NEC 250.20 for the Journeyman Exam: Which Systems Must Be Grounded, Ungrounded Systems and the Main Bonding Jumper

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Where this fits

Part II of Article 250 answers the first grounding question: is this system grounded at all, and which conductor? Transformers and generators that create a new system are on the separately derived systems page. Electrodes are on the grounding electrode system page. Conductor sizes are on the grounding conductor sizing page.

250.20: which AC systems must be grounded

Under 50 V. Grounded when supplied by a transformer whose supply system is more than 150 V to ground, when the supply is ungrounded, or when run outdoors as overhead conductors.

50 V to 1000 V. Grounded in any of these three cases:

1. The system can be grounded so the maximum voltage to ground on the ungrounded conductors is not more than 150 V.
2. A 3-phase, 4-wire wye system where the neutral is used as a circuit conductor.
3. A 3-phase, 4-wire delta system where the midpoint of one phase winding is used as a circuit conductor.

Over 1000 V. Different grounding requirements apply. This page covers systems through 1000 V.

Worked example: grounded or not?

System	Must it be grounded?	Which case
120/240 V single phase, 3-wire	Yes	120 V to ground (case 1)
208Y/120 V, 3-phase, 4-wire	Yes	120 V to ground (case 1), and the neutral is used (case 2)
480Y/277 V with neutral used	Yes	Case 2
240/120 V high-leg delta, 4-wire	Yes	Midpoint used as a circuit conductor (case 3)
480 V, 3-phase, 3-wire delta	No	No neutral, cannot be grounded to 150 V or less

For the 480 V delta, test each case. Grounding one phase would leave the other two phases at 480 V to ground, so case 1 fails. It has no wye neutral and no delta midpoint used as a circuit conductor, so cases 2 and 3 fail too. It may still be grounded, for example corner grounded or through a high-impedance ground. It is just not required.

250.21: when the system is not required to be grounded

• Examples not required to be grounded. Electric systems used only for industrial electric furnaces, separately derived systems used only for rectifiers that supply adjustable-speed industrial drives, and other systems that 250.20(B) does not require to be grounded. Health care isolated power systems are a different category: other provisions, including 250.22 and Article 517, require them to stay ungrounded.
• Ground detectors. An ungrounded AC system covered by 250.21(B), operating from 120 V through 1000 V, has ground detectors, with the sensing equipment connected as close as practicable to where the system receives its supply.
• Marking. The source or first disconnecting means is legibly marked as an ungrounded system, with the operating voltage between conductors. The label is durable enough for the environment.

250.24: grounding a service-supplied system

• Where the grounding electrode conductor connects. At any accessible point from the load end of the service drop or service lateral up to and including the terminal or bus where the grounded service conductor connects at the service disconnecting means.
• No load-side grounding connection. After the service disconnect, the grounded conductor is not connected again to the equipment grounding conductors or to grounding electrodes, except where Article 250 allows it.
• The grounded conductor comes to the service. Even if the building has no line-to-neutral loads, the grounded conductor is run to each service disconnecting means and bonded to the enclosure. Its minimum size is from Table 250.102(C)(1).

250.26: which conductor gets grounded

System	Grounded conductor
Single phase, 2-wire	One conductor
Single phase, 3-wire	The neutral
Multiphase with one wire common to all phases	The common conductor (the neutral of a wye)
Multiphase with one phase grounded	One phase conductor (corner grounded delta)
3-phase, 4-wire delta with the midpoint of one phase winding used as a circuit conductor	The midpoint conductor (the neutral), not the high leg

250.28: main and system bonding jumpers

• Material. Copper, aluminum, copper-clad aluminum or another corrosion-resistant material.
• Function. At the service, the main bonding jumper connects the grounded service conductor to the equipment grounding conductors and the service enclosure. A system bonding jumper does the same job for a separately derived system.
• Form. A wire, bus, screw or similar conductor. A bonding screw has a green finish that is visible with the screw installed.
• Size. A wire-type jumper is not smaller than Table 250.102(C)(1). Where the supply conductors are larger than the table covers, it is not less than 12.5 percent of the area of the largest ungrounded supply conductor, or of the equivalent area of parallel conductors.

250.36: impedance grounded systems (480 V to 1000 V)

A grounding impedance, usually a resistor, limits ground-fault current to a low value. It is permitted for 3-phase AC systems of 480 V to 1000 V where:

• Maintenance and supervision ensure that only qualified persons service the installation.
• Ground detectors are installed on the system.
• Line-to-neutral loads are not served.

Check yourself

1. Must a 208Y/120 V system be grounded? Yes.
2. Must a 480 V, 3-wire delta be grounded? No, but it may be, and if not grounded it needs ground detectors and marking.
3. In a 240/120 V high-leg delta, which conductor is grounded? The neutral from the center tap.
4. Can a high-impedance grounded system serve line-to-neutral loads? No.

FAQ

Sources checked October 2026

• PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
• NFPA 70, National Electrical Code, 2026 Edition, Article 250 Part II