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Issue 37 – Topic High-Leg Delta (High Leg System)
High leg delta, also known as wild-leg, stinger leg, bastard leg or high leg is a type of electrical service connection for three phase electric power installations. It is used when both single and three phase power is desired to be supplied from a three phase transformer. It is often found in older commercial buildings and warehouses.
The NEC Code requires that the high leg be identified by an orange color.
The high leg or phase with the higher voltage as measured to neutral has traditionally been designated Phase B. A change to the 2008 NEC now allows the high leg of a four-wire three-phase delta service to be labeled as the C phase instead of the B phase. This change was made because utilities usually require the high leg to be the C phase in the metering equipment.
You can see in the image, the high leg of the service starts as the C phase at the meter and terminates in the panelboard as the B phase.
Most electricians still prefer to keep the high leg on the B phase in the panelboard however it is permitted to be the C phase since the 2008 change.
The three-phase power is connected in the delta configuration, and the center point of one phase is grounded. This creates both a three phase and a single phase supply in one system.
The image below shows the voltage readings across the legs of the system. Any two legs of the system A-B, C-A & C-B give us 240 volts. A to neutral or C to neutral give us 120 volts. B, the high leg to neutral gives us 208 volts.
Another method requires two transformers. One transformer is connected to one phase of the overhead primary distribution circuit to provide the lighting side of the circuit (this will be the larger of the two transformers), and a second transformer is connected to another phase on the circuit and its secondary is connected to one side of the lighting transformer secondary, and the other side of this transformer is brought out as the high leg. The image below shows this setup.
In some ways the high leg delta service provides the best of both worlds. A line to line voltage that is higher than the usual 208 volts that most three phase services have. A line to neutral voltage on two of the phases, sufficient for connecting appliances and lighting. Large pieces of equipment will draw less current than with 208 volts, requiring smaller wire and breaker sizes. Lights and appliances requiring 120 volts can be connected to phases A and C without requiring an additional stepdown transformer.
High leg delta is not the most common system out there but it is around. So its important for us to have some backround on how it operates. This comes into play when quoting electrical equipment used on this system, especially those of us that quote gear. Its important to read the panel and feeder schedules and ask the contractor if we’re unsure. Sometimes a call to the local utility where the job is located cant hurt either. After all information is free. There can be some different steps needed when configuring equipment for this voltage system so reach out to you reps as needed.
Thanks for reading this weeks article.
See everyone next week.