A 120/240 V high-leg delta service hides a trap: two legs give a clean 120 V to neutral, but the third — the wild leg — sits at about 208 V to ground and will destroy any 120 V load wired to it. This calculator shows exactly what that high leg reads and reminds you of the NEC marking rule that keeps it safe.
How it works
The center-tapped winding sets the neutral halfway across one phase, leaving the opposite apex of the delta elevated:
normal legs to neutral = V_line-to-line / 2
high leg to neutral = V_line-to-line × sin(60°)
= V_line-to-line × 0.866For a 240 V delta that is 120 V on the two good legs and 207.8 V on the high leg. The three line-to-line voltages remain equal, so three-phase equipment is unaffected — the elevated reading appears only between the high leg and neutral.
Example and notes
On a 240 V delta the high leg reads about 208 V to ground. NEC 110.15 and 408.3(F) require that conductor to be phase B in the panel and marked orange so no one lands a 120 V receptacle on it. If you enter a single-pole device rating, the tool confirms whether it can survive the high-leg voltage, but the correct practice is never to connect 120 V single-pole loads to the high leg at all.
Why the high leg exists and where you find these services
The 4-wire high-leg delta was a common commercial and light-industrial service in the United States before the wye service became dominant. It arose as a practical compromise: a single transformer bank could supply both 240 V three-phase power for motors and 120 V single-phase power for lighting from the same service entrance, avoiding the need for separate transformer sets.
You still encounter it in:
- Older commercial buildings, machine shops, and auto-repair garages that pre-date widespread wye service
- Rural industrial sites where the utility only provided delta distribution
- Legacy panelboards in small manufacturing where the three-phase equipment was installed before the building was expanded with 120 V circuits
The service is fully functional for any pure three-phase load — motors, compressors, HVAC equipment — because line-to-line voltage is identical on all three pairs of conductors. The hazard is entirely on the neutral side.
The geometry behind the 208 V reading
The 208 V figure is not arbitrary — it comes directly from the geometry of an equilateral triangle. Picture the three-phase delta as an equilateral triangle, with each side representing 240 V. The center-tap of one side sits at the midpoint of that side, which by geometry is the altitude foot of the opposite vertex. The distance from that altitude foot to the opposite vertex is the height of the equilateral triangle, which equals the side multiplied by sin(60°) = √3/2 ≈ 0.866.
So: 240 × 0.866 = 207.8 V, or roughly 208 V. The same relationship applies to any delta voltage — a 480 V delta would produce a high leg of about 416 V to neutral.
NEC marking requirements in practice
The Code requires that the high leg be durably identified in any panelboard or switchboard where the neutral conductor is present (NEC 408.3(F)). The conventional marking is orange, though the Code requires only that it be identified — the orange convention comes from longstanding industry practice. When you encounter an orange-marked breaker slot or bus bar in a panel, the first thing to do before placing any 120 V circuit on it is verify the panel is not a high-leg delta service.