Several of my engineering contacts have asked me about my interpretations of the new CE Code Ampacity Table D17. This blog post contains my interpretations as of May 19, 2020. Your local Authority Having Jurisdiction may have a different interpretation, so please confirm your AHJ’s position prior to making a cable sizing determination. Ampacity calculation tables and code rules are also constantly evolving. Refer to the latest CE Code version and go to Table D17 for your 5KV to 46KV shielded cable ampacities.
CE Code Table D17 is a new 5KV to 46KV cable ampacity table that changes how designers size medium voltage cables. Substantial work has been done over the last few code cycles to harmonize the Canadian Electrical Code (CE Code) with the US National Electrical Code (NEC). Part of this process has been to align the CE Code ampacity calculation methods with NEC methods.
For 75oC rated circuits, multiply the 90oC based Table D17 ampacity by 0.886.
A bit of history first: In 2012, CE Code ampacity Tables 1 to 4 were changed to base ampacities upon the temperature ratings of the circuit (4-006 Temperature Limitations Rule), not just the cable. Then, in 2015, the previously non-voltage-specific Tables 1 to 4 became limited to not more than 5000V. Table D17 first appeared at this point, providing ampacity tables for 5KV to 46KV cable applications. In 2018, CE Code Tables 1 to 4 changed to be limited to not more than 5000V and unshielded. Today, Table D17 applies to your 5KV shielded cables and higher voltage applications.
Table D17 consists of 14 tables representing different medium voltage cable applications, including direct buried cables, buried conduit, concrete encased duct banks and cable trays. They differentiate ampacities into two new groups: 5KV to 15KV, and 25KV to 46KV. Prior to D17, there were no special allowances for higher voltage applications like 25KV. These tables bring the CE Code ampacities closer to the IEEE835 tabulated ampacities.
There can be significant ampacity benefits with Table D17 while other times the opposite. The tables will continue to be improved with each CE Code cycle correcting inconsistencies and adding more variations. I suggest reviewing old designs per the latest tables as they might result in a different cable size than used in the past projects. Refer to my appended example calculations.
Note that Table D17 has a list of Conditions of Use that the ampacity numbers are based upon. If your application doesn’t meet all those assumptions, Table D17 does not apply. Rule 4-004(G) allows for an IEEE 835 ampacity calculation in this case. You may need to brush up on your ampacity calculation software skills as there are now a lot of applications that don’t fit an existing CE Code table. Common applications requiring an IEEE 835 calculation are underground applications with more than 2 conductors per phase, cables buried deeper than 915mm, and different separations between conductors.
Caution: Burial Depths
The deeper a cable is buried, the more it will need to be derated – the NEC rule of thumb is 6% deration per additional foot of burial depth.
In summary, Table D17 helps designers size medium voltage cables for applications that meet the table’s Conditions of Use. With tables for all applications now located in one handy section, designers will find it easy to compare how different installations affect cable sizing. If the application falls outside the table Conditions of Use, or no applicable table is present, an IEEE835 ampacity calculation can be performed. Confirm your AHJ’s position prior to making a cable sizing determination as they may have a different interpretation of this new CE Code table.