Replies

1. 
   Luka | Aug 21, 2002 04:30am | #1

   I would not count on the post in concrete as ground.

   The required size for grounding rods varies with your location.

   When I put my own 200 amp electrical service in here, the required grounding was two bars - 5/8 inch by ten foot rebar, driven in to leave 4 inches showing for clamping purposes. They had to be 6 feet from each other.

   For your purpose, I am sure you don't need the same as a full service box. But I would go with at least an 8 foot 1/2" rebar.

   I'm not an electrician. I'm sure the forlorn dood, or one of the other real electricians, will be along any minute to correct me, and offer better advice.

   : )

   Quittin' Time

1. 
   toddlen1875 | Aug 21, 2002 04:39am | #2

   i would use a real ground rod .  and for the love of god ,  make sure that exterior  receptacle is gfi protected.  

2. 
   4Lorn1 | Aug 21, 2002 06:03am | #3

   Yes and no.

   Your panel is grounded, if it is properly installed, through the neutral/ground bar which is grounded by the neutral connection back to the transformer, the ground system connected to that transformer (Usually every transformer and second pole has a ground rod.) and to a lesser extent the ground rod at the end of the GEC (Grounding Electrode Conductor). These systems work well because they are all connected together, have many points of contact and are dispersed over large areas.

   Due to the vagary of ground rod installations, variable moisture content, soil chemistry and make up ground rods are not considered to be reliable ground points as far as equipment grounding is concerned. The NEC specifically disallows the use of earth grounds, rods, wells and Ufer grounds as the sole method of grounding for a branch circuit.

   Can you get a ground point by hammering in a ground rod? Yes. In the field I have sometimes stobbed in a ground rod to derive a 120v source to run tools on a site that has higher voltage three phase power. Is it reliable? No. Grounds are there to save your life, equipment and allow the timely tripping of the breaker. An unreliable one has the potential to be more dangerous than a circuit known not to have one.

   Run a new cable. This might not be so hard as it sounds as you can, check with the AHJ, run a UF cable at 12" underground as long as the cable is protected by a GFI. If you can't GFI it or meet the other requirements run it at 18" in PVC conduit. Without the conduit its 24".

   At least GFI the circuit, before it goes underground preferably, and use only ungrounded receptacles on the run. These have only the two vertical slots so a three prong plug will not work. They won't ground plugged into them but the GFI will provide some protection if the current runs astray and the lack of a ground hole will alert people about the lack of the ground connection and prevent using equipment that needs a ground to safely operate. Don't use an adapter plug to get around this.

   I would go for plan 'A'. Safe, effective and fairly easy to do.

1. 
   rjw | Aug 21, 2002 06:25am | #4

   The NEC specifically disallows the use of earth grounds, rods, wells and Ufer grounds as the sole method of grounding for a branch circuit.

   Ufer grounds? Typo? Or just another area of my ignorance exposed?

1. 
   CPopejoy | Aug 21, 2002 08:07am | #5

   To Bob-- A Ufer ground is a length of rebar in a concrete footing or foundation.  It's named after a Mr. Ufer who developed it during WWII as a means to effectively ground munitions bunkers in the American Southwest, where conventional ground rods provided poor connection to the dry, sandy soil.  Static electricity buildup or lightning is bad news around bombs.  The Ufer electrode (20 feet of 1/2" or larger rebar, or 20 ft of #4 bare copper wire (solid) placed about 4" above the earth in the concrete footing will do it.  Makes for a very low resistance connection to earth potential. 

   To the original poster--the idea behind the grounding socket in an outlet is to provide an independent, low resistance path back to the breaker panel ground bar.  If there's a short in an appliance (or tool or light fixture), where a current-carrying wire touches the tools or fixtures metal case, the equipment grounding conductor (the ground wire) will allow enough current to flow back to the panel so that the circuit breaker will trip, and quickly.  For a 20 amp circuit breaker, this will take 60 to 80 amps for a tenth of a second.  If you connect that ground pin in the outlet to a ground rod at the lightpost, there's no way that the earth will conduct enough current back to the main panel to trip the circuit breaker.  So, DO NOT use a separate ground rod or source; you have to run a new cable, one with a grounding conductor.   This is true for any branch circuit. 

   For a feeder from the service panel in one building to a subpanel in another building, with no metallic paths between the buildings, you are allowed by Code to establish another ground point of reference at the subpanel and not run an equipment grounding conductor between the panels.  This is because the subpanel branch circuits are protected against ground faults by the equipment grounding conductors that run back to the subpanel ground bar.

