National Electrical Code

Feeder Taps Greater Than 1,000V

Aug. 11, 2017

There is a drastic difference between tap rules for circuits not more than 1,000V and circuits over 1,000V.

In the 2017 National Electrical Code (NEC), there are plenty of tap rules in Sec. 240.21 and Sec. 240.92 for circuits not more than 1,000V. The same is true for transformer secondary conductor rules. Of course, if the tap rules and secondary conductor rules are used, then all of the qualifying criteria and conditions must be met for both taps and secondary conductors. However, all of these tap rules and secondary conductor rules only apply to circuits not more than 1,000V. There are no similar tap rules or transformer secondary conductor rules for circuits greater than 1,000V.

For medium- and high-voltage taps, it is evident there isn’t an allowable reduction in the size of conductors. The only mention of a tap for circuits greater than 1,000V is in Sec. 240.101(B) where it states: “(B) Feeder Taps. Conductors tapped to a feeder shall be permitted to be protected by the feeder overcurrent device where that overcurrent device also protects the tap conductor.” So when tapping a medium- or high-voltage conductor, the tap would effectively be required to be the same wire size as the tapped conductor. But it can get more complicated than just making the tap the same size as the tapped conductor. For instance, short-circuit calculations and protective device settings would need to be developed to ensure that a fault on a tap conductor can indeed trip the overcurrent device.

As far as medium- and high-voltage transformer secondary conductors are concerned, it is very common for voltages of 4.16kV and 13.8kV to employ cable bus or bus duct from the secondary terminals of the transformer to the main overcurrent protective device in the downstream switchgear without overcurrent protection at the transformer secondary terminals. This is allowed by Sec. 240.100, which allows these circuits to be designed under engineering supervision. The engineer will do short-circuit calculations and protective device settings to ensure that the secondary conductors are protected.

It should be noted, there isn’t a definition for “engineering supervision” in the Code. However, it is typically interpreted to mean supervision by a qualified, licensed, professional engineer.

About the Author

Eddie Guidry | Senior Fellow

Eddie Guidry is a senior fellow with Fluor Enterprises, Inc., Sugar Land, Texas. He is highly skilled in electrical and control systems for industrial construction, design, and engineering (both domestic and abroad). The majority of Eddie’s 38 years of experience includes upstream and downstream portions of petrochemical plants and refineries. He is also heavily experienced in water and wastewater treatment facilities. Eddie is very active in the development of U.S. national (ANSI) and international codes and standards, and has been a principal member of the National Fire Protection Association (NFPA) National Electrical Code (NEC) committee since 1999 and NFPA’s National Advisory Committee on Electrical Safety Research since 2008. Eddie, who has also developed and conducted many electrical courses and seminars over the years, currently holds the corporate Master Electrician license for Fluor Corporation in the State of Texas and is also an ICC/IAEI certified electrical inspector.