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#12219 08/04/02 12:11 AM
Joined: Apr 2002
Posts: 257
I am working on a job where we are retrofitting panels in an old school and pulling new feeders.

The service is 120/208 3-Phase.

The specs call for us to use booster transformers to feed one distribution panel making it 240V 3-Phase (No Neutral).

There was an existing section of the MDP that was fed via transformers to accomplish the same thing.

The specs call for (3) 7.5 KVA transformers (heavy little suckers) to be used to "boost" the 208V to 240V.

The transformers have one primary winding (H1 & H2) and two secondary windings labeled X1-X2 & X3-X4.

The label reads: Primary Secondary
240V 32V
120V 16V

I have never done this before and was wondering if any of you have and if you could tell me how to accomplish this. Or,in other words, which X's, H's and lines do I connect together? By the way, this is on a 400A feeder.

#12220 08/04/02 12:42 AM
Joined: Apr 2002
Posts: 558

All of the Buck-Boost transformers I have used come with wiring diagrams for different voltages and configurations. I know the Cutler Hammer distribution catalog has about 16 pages with charts and wiring diagrams. I don't have anything on this computer to send you right now but might be able to scan a diagram and send it to you. Your transformers should also have H3 & H4.


Curt Swartz
#12221 08/04/02 01:36 AM
Joined: Apr 2002
Posts: 2,527
You might look over − page 11. Ask the person who specified the transformers in the first place.

Post again if you need a suggestion.

#12222 08/04/02 09:51 AM
Joined: Jul 2002
Posts: 175
The principle of bucking or boosting involves connecting the primary and secondary windings of each of the transformers as an auto-transformer.

They would be series-aiding for boost operation, and series-opposing for bucking.
This is a single phase example-

[Linked Image from]

There are many different three phase connections. You should follow the manufacturers recommendations.
Here is a typical example of how it could be done.

[Linked Image from]


#12223 08/05/02 01:32 AM
Joined: Oct 2000
Posts: 2,723
Likes: 1
Broom Pusher and

Take a look at the Schematics I have in the Technical Reference Section regarding Buck / Boost Transformer connections using a typical Isolated Transformer, which is located at:

Buck/Boost/Choke + Polarity Test drawings for Transformers

In a nutshell, what you will be doing is creating an Autotransformer out of an Isolated Transformer. The connection scheme will have the coil arrangements in a Series Additive setup.

A quick description:


[*] Connect the "Split Coils" on each side in Series (for the "Higher" listed Voltage). This means connect H2 and H3 together, also hook X2 and X3 together. Do this on both Transformers.

[*] Connect the "H4" end of the Primary coil to the "X1" end of the Secondary coil.

[*] Feed this side with one Ungrounded Conductor - for example, this would be Phase C on the "Right Side" Transformer - for the "Left Side" Transformer Phase A would be used.

[*] Connect an "Output" Conductor to "X4" for the Load side connection (Transformer Output).

[*] Connect "H1" to an Ungrounded Conductor which will be "Common" for this arrangement - in this example use Phase B for the common Ungrounded Conductor.

[*] For an Open Delta arrangement, the above would be done using Two Identical Transformers.

[*] For a Wye Connection, Three Identical Transformers would be used, and the Common Conductor would be the Grounded Conductor. Primary coils would be setup in Parallel (lower Voltage).

Use Ed's example drawing above [the Open Delta one] for applying these figures to.

Using the 7.5 KVA 120/240 VAC x 16/32 VAC Transformers, you will end up with 240 VAC output [208 VAC + 32 VAC], but with a maximum continuous KVA available output of 37.5 KVA per Transformer.

Let us know if you have further questions and how things turn out.

Scott S.E.T.

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
#12224 08/05/02 11:10 PM
Joined: Apr 2002
Posts: 2,527
If you want to use 120V and 32V ratings and all three 7½kVA drytypes, it should serve a 240V 3ø load of 234 amperes. Figure E on page 40 of the cited link is probably suitable for the application. It may be a good idea to well mark the 240V panelboard where facility folks are used to finding 208V. It may be a good idea to gut the neutral bus so that someone doesn’t think installing a single pole breaker mistakenly wanting 120V.

[This message has been edited by Bjarney (edited 08-06-2002).]

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