How many times do I have to bond the metal water piping system and structural metal electrodes? As many times as it takes.
The grounding and bonding requirements of the NEC might just be the least understood and most incorrectly interpreted section of the Code. In fact, some of the same questions pop up time and time again, including: When am I required to use a concrete-encased electrode in the grounding electrode system? When can the metal frame of a building or structure serve as a grounding electrode? Can a metal underground well casing serve as a grounding electrode? Where and how do I bond each separately derived system within the building?
In an effort to alleviate some of these misunderstandings, the Code committees agreed upon several changes to Art. 250 in its 2005 revision cycle. Although the changes noted in this article are by no means a complete list of revisions, it's a good starting point in your quest toward complete comprehension of this section.
Grounding electrode system (250.50). With respect to the mandatory use of grounding electrodes, the phrase “if available at each building or structure served” has been replaced with “are present at each building or structure served.” The intent of the new wording is clear. For new buildings or structures, it's now mandatory for you to bond the grounding system to the steel reinforcing bars or rods present in the foundation or footings and use them as a concrete-encased electrode. The good news is an exception to this rule exempts existing buildings and structures where access to the concrete encased electrode would require you to demolish or disturb existing foundations.
Since this change was implemented, many local authorities having jurisdiction (AHJs) and state agencies have notified electrical contractors working in their areas that they'll be strictly enforcing this rule on future projects. One interesting side note is that a number of localities in California and the Southwest have required rebar to be stubbed-out of footings since as far back as 1981.
As the electrical contractor, the challenge arises when the general contractor or other subcontractor pours the concrete for the foundation or footing prior to any electrical bonding connection being made to the steel reinforcing bars or rods. It's not uncommon for this to occur before you're even awarded a contract. Just as it would be difficult for an AHJ to enforce NEC rules on a GC who's not performing the electrical work, it falls on you to establish a good working relationship with the general contractor and/or concrete subcontractor to ensure any pre-work required to accommodate the use of concrete-encased electrode is sufficiently addressed from the start of the project.
One method you can use to accomplish this task is to provide the general contractor or concrete subcontractor with a drawing or sketch showing a piece of rebar (minimum ½-inch outer diameter) connected to other rebar members (in a typical footing) and stubbed out 1 to 2 feet beyond the footing surface. This connection can be easily made by the same rod-buster who is charged with the task of securing all the other rebar together prior to the concrete pour.
Grounding electrodes (250.52). Revisions to 250.52(A)(2) now clearly identify four ways in which you can use the metal frame of a building or structure as a grounding electrode:
10 feet or more of a single structural metal member is in direct contact with the earth or encased in concrete that is in direct contact with the earth.
The structural metal frame is bonded to one or more of the grounding electrodes as defined in 250.52(A)(1), (A)(3), or (A)(4).
The structural metal frame is bonded to one or more of the grounding electrodes as defined in 250.52(A)(5) or (A)(6) that comply with the requirements of 250.56.
Other approved means of establishing a connection to earth.
The intent of this additional wording to the Code is to outline some acceptable requirements for use of structural metal members as grounding electrodes — and to ensure their use as part of the grounding electrode system by properly bonding the structural metal members to other electrodes.
Additional wording was also added to 250.52(A)(7), which now allows you to use underground metal well casings as a grounding electrode. This rule change may be a blessing in disguise for the effectiveness of the grounding electrode. Consider a metal well casing, which may be 4 to 8 inches in outer diameter, driven to depths between 75 to 400 feet, in direct contact with an underground water supply and moist soil. Does it get any better than this? It may arguably be the most effective ground electrode of any of the choices listed in 250.52(A)(1) through (A)(7).
This revision permits the use of this electrode — provided that it's not effectively bonded to a metal water pipe. For many installations, where PVC water piping materials are used to connect the well casing to the structure, this may be the most effective electrode you'll ever find.
But be mindful of the requirements of 250.50 when working with well casings. If any of the electrodes listed in sections 250.52(A)(1) through (A)(6) are present, you must consider them primary electrodes and bond them together to form the grounding electrode system. In this case, the metal well casing is an electrode that supplements the primary electrodes noted in 250.52(A)(1) through (A)(6).
