You must power a new office in Warehouse 5. To do this, you need a 3kVA transformer (277V single-phase to 120V, 2-wire) for lighting and a receptacle for the coffeepot and computer terminal. So how do you connect the transformer?
Within the junction box, the "H" connections denote the higher-voltage winding of a transformer, and the "X" connections identify the lower-voltage windings. This model has two 21/2% "FCBN" taps; full current below normal. Why the taps? If the primary voltage is consistently low (about 270V: 2.5%, using leads H1-H3), or 263V: 5%, using leads H1-H2), you can change a tap to help maintain the 120V on the secondary. Even if the primary is a normal 277V (using leads H1-H4), you can change taps to maintain 120V on the secondary. Configure the two separate windings on the "X" side for 120V by placing the windings in parallel. To get 240V, put them in series (splice H2 to H3; splice H1 and H4 to the load). These connections are application-specific; consult the manufacturer's labeling and literature before energizing any connection.
They do not automatically maintain the secondary voltage. The pigtail leads splice to wiring from the raceways with ring-terminal crimp lugs and machine screws, to which you add insulating materials (caps and/or appropriate splicing tape).
Install a 10A dual-element fuse in a spare single-pole switch in 480Y/277V lighting panel on the opposite wall of the warehouse. The primary-side fuse provides suitable secondary-side overcurrent protection; because of its configuration for 2-wire operation. To minimize voltage drop, place the transformer nearest the load (office) end of the circuit.
Losses vary with the square of the respective winding currents; this transformer location cuts them by about 81%. Grounding is simple in this case: Bond the transformer secondary to its supporting steel column.
Suppose you must install another transformer (15kVA 480V 3-phase, 3-wire to 240V 3-phase, 3-wire), serving five small adjustable-frequency drives (AFDs) for a packaging assembly. Connections to this type of transformer are basic-three wires into the high side (H1, H2, and H3) and three wires out from the low side (X1, X2, and X3).
No. 10 AWG primary-side wiring (22.6A at 480V) and secondary-side No. 8 (45.1A at 240V) will give you full use of the transformer capacity. Use ring-terminal crimp lugs and machine screws on connection tangs, with quarter-inch Belleville washers to compensate for the varying thermal expansion of different conductors.
Flexible metal conduit isolates vibration, but what about equipment grounding? NEC Sec. 351-9 permits three-quarter-inch flex for equipment grounding for an overcurrent device up to 60A. The high-side overcurrent device (in this case, a 3-pole, 25A thermal-magnetic breaker) provides transformer secondary-side short-circuit and overload protection. The individual AFDs need corresponding overcurrent protection and disconnecting means.
For a delta-delta, Sec. 240-3(i) permits omission of a dedicated overcurrent device for transformer protection. Sec. 250-5(b) permits the 240V 3-phase, 3-wire transformer secondary to operate without system grounding. With this voltage configuration, any overcurrent devices must have at least a 240V rating, and using "slash-rated" 120/240V devices is potentially hazardous.
