Wiring a Building Switchboard from an Outdoor Transformer
The main question that must be asked when wiring a building switchboard from an outdoor transformer is: Does there have to be a main disconnect when all applicable electrical switchboard rules are considered? Yes! There is no way to avoid a main disconnect. However, even if there is a main overcurrent device located at the transformer, this should not cloud your judgment when considering all electrical switchboard requirements. So the best advice that can be given is to analyze each switchboard rule independently (disconnecting the building, transformer protection and feeder elements), and then correlate the analysis based on the worst case.
Wiring a Building Switchboard: Disconnecting the Building Rule
First of all when wiring a building switchboard, the building disconnecting must meet the requirements of Sec. 225-8 (b) – which states: that when more than one building or other structure is on the same property and under single management, each building or other structure served shall be provided with means for disconnecting all undergrounded conductors.
The disconnecting means shall be installed either inside or outside of the building or structure served, at a readily accessible location nearest the point of entrance of the supply conductors. Ultimately, disconnects shall be installed in accordance with the requirement of Sections 230-71 and 230-72.
A remote disconnect at the transformer could not be used to comply with this requirement; however, up to six building disconnects could be used for this purpose. This is because the cross-reference to Sec. 230-71 incorporates the familiar six-service disconnect rule for building disconnects. Therefore, in terms of applicable requirements for disconnecting the building, the building switchboard may include up to six disconnecting means.
Wiring a Building Switchboard: The Transformer Protection Aspects
While wiring a building switchboard, let’s assume this is a medium-voltage transformer in a commercial occupancy that will not generally be judged to have the qualified personnel in place to “monitor and service the transformer installation” required to meet the allowances for supervised installations in Sec. 450-3(a)(2). Therefore, the transformer must be protected for unsupervised installations per Table 450-3(a)(1).
Assuming a 6% or lower impedance in the transformer, the primary fuses cannot exceed the next higher standard size above 300% of the rated current.
Wiring a Building Switchboard: The Feeder Elements
The service conductors supply the medium-voltage main switch ahead of the transformer primary. The conductors connected to the transformer secondary are feeder conductors, as are the busbars within the switchboard. Whatever overcurrent protection arrangement are made, they must protect both the busbars and the feeder conductors in accordance with Article 230.
If the conductors are suitably protected at the transformer, then up to six disconnects may be installed in the building switchboard without an additional main device. These disconnects and their associated overcurrent protection may be sized to the load requirements of their derived feeders, and the sum of those ratings may exceed that of the bus. Of coarse, the calculated total load must not exceed the 3000A rating of the main bus.
