For supplying three-phase, 240-volt loads with small amounts of 120/240-volt, single-phase load. No problem from third harmonic overvoltage or telephone interference. With a disabled unit, bank can be reconnected in open-delta for emergency service.

This connection is often used to supply a small single-phase lighting load and three-phase power load simultaneously. As shown is diagram, the mid-tap of the secondary of one transformer is grounded.

Thus, the small lighting load is connected across the transformer with the mid-tap and the ground wire common to both 120 volt circuits. The single-phase lighting load reduces the available three-phase capacity. This connection requires special watt-hour metering and is not available from all utilities.


The transformer with the mid-tap carries 2/3 of the 120/240-volt, single-phase load and 1/3 of the 240-volt, three-phase load. The other two units each carry 1/3 of both the 120/240- and 240-volt loads.

High circulating currents will result unless all units are connected on same regulating taps and have same voltage ratios. Bank rating is reduced unless matching impedance transformers are used. The secondary neutral bushing can be grounded on only one of the three transformers.

When three transformers are operated in a closed-delta bank, care should be taken to make certain the impedances of the three units are practically the same. Transformers having more than 10% difference in impedance rating should not be operated together in a closed-delta bank unless a reactor is used to increase the impedance of the unit having the lower impedance rating to a value equal to the other units.

If the voltage ratio of all three of the transformers is not the same, there will be a voltage tending to circulate current inside the delta. The current will be limited by the impedance of the three transformers considered as a series circuit.

It is a good practice, before applying voltage to three transformers in closed delta, to insert a fuse wire between the leads coming from the high-voltage bushings of two transformers closing the delta bank. The fuse wire should be of sufficient size to carry the exciting current of the transformers.

The use of this fuse wire offers a very simple means of making certain the transformers have the proper polarity.
This connection should not be used with CSP transformers if used to supply a combined three-phase and three-wire single-phase load due to unequal voltage division of single-phase load when the tapped transformer breaker is opened.

NEC 2002: 110.15 High-Leg Marking.
On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded to supply lighting and similar loads, the conductor or busbar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point on the system where a connection is made if the grounded conductor is also present.

NEC 2002 Handbook:
Added for the 2002 Code, this section now contains a requirement that appeared in 384-3(e) of the 1999 NEC. This requirement was moved to Article 110, where the application becomes a more general requirement.

The high leg is common on a 240/120-volt 3-phase, 4-wire delta system. It is typically designated as “B phase.” The high-leg marking is required to be the color orange or other similar effective means and is intended to prevent problems due to the lack of complete standardization where metered and non-metered equipment are installed in the same installation. Electricians should always test each phase relative to ground with suitable equipment to determine exactly where the high leg is located in the system.

NEC 2002: 408.3 / Support and Arrangement of Busbars and Conductors / (E) Phase Arrangement
The phase arrangement on 3-phase buses shall be A, B, C from front to back, top to bottom, or 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.

Exception: Equipment within the same single section or multisection switchboard or panelboard as the meter on 3-phase, 4-wire, delta-connected systems shall be permitted to have the same phase configuration as the metering equipment.

FPN: See 110.15 for requirements on marking the busbar or phase conductor having the higher voltage to ground where supplied from a 4-wire, delta-connected system.

NEC 2002 Handbook:
The high leg is common on a 240/120-volt, 3-phase, 4-wire delta system. It is typically designated as “B phase.” Section 110.15 requires the high-leg marking to be the color orange or other similar effective means of identification. Electricians should always test each phase to ground with suitable equipment in order to know exactly where this high leg is located in the system.

The exception to 408.3(E) permits the phase leg having the higher voltage to ground to be located at the right-hand position (C phase), making it unnecessary to transpose the panelboard or switchboard busbar arrangement ahead of and beyond a metering compartment. The exception recognizes the fact that metering compartments have been standardized with the high leg at the right position (C phase) rather than in the center on B phase.

See also 110.15, 215.8, and 230.56 for further information on identifying conductors with the higher voltage to ground. Other busbar arrangements for making additions to existing installations are permitted by 408.3(E).

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