In my basement, the main service panel is on one side of the house and they
had to run about 25 ft through the joists to tie the ground into a copper
water line. I need to add a couple of circuits in the basement. Is it
acceptable to run the romex through the same holes as the copper ground?
The holes are plenty big. It would save me drilling about 20 new holes.
Why not? It would make your house less weakened by not drilling new
holes. As long as you don't run the romex thru the pipe<g> there
should be no problems. Double-check your local code guy. Adding
circuits in the basement means you will have to GFCI the outlets or at
New code requires it in most cases. You might as well cover the
existing ones while you are at it. Theoretically you do not but the
inspector could decide he thinks you are making changes and have to
bring it all up to date. If you are having it inspected or not. Home
inspector could ding you on resale as well. If they are on their own
circuit then changing to a gfci breaker in the box will do it.
Then why does the U.S. electrical code consider concrete, brick or
tile walls as grounded when considering working clearance requirements
in front of panels? See Table 110.26(A)(1)
Why is a Ufer (concrete encased electrode) ground such a good ground for
http://tinyurl.com/27wg7bd and http://tinyurl.com/27bjtaa
I don't know anything about the electrical code, but I do know facts.
Go get your ohm meter and measure the resistance between a dry concrete
floor and ground. If you can't get a reading, put a few drops of water
on the concrete. Still if the slab is dry, there will be a very high
Actually I do remember part of the code, but it may just be for FL? The
rebar in the footing is all electrically tied together and serves as the
ground instead of the ground rods. That isn't too bad since the footer
is on ground that will normally be damp. But up a foot, on top of a
layer of stone, the concrete floor normally stays dry, if it's done
If you are so confident about this assessment, then grab a 12 gauge conductor
that is long enough to reach from one of the input terminals of a power meter and
touch it to the nearest concrete slab while having it wrap around your arm from
to wrist (insulated wire is okay).
If the meter is above a concrete slab, touch that terminal while having bare
You might find the results shocking. :)
I would have no problem at all taking a 12 gauge hot wire with no
breaker and touching it to a dry concrete floor. If I can find an easy
spot on the garage floor that isn't epoxy coated maybe I'll video it for
What is the point of having insulated wire wrapped around my arm? What
is that supposed to do?
Yes I may indeed! But only a fraction of the shock having one hand
grounded and accidentally touching a hot terminal with the other hand.
Reminded me of a cabin we would go to as kids. Had to plug in the
refrigerator when we got there. Then it was a 50/50 chance if it would
give us shocks when barefoot, if it did then we would turn the cord to
switch the polarity.
That test would require an expensive specialized tester. The
U.S. code says one is supposed to drive a ground rod, test it, and drive
a second if the first one doesn't show 25 ohms or less to the earth.
The instructor asked how many electricians had one of those testers. No
one out of fifty or so did. He gets similar responses where ever he
goes. People just drive two ground rods and are done according to code.
Herbert Ufer developed this during WWII in Arizona. The Army
needed a better way to ground for the bomb bunkers. I guess ground rods
would have been impractical in meeting the Army's requirements in the
. Using rebar in concrete
solved the problem. Resistance to earth was under 5 ohms even years
later. There is some info here about halfway down the page:
Dry or damp, my experience is that concrete is often worse than wood.
I have seen plenty of concrete acting electrically as if it was damp while
wood that has been on top of the same concrete in the same spot of the
same basement for years behaved as dry wood.
I would consider concrete baked to be *known* to be dry about as good as
wood that is "merely looking good and dry".
I would be more concerned about what the resistance is once the
contact area gets to be that of a human hand or a human foot. An ohmmeter
probe has contact area smaller than that of human hands and feet by about
3 orders of magnitude. And perspiration is a lot more conductive than
most tap water.
One more thing, slightly important: Getting less than 100 or 50 mA
conducting through your body does not mean that you will certainly survive
the shock. Many sources say that 100-1000 milliamps is a deadly range due
to causing ventricular fibrillation, and some say that range goes down to
50 mA. But I would not consider 40 mA safer than a few amps. I have
heard of the rare failure to survive getting shocked by 30 mA neon sign
transformers, and I don't think that survived full-current shocks by those
frightening things are all that common either.
