How Do You Connect a Bus Duct to a Transformer?
By DHYN Engineering Team · September 28, 2026
Quick answer: In most projects the bus duct meets the transformer through a flanged end: the duct's connection flange bolts to the transformer's secondary terminal box, with flexible copper braids or a dedicated flexible section in between to absorb vibration and installation tolerance. One rule matters above all: never bolt rigid busbars straight onto the transformer bushings.
The transformer to bus duct connection looks simple on a drawing and causes a surprising share of site problems. Here is how it is supposed to work, and where it goes wrong.
The standard connection: flange plus flexible links
The transformer leaves the factory with a terminal box or bushing flange on the low voltage side. The busway ends in a matching flanged end, sometimes called a transformer connection unit, that bolts to it with a gasket between. Between the busway bars and the transformer terminals sit flexible copper braids or laminated shunts. They carry the full rated current, but mechanically they are soft, and that softness is the whole point. A typical flex link absorbs 10 to 20 mm of misalignment in any direction, which is the difference between a connection that bolts up in an hour and one that needs the transformer re-leveled.
Why rigid connections fail
A transformer hums. Magnetostriction in the core produces vibration at twice the line frequency, 100 or 120 Hz, every second of every day it is energized. Bolt a rigid copper bar to the bushing and that vibration walks the joint loose over months, then the contact resistance climbs and the joint cooks. Thermal expansion does similar damage from the other direction: a 30-meter busway run grows roughly 20 mm between a cold warehouse and full load, and if the transformer end cannot move, that force lands on the bushings. On oil-filled units that is how you get weeping gaskets.
Outdoor transformer connections
Pad-mounted and outdoor oil transformers bring water into the picture. The flange gasket has to maintain the duct's IP rating at the connection, the housing should fall away from the flange so condensation drains outboard, and the section nearest the transformer deserves the highest protection class on the run. Where the duct runs outdoors for any real distance, a resin cast busway is usually the right answer, since sandwich construction is not meant to live in the rain.
Low voltage versus medium voltage
On the LV side, a sandwich bus duct at 400 V or 690 V, from 250 A up to 6300 A, connects straight into the transformer terminal box as described above. Step up to 13.8 kV generator step-up or station service transformers and the construction changes completely: you are into non-segregated or isolated phase bus duct with porcelain or epoxy insulators, which is a different product family covered in our isolated phase vs non-segregated bus duct article.
Field mistakes we keep seeing
Forcing alignment with come-alongs until the flanges meet, which preloads the joint and guarantees a hot spot. Skipping the re-torque after the first heat cycle. And the classic: connecting everything beautifully, energizing, and discovering the phase rotation is backwards because nobody checked the bushing labels against the duct markings. Five minutes with a rotation meter before energizing beats a very awkward phone call.
Send us the transformer outline drawing with your inquiry and we will design the flanged end to match it at the factory, which is where this connection should be solved.
Need bus duct for a real project?
DHYN ELEC GROUP designs and manufactures bus ducts, transformers and switchgear. Send us your single-line diagram and we will come back with a design and a quotation. Contact us or message us on WhatsApp.