Technical Article

NEC Basics: Solidly Grounded, Service-Supplied AC Systems Above 1 kV

November 01, 2023 by Lorenzo Mari

Learn about single-point and multi-grounded neutrals and service-supplied AC systems above 1 kV.

Part X of Section 250 deals with grounding alternating-current (AC) systems and circuits above 1 KV. Solidly grounded systems have the neutral connected to the ground without an intentional impedance, producing high-magnitude ground-fault currents–of several thousand amperes–that may exceed the three-phase fault current.

Employing high voltage to distribute and use electrical energy represents economies by reducing current, conductor size, and I²R losses. Lessening the current decreases voltage drop, letting equipment work properly and last longer.

Image used courtesy of Adobe Stock

Typical industry practice employs solid grounding for voltages up to 1kV, low-impedance grounding for the 1 kV - 15 kV range, and solid grounding above 15 kV.

National Electrical Code Section 250.180 General Requirements for Systems Over 1kV

Grounded systems above 1 kV must comply with sections 250.1 through 250.178. Sections 250.182 through 250.194 modify and supplement the previous sections.

National Electrical Code Section 250.182 Derived Neutral Systems

This section permits obtaining a neutral point from a grounding transformer in systems above 1 kV.

Grounding transformers produce a neutral connection on an ungrounded three-phase system, giving a path for ground-fault zero-sequence currents. They also permit the exciting current’s triple-harmonics flow in an ungrounded transformer.

Two typical configurations of three-phase grounding transformers, or three-phase grounding banks, are:

Interconnected star or zig-zag, with or without secondary winding
Wye-delta

Figure 1 shows two coils wound in each leg of the magnetic core–with the same number of turns. One is named the outer coil (zig), and the other the inner coil (zag). The phases of the ungrounded system attach to the external coils, and the internal coils form the neutral.

Figure 1. Zig-zag three-phase grounding transformer, core-type construction. Image used courtesy of Lorenzo Mari

The zig-zag connection presents a high magnetizing impedance to balanced positive- and negative-sequence currents and a much lower leakage impedance to triple-harmonics and zero-sequence currents flow.

A zig-zag transformer with a neutral connection to the ground provides a path for zero-sequence currents originating in ground faults. It has a minor impact on positive- and negative-sequence currents.

Connect a wye-delta grounding transformer like a conventional power transformer. The grounding transformer does not necessarily supply a load on the delta-connected secondary.

With balanced three-phase voltages, only the excitation current flows. The voltages induced in the secondary are balanced, and the total voltage around the delta windings is zero.

Under a single line-to-ground fault, one of the primary winding voltages collapses, inducing zero voltage in its secondary winding. The net secondary voltage is not zero anymore, allowing the flow of a large zero-sequence current in the closed delta. This zero-sequence current reflects the primary, flowing to the fault spot.

Figure 2 shows a wye-delta grounding transformer.

Figure 2. Zero-sequence current flow in wye-delta grounding transformer. Image used courtesy of Lorenzo Mari

Connecting the grounding resistor across the broken delta in high-resistance grounding will limit the fault current to the design value.

National Electrical Code Section 250.184 Solidly Grounded Neutral Systems

Solidly grounded neutral systems can be:

Single-point grounded
Multigrounded

This rule permits multi-grounded neutrals but does not oblige neutrals to be multi-grounded.

Neutral systems grounded at various points–multi-grounded–ensure keeping the connection to the ground under certain events. The resistance of multiple grounds varies approximately inversely to their number. A more significant number of links will open automatic protective devices sooner in case of failures, providing superior safety.

Figure 3 shows a multi-grounded neutral system.

Figure 3. A multi-grounded neutral system. Image used courtesy of Lorenzo Mari

It is essential to clarify that a grounded phase–like a corner-of-the-delta grounding system–is not neutral and does not meet the requirements of this section.

Section 250.184(A) Neutral Conductor

250.184(A)(1) Insulation Level

The minimum insulation level of the conductor must be 600 V.

Exception N° 1. It is acceptable to use bare copper conductors in the following locations of multi-grounded systems:

Service entrance
Service lateral
Direct-buried portions of feeders

Exception N° 2. Using bare copper conductors as neutrals in overhead portions mounted outside premises is acceptable.

Exception N°3. Using bare copper conductors as neutrals is acceptable if they are isolated from the ungrounded conductors and protected from damage.

