A high leg is a specific conductor in a three-phase delta transformer system that delivers approximately 208 volts to neutral instead of the standard 120 volts. Its purpose is to provide three-phase 240-volt power alongside single-phase 120-volt power from a single transformer setup, making it a cost-effective way to serve buildings that need both. You’ll find high-leg delta systems most often in older commercial buildings, rural installations, and facilities like machine shops or small factories across North America.
How a High-Leg Delta System Works
In a standard high-leg delta configuration, a transformer produces three hot wires (phases A, B, and C) plus a neutral. The neutral is created by center-tapping one of the transformer windings, which splits that winding into two equal halves. This center tap gives you 120 volts between phases A and C and the neutral, just like the familiar circuits in a residential panel.
The third phase, designated as B, sits at a different electrical position relative to that center tap. Because of the geometric relationship between the three phases in a delta arrangement, the voltage from phase B to neutral works out to 120 volts multiplied by the square root of 3 (roughly 1.732), which equals approximately 208 volts. Meanwhile, the voltage between any two hot phases remains 240 volts, just like the other legs. This is what makes phase B the “high leg”: it produces a higher-than-expected voltage when measured to neutral.
What Each Voltage Level Powers
The beauty of this arrangement is its versatility. A single transformer bank delivers three distinct voltage levels for different types of equipment:
- 120 volts (phase A or C to neutral): Standard single-phase power for lighting, receptacles, office equipment, and control circuits.
- 240 volts (any phase to phase): Single-phase power for larger loads like HVAC compressors, welders, or water heaters.
- 240 volts (all three phases): Three-phase power for motors, industrial machinery, and heavy commercial equipment.
The 208-volt high leg itself can be useful for certain loads rated for that voltage, but it cannot safely feed standard 120-volt equipment. Its primary role in the system is completing the three-phase circuit so that 240-volt three-phase motors and equipment can run, while the other two phases handle everyday 120-volt needs through the neutral connection.
Why It Exists Instead of Other Systems
High-leg delta gained popularity because it’s economical for utility companies and building owners. Supplying a small commercial building with both single-phase 120-volt power and three-phase 240-volt power normally requires either a more expensive wye transformer or additional transformer banks. A high-leg delta system accomplishes both with a simpler transformer arrangement, which is why utilities historically favored it for rural areas and smaller commercial customers where the load didn’t justify more complex infrastructure.
Modern construction tends to favor 120/208-volt wye systems, which provide consistent 120-volt access on all three phases without any “wild” legs. But high-leg delta systems remain extremely common in existing buildings, and electricians regularly encounter them during renovations, panel upgrades, and troubleshooting.
Identification and Code Requirements
Because the high leg looks identical to the other two phases at a glance but carries a very different voltage to neutral, proper identification is critical. The National Electrical Code (NEC Section 110.15) requires the high-leg conductor to be marked with a durable orange finish or another clearly effective method. This orange marking must appear at every connection point in the system where the neutral is also present.
The NEC also requires (Sections 408.3(E)(1) and 409.102(B)) that the high leg always terminate on the “B” phase position in panelboards and load centers. This isn’t arbitrary. Placing it consistently in the B position means electricians across the country know exactly where to expect the 208-volt conductor, reducing the chance of accidentally wiring a 120-volt circuit to the wrong phase.
What Happens When Wiring Goes Wrong
Improper wiring is the most common and most damaging risk with high-leg delta systems. If someone accidentally connects a 120-volt load to the high leg, that equipment receives approximately 208 volts instead. The results are often immediate: blown electronics, destroyed lighting, failed control circuits, and tripped breakers. Standard receptacles, building controls, and programmable logic controllers are all at risk.
In one documented case, control cabinet power was mistakenly fed from the high leg during a panel expansion. Sensitive electronics designed for 120 volts were exposed to roughly 208 volts, which triggered repeated nuisance trips and corrupted the control logic. These failures can be expensive to diagnose because the wiring may look correct at first glance if the orange identification was missing or faded.
Incorrect phase assignment in panelboards can also cascade into broader problems: blown fuses, damaged transformers, and unexplained equipment failures throughout a building. This is precisely why the orange marking requirement exists and why experienced electricians always verify phase voltages with a meter before working on an unfamiliar panel, especially in older buildings where original labeling may be long gone.
The Voltage Math Behind the High Leg
If you’re curious why the number lands at 208 volts specifically, it comes down to how three-phase power works geometrically. The three phases in a delta system are spaced 120 degrees apart electrically. When you center-tap one winding to create a neutral, phases A and C each sit 120 volts away from that neutral in opposite directions. Phase B, however, is offset at an angle that places it farther from the neutral point.
The exact distance is calculated by multiplying the phase-to-neutral voltage (120V) by the square root of 3, which is approximately 1.732. That gives you 120 × 1.732 = roughly 208 volts. This isn’t a defect or an accident. It’s an unavoidable consequence of tapping a neutral from only one winding of a three-phase delta transformer. The other two phase-to-phase voltages remain perfectly balanced at 240 volts.