Basics of Transformer Installation

Oil Immersed Transformer

You have to supply electricity to the new offices in Warehouse 5. To do this, you need a 3kVA transformer (277V single phase to 120V, 2-wire) for the lighting and sockets for the coffee maker and computer terminals. So how do you connect the transformer? In the terminal box, the "H" wire indicates the high-voltage winding of the transformer and the "X" wire indicates the low-voltage winding.

You have to supply the new office in Warehouse 5. To do this, you need a 3kVA transformer (277V single phase to 120V, 2-wire) for the lighting and sockets for the coffee maker and computer terminals. So how do you connect the transformer?

In the terminal box, the "H" connection indicates the high voltage winding of the transformer, and the "X" connection indicates the low voltage winding. This model has two 21/2% "FCBN" taps; the full current is lower than normal. Why taps? If the primary voltage is consistently low (approximately 270V: 2.5% with leads H1-H3), or 263V: 5% with leads H1-H2), you can change a tap to help maintain 120V on the secondary. 

Even if the primary is a normal 277V (with leads H1-H4), you can change the tap to maintain 120V on the secondary. by connecting the windings in parallel, The two separate windings on the "X" side are configured for 120 V. To obtain 240 V, connect them in series (connect H2 to H3; connect H1 and H4 to the load). These connections are application-specific; consult the transformer manufacturer.

H-Class Dry-Type Power Transformer        

They do not automatically hold the secondary voltage. The pigtail is connected to the wiring via ring terminal crimp lug and machine screws from the raceway, to which you can add insulation (caps and/or appropriate splicing tape).

A 10A double-element fuse is installed in the spare single-pole switch in the 480Y/277V lighting panel on the wall opposite the warehouse. The primary side fuse provides suitable secondary side overcurrent protection; as it is configured for 2-wire operation. To minimize voltage drop, place the transformer closest to the load (office) end of the circuit.

The losses vary with the square of the current in each winding; this transformer position reduces them by approximately 81%. Earthing, in this case, is simple: connect the transformer secondary to its supporting steel column.

Suppose you have to install another transformer (15kVA 480V 3 phase 3 wire to 240V 3 phase 3 wire) to provide five small inverters (AFD) for the packaging assembly. The connection to this type of transformer is basic - three wires to the high side (H1, H2, and H3) and three wires coming out of the low side (X1, X2, and X3).

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 SG (B) 10 series Non-encapsulated H-class Dry-type Power Transformers

The 10 AWG primary side wiring (22.6 A at 480 V) and the 8 AWG secondary side (45.1 A at 240 V) will allow you to make full use of the transformer capacity. Ring terminal crimp tabs and machine screws are used on the connection shank legs, and quarter-inch beveled washers are used to compensate for the different thermal expansion of the different conductors.

Flexible metal conduit isolates vibration, but what about equipment grounding?NEC sec. 351-9 allows a three-quarter inch bend for equipment grounding of overcurrent equipment up to 60A. High voltage side overcurrent devices (in this case 3-pole, 25A thermomagnetic circuit breakers) provide protection against short circuits and overloads on the secondary side of the transformer. Individual AFDs require corresponding overcurrent protection and disconnecting devices.

For delta-delta, sec. 240-3(i) allows the omission of a dedicated overcurrent device for transformer protection. 250-5(b) allows the 240V 3-phase 3-wire transformer secondary to be operated without system grounding. With this voltage configuration, any overcurrent device must be at least 240V rated, the use of "slash rated" 120/240V devices is potentially dangerous.

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