My house is Knob & Tube, until I get it rewired. In the meantime, previous owners have installed a subpanel in the basement that is ungrounded.
I will be installing/sledging a new ground rod 10′ x 5/8″ with a clamp in order to ground at least those circuits. I know I need at least 6ga for the ground – it will be in the rafters in the basement so it doesn’t need a conduit or a raceway. Is it NEC approved to use stranded wire? I was hoping to use THHN for the extra protection and visibility, but can’t find solid 6ga THHN (green) by the foot locally.
Thanks for any recommendations and answers.
Replies
You won't find #6 THHN in any color but black. Buy a roll of green tape and spiral wrap the wire to designate as a ground.
You could check with your local utility company for the length of solid wire you need. They generaly use #4 solid to ground thier overhead poles. they will give you the name of a supplier of sell you the piece you need.
Dave
if its the wire that goes from the box to the rod, most of those are plain solid copper with no outside coating, just wire. Also in this area if the main wires are seperate, not bundled, they have to be in conduit or raceway no matter where they are. In other words if not romex then must be in conduit
Here's one source for wire up to 1000 MCM with colored insulation:
http://www.fourstarwire.com/shop/subcategory.asp?category=Building%20Wire
At home I probably have a flyer I got from another vendor.
-- J.S.
Interesting John. I wonder what a 500' spool of #6 stranded green cost?
Virtually everything we do is in conduit, raceways, and ducts. Green tape has got to be way cheaper than special ordering colored wire in the larger gauges.
Never the less, I get your point. Just because we don't use it, doesn't mean it is not out there.
Dave
P.S. I bookmarked the site, thanks.
Hey Dave,
Here in Virginia, even Home Depot sells #6 THHN stranded in green by the foot or spool. There is something to the economy of buying just a spool of black and rolls of green and white tape for larger jobs, unless the run is 400+ feet long.
Frank DuVal
>>My house is Knob & Tube, until I get it rewired. In the meantime, previous owners have installed a subpanel in the basement that is ungrounded.
How is the subpanel wired? Does it have a floating neutral and the grounds and neutrals kept separate back to the main panel?
Generally, subpanels in the same building as the main are not separately grounded, but there are exceptions.
If you don't understand that question, consider hiring an electrician.
And the type of wire used for the grounding electrode can be solid or twisted, insulated or not, it depends on local codes and practices.
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Thanks for the reply; sorry mine is so tardy - out of town.
By Knob and Tube, I meant that the main panel has fuses! House is 1931, original main panel. There is no ground. The subpanel is in the basement. It serves everything in the house that has been updated. It has conventional circuit breakers with NM cable. Somebody did ground the supanel to the galvy water pipes, but I did the front yard irrigation last summer and discovered that the service off the water main at some time was replaced with PVC. That means the subpanel is effectively ungrounded and there are no grounds going back to the main fuse box/panel.
So I was going to drive a 5/8"x10' ground rod outside the house. NEC says one 1/2" copper rod with 8' of veritical soil contact is adequate as a ground rod, but I like to go the extra step. This rod will be next to the existing main box/panel, so that when I do get a new service entrance, the ground rod will be right there.
I wanted to run the grounding electrode in the basement rafters and then just drill a hole through the house and pop out right next to the new rod. If all that is clear, I'd still want to run green jacketed wire. Does is have to be solid?
>>By Knob and Tube, I meant that the main panel has fuses! House is 1931, original main panel. There is no ground.
A main panel with fuses might have knob and tube, or it might have "rag" wire. In my area, 1 1931 house would mostly have rag.
There are 2 types of "grounding" in electrical systems. The ground wires from outlets to the panel, and the grounding electrode from the panel to actual ground.
There should be a wire going from the neutral bar in your fuse panel connected either to a plumbing pipe, the water main coming in or a driven ground rod. (I'm pretty certain they weren't grounding to rebar in the concrete back then.)
In my area, for that vintage house that wire would be bare copper. It is an important part for the safety of the system.
You also have to be sure that your subpanel and the main panel boxes are electrically bonded together
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Thanks to Jeff and David and Jim and Rich and Steven and Mark and Jason and Shep and Jen and Mike and Joe and Bill and Ken for their offers!
Several donations have arrived! Thanks and God bless!
NEC 250.62 specifically says the grounding electrode conductor can be solid or stranded, insulated or bare.
