Cathodic Protection Systems and the NEC

Jun 1, 2008 12:00 PM, By Whitt L. Trimble and Eddie Guidry, Fluor Enterprises, Inc.

Conflicts and solutions for complying with grounding revisions of the NEC for cathodically protected facilities.

During the 2005 National Electrical Code (NEC) review cycle, a relatively simple revision was made that created a significant change on the cathodic protection (CP) systems of petroleum and chemical facilities (plants). The 2005 NEC made it clear that there must be an interconnection of all concrete-encased steel reinforcing bar (rebar) to the facility grounding electrode system.

Fig. 1. Cable stubbed out of concrete foundation.

For facilities using copper grounding electrode systems (and also having cathodic protection as part of their corrosion mitigation and integrity programs), this revision may require a facility to change their entire cathodic protection design and operating philosophy. It will definitely require greater coordination and cooperation between the various engineering and construction disciplines.

2005 Code changes

In NEC Sec. 250.50, it is required that all grounding electrodes present be bonded together to form the grounding electrode system. The types of grounding electrodes that must be bonded together, if present, are listed in 250.52(A)(1) through (A)(6). They include: 1) metal underground water pipe, 2) metal frames of buildings or structures, 3) concrete-encased electrodes (usually rebar), 4) ground rings, 5) rod or pipe electrodes, and 6) plate electrodes. This article will focus on concrete-encased electrodes, copper ground rings, and the potential problems this may create by bonding without consideration of potential corrosion.

Prior to the 2005 edition of the NEC, the first paragraph of Sec. 250.50 started with the phrase: “If available on the premises…” all of the types of grounding electrodes listed in the preceding paragraph were required to be bonded together. More times than not, industrially oriented engineers and designers took the phrase “if available” to mean that since the concrete structures were usually poured before the electricians were onsite, then the rebar wouldn't be available. Therefore, they assumed it wasn't necessary to bond the rebar to the other grounding electrodes and make the rebar part of the grounding electrode system. In other words, the way the NEC used to be worded seemed to give the designer some latitude as to whether to bond all of the potential grounding electrode types together.

Right or wrong, this was — and still is — the typical attitude when designing plant grounding electrode systems. If you're a design engineer who tries to follow the NEC, however, removal of the words, “If available on the premises…” has a rather profound meaning. Now it becomes clear that one must bond the copper ground ring to the concrete-encased rebar (and all other electrodes), unless the rebar is in an existing structure. With a literal interpretation, all concrete structures within a process unit, with the equivalent of more than 20 feet of ½-inch (or larger) rebar in it, must be bonded to the copper ground ring and all other electrodes. This includes pump foundations, pile caps, spread footings, concrete piers, and tank bottoms. For new process units, this adds considerable initial costs and probably even more so later, due to the resulting corrosion currents.