There are (3) ways to go on this scenario - all are based on what is acceptable for the Equipment involved.OPTION #1
This option is the most simplest: Tapped Autotransformer.
Using a 277V wound Autotransformer, with a tap for 208V:
- Connect the Transformer across a 277V 1 Phase 2 Wire "L-N" Circuit.
- Place the Grounded Conductor on the "Common" side of the Autotransformer.
- From the 208V Tap, bring out an Ungrounded Conductor.
- From the "Common" side connection, bring out the Grounded Conductor.
- From numbers "3" and "4" above, a 208V 1 Phase 2 Wire Circuit is created.
Insert OCPDs as necessary. At minimum, OCPD for the Primary side Feeder.
Include the EGC with the Primary Circuit, and bring it out for the 208V Secondary Circuit.
Do not bond the Grounded Conductor!!!
Single Phase Isolated Transformer with 208V Secondary.
As mentioned by Ghost307
, use an Isolated transformer with a Primary Voltage rating that is readily available.
Feed the Primary side from the available System:*
For a 277V Primary: One Ungrounded Conductor, plus the System Grounded Conductor.*
For a 480V Primary: Two Ungrounded Conductors.
Run the EGC to the Transformer case; bond it to the case.
On the Secondary side, bond Terminal "X2" to a local Grounding Electrode System.
Additionally, install a Jumper from "X2" to the Transformer case.
Derive an SDS from the Secondary side for the new 208V 1 Phase System as follows:
- From Terminal "X1", bring out the 208V System's Ungrounded Conductor.
- From Terminal "X2", bring out the 208V System's Grounded Conductor. Identify this Conductor as a System Grounded Conductor, using either White or Gray Coloring.
- From the "Jumper" at Terminal "X2", bring out an EGC. Identify with Green Coloring.
The Secondary Feeders _MAY_ be protected by the Primary Feeder OCPD, or you may choose Over Current Protection for both the Primary Feeders and the Secondary Feeders.
Since the output Voltage is only a single Voltage (208V only), the Secondary Feeders may be protected by the Primary Feeders' OCPD as described below:A:
If the Primary rating is 9 Amps or more: Maximum OCPD on Primary side is 125% the FLA of the Primary Winding(s).B:
If the Primary rating is less than 9 Amps: Maximum OCPD on Primary side is 167% the FLA of the Primary Winding(s).C:
If the Primary rating is less than 2 Amps: Maximum OCPD on Primary side is 300% the FLA of the Primary Winding(s).
The option exist to run the Secondary side as an Ungrounded System. This does not eliminate the need of bonding the Metallic Equipment connected to the Secondary side Circuitry to a local GES.
The ONLY difference between a Grounded System -vs- an Ungrounded System is installing a Main Bonding Jumper; or in this case, a "System Bonding Jumper", between the SDS and a local GES.
For more information on Grounded -vs- Ungrounded Systems, refer to the Transformer Schematics posted at the Technical Reference section.
3 Phase Isolated Transformer - 480V x 208Y/120V:
This option would be the most involved, but may be the most cost effective!
This type of Transformer is an off-the-shelf item, readily available from nearly all Wholesalers.
The costs and availability might result equal to a Single Phase setup.
This option allows for future expansion and usage.
Setup would be similar to "OPTION #2" above, with the following revisions:A:
Primary side fed with 480V 3 Phase 3 Wire Circuit + EGC.B:
Primary side Over Current Protection only covers Primary Feeders.C:
Secondary side shall be Grounded. Ground at Terminal "X0".D:
Secondary Feeders to be Protected by OCPDs. Primary OCPD may not protect Secondary Feeders.E:
Secondary side will consist of (3) Ungrounded Conductors, and (1) EGC bonded to the GES and the "X0" Star Point.F:
Secondary side Grounded Conductor _MAY_ be brought out from Terminal "X0" to a Panelboard / Disconnect if wanted.
If the Panelboard or Switch (Disconnect) will be feeding only L-L / L-L-L connected loads, the System's Grounded Conductor does not need to be included - provided the Main System Bonding Jumper exists at the Transformer, and there is a suitable EGC between the Transformer and the Panel or Switch.
As far as "What Would I Do", I would do one of the three options listed above!
BTW, Abbreviations used:
* EGC = Equipment Grounding Conductor.
* GES = Grounding Electrode System.
* OCPD = Over Current Protection Device.
* SDS = Separately Derived System.
* L-N = Line To Neutral.
* L-L = Line To Line.
* L-L-L = Line to Line To Line (3 Phase 3 Wire)