Here we use stranded copper, bare, #4 I think but maybe it's #6. Outside the building it has to be in pipe so I put the ground rods (2 required) under the house.
What is your overall setup? Is the main pannel grounded?
The "ideal" is that you have ONE and ONLY ONE common "ground point" and that is at the main pannel.
A single driven electrode is not enough, but if you have at least 10ft of metallic water pipe in the ground that can be used along with the driven electrode.
Now what you are proposing would be better than "nothing".
But I would look at getting this more near the the code.
Put in the electrode and connect it with the water pipe (or a second electrode) and then run it to the main pannel and then to the sub-pannel.
Check my reply to Bob Walker for more specifics.
<A single driven electrode is not enough>
I may have to check my inspector's copy of the NEC again, but I reallly thought it said 1/2" copper driven for at least 8' of soil contact was adequate as a grounding electrode. The inspector agreed, but he's as eager to learn as I am, so if you think it's necessary, by all means, set us straight!
Thanks.
Two rods required here, they're 8 feet long and I left a foot showing on each, they have to be at least 6 feet apart.
One is ok IF it tests below some specified limit, which I don't want to look up and most people don't/can't do and thus put in the 2nd electrode. Commonly the water pipe is used, but if it is plastic that won't work so you need a 2nd driven electrode.
(Note there are a number of different options and types of electrode and some of them don't need the 2nd one, but not applicable to simple residential retro fits.)
But you still want to connect a ground wire to any metal water pipes that you have to "bond" them to the system ground. The idea is that all metal in the house be at the same potential.
> <A single driven electrode is not enough>
> I may have to check my inspector's copy of the NEC again, but I reallly thought it said 1/2" copper driven for at least 8' of soil contact was adequate as a grounding electrode.
What code says is that one rod is enough if it tests at 25 ohms to ground or less. If not, you have to drive a second rod at least 6 ft. away. If you don't do a whole lot of new electric services, it can be more cost effective simply to drive the second rod rather than own the special tester. It can also a lot quicker for the inspector to look at two rods than to convince him that you did the test correctly.
-- J.S.
John, Assuming one rod is inadequately grounded due to, I assume, local soil conditions, why assume a second rod will change the situation?
If one is not enough, and the second is only 6' away, what guarantee is there that it will be sufficient?
Or is it just sufficient overkill that it works?
Joe H
True, there's no guarantee that a second rod will get the combined resistance under 25 ohms. Requiring a second rod but no test is a decision the code writers had to make, just like they chose 25 ohms instead of 10 or 50 or any other number. It's a judgement call as to how much effort and expense to demand from the builder balanced against how much good extra ground rods would do. Of course, the less conductive the ground is, the less danger there is from current flowing through it.
-- J.S.
A number of studies have shown that in many cases ground rods are about useless for return currents. With average soil conditions they are a poor second, redundant, return path to the transformer. They don't hurt, a virtue of the mathematics associated with Ohm's law, but they don't help much either.
They have some utility as a connection to the earth for ultra-high voltage currents like a lightning strike. The earth itself being their desired final destination. Under ideal conditions ground rods and grids can effectively connect the voltage spikes to the water table and the earth.
The two rod limit is is a bow to these limitations. At times even extensive ground grids cannot help when poor soil conditions are present. I have seen cases where a steel cased well was sunk several hundred feet to get to the water table and a good connection to earth. The site was on a sand hill. This was at a building where delicate electronics, under pain of loosing thousands of dollars a minute, absolutely and positively had to be protected from the lightning so common in Florida.
If I remember correctly, the poor conductivity of the earth is a reason that the grounding electrode conductor can be significantly smaller than the service entrance conductor (i.e. a #6 copper ground wire for a 1/0 copper service cable wire) because a #6 will handle all of the power that the earth is likely to be able to absorb.
4Lorn1 can correct me on this if necessary...
My understanding is that you are correct on this. For the most part a #6 copper conductor will handle all a ground rod, or a set of ground rods, is likely to take.
> A number of studies have shown that in many cases ground rods are about useless for return currents.
True, that's not the purpose of linking building electrical systems to true earth ground. The planet probably won't conduct enough electricity to trip a breaker, but it can conduct more than enough for a fatal shock. That's why you want to short out any voltage difference that may exist between the dirt and your metal boxes and conduit.
-- J.S.