Recently, an inspector and contractor called me concerning a 277/480-volt feeder circuit that was designed to comply with the 2008 National Electrical Code (NEC) and supplied a building from the service equipment located in a substation switchboard room. In this installation, the grounded conductor was to serve as a neutral plus an equipment-grounding means. The plan was to install 10 disconnecting means in the general-purpose panelboard located in the building supplied by a feeder that is determined by the connected load.
The callers wanted my opinion on the type of panelboard, number of disconnects permitted, and the bonding and grounding requirements. They were in disagreement as to how this feeder had to be installed, considering single management of the building with proper supervision and qualified personnel handling all maintenance and switching operations. They were interesting questions with a few surprising answers.
An outside feeder circuit
I pointed out that Article 225 of the NEC contains special requirements for an outside feeder circuit. For 600 volts or less, a designer uses the references from Article 225.3(B) to determine the load served and Article 225.5 for calculating the size conductors. If a feeder circuit has a voltage of more than 600 volts, the conductors must be designed and installed by the provisions listed in 225.5, plus its references. The panelboard and disconnecting requirements are laid out in 225.32 through 225.36, while the bonding and grounding techniques can be obtained in 250.32.
The callers informed me that the feeder circuit supplied a connected load of 1,200 amps that is operated continuously. For determining the calculated load, I referred them to 215.2(A)(1), which requires the 1,200 amps to be multiplied by 125 percent. This totals 1,500 amps.
Instead of five conduit runs of RMC with 3-250 kcmil, AWG, THWN copper conductors in each run, six runs are necessary. Included in each run is a grounded-neutral conductor sized per 220.61 and 215.2(A)(1), Ex. 2. All conductors are enclosed and routed in the same conduit system in accordance with 300.3(B) or 300.5(I).
According to their information, a 1/0 AWG, THWN, copper neutral conductor was sized per 220.61. Based on a 1,200-amp device, a 3/0, AWG, THWN copper conductor was selected from Table 250.122 and routed in each conduit to handle the neutral load and serve as an equipment-grounding conductor (EGC), too. The continuous load calculation required a 1,500-amp overcurrent device to be placed in the switchboard ahead of the feeder circuit. I suggested, based on a 1,500-amp protective device, that a 4/0 copper EGC sized from Table 250.122 be pulled in each conduit along with the neutral and ungrounded phase conductors per 250.32(B). Since the building is a new installation, the Ex. to 250.32(B) prohibited the use of the neutral being used as both a current-carrying conductor and an EGC.
I mentioned that the panelboard must be rated for use as service equipment in accordance with the provisions of 225.36. This section is very clear that the disconnecting means written about in 225.31 must be suitable for use as service equipment, and therefore, a general-use type is not permitted. For additional information, review the requirements pertaining to service equipment outlined in 230.66.
I also pointed out, unless a building falls under certain conditions of use, the disconnecting means for a feeder supply is limited to no more than six disconnects. These can be six switches or circuit breakers mounted in one enclosure or individual enclosures.
As outlined in 225.33(A) and 225.34(A), grouping of these devices in one location allows the disconnection of the feeder circuit conductors and other components with six throws of the hand. And, of course, for easy and fast identification, 408.4 requires each disconnecting means to be identified as to which circuit and load they individually protect and serve.
A handy exception
When a feeder circuit is installed under single management, documented safe switching procedures are established and maintained for disconnecting purposes, and qualified individuals monitor the installation as planned, the disconnecting means can be located in the substation. In other words, the building panelboard can be equipped with 10 disconnecting means as permitted by the Ex. 1 to 225.32. However, for safety reasons, 250.32(D) requires the EGC to be routed with the neutral and ungrounded phase conductors and terminated as 408.40 demands.
The 2008 NEC is clear that, for new installations, only the grounded conductor can be utilized as a current-carrying neutral. The panelboard is required to be suitable for use as service equipment and can possibly house 10 disconnecting means. A separate EGC must be routed with the circuit conductors and the connected load calculated for continuous operation.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.