ERICO Practical Guide to Electrical Grounding - Manual (Page 19)

11
The third component of the grounding electrode system is
concrete-encased electrodes. These are usually referred to
as “rebar,” which is short for reinforcing bar (Fig. 1-28 and
1-29). Rebar is used to add strength to poured concrete
installations and by its nature tends to be an excellent
grounding electrode. This is because the rebar is
surrounded by concrete which has a lower resistivity than
the earth. This, coupled with the fact that concrete absorbs
moisture from the surrounding earth, makes the concrete-
encased electrode an excellent grounding electrode. See the
discussion on page 17 on “Ufer” grounding. The NEC
requires that the concrete-encased electrode be covered by
at least 2 inch (50 mm) of concrete and consist of at least 20
feet (6.4 m) of reinforcing bars of not less than 1/2 inch in
diameter (No. 4 rebar) located near the bottom of a concrete
footing or foundation. Contractors should look closely at
the material used for the reinforcing bars. The rebar is often
covered with a nonconductive coating, such as epoxy,
which do not make them suitable for grounding electrodes.
The NEC also permits at least 20 feet (6.4 m) of bare
copper, not smaller than No. 4 AWG, to be used as a
substitute for the rebar for a grounding electrode.
(Fig. 1-30) Connections of the grounding electrode are
critical to maintaining the integrity of the grounding
system. Section 250-115 requires that where the grounding
Chapter 1: Building and Service Entrance Grounding
HA
HB
VN
HA
HS
HC
HT
VS
VS
VF
VB
VG
VT
VV
Fig. 1-27
Fig. 1-29
Copper Wire As Concrete
Encased Electrode
Fig. 1-30
Finished
Surface
Concrete
Foundation
Foundation Rebar
(See Note Below)
CADWELD Type RR or RD
Foundation Rebar Ground Connection
Fig. 1-28
Grounding Book 4/14/99 10/5/99 6:01 PM Page 11 (Black plate)