What Is a 4 Wire Delta Connected System?
A 4 wire delta, also called a high leg delta, red leg, wild leg, or stinger, is a 3 phase 120/240 volt service where one winding of the transformer bank is center tapped to create a neutral. Two of the three phase conductors measure 120 volts to that neutral. The third measures approximately 208 volts to neutral, which is where the name high leg comes from.
The voltages you will actually measure
Phase to phase is 240 volts on all three combinations. Phase A to neutral is 120 volts. Phase C to neutral is 120 volts. Phase B to neutral is 208 volts, which comes from the geometry of the delta: the center tap sits at the midpoint of one winding, and the opposite corner of the triangle is 240 times the square root of 3 divided by 2, or about 207.8 volts, away from it.
That gives you a service that can supply 3 phase 240 volt motor loads and single phase 120/240 volt lighting and receptacle loads from one transformer bank. It was common in older commercial and light industrial buildings, especially shops with a mix of 3 phase machinery and ordinary 120 volt convenience power. Utilities are phasing it out in favor of 120/208 wye, but there are still plenty of them in service.
The code rules that go with it
Section 110.15 is the marking rule. On a 4 wire delta connected system where the midpoint of one phase winding is grounded, the conductor with the higher phase to ground voltage shall be durably and permanently marked by an outer finish that is orange in color, or by other effective means. That marking is required at each point on the system where a connection is made if the grounded conductor is also present.
Section 230.56 carries the same requirement for service entrance conductors. Section 408.3(E) is the busbar rule: in switchboards, switchgear, and panelboards supplied from a 4 wire delta connected system, the phase busbar or conductor having the higher voltage to ground shall be the B phase, that is, the middle position in an A, B, C arrangement viewed from the front.
Orange for the high leg is nearly universal in the field, but note that 110.15 permits other effective means, and some jurisdictions and older installations used red. Never assume by color alone. Meter every leg to the neutral before you land anything.
What goes wrong on these systems
The classic failure is a single pole 120 volt breaker landed on the B phase busbar. The load sees 208 volts instead of 120 and it fails immediately, usually loudly. Section 408.3(E) exists specifically to make the high leg land in a predictable place so this is avoidable, but panel schedules on old installations are often wrong.
The second issue is neutral loading. Only the two 120 volt legs carry neutral current, so the single phase load is spread across two phases instead of three, and the transformer bank is unbalanced by design. Section 220.61 governs the neutral feeder calculation and the high leg contributes nothing to it.
The third is that many modern panelboards are not rated for a high leg application, and some are marked to prohibit it. Check the panel label before ordering, and verify the interrupting rating and the bus rating for a 3 phase 4 wire delta service specifically.