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Electrical Supply Configurations

Following are the typical transformer configurations that are used to supply 60 Hz power
at 600 volts or less in the United States. While these generally describe the utility supply, large industrial facilities may receive power at higher distribution voltage levels and
derive the lower voltages internally. The voltages in the chart are the standard nominal supply values.

Several of the systems are rarely used for new installations, although they are still found
in existing facilities. Other arrangements exist that are used only occasionally or for
special purposes, and these have not been included at this time.

The colors green, green/yellow, white, natural gray, orange, brown & yellow are reserved for specific conductors as defined by the NEC. All other conductors may be any color
except these. While the chart shows typical color usage, some installations will differ.

 

SINGLE-PHASE

THREE-WIRE

Nominal Voltages
Phase-Phase Phase-Neutral
240 120

This is the most common supply for residences and small commercial facilities. It is also used for the offices in industrial facilities, where it may be derived from a higher available voltage by means of a local transformer.

TWO-WIRE

Nominal Voltages
Phase-Phase Phase-Neutral
- 120
- 277

Used infrequently for residential service or industrial single-phase loads.

TWO-WIRE ISOLATED

Nominal Voltages
Phase-Phase Phase-Neutral
120 -
240 -

Used to prevent ground fault arcs in hazardous atmosphere areas in hospitals and other similar applications. It may also be used as part of a power quality solution for sensitive loads. Isolated sources are generally derived locally near the point of use.

THREE-PHASE, THREE-WIRE

CORNER-GROUNDED DELTA

Nominal Voltages
Phase-Phase Phase-Neutral
240 -
480 -
600 -

Used occasionally in industrial facilities with only three-phase loads. No neutral is available.

UNGROUNDED DELTA

Nominal Voltages
Phase-Phase Phase-Neutral

240

-
480 -
600 -

Used occasionally in industrial facilities with only three-phase loads, or where isolation is required. No neutral is available.

OPEN DELTA

Nominal Voltages
Phase-Phase Phase-Neutral
240 -
480 -
600 -

Similar to full delta, but used less frequently, for smaller three-phase loads.

UNGROUNDED WYE

Nominal Voltages
Phase-Phase Phase-Neutral
480 -
600 -

Used occasionally in industrial facilities with only three-phase loads, or where isolation is required. The center point may be grounded through a high impedance, but no neutral is available.

THREE-PHASE, FOUR-WIRE

GROUNDED WYE

Nominal Voltages
Phase-Phase Phase-Neutral
208 120
480 277
600 347

This is the most common system for large commercial office buildings at 208 volts, or industrial facilities at 480 volts with 277 volt lighting. All three phases can supply phase-to-neutral loads.

CENTER-TAP GROUNDED DELTA

Nominal Voltages
Phase-Phase Phase-Neutral
240 120 (phases A & C)
  208 (phase B)

Used for commercial or industrial facilities with primarily three-phase loads. The high leg (phase B) must be identified, and is not usable for phase-to-neutral loads.

CENTER-TAP GROUNDED OPEN DELTA

Nominal Voltages
Phase-Phase Phase-Neutral
240 120 (phases A & C)
  208 (phase B)

Similar to full delta, but used for smaller systems with minimal three-phase loads.

   
   
   

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