250.184(A)(2) Ampacity

The ampacity must not be less than:

The load carried by the conductor
33.33 % of the ampacity of the ungrounded conductors

Exception: It is permitted to size the ampacity to a minimum of 20% of the ampacity of the ungrounded conductors under engineering supervision in industrial and commercial premises.

Section 250.184(B) Single-Point Grounded Neutral System

This system grounds the neutral at one point–do not make additional neutral-to-ground connections downstream. A typical point to connect the system’s neutral to the ground is at the source transformer.

The rules to observe are:

Supply the system from one of the following:

◦ A separately derived system
◦ A multi-grounded neutral system. Connect an equipment grounding conductor to the multi-grounded neutral conductor at the source.

Furnish a grounding electrode per Part III of this Article.
Following Part III of this Article, provide a grounding electrode conductor to connect the system neutral conductor to the grounding electrode.
Provide a neutral conductor only to supply phase-to-neutral loads.
Employ insulated conductors for the neutral and connect it to the earth at only one location.
Provide an equipment grounding conductor to each structure, building, and enclosure.
Run the equipment grounding conductor with the phase conductors – in feeders and branch circuits – and abide by the following instructions:

◦ Must not carry a continuous load.
◦ It can be bare, covered, or insulated.
◦ Must have enough ampacity to sustain the maximum fault current duty.

Install a bonding jumper connecting the grounding electrode conductor and the equipment grounding conductor.

Section 250.184(C) Multigrounded Neutral System

This system comprises multiple neutral connections to the ground–at the source and over extensive distances, typically outdoors.

Typical applications are distribution systems supplying several buildings and structures.

The following rules apply to this type of system:

It is permitted to ground the neutral conductor at several points simultaneously–only in solidly grounded neutral systems. The locations acceptable for grounding are:

◦ Transformers feed conductors to buildings and other structures.
◦ Underground circuits with the neutral conductor exposed.
◦ Outdoor overhead circuits–like between poles.

Connect the neutral conductor to grounding electrodes at transformers and other locations.
Install at least one grounding electrode every 400 m and connect it to the neutral conductor. Keep a maximum distance of 400 m between electrodes.
Ground the shielding of a multi-grounded shielded cable system at each joint exposed to personnel contact.

Exception: Removing the cable jacket of an uninterrupted conductor longer than 400 m is not required if the only purpose is bonding the neutral conductor to a grounding electrode.

Figure 4 shows a four-wire common-neutral multi-grounded distribution system supplying power to several buildings. This system shares the neutral between the primary and secondary distribution systems. The neutral is grounded at the distribution transformers, at 400 m intervals where there are no transformers, and at service equipment–connected to the building’s grounding electrode system.

Figure 4. A four-wire multi-grounded common-neutral system. Image used courtesy of Lorenzo Mari

National Electrical Code Section 250.186 Grounding Service-Supplied AC Systems

Section 250.186(A) Systems with a Grounded Conductor at the Service Point

Do the following when an AC system grounded anywhere has a grounded conductor at the service point:

Install a grounded conductor per sections 250.186(A)(1) through (4) and route it with the ungrounded conductors to each service disconnecting means.
Connect the grounded conductor to its terminal or bus at each disconnecting means.
Connect a main bonding jumper between each service; disconnecting means enclosure and the grounded conductor.
Compute the size of the grounded conductor(s) per sections 250.184, 250.186(A)(1), and 250.186(A)(2) and select the largest result.

Figure 5 shows a grounded conductor directed along with the ungrounded conductors to each service disconnecting means and main bonding jumpers connecting the grounded conductor to the enclosures.

Figure 5. A grounded conductor brought to each service disconnecting means. Image used courtesy of Lorenzo Mari

Exception: It is permitted to connect the grounded conductor(s) to the assembly’s common grounded conductor(s) terminal or bus when two or more service disconnecting means are located in a single assemblage listed for use as service equipment. The array must include a main bonding jumper between its enclosure and the grounded conductor(s). See Figure 6.

Figure 6. Conductor connections in a single assembly are listed for use as service equipment. Image used courtesy of Lorenzo Mari

250.186(A)(1) Single Raceway or Overhead Conductor

Abide by the following rules:

Compute the grounded conductor’s minimum size per Table 250.102(C)(1), entering with the size of the biggest ungrounded conductor or equivalent area for parallel conductors.
Its size is not required to be bigger than the largest service-entrance conductor(s).

