Transformer secondary conductor protection for voltage levels less than 1,000V is adequately covered in Art. 240 of the National Electrical Code (NEC) – more specifically, Sec. 240.21(C)(4). There are several options for protecting the secondary conductors at low voltage levels (see Parts II and VIII of Art. 240). However, protecting transformer secondary conductors that operate on systems greater than 1,000V is less clear.
In industrial facilities, it is common to have bus duct or cable bus supplying power from the secondary side of a transformer to switchgear at the 4,160V and 13.8kV levels. But it’s a common mistake on the design engineer’s part to utilize the secondary conductor rules found in Art. 240 for voltage levels above 1,000V.
Article 240 addresses conductor protection for installations greater than 1,000V in Part IX. As outlined in Sec. 240.100, conductors operating at greater than 1,000V need to be protected at the point where they receive their supply. Although this is the same rule we find for low-voltage conductors in Sec. 240.21, the second part of that same sentence goes on to say that the conductors can be protected at an alternative location when designed under engineering supervision. Presumably this could mean that the overcurrent protection device(s) could be located in the switchgear that is being supplied by the conductors. What exactly does this mean to the design engineer?
Transformer secondary conductors on systems greater than 1,000V can be installed unprotected at the source only where studies have been performed under engineering supervision to ensure that the conductors will not be damaged if a through fault or an overcurrent condition occurs. In other words, instead of having black and white NEC rules like those found for installations less than 1,000V, the onus lies with the design engineer and the authority having jurisdiction (AHJ) to ensure that the installation is safe.
A common method utilized by the design engineer is to create a relay coordination study. The relay coordination study will indicate how the transformer secondary conductors are adequately protected. Another common method of protecting transformer secondary conductors on systems greater than 1,000V is to include them within the transformer differential relay zone of protection.
As a footnote, it’s important to note the term “engineering supervision” isn’t defined in the Code at this time. It is used throughout the Code, and several unsuccessful attempts over the years have been made to define this term. It’s been my observation that the most prevalent interpretation means calculations must be performed under the supervision of a licensed, professionally registered engineer. As with any other Code interpretation though, until this term is defined by the NEC, it is ultimately up to the AHJ to determine what it means.
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