Why Does the High Leg Measure 208V Instead of 120V?

Electrical Technology

Why Does the High Leg Measure 208V to Neutral While Normal Hot-to-Neutral (L–N) Measures 120V?

In North America, 208V is commonly available in two configurations of single-phase and three-phase supply systems:

Wye-Wye (Three-phase, four-wire system): Provides both 120V and 208V, suitable for single-phase and three-phase loads.
Wye-Delta or Delta-Delta (including Open-Delta, three-phase, four-wire system): Provides 120V, 208V, and 240V for both single-phase and three-phase applications.

In a 120V/240V supply system, the voltage between a phase (hot conductor) and neutral measures 120V single-phase, while the voltage between two hot conductors (phases) measures 240V single-phase.

The question then arises: if the voltage between hot and neutral is 120V, why does the voltage between the high-leg (hot leg) and neutral measure 208V? How is this possible?

Hot to Neutral = 120V

In this system, the utility provides 240V delta as the base supply. A center tap is taken from one transformer winding, which crates a neutral point. From either end of that winding to neutral, you measure 120V.

A normal hot (A or B phase) + neutral gives you 120V single phase because of the center-tap transformer winding. The configuration is shown in the above fig.

Mathematically, voltage between A-N or B-N is equal to 120V because the 240V (line to line) is split in hot to neutral in the middle.

½ 240V = 120V – Single-Phase

High Leg to Neutral = 208V

The high-leg voltage to neutral is approximately √3 × 120V ≈ 208V. This is because the neutral point splits the phase-to-phase voltage (240V) into two 120V segments for the other two phases, but the high leg’s voltage to neutral is derived from the delta’s geometry, resulting in a higher voltage.

As shown in the above figure, the third winding of transformer (phase C) in the delta is connected to the midpoint of the opposite windings (A and B). Because of this geometry, the C phase (high-leg or wild-leg) is not symmetrically placed with respect to neutral.

When you measure from C phase to neutral, the voltage is higher because:

V_CN = (240² − 120²) ≈ 208 V

√3 240V = 208V

√3 120V = 208V

This comes directly from applying the Pythagorean theorem in a phasor diagram. That’s why the system is called high-leg delta because one leg is “higher” to neutral than the others.

Mathematically, in a high leg power distribution system:

Voltage between Hot and Neutral = ½ 240V = 120V – Single-Phase
Voltage between High leg and Neutral = ½ 240V √3 = 208V – Single-Phase
Voltage between two Hot (Line to Line) = 120V 2 = 240V – Single-Phase
Voltage between All three Hots = 240V – Three-Phase

Good to Know:

The winding center tap provides 120V on A–N and C–N.
The C phase is 120° away, so its vector to the neutral midpoint works out to ~208V.

Why it’s Designed this Way?

If we can easily obtain and use a 208V single-phase supply from a Wye-Wye 120V/208V, three-phase four-wire system, then why design a different and more complex High-Leg Delta system to achieve the same 208V supply

The key advantage of the High-Leg Delta system is that it provides three different voltage levels within a single system: 120V, 208V, and 240V. This flexibility allows it to serve both lighting loads (120V), certain equipment requiring 208V, and heavy machinery or appliances that operate at 240V.

For example, high leg delta system allows:

120V single-phase loads (line-to-neutral, but only from A or B to N).
208V single-phase loads (Hot leg (C) to neutral).
240V single-phase loads (line-to-line).
240V 3-phase loads (using all three delta phases).

The High-Leg Delta configuration typically placed the high leg on Phase B in switchboard and panelboard. On the other hand, the ANSI standard requires the high leg to be connected on Phase C in meter box, and it must be identified with orange color for proper marking and safety compliance (NEC – 110.15, 230.56, 408.3(E)(1), 409.102)(B), & 408.3(F)(1)).

Good to Know:

Generally, The “high leg” (208V to neutral) is never used for 120V loads; it’s only used as part of the 3-phase system.
High leg delta configuration can also be used for 240, 415 & 480V supply systems.
By code (NEC – 110.15), the high leg must be colored orange and must be located on “B” phase (NEC – 230.56) in panelboards (409.102)(B), and on C phase on meter box required by manufacturer to prevent accidental 120V connections. Refer to (NEC – 408.3(E)(1) and 408.3(F)(1)).
High leg must be identified using the exact value of voltage and phase with a note of caution. NEC 408.3(F)(1).

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