Codes & Standards
D Tables and the Notwithstanding clause – Code File, October 2022
November 1, 2022 By David Pilon
November 1, 2022 – To understand the purpose of the CE Code’s D Tables, we must first understand how temperature and environment affect a cable’s rating. For instance, when we run a cable through a high ambient-temperature area, we look to Table 5A to adjust the ampacity of the cable for that installation.
Three tables, in fact, are used to adjust cable ampacity based on various factors—Tables 5A, 5B and 5C. Direct burial, however, is not one of them. For that, we turn to Rule 4-004(1)(2) in the CE Code where, via the D Tables, we find direction for direct burial, underground or raceway installations.
You have a few options at your disposal for these types of installation. You could use the D Tables—which are based on the Neher-McGrath methodology, as applied to IEEE 835—or use IEEE 835 to provide your calculations and prove your cable size selection (CE Code, Appendix B).
When the installer is doing the design, the D Tables provide the minimum cable rating. In some instances, cost savings will be realized on a reduced cable size; in others, the savings will come from not having to dig up and replace cables due to a reduction in ampacity.
For example, I am building a 1200A service. Rule 4-006 tells me I need to use the 75 C column in the Tables. Further, I want to use aluminum, as it is easier to work with and more cost-effective. If I use multi-conductor cable, Table 4 says I need four (4) runs of 500-kcmil cable with an ampacity of 310A each.
Now I take direct burial into account, and discover I may need to use a different size of cable to perform the same work. I turn to the D Tables, which are specific as to the type of installation and its location (e.g. underground).
Now I look at the configuration and spacing requirements on pages 764-765 of the CE Code, Appendix D. I am using four (4) parallel cables (Detail 4 and Table D10B). Note that these tables are at 90 C and, as such, you must adjust to 75 C by multiplying the ampacity by “.886”. In this scenario, the 600-kcmil rated at 345A x .886 = 306A. Therefore, I am going to need 600-kcmil cables to meet the ampacity requirements.
Now for the notwithstanding clause, which causes the most confusion!
Rule 4-004(17) states that where the lower-ampacity portion of the cable installation—consisting of not more than four (4) conductors in total—does not exceed 10% of the circuit length or 3 metres (which ever is less), the higher ampacity shall be permitted.
So, a 600-kcmil multi-conductor cable in free air at 75 C is 340A; but, when installed as per Detail 4, it has a 75 C rating of 306A. In this case, the lower ampacity is 306A so, should the underground portion exceed 3 metres or 10% of the run, then the lower ampacity (the 306A) comes into play.
Sometimes, however, the numbers flip. Take, for example, a 200A farm service where we need to run cable across the yard. Again, we are using aluminum. Table 4’s 75 C column tells me I need 250-kcmil cable for this service. Table D10B tells me I can do this installation with 2/0 aluminum with a 75 C rating of 204A.
Table 4 rates 2/0 cable at 75 C at 135A. The D Tables state that aluminum 2/0 cable is good for 204A when direct buried. So, again, we look at the notwithstanding wording.
In this situation, the lower-ampacity portion of the installation is the free air portion, so we need to look at how much cable is in that area. If more then 10% of the run—or 3 metres—is in the free air location, then the lower ampacity of 135A has to be used.
There are ways to make this work, so be sure to contact your local authority having jurisdiction for guidance.
David Pilon is manager, Electrical Inspections, at Technical Safety Authority, Saskatchewan (TSASK). He also serves as vice-chair of the Canadian Certified Electrical Inspector (CCEI) committee of the International Association of Electrical Inspectors (IAEI), Canadian Section. He can be reached at firstname.lastname@example.org.
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