   In other words, the point is that the ground terminal on outlets, etc must be solidly connected by a low impedance path (like a wire or metal conduit) to the grounding point of reference at the panel from which the branch circuit originates, so the breaker in that panel can clear any ground fault.  Just connecting the outlet's ground terminal to earth potential somewhere is no good.

   Good luck.

   Cliff

2. 
   4Lorn1 | Aug 21, 2002 08:07am | #6

   Ufer, I'm not sure of the spelling but I think I'm close, is essentially a metal ground rod or grid that is encased in concrete. Tying to the reinforcing bars in a large enough size slab is essentially the same as a Ufer, capitalized because my understanding is that it is named after the guy who did the research, ground. The concrete holds water and remains alkaline and so it maintains a better electrical connection with the earth than a plain piece of metal would.

   Looks like CAP beat me by a minute or so, the time it took me to compose my reply, and he seems to have more information than I do On Mr. Ufer. I vaguely remember some of what he said, sounds right, but don't remember many of the details.

   Edited to add the last block.

   Edited 8/21/2002 1:13:30 AM ET by 4LORN1

1. 
   user-178115 | Aug 21, 2002 08:40am | #7

   4LORN1:

   Asking the same question "Am I Grounded?", but in a different case.  Having read some of your previous postings would appreaciate your advice: two years ago an electrician/neighbor put a new 200 Amp service in my 1860's farmhouse.  Only one ground is visible--leading from main panel to the incoming water service; clamped on both sides of the water meter.  Is this adequate--only asking because he originally left it unattached for @ three weeks until he could get back with the clamps, and I wasn't impressed with some other things (e.g.: put two lights down in the cellar but didn't run it to a light switch at the top of the stairs; ran a feeder to an existing subpanel, but the ground wire was connected to nothing (is this normal?).

   Regards,

   Rework

1. 
   4Lorn1 | Aug 22, 2002 05:38am | #9

   I would add a ground rod if there is no grounding conductor is run from the meter or the neutral of the main panel. A piece of #6 solid run from the main panels neutral/ground bar to a bronze clamp attached to a 10' copper plated ground rod. It can only help. Grounding to the water line, glad to hear they clamped it on both sides of the meter in case the meter is removed, is old school and while it is, assuming the lines are conductive, a better ground due to the potentially large contact area, it can be very unreliable if water lines leading to the house are replaced with plastic.

   I'm surprised that the electrician didn't provide a way to illuminate the stairs from the top of the stairs. This is usually a code requirement for obvious safety reasons. A set of three way switches would be my preference.

   Hmmm. ground wire not connected at a subpanel. The only thing that comes to mind that wouldn't be a problem would be if the subpanel only had 240v loads. In this case the unused conductor would be the neutral. If it is a spare wire they might have run a four wire cable, smart choice if any 120v loads are to be added in the future (It saves having to run a new cable.), and left the neutral disconnected. If they ran a four wire cable why wouldn't they go ahead and land a neutral on both ends. If not connected why would this conductor be visible and not be tucked neatly into the back of the box? Lots of questions and not a lot of answers on that one.

   Have a friendly electrician check this out. A quick visual check will verify the installation. It shouldn't take but 10 minutes, a screwdriver and a flashlight. Ask around some electricians will do this sort of thing for free, a few beers and a handshake or a nominal fee.

2. 
   rjw | Aug 21, 2002 01:41pm | #8

   Cap & 4Lorn1

   Thanks!  I "knew" that, only I completely forgot that I knew that, so I'll charitibly chaulk it up to "senior moment."

   BTW, I've studied this stuff a bunch, and you guys have described it so clearly that I finally feel like I'm getting a grasp on the subject.  Great job and thanks again!

3. 
   darkchalk | Aug 22, 2002 10:17pm | #10

   To Bob Walker, 4LORN1 et el: Thanks for the advise. Sounds like the best and definite thing to do is run a new cable. Thanks again everybody.

1. 
   rjw | Aug 22, 2002 10:31pm | #11

   Thanks, but it's 4Lorn1 and CAP who get the credit here, not me.  I've just been along to pick up some of their expertise

4. 
   r_ignacki | Aug 23, 2002 12:16am | #12

   YES, you are grounded.

   sorry, but playing with electricity means you are grounded. NO stereo, phone, get  home by 9: pm, and no going to the beach this weekend.

   no turn left unstoned  

1. 
   Luka | Aug 23, 2002 08:09pm | #13

   Good job guys.

   : )Quittin' Time