrected in accordance with Table 5B. With more than 4 cables in contact, the rule led us to the lower allowable ampacities of Tables 2 and 4.

For single-conductor cables in free air, Rule 4-004 of the 2012 CEC is identical to the 2009 version for spacing apart of at least the diameter of the larger adjacent cable.

But here is where the similarity between the 2012 and the 2009 CEC ends:

For single conductors between 25% and 100% of the larger adjacent cable diameter, we must now use Tables 1 or 3 with correction factors from Table 5D.

For up to 4 single-conductor cables spaced less than 25% of the larger adjacent cable diameter, we must now use Tables 2 or 4 with correction factors from Table 5B.

For more than 4 single-conductor cables spaced less than 25% of the larger adjacent cable diameter we must now use Tables 2 or 4 with correction factors from Table 5C for total number of conductors. (Table 5C correction factors do not apply to runs shorter than 600 mm).

I would be remiss in not mentioning a very important change —new Rule 4-006. This rule requires that when equipment (such as circuit-breakers) is marked for maximum termination temperatures, allowable ampacities must be based on the corresponding conductor temperature columns in Tables 1 to 4. Remember, this applies to both ends of every conductor or cable. If the equipment is unmarked, the 90° C ampacity column applies.

As with earlier articles, you should always check with electrical inspection authority in each jurisdiction for a more precise interpretation of any of the above.


Read more by Leslie Stoch

Tags:  Canadian Code  July-August 2012 

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