Delta High Leg Can Now Be the “C” Phase

Summary: 2008 NEC allows the high leg of a delta high leg system to be labed as the “C” phase instead of the “B” phase.

A change to the 2008 NEC now allows the high leg of a delta high leg system to be labeled as the “C” phase instead of the “B” phase as was the traditional phase designation.  The high leg must be identified by an orange color (it was often referred to as a red-leg delta) or by other effective means and is usually the B phase. However, to accommodate meter configuration the high leg is permitted to be the C phase where metering is part of the switchboard or panel board. The Code change in this section requires legible, permanent field marking of the switchboard or panel board. 

Below is the traditional arrangement of the high leg delta system with the “B” leg as the high leg.  The high leg has a phantom 208 volts phase to neutral which is seldom used.  The difference between the first and second diagrams is the designation “CLOSED” and “OPEN”.  This refers to whether or not the delta is formed with three transformers (CLOSED delta) or two transfomers (OPEN ansformer.

The 2008 NEC now allows the high leg to be the “C” leg, which then allows the first two busses in a switchboard to be across the 120/240 volt side of the delta.  Single phase meters can then connect to these two busses without having to bridge the center bus.

 

 

 

 

 

 

 

 

 

 

One Response to “Delta High Leg Can Now Be the “C” Phase”

  1. Grant Register Says:

    NEC 2008 408.3, (E) Phase Arrangement. The phase arrangement on 3-phase buses shall be A, B, C from front to back, top to bottom, left to right, as viewed from the front of the switchboard or panelboard. The “B” phase shall be that phase having the higher voltage to ground on 3-phase, 4-wire, delta-connected systems. Other busbar arrangements shall be permitted for additions to existing installations and shall be marked.

    You Article entitled [Delta High Leg Can Now Be the “C” Phase] states that the code allow “C” phase to be the high leg, but how do you reconcile the above Article also in the 2008 NEC? You help would be appreciated.

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