250.186(A)(2) Parallel Conductors in Two or More Raceways or Overhead Conductors

Do the following if the ungrounded service-entrance conductors are mounted in parallel in two or more raceways or overhead conductors:

Install the grounded conductors in parallel.
Size the grounded conductor in each raceway or overhead per Table 250.102(C)(1) based on the total cross-sectional area of the ungrounded conductors in parallel.
Install a minimum size of N° 1/0 AWG. Section 310.10(G)(1) permits connecting conductors in parallel in sizes N° 1/0 AWG and larger.

Example: A three-phase, three-wire service consists of four conductors size N° 350 kcmil per phase connected in parallel. There is no need for a neutral conductor. Compute the minimum size for the grounded conductor in each conduit.

Solution:

a. Compute the total cross-sectional area of the ungrounded conductors.

4 x 350 kcmil = 1 400 kcmil

b. Enter Table 250.102(C)(1) in the line “Over 1 100” and see Note 1.

c. Compute the 12.5% of the total cross-sectional area.

12.5% x 1 400 kcmil = 175 kcmil

d. Compute the equivalent cross-sectional area per conduit.

175 kcmil / 4 = 43.75 kcmil

e. Go to Table 8, Chapter 9, and find a conductor with a cross-sectional area equal to 43.75 kcmil or the next larger.

The cross-sectional area of conductor size N° 3 = 52.62 kcmil

f. Look for the cross-sectional area of the conductor size N° 1/0 AWG, compare it with conductor size N° 3 AWG, and pick the largest.

Conductor N° 1/0 AWG = 105.6 kcmil

Pick conductor size N° 1/0 AWG as the smallest size for the grounded conductor in each conduit.

250.186(A)(3) Delta-Connected Service

The minimum ampacity of the grounded conductor for a three-phase, three-wire delta service must be equal to or larger than that of the ungrounded conductors.

250.186(A)(4) Impedance Grounded Systems

Install the impedance grounded systems per Section 250.187.

Section 250.186(B) Systems without a Grounded Conductor at the Service Point

Do the following when an AC system grounded anywhere does not have a grounded conductor at the service point:

Install and route a supply side bonding jumper with the ungrounded conductors to each service disconnecting means.
Connect the bonding jumper to the equipment grounding conductor terminal or bus at each disconnecting means.
Install the bonding jumper following sections 250.186(B)(1) through (3).

Figure 7 shows a supply-side bonding jumper routed with the ungrounded conductors and connected to the equipment grounding conductor terminals at each service disconnecting means.

Figure 7. A supply-side bonding jumper connected to the equipment grounding conductor terminals at each service disconnecting means. Image used courtesy of Lorenzo Mari

Exception: It is permitted to connect the supply-side bonding jumper to the assembly’s common equipment grounding terminal or bus when two or more service disconnecting means are located in a single assemblage listed for use as service equipment.

250.186(B)(1) Single Raceway or Overhead Conductor

Abide by the following rules:

Compute the supply-side bonding jumper’s minimum size per Table 250.102(C)(1), entering with the size of the biggest ungrounded conductor or equivalent area for parallel conductors.
Its size is not required to be bigger than the largest service-entrance conductor(s).

250.186(B)(2) Parallel Conductors in Two or More Raceways or Overhead Conductors

Do the following if the ungrounded service-entrance conductors are mounted in parallel in two or more raceways or overhead conductors:

Install the supply-side bonding jumper in parallel.
Size the supply-side bonding jumper in each raceway or overhead per Table 250.102(C)(1) based on the total cross-sectional area of the ungrounded conductors in parallel.
Install a minimum size of N° 1/0 AWG.

250.186(B)(3) Impedance Grounded Systems

Install the impedance grounded systems per Section 250.187.

Takeaways of Solidly Grounded, Service-Supplied AC Systems Above 1 kV

Solidly grounded systems have the neutral connected to the ground without an intentional impedance.
It is permitted to obtain a neutral point from a grounding transformer.
The two solidly grounded neutral systems are single-point grounded and multi-grounded.
The rules to ground service-supplied AC systems depend on the presence or absence of a grounded conductor at the service point.
A supply-side bonding jumper must be run with the ungrounded conductors to each service disconnecting means if there is no grounded conductor at the service point.

Please visit the following articles for more on Lorenzo Mari’s NEC Basics series.