Grounding and Bonding — Part 2

Jul 28, 2011 1:16 PM, By Mike Holt, NEC Consultant

One of the revisions to the 2011 NEC involves a new definition for a common term: bonding jumper, supply-side. A supply side bonding jumper is a conductor on the supply side or within a service or separately derived system to ensure the electrical conductivity between metal parts required to be electrically connected (Fig. 1).

Fig. 1. A supply-side bonding jumper is a conductor on the supply side or within a service to ensure the electrical conductivity between metal parts required to be electrically connected.

The method used to size bonding jumpers depends on their location in the circuit, which can be a point of confusion. Generally speaking, if bonding conductors are:

On the load side of the service main overcurrent device, they are sized according to Table 250.122, based on the rating of the overcurrent device.
On the supply side of a service main overcurrent device or ahead of the overcurrent protection device on the secondary of a separately derived system, they are sized using Table 250.66 and the 12.5% rule in 250.102(C).

Because of the deletion of the definition “grounding conductor” from Art. 100, a revision was required in the note to 250.4(A)(1) that had previously used this term. The revised reference to bonding and grounding electrode conductors now provides a much more specific Code application. This note now makes it clear that the grounding conductor being referred to is the grounding electrode conductor, which should not be any longer than necessary.

The description of currents that aren’t considered to be objectionable has been changed in 250.6(C). Temporary currents from “abnormal conditions, such as ground faults,” aren’t to be classified as objectionable current.

The NEC previously used the term “accidental” to describe conditions that resulted in currents not deemed objectionable. Ground faults are listed in this section as an example of a condition that results in this current, but only under abnormal conditions.

Objectionable current

Objectionable neutral current occurs because of improper neutral-to-case connections or wiring errors that violate 250.142(B). The reason this current is considered objectionable is because when it flows on or through metal parts, it can cause improper operation of electronic equipment, fires, electric shock, and even death.

Fig. 2. The absence of a zero volt reference may be an indication of the presence of objectionable neutral current.

When a system is properly bonded, the voltage between all metal parts will be zero. When properly grounded, the voltage between ground and those parts is also zero. However, the absence of a zero volt reference may very well be an indication of the presence of objectionable neutral current (Fig. 2).

Objectionable neutral current will flow when the neutral conductor:

Is connected to the metal case of a panelboard that’s not used as service equipment.
Is connected to the metal case of a disconnecting means that’s not part of the service equipment.
Is connected from one system to a circuit of a different system due to wiring errors (Fig. 3).

It will also flow on metal parts if:

The circuit equipment grounding conductor is used as a neutral conductor.
The neutral conductor is connected to the circuit equipment grounding conductor on the load side of the system bonding jumper for a separately derived system.

The removal from Art. 100 of the term “grounding conductor” also had an effect in 250.8(A), which now makes it clear that its conductor termination requirements apply to bonding jumpers, equipment grounding conductors, and grounding electrode conductors.

Part II. System grounding and bonding

The 2011 NEC addresses the location of ground detection sensing equipment for ungrounded systems as well as adding marking requirements for ungrounded systems.

Fig. 3. Danger: The 120V/208V panelboard (de-energized) can have dangerous voltage from the 277V lighting circuit because of the crossed neutrals.

Ground detectors are required for ungrounded systems that operate between 120V and 1,000V [250.21]. The location of the sensing equipment has never been explicitly stated in the NEC until now, leaving some people confused as to the intent of the requirement.

With this revision, these sensing devices must be installed as close as practicable to the point where the system receives its supply. Previously, you could install this equipment at the branch circuit level. Doing so would leave all of the feeder circuits without the detection, allowing them to operate under a ground fault without any indication.

Several revisions to 250.24(C) make it easier to navigate. The Code now spells out what the rules are for sizing the grounded conductor when dealing with service conductors in a single raceway and when parallel service conductors are installed in multiple raceways. A new subsection (3) was also added to clarify that the grounded conductor of a delta-connected, corner-grounded 3-phase, 3-wire service must have an ampacity at least equal to that of the ungrounded conductors.

