Search
Home / Technical Articles / Protection / What does bonding (bonded) mean and what does NEC article 250 say about it?

The National Electrical Code (NEC) covers grounding and bonding in several articles, but the primary coverage is in Article 250. Typical commercial electrical systems are grounded systems.

Bonding
Bonding (photo credit: publicism.info)

Within the system, some things are grounded… some things are bonded. The terms grounding and bonding are used throughout the NEC. Bonding and grounding requirements overlap somewhat, but they all have a common interest – that of preventing electrical shocks and electrical fires.

There is a distinct difference between grounding and bonding. Each serves a different purpose. However, in most cases equipment that is grounded is also bonded by being connected together.


What Does Bonded (Bonding) Mean?

Bonded (bonding) is defined in NEC Article 100 as: Connected to establish electrical continuity and conductivity. See Figure 1 below which shows two metal boxes bonded together with the metal raceway installed between the two boxes.


Bonding Conductor (Jumper)

Bonding conductor or jumper is a reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected. This definition was expanded in the NEC to include bonding conductors as these conductors are often 20 ft (6 m) or longer as used in NEC Chapter 8.

Figure 1 shows two metal boxes bonded together by a conductor installed in the raceway. This bonding jumper would be installed if the raceway were nonmetallic.

Two methods for bonding the metal parts together
Figure 1 – Two methods for bonding the metal parts together. One method is using the metal raceway. The second method is to install a separate equipment grounding conductor between the two metal boxes, as would be required if a nonmetallic raceway had been used. An equipment bonding jumper must be sized large enough to handle any fault current that it may be called on to carry.

Bonding Jumper, Equipment

Equipment bonding jumper is the connection between two or more portions of the equipment grounding conductor. A conductor installed in the raceway between the two boxes in Figure 1 would be classified as an equipment bonding jumper.


Bonding Jumper, Main

Main bonding jumper is the connection between the grounded circuit conductor and the equipment grounding conductor at the service. A main bonding jumper is clearly shown in Figures 1 and 3.

Terminology of service grounding and bonding
Figure 2 – Terminology of service grounding and bonding (click to enlarge)

Figure 3 below illustrates a main bonding jumper that is installed at the service equipment and a system bonding jumper that is installed for separately derived systems. As can be seen, these bonding jumpers perform identical functions: providing a low-impedance path for ground-fault currents to return to their source.

Effective Ground-Fault Current Path is an intentionally constructed low-impedance conductive path designed to carry fault current from the point of a ground fault on a wiring system to the electrical supply source.

Bonding Jumper, Supply-Side:

Bonding jumper supply-side is a conductor installed on the supply side of a service or within a service equipment enclosure(s), or for a separately derived system, that ensures the required electrical conductivity between metal parts required to be electrically connected. This definition was added to the NEC.

It describes a conductor used to bond metallic parts of services and separately derived systems together on the supply side of an overcurrent device.

Effective Ground-Fault Current Path. An intentionally constructed low-impedance conductive path designed to carry fault current from the point of a ground fault on a wiring system to the electrical supply source.
Effective Ground-Fault Current Path. An intentionally constructed
low-impedance conductive path designed to carry
fault current from the point of a ground fault on a wiring system
to the electrical supply source.

Because no overcurrent device is located at this position, sizing of the Supply Side Bonding Jumper is selected from NEC Table 250.66. Specific reference is made in NEC 250.30 for separately derived systems and in NEC 250.102(C) for services.


Bonding of Electrically Conductive Materials and Other Equipment

Normally non-current- carrying electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.* NEC 250.(A)(4)


Bonding of Electrical Equipment

Normally non-current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path* 250.4(A)(3)

Why Bond?

NEC 250.4(A)(3) states that Normally non-current-carrying conductive materials enclosing electrical conductors or equipment, or forming part of such equipment, shall be connected together and to the electrical supply source in a manner that establishes an effective ground-fault current path.

This refers to the metal race-ways, metal cables, boxes, panelboards, nipples, transformers, and any other electrical materials and equipment related to the electrical installation.

Main bonding jumper installed in service
Figure 3 –  Main bonding jumper installed in service. System bonding jumper installed in separately derived system. Both provide a path for ground-fault current to return to source

NEC 250.4(A)(4) states that Normally non-current-carrying electrically conductive materials that are likely to become energized shall be connected together and to the electrical supply source in a manner that establishes an effective ground- fault current path.

This refers to items not directly associated with the electrical system, such as metal frames of buildings, metal gas piping, metal water piping, and similar metallic materials.

After proper grounding has been taken care of, it is also the responsibility of the electrician to make sure that all conductive materials are properly bonded together. This includes all metallic piping, sheet metal ducts, metal framing, metal partitions, metal siding, and all other items that could come in contact with electrical system wiring or circuits and with a person or animal.

An electrician makes sure that all electrical raceways, panelboards, and other electrical equipment are prop- erly grounded and bonded in conformance to the NEC.

The issue of grounding and bonding ductwork and metal siding is another problem. For the most part, these building components become grounded and bonded to the electrical system by chance. These components are installed by other trades.

Ideally, except in special cases where isolation is necessary, bonding of all conductive materials should be the goal.


Grounding Service NEC Code 250.24(A)(1)

Electrical code expert Mike Holt discusses terminating the grounding electrode conductor and 2011 Code compliance with 250.24(A)(1).

Grounding vs Bonding


References //

  • National Electrical Code (NEC) Eldition 2014
  • Electrical Wiring, Commercial by Ray C. Mullin, Phil Simmons

SEARCH: Articles, software & guides

Premium Membership

Premium membership gives you an access to specialized technical articles and extra premium content (electrical guides and software).
Get Premium Now ⚡

About Author

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

2 Comments


  1. David E Leigh
    Jul 22, 2017

    The concept of “direct injection of noise” is mentioned here and in the business of Intrinsic safety. It really is just a concept the validity of which can be debated until tests show it to be true. My personal opinion is over the 35 years of investigations that there is no truth in it.


  2. Francis Griffith
    Jul 21, 2017

    Edvard, This article needs to be expanded to cover ‘bonding & earthing (grounding)’ for EMC purposes.
    At the moment the article talks about SAFTETY earthing, but, what about FUNCTIONAL earthing?
    Although the two are very close in purpose – minimization of potential differences , the frequencies involved are different so require different understanding and approach.
    One example is EMC earthing of VSD motor cables, here the armour(braid) – maybe conduit, forms the FUNCTIONAL earth – this is a primary function, SAFTEY earthing comes about as a second effect. Also the order in terminating is very important for VSD motor cables Motor PE – armour(shield/braid)/conduit – VSD PE IN THAT ORDER. Then cross-bond to cubicle earth. If we follow normal SAFETY earthing the armour(shield/braid)/conduit would be connected directly to the cubicle earth bar- no good direct injection of ‘noise’ straight into the earth system.
    So some discussion of where FUNCTIONAL earthing marries up with SAFTEY earthing without causing discussions as to the right way.

Leave a Comment

Tell us what you're thinking... we care about your opinion!