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#128889 - 03/11/04 12:46 PM 120/208 and 277/480V
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden

I think the two voltages in the subject line are the standard voltages for industrial use in the US. What I don't see is the connection between them. It seems that if you use a transformer bank consisting of standard 120/240V single phase transformers you could get 120/208 and 240/416V. I infer that the 277/480V transformers must be of a separate type, right? Is the 120V taken from the same transformer as the 277V or are these two entirely separate systems?

(I considered placing this message in the General area, but as it is theory to some degree it landed here.)

#128890 - 03/11/04 04:37 PM Re: 120/208 and 277/480V
crash Offline

Registered: 10/21/03
Posts: 130
Loc: Calgary Canada
You would use a bank of three, 480V to 120V transformers (4 to 1). The high voltage side would be hooked up in Delta. The low voltage side would be hooked up in Wye.

#128891 - 03/11/04 08:07 PM Re: 120/208 and 277/480V
Scott35 Offline

Broom Pusher and

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA

You are right about the 1Ø Transformers being different between a 208Y/120 3Ø 4W bank, and a 480Y/277 3Ø 4W bank.

On the 208Y/120 3Ø 4W bank, each Transformer is (usually) the commonly used 6900/13800V × 120/240V 1Ø Split Coil types. The Secondary Windings (of each Transformer) are connected in Parallel, so the output Voltage is 120VAC. All 3 Pots have the Secondary Windings setup in Parallel, so each one will have 120VAC output.

On the 480Y/277 3Ø 4W bank, each Transformer may be:

A: 6900/13800V × 277V 1Ø 2 Wire type Pot, with Split Coil Primary Windings and a Single (Non-Split) Coil Secondary Winding,


B: 6900/13800V × 480/277V 1Ø 2 Wire type Pot, with Split Coils on both sides (Primary and Secondary).

If option "B" is used, the Secondary of each Pot is connected for an output Voltage of 277VAC.

The physical appearance of the two different types of Pots are:

A: On the 120/240V types, there are three (3) small Bushings on the upper "face" of the Enclosure, which are the Output Terminals.
"X0" - the "Center Tap Point" is between Terminals "X1" and "X2". "X0" is only used on the 120/240V 1Ø 3W systems.

B: The 277V (or 480/277V) types have only two Bushings on the "face", which are about 50% larger than the ones on the 120/240V Pots.

Connection procedures for each of the different classes are as follows:

To form the Star point of the Wye, the "X1 Output Terminal" of each Pot (Transformer) is connected together. The Common Grounded Conductor is derrived from this Star point.
The "X2 Output Terminal" from each Pot become the individual Ungrounded Conductors.

On Delta Connected banks, the Secondary Windings on each Pot are connected in Series Adding, as to yield the higher of the two possible Voltages for the Output.

For a 120/240V 3Ø 4W Delta, one Pot uses the "X0" Center Tap Bushing, and that Circuit Conductor becomes - of course - the System's Grounded Conductor.

Let me know if you have further Q's. I'll check the Transformer drawings I have on-line for something which shows details to cover what's described here.

Man, it has taken me nearly 30 Minutes to write this message! Every 2 Minutes, the Phone rings, someone comes to the door, yadda-yadda-yadda!!!

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#128892 - 03/12/04 08:19 AM Re: 120/208 and 277/480V
C-H Offline


Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Thanks guys!

I didn't think of that option. Is it common to feed only 277/480V to the building and then use transformers to step down to 120V? It seems like a very useful setup in cases where the cable runs are long.

You're a fast typer if you managed to get that down in thirty minutes while talking to people! I'll read and draw...

#128893 - 03/12/04 03:13 PM Re: 120/208 and 277/480V
crash Offline

Registered: 10/21/03
Posts: 130
Loc: Calgary Canada
C-H: Actually around here it is quite common to see buildings feed with a 600/347 service. The voltage is, as you stated, stepped down to 120/208V using a Delta Wye connection.

#128894 - 03/13/04 08:50 AM Re: 120/208 and 277/480V
Bjarney Offline

Registered: 04/10/02
Posts: 2561
Loc: West-Southern Inner-Northeast ...
Scott — Minor nit, here. 6,930 Vø∙n corresponds to 12,000 Vø∙ø. 13,800 Vø∙ø is associated with 7,970 Vø∙n. [Forgive me, Scott.]

Donald Beeman’s 1955 Industrial Power Systems Handbook recommended solidly grounded 208Y and solidly grounded or resistance-grounded 480Y for commercial/small-industrial building-distribution voltages. The book sort of ‘poo-pooed’ 240V ∆ as essentially ‘stone age’, but as we can see, ~50 years later there’s still a lot of it for small 3ø loads.

A modern iteration of Beeman is IEEE’s standard 141-1993, Recommended Practice for Electric Power Distribution for Industrial Plants {a/k/a the ‘Red Book’}.

Where 480Y is common below the 49th parallel, 600Y is equally status quo above the 49th. {208Y is popular on both sides—sort of an odd coincidence.} 120/240V 1ø 3w is extensively used in homes above and below 49°.

The 60Hz, 208 versus 240V ‘gap’ also shows up at 4.16-4.8, 12.0-13.8 (or 14.4) and 115-138kV levels on the North American continent. Voltage ratios of 2 and √3 seem to have evolved where inherently a factor of 2 relates to equal series/parallel windings and √3 relates to ø-ø/ø-n quantities.

[This message has been edited by Bjarney (edited 03-13-2004).]


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