Separately derived systems

Section 250.30 has been reorganized and includes many revisions and notes to clarify the grounding and bonding requirements of separately derived systems (SDS). Following are some highlights:

A neutral-to-case connection must not be made on the load side of the system bonding jumper, except as permitted by 250.142(B).
An unspliced system bonding jumper must be installed at the same location where the grounding electrode conductor terminates to the neutral terminal of the SDS — either at the SDS or the system disconnecting means, but not at both locations [250.30(A)(5)].
Where the system bonding jumper is installed at the source of the SDS, the jumper must connect the neutral conductor of the SDS to the supply-side bonding jumper and the metal enclosure of the source (transformer case).
If the system bonding jumper is installed at the first disconnecting means of an SDS, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal enclosure of the disconnecting means (Fig. 4).
If the SDS and the first disconnecting means are in separate enclosures, a supply-side bonding jumper must run to the SDS disconnecting means. The supply-side bonding jumper can be a nonflexible metal raceway, a wire, or a bus.

Fig. 4. If the system bonding jumper is installed at the first disconnecting means of an SDS, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal enclosure of the disconnecting means.

If the supply-side bonding jumper is:

Of the wire type, size it per Table 250.66, based on the area of the largest ungrounded SDS conductor in the raceway or cable.
A bus with a cross-sectional area no smaller than required by Table 250.66.
Installed at the disconnecting means instead of the source, the requirements of 250.30(A)(3) apply.

Install the grounding electrode as near as practicable — preferably in the same area as the system bonding jumper [250.30(A)(4)]. Size its conductor per 250.66, based on the area of the largest ungrounded conductor of the derived system [250.30(A)(5)].

Buildings or structures supplied by a feeder or branch circuit

The exception dealing with existing installations has been clarified, and a new subsection addresses buildings supplied by an SDS.

The 2008 NEC contained a significant change to 250.32 by requiring that an equipment grounding conductor be installed for buildings or structures supplied by feeders or branch circuits. With this change, an exception added to the 2008 Code clarified that existing premises wiring systems need not comply with this new rule.

Because most NEC changes don’t come with an exception that gives exemption for existing installations, confusion ensued. For example, when the AFCI requirements were expanded, there wasn’t an exception for existing buildings, because the Code isn’t retroactive. Why not? Because, as 90.2(A) tells us, the NEC is an installation standard, not a maintenance standard.

So when the exception was added, many Code users had no idea what it really pertained to. The change in this NEC revision cycle, however, clarifies that this exception applies only to existing structures that met the previous Code requirements and continue to meet the previous requirements, such as not having continuous grounded metal paths between the two buildings or structures.

Previous NEC editions required a building or structure supplied by a feeder or branch circuit to be supplied with an equipment grounding conductor. When a building or structure is supplied by an SDS, with no overcurrent protection at the source, there’s no equipment grounding conductor by definition. This revision clarifies that when this occurs, a supply-side bonding jumper must be run to the building or structure and installed per 250.30(A).

Some other key points from all these revisions include:

To quickly clear a ground fault and remove dangerous voltage from metal parts, the building or structure disconnecting means must be connected to the circuit equipment grounding conductor, which must be one of the types described in 250.118 [250.32(B)(1)].
If the supply circuit equipment grounding conductor is of the wire type, size it according to 250.122, based on the rating of the overcurrent device.
You can’t connect the supply circuit neutral conductor to the remote building or structure disconnecting means [250.142(B)]. However, the neutral conductor can continue to serve as the ground-fault return path for the structure disconnecting means for existing installations in compliance with previous editions of the Code where there are no continuous metallic paths between structures, and some other conditions are met [250.32(B)(1)Ex].

When applying the revised requirements of Part II, it’s critical that you don’t confuse the grounding path with the bonding path — and that you watch those neutral connections carefully. As a rule, the neutral should never be connected to the enclosure or equipment grounding conductor anywhere except in the service disconnects and the secondary side of separately derived systems. Objectionable neutral current presents a real danger and can damage equipment as well as cause fires and electric shock or electrocution.

In the final part of this series, we’ll turn our attention to Parts III, IV, and VI of Art. 250.