Separately derived systems. A single sentence addressing the bonding of separately derived alternating current systems [250.30(A)(6)] seems simple enough. It states that structural steel that forms the framework of a building, and metal water piping, both in the area of the separately derived system, shall be bonded in accordance with 250.104(D). But beware of the requirements outlined in 250.104(D). They may surprise you.
As per 250.104(D), for every separately derived system you install (mostly transformers), you'll need to ensure that the metal water piping system and structural steel members in the area served by the separately derived system are bonded together — regardless of the fact that you've already bonded them together when making your grounding electrode connections for the service grounding portion of the job. This must be done every time you add another separately derived system to the project.
One may argue that 250.30(A)(7) requires you to use only a metal water pipe electrode as specified in 250.52(A)(1) or an effectively grounded structural metal member as specified in 250.52(A)(2), but not both. Typically, you would choose the one closest to the separately derived system. Similar to lightning protection systems, best practices dictate that your most effective installation uses the shortest possible length of conductor. This section of the Code clearly does not require you to use both of these electrodes.
What you have to remember in this situation is that you're dealing with two separate issues. With respect to the grounding electrode, as specified in 250.30(A)(7), one electrode is adequate. With respect to bonding, you must do whatever section 250.104(D) instructs you to do — regardless of what you did with the electrodes as per 250.30(A)(7).
As per 250.104(D)(1), you must bond the grounded conductor of each separately derived system to the nearest available point of the metal water piping system in the area served by each separately derived system. You must make this connection at the same point on the separately derived system where you connected the grounding electrode conductor. Size each bonding jumper in accordance with Table 250.66 based on the largest ungrounded conductor of the separately derived system. The bonding requirements for structural metal [250.104(D)(2)] mirror those noted above for metal water piping systems.
Where a common grounding electrode conductor is installed for multiple separately derived systems as permitted by 250.30(A)(4), and exposed structural metal is interconnected to form the building frame or interior metal piping exists in the area served by the separately derived system, you must bond the metal piping and the structural metal members to the common grounding electrode conductor.
There are exceptions noted for all three items of 250.104(D), which can be found on page 70-108 of the 2005 NEC.
As per 250.30(A)(2), when installing a bonding jumper of the wire type with the derived phase conductors from the source of a separately derived system to the first disconnecting means, you must size it in accordance with 250.102(C), based on the size of the derived phase conductors. The requirements of 250.102(C) direct you to Table 250.66 when sizing this type of conductor.
Let's look at a sample calculation for more perspective. Suppose you're working with a 112.5kVA, 480V, 3-phase, 3-wire primary rated transformer with a 208/120V, 3-phase, 4-wire wye connected secondary. The secondary (separately derived system) features one 600kcmil THHN-insulated conductor per phase, which are connected to the 75°C terminals at the transformer. What size equipment-bonding conductor is required in this instance?
You size the equipment-bonding jumper based on the largest phase conductor (i.e. 600kcmil conductor). Referring to Table 250.66, you see that a 1/0 AWG copper conductor is required in this situation. Although there is no Code requirement for color coding or marking of this conductor, it's probably a good idea to identify it with a distinctive green or green/white marking at the terminating point as well as along the entire accessible portion of this conductor.
Owen is the owner and president of National Code Seminars in Pelham, Ala.
If present, use the concrete-encased electrode(s) in a new building or structure as one of the system grounding electrodes. If this connection isn't made before the concrete is poured, you may find yourself removing a portion of the new concrete to gain access and make the connection.
If conditions permit, you may use a driven structural metal member (driven at least 10 feet into the earth) as one of the system grounding electrodes. Make sure and follow all the requirements outlined in 250.52(A)(2).
If conditions permit, you may use a metal well casing (with no metallic water piping connected to it) as a supplemental grounding electrode. Refer to 250.52(A)(7).
Ensure the metal water piping system(s) and the structural metal member(s) that are interconnected to form the building frame, in the area served by each separately derived system, are effectively bonded to the grounded conductor of the separately derived system(s) at the same point as the connection to the grounding electrode conductor.
When installing the equipment-bonding jumper (grounding conductor) from the source of the separately derived system to the first disconnecting means, size it based on the largest derived phase conductor (from the separately derived system to the first disconnecting means). Follow the requirements of 250.102(C), and use Table 250.66 to determine the proper conductor size. See 250.30(A)(2).