MV vs. LV Ampacity and the NEC
The National Electrical Code is now in its 2020 iteration (You can access this copy for free online by creating an account here). The code is always improving and innovating. However, it is important to understand the limitations of the code, particularly in regards to cable ampacity.
One major change in the 2020 edition of the code is the separation of cable ampacities, the evaluation of how much current a particular conductor can carry under specific conditions, into two separate code sections. Now, NEC Article 310 addresses ampacities for conductors rated up to 2kV while NEC Article 311 addresses ampacities for medium voltage conductors (cables rated above 2kV and up to 35kV). The change was important for two reasons. First, the sheer amount of information present in the old version of Article 310 where the sections were combined was too much. It was difficult to understand what guidelines and tables applied to one voltage versus the other and this new separation makes the information easier to digest. The second reason, and the more important reason, is that the code treats medium voltage and low voltage differently. Medium voltage conductors and equipment are more technically challenging than low voltage. Systems involving medium voltage frequently utilize more sophisticated relaying and protection, involve higher up-front costs that make advanced engineering more important, and tend to be employed with systems that cover greater geographic footprints.
A separation of medium voltage and low voltage ampacities brought the code in the right direction. However, what is now more apparent than ever is the differences in detail between the sections. Article 311-Medium Voltage, accounts for many conditions of use, including the following:
The section isn't perfect, with the document itself noting that some of the ampacity tables apply only to single point grounded systems (no adjustment factors for circulating currents in shield), only 100% load factors, and only the standard 90 rho value for thermal resistivity. That said, the quality of information is leaps and bounds ahead of what can be found in the low voltage section.
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Under Article 310 of the NEC, one can find only a handful of useful tables, with most people ever only using the table now known as 310.16 and formerly known as 310.15(B)(16). Table 310.16 in the 2020 NEC is meant to be used for conductors in raceway, cable, or directly buried. In other words, the table is meant to apply to virtually all standard conditions of use that could exist-that is a tall task. In reality, ampacities vary considerably for conduit in air, direct burial, and conduit in the ground. If you don't believe me, check out Article 311 for reference.
Why does the code do this? Safety, of course. 310.16 is the standard being used by all sorts of people, many of whom are not engineers. 310.16 is unlikely to lead to an unsafe installation if applied with appropriate derating factors. On the other hand, medium voltage tables can easily be misapplied, leading to severe overheating of cables and eventual damage or safety concerns.
However, This isn't the end of the story. Ampacity tables are not limited to just the tables provided in the NEC for a fully engineered application. The NEC even says so in 311.60, referencing its source material, the much more expansive IEEE 835 Ampacity Tables. IEEE 835 provides thousands of pages of ampacity tables based on conditions of use, voltages, and metal types. Additionally, more accurate and specific ampacity studies can be completed using Neher-McGrath software packages such as CYMCAP, ETAP, or AmpCalc. Both IEEE 835 and these calculations should only be used under experienced professional engineering supervision, as noted by the NEC.
So think twice about your ampacity choices and when detailed engineering is the right solution!