250.50 Concrete-encased Electrode and the 2005 NEC Changes

For many years the concrete-encased electrode (ufer) has been a topic of much discussion among electricians, inspectors, and engineers. It seems the terminology of the wording “if available” as used in article 250 has contributed to an inconsistent understanding, intent, and enforcement of this code section. Concrete-encased electrodes normally have low resistance and are considered very reliable in areas with sandy type soil conditions like Florida.

Going back to 1942 in Arizona, the concrete-encased electrode installed in 24 buildings was tested by a man named H.G. Ufer. Hence the name “Ufer” is commonly used for the term concrete-encased electrode. The resistance values were tested bimonthly over an 18 year period in two locations. The first was near Tucson, Arizona, which is normally hot and dry with sand and gravel soil. The second location was near Flagstaff, Arizona, where the soil conditions are clay, shale gumbo, loam with some soft limestone. In 1960 the maximum reading was 4.8 ohms and the minimum was 2.1 ohms. The average value of the 24 installations was 3.57 ohms. Based on this 18 year test period and report in an IEEE Conference Paper, CP-61-978 by H.G. Ufer, CMP-5 accepted a concrete-encased electrode that shall consist of at least 20 ft of bare copper not smaller than #4 AWG encased in 2 inches of concrete located near the bottom of the footing or foundation.

To bring things closer to home, in a north central Florida county back in 1998, an analog earth resistance tester was used at several different job locations in different sub-divisions through out the county. A continuous 20 ft length of 5/8 rebar, with 2 ft turned up from the footing to allow the GEC to be attached outside the footing, the maximum reading was 7.8 ohms, and the minimum reading was 3.2 ohms. In contrast, tests were made on electrical services with only the GEC attached to one 8 ft ground rod driven at or just below grade. The readings ranged from a minimum of 400 ohms to a maximum of 10,000 ohms. The average readings seemed to be around 1850 to 2000 ohms.

In 2005, Article 250.50, the word “available” was removed and replaced with the word “present”. This is consistent with the requirements of the NEC Manual of Style 3.2.1, which states, “The NEC shall not contain references or requirements that are unenforceable or vague”. This clarification should help us to understand that since the concrete-encased electrode (ufer) is present because it is an inherent part of building construction, it shall be used as part of the grounding electrode system. A new exception was also added to exempt existing buildings or structures since the rebar in the footer or foundation is not required to be made available. This is consistent with the intent of a formal interpretation.


NEC Formal Interpretation 78-4
Reference: Article 250.50
Question: Is it the intent of 250.50 that reinforcing steel, if used in a building footing, must be made available for grounding?
Answer: No.
Issue Edition: 1978
Reference: 250-81
Issue Date: March 1980

Since the purpose of the grounding electrode system according to 250.4(a)(1) is to make connection to earth to:
1. Limit voltage imposed by lightning
2. Limit voltage imposed by line surges
3. Limit voltage imposed by unintentional contact with higher-voltage systems
4. Stabilize voltage to earth during normal operation.

Based on the proven test results, and the reliability of the concrete-encased electrode, we should all make a special effort to ensure as intended by 90.1; “The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity”.

Mike, thanks for that excellent information. I will surely reference this post again and again. I am also going to send people to it, that have questions on the subject.

Thanks, the article was also sent to Building Officials Association of Florida to maybe be put in the Newsletter.

I'm not sure I like the intent of this new change. A concrete-encased electrode can be present but not available, so what do we do in this situation? If bonding the rebar to the GEC isn’t necessary for "minimum" safety enough to require it for existing buildings, why is it anything but a design option at all?

The codes allowance of optional electrodes implies that any will suffice for grounding purposes, but with this change, it implies preference to the concrete-encased electrode.

I too have read many studies on the low resistance qualities of "Ufer" electrodes, however I have never really seen any significant data on how that lower resistance provides the benefits of those items listed in 250.4(A)(1). I don't believe there is any evidence that a structure with a concrete-encased electrode is any more protected against lightning or any other high energy event verses one with only ground rods.

I can't think of a more reliable grounding electrode that would be copmmonly used in Florida. Water lines are virtually always plastic, a couple rods stuck in the sand are not going to do much and I haven't seen a residential builder willing to run a ground ring.
If nothing else, at least the Ufer will create a ground plane under the house and bond all the concrete surfaces. With the current building code this becomes a Faraday cage around the house if it is the typical Fla CBS.
All the code is saying is that we should be using this steel in new construction, not that we have to chip up poured concrete.

Well, I think you just both hit the nail on the head.

Yes, the purpose of the change is to clarify that the intent of the code is to have the concrete encased electrode on any new building. So, this is telling the builder and the electrical contractor that the footer steel WILL be utilized. As always it is up to the contractor to comply, or depending upon the jurisdiction there may be consequences if the same contractor does not utilize the UFER (repeatedly, time and time again).

FYI-

UL listed rebar ground clamps can be found at www.bptfittings.com/catalog/rb-16.shtml

Build three structures next to each other and install a typical electrical service to each. For the first structure, a concrete-encased electrode is used with a measured resistance of 6-ohms. The second structure gets a single ground rod measured at 250-ohms. The third structures has no grounding eloectrode system.

Monitor the three systems for 10 years using a double blind method. I would be willing to bet that a trained professional would not be able to match the data with each structure.

Basically, there would be no discernible difference in the operation or safety of each system.

Don't get me wrong. I am a consumer advocate. I proposed expaned use of GFCI, AFCI, and even voltage drop requirements for the 2005 NEC. There are many better ways the purpose of our code can be met without adding more conservative grounding methods.

Bryan is a man who has never had the snot knocked out of him by standing on a terazzo floor barefooted and touching the frame of the fridge with a marginal GES. (my first night in this home)
I want ZERO ohms if I can get it and I certainly want the floor bonded to the GES. You get that with a Ufer.
BTW day 2 was spent driving new rods, running a new GEC, bonding abandoned pipe in the slab and anything else I could think of to get closer to that zero number.

I think like it or not, and whatever our personal opinions are,

the UFER is here to stay.