Technical Article

National Electrical Code 2023 Basics: Grounding and Bonding Part 5

December 06, 2022 by Lorenzo Mari

Learn about the rules involving the system bonding jumpers in grounded 1 kV or less separately derived AC systems.

To catch up on Lorenzo Mari’s series on grounding and bonding, please follow these links:

A separately derived system is a source of power other than the service. It can be a standby generator, a motor generator set, an inverter, a transformer, and the like. The most common system is the transformer. NEC Section 250.30(A)(1) regulates the system bonding jumper on grounded separately derived AC systems.

What Is a Separately Derived System?

Article 100 defines the term Separately Derived System and says the following:

It is an electrical source.
It is not a service.
It has no connection(s) to the circuit conductors of other sources–except those required to comply with grounding and bonding rules.

Figure 1 shows a generator-type separately derived system. A standby generator, used in emergency or backup when a utility service interruption occurs, is a separately derived system if the regular and backup supplies do not share the grounded conductor.

Figure 1. Separately derived system. Image used courtesy of Lorenzo Mari

In this case, the transfer switch is 4-pole, shifting all three phases and the grounded conductor–the neutral.

Equipment grounding conductors, metal enclosures, metallic raceways, and the earth may connect both sources.

Figure 2 shows a generator-type system that is not separately derived. The 3-pole transfer switch shifts the three phases while the two sources share the grounded conductor.

Figure 2. Non-separately derived system. Image used courtesy of Lorenzo Mari

To summarize, in a 3-phase generator-type system:

1. Inspect the transfer switch.

2. If the transfer switch is 4-pole–switching the grounded conductor–the system is separately derived.

3. If the transfer switch is 3-pole–not switching the grounded conductor–the system is not separately derived.

In a transformer-type system, to qualify for a separately derived system, confirm that there is no jumper joining the primary and secondary windings other than grounding connections.

It is typical to have a connection from the service-grounded conductor through the main bonding jumper and equipment grounding conductor to the transformer’s frame. Also, it may have a link from the transformer’s structure to the neutral of the separately derived system through the system bonding jumper.

Refrain from mistaking a strap placed to bond the transformer core to the case with the system bonding jumper. See Figure 3.

Figure 3. A strap bonds the transformer core to the casing. Image used courtesy of Schneider Electric

A remarkable fact is that an autotransformer cannot be a separately derived system because the primary and secondary windings have a common conductor.

Section 250.30. This section takes care of the grounding rules for separately derived AC systems.

It requires that separately derived systems comply with sections 250.20, 250.21, or 250.26, as appropriate. Moreover, they must comply with sections 250.30(A) for grounded systems or 250.30(B) for ungrounded systems.

This section also states that multiple comparable power systems connected in parallel, creating one supply system, shall be considered a single separately derived system and installed in conformity with Section 250.30.

Section 250.30(A) Grounded systems. A grounded, separately derived system will follow sections 250.30(A)(1) through 250.30(A)(8).

This section prohibits additional connections of the grounded conductor to the ground or grounded bodies on the load edge of the system bonding jumper, except under particular rules in other sections of Article 250.

The incorrect practice of duplicating the system bonding jumper by placing it on the source and the first disconnect means can create a current parallel to the grounded conductor through non-current-carrying metallic elements. This fact represents a potential danger for people. Do it only in buildings supplied by outdoor separately derived systems avoiding parallel paths to the grounded conductor.

250.30(A)(1) covers the system bonding jumper. The main rules are:

It must be unspliced.
It must conform with 250.28(A) through (D).
It should be positioned at a sole point between the source and the first disconnecting means or overcurrent protection device.
It may be located at the source of a system without disconnecting means or overcurrent protection devices, per sections 250.30(A)(1)(a) or 250.30(A)(1)(b).
It should stay within the originating enclosure.
If located outside the building, it must comply with section 250.30(C) “Outside Source.”

Figures 4 and 5 show the installation of system bonding jumpers between the source and the first disconnecting means or overcurrent protection device. Figure 4 shows the connection at the first overcurrent protection device and Figure 5 at the source.

Figure 4. System bonding jumper at the first disconnecting means of a generator-type separately derived system. Image used courtesy of Lorenzo Mari

Figure 5. System bonding jumper at the source of a transformer-type separately derived system. Image used courtesy of Lorenzo Mari

Exception N° 1 permits connecting a single system bonding jumper at the tie point of arrangements installed in line with Section 450.6, “Secondary Ties.”

It usually applies to two separately derived systems supplying double-ended equipment with both neutrals joined. See Figure 6.

Figure 6. Single system bonding jumper at the grounded conductor tie point. Image used courtesy of Lorenzo Mari

In this scheme, a fault on the primary system will not interrupt the service to the loads.

It is convenient for places where continuity of supply is critical, such as high-rise office buildings, hospitals, and institutional buildings–mainly using three-phase service.

Exception N° 2 permits the installation of two system bonding jumpers at the source and first disconnecting means in buildings supplied from an outdoor separately derived system. The prerequisite is to avoid establishing a parallel path to the grounded conductor. This exception does not consider the earth as a parallel path.

Figure 7 shows two system bonding jumper connections without establishing a current path parallel to the grounded conductor. Notice that the route through the earth does not apply as a parallel path.

Figure 7. System bonding jumpers in two locations. Image used courtesy of Lorenzo Mari

250.30(A)(1)(a) requires the system bonding jumper–when installed at the source–to connect the grounded conductor to the:

Supply-side bonding jumper.
Normally non-current-carrying metal enclosure.

Figure 8 shows the ground-fault current circuit with a system bonding jumper installed at the source.

Figure 8. Ground-fault current circuit with bonding jumper at the source. Image used courtesy of Lorenzo Mari

250.30(A)(1)(b) requires the system bonding jumper–when installed at the first disconnecting means–to connect the grounded conductor to the:

Supply-side bonding jumper
Metal enclosure of the disconnecting means
Equipment grounding conductor(s)

Figure 9 shows the ground-fault current circuit with a system bonding jumper installed at the first disconnecting means.

Figure 9. Ground-fault current circuit with bonding jumper at the first disconnecting means. Image used courtesy of Lorenzo Mari

Key Takeaways of System Bonding Jumpers

A separately derived system is a source of power – not a service.
The most common separately derived system is the transformer.
Separately derived systems do not share any conductor with other supplies. Transfer devices must not transfer the grounded conductor.
Install the system bonding jumper at only one place between the source and the first disconnecting means or overcurrent protection device.
A building supplied from an outdoor separately derived system may have two system bonding jumpers if there are no parallel paths to the grounded conductor.