Power systems under 1,000v installed in premises wiring can be in a few different voltage and phase configurations. Examples include 240/120V, single-phase, 3-wire and 208Y/120V, three-phase, 4-wire. Many of these systems include an intentionally grounded conductor, which is often a neutral conductor connected to a neutral point. 

In grounded systems, there is a reference established to ground and the grounded conductor. Often grounding a system results in the same voltage to ground from each of the ungrounded conductors the system supplied. As an example, in a 208Y/120V system, the voltage to ground from each ungrounded phase conductor is 120V. Another example is a 480Y/277V system, in which the voltage to ground from each ungrounded phase conductor is 277V. 

There are three-phase grounded systems that produce three ungrounded phase conductors and a neutral, but one ungrounded phase conductor has a higher voltage to ground. They are generally known as high leg, delta-connected systems and can be supplied from open delta bank transformer configurations or a closed delta-connected arrangement. Open delta systems provide about 65% less kilovolt-ampere capacity than those where the delta is closed with an additional (third) transformer in the bank. Let’s look closer as some requirements in the National Electrical Code.

High leg requirements

The “high leg” in electrical systems has been referred to as the wild leg, the stinger leg, the bastard leg and so on. The NEC uses the terms 4-wire, delta-connected systems, or the simple term, high leg. 

In a three-phase, 4-wire, delta-­connected system, Section 250.26(5) indicates that the grounded conductor is connected to the neutral point of one transformer in the three-phase bank. To simplify this for better understanding, a typical 240V, high leg, delta­connected system produces 240V from phase to phase around the delta and 120V from phase to ground on the transformer that has a grounded midpoint. 

From the “B” phase of such systems, the phase-to-ground voltage is 208V. The high leg voltage results in the phase-to-ground voltage (120V) multiplied by 1.73. Because the grounded midpoint of two (A and C) of the ungrounded phases creates a 120V output, the neutral created is often used as a circuit conductor to supply 120V loads; therefore, grounding these systems is required and not optional. (See image.)

Markings

Section 110.15 requires identifying the systems’ high leg by durably and permanently marking with an orange outer finish or identified by other effective means. Section 230.56 requires high leg service conductors to be identified in the same manner. 

Section 110.15 says this means of identification shall be placed on the high leg at each point on the system where a connection is made if the grounded (neutral) conductor is present. Section 408.3(E) indicates that the high leg of a system is generally required to be the “B” phase, except for high leg connections in some metering equipment or multisection switchboards, switchgear or panelboards with a metering section, according to 408.3(E)(1) Exception. 

Often the serving utility company publishes requirements for electric service that provide direction about which conductor is the high leg through their metering equipment. The exception relaxes the “B” phase requirement in this equipment. However, once the wiring extends beyond the metering equipment, the high leg bus in downstream equipment must be the “B” phase and marked. 

Section 408.3(F)(1) now includes a specific marking requirement for switchboards, switchgear or panelboards containing a high leg AC electrical system. The enclosure(s) shall be durably and permanently field-marked with the following text: “Caution ____ phase has ____ volts to ground.”

The markings are intended to warn personnel about the higher voltage present on the equipment’s “B” phase. Qualified people working on such equipment understand that utilization equipment, such as luminaires and appliances, can be severely damaged and often destroyed by the higher voltage. 

Another important factor is the equipment installed for delta-connected, high leg systems. Panelboards, switchboards and switchgear must be rated for the voltage supplied. In this case, the equipment should bear a label indicating the rated voltage and phase configuration of the equipment. An acceptable example is 240/120V, three-phase, 4-wire. Equipment marked as 208Y/120V, three-phase, 4-wire would not be suitable for use on the high leg system.

Michael johnston