High-leg delta

High-leg delta (also known as wild-leg, stinger leg, bastard leg, high-leg, orange-leg, red-leg, dog-leg delta) is a type of electrical service connection for three-phase electric power installations.

The three-phase power is connected in the delta configuration, and the center point of one phase is grounded.

[1] By convention, the high leg is usually set in the center (B phase) lug in the involved panel, regardless of the L1–L2–L3 designation at the transformer.

The line-to-line voltage magnitudes are all the same: Because the winding between the a and c phases is center-tapped, the line-to-neutral voltages for these phases are as follows: But the phase-neutral voltage for the b phase is different: This can be proven by writing a KVL equation, using angle notation, starting from the grounded neutral: or: If the high leg is not used, the system acts like a split single-phase system, which is a common supply configuration in the United States.

Thus, large pieces of equipment will draw less current than with 208 V, requiring smaller wire and breaker sizes.

The distribution transformer output is 200 V line-to-line and 100 V line-to-neutral, while the high-leg to neutral voltage is 173 V. This provides 200 V for both three-phase and split-phase appliances.

Current practice is to give separate services for single-phase and three-phase loads, e.g., 120 V split-phase (lighting etc.)