Now, for an even more important thing: Shocks of a few milliamps can be
bad, even if you feel safe betting your life on lack of electrocution by a
The shock can still jolt you into contacting a source of a worse shock,
or more likely can jolt you into falling and/or
throwing/hitting/bumping-into things and breaking things including your
body. A 1 milliamp shock can startle a few people. A 3 milliamp shock
probably startle many people. 5 milliamps can cause involuntary muscle
contractions, and 10 milliamps usually does and also usually produces a
startling amount of outright major pain.
<I snip from here>
- Don Klipstein ( firstname.lastname@example.org)
Yes it is, but with perspiration the concrete wouldn't be dry like I
said. I think it's mostly the salt in perspiration that makes it more
Well I don't know the mA of the hundreds of shocks I've received, and
I'm not saying any of this is safe, but at least 5 times I got the 35KV
from a large color monitor. Enough for it to have arced about an inch
before it got me. And a few times accidentally touched the horizontal
output transistor with one hand and the other hand to ground. I forget
the voltage there, over 1000v and high frequency. The high frequency
really gives it a bite! I had a customer call me to repair their
pinball machine in their basement. The complaint was they were getting
shocks. This model from the 60's is among the few that used line
voltage for the coin switches. I got there and a kid with bare feet was
playing it. I pulled the plug and got him out of there. Then the
mother came down and told me it only happens when they come in from the
pool soaking wet! Holy shit! I found and repaired the short from line
voltage to the metal door, to the hinge, and to other brackets where
they were getting shocked. Then I replaced the lamp cord and plug (not
polarized) with a grounded cord. Ran a ground wire all over the machine
to almost every place a dangerous situation may develop. And before
leaving told the whole family to never touch it unless they are dry and
Either that was a capacitive source (brighter lightning-like spark) or
the current was a couple milliamps. The capacitive source is a different
story - the quantity of concern is joules. I seem to think that a
somewhat common figure for threshold of significant or high chance of
electrocution from that is 10 joules, and my father made me not store more
than 1 joule in a high voltage capacitor until I was 17 or so. I seem to
think that in a monitor or a TV set, a couple to a few tenths of a joule
is stored. (I would not bet my life on surviving whatever that actually
Sounds to me about 26-30 KV. There is a need for lower voltage - to
reduce production of X-rays and to make the X-rays easier to block with
a thick CRT face made of leaded glass.
High frequency reduces the bite. I describe that in:
- Don Klipstein ( email@example.com)
I've never seen one and don't know much about them. They're
called ground resistance testers. Prices on a quick search ran from
about $700 to over $5000. The test is from the ground rod or electrode
to the earth. I ran across this while looking: http://tinyurl.com/csqbv9
National, I think. It was one of several alternative for a long
time. There was a push to get the concrete guys to put them in in all
new construction. Electricians are supposed to use everything that's
I work on irrigation systems which are probably more prone to
lightning than say houses. We've been using the ufer ground for some
time. Besides, it's easier to stick an extra rebar in concrete than
drive a rod.
I asked because I tried using an ohm meter on my newer service. I
could't get accurate readings with any of my VOM's. My guess was that
there was some slight chemical reaction between the rod and the earth
making a sort of battery. Switching polarity of the ohm meter gave
different strange readings. Basically it looked like I had over 30k
ohms! I don't suggest others do this, but I disconected the ground wire
from the rods. Then I took an extra 30 amp breaker and ran the hot
outside. Turned on the breaker. Measured 120VAC from hot wire to
ground. Touched hot to ground rods and got a spark and the breaker
blew. I didn't need to do the math, it's well under 25 ohms! When
testing the ground in a home, I normally use a 100 watt light bulb from
hot to ground. If it's not full brightness I say it's bad. I don't
think those handy little outlet testers put enough load to really test
the ground. Maybe they do now, but back 30 years when I first bought
one I realized it's ground test was useless.
Wouldn't the electrician need to come out before the pour and bond all
the rebar together, then run one rebar out for the service? I was told
it all had to be connected, or at least real connectors all the way
around the perimeter of the footer and to the wire mesh, the little
"twist ties" the concrete guys use are not enough. Copper wire and
brass/bronze clamps in a big loop.
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