electrical question- use a concrete encased electrode for a gate operator ground?

stoneaxe · Nov 11, 2014

My gate has 6x6 steel posts with a rebar cage welded on (about 20 lineal feet of bar), set in a 32" cube of concrete- there is a # 6 copper wire attached to the rebar with clamp that exits the concrete. - can I use this to ground the operator which is 2' away? Sorta scared to drive a rod, there is a buried primary line very close.

I did a little reading and it seems like this is an OK ground, but need expert advice- thank you!

bridgedog · Nov 11, 2014

The concrete itself is non conducting. That said if the concrete and the surrounding soil is damp it would ground fine. there is a possibility that whoever installed the block also thought to put a grounding rod in the soil below the block since he took the time to have a copper wire come out of the slab.

Lakeside53 · Nov 11, 2014

Maybe, but your cage isn't optimal. 20 plus feet of rebar in the lower potion of the concrete footing is is actually the preferred ground now - called an "ufer ground" and required for new construction around here. If in doubt, call your AHJ (Authority Having Jurisdiction) in your local electrical code dept. If you are concerned about driving a rod (get a locate done...) there are alternatives that meet the NEC code, with local overrides as applicable - like a modest sided steel plate buried not all that deep (IIRC.. about 144 (?) sq inches.)

Worst case, get the ground to earth impedance measured.

Tony Quiring · Nov 11, 2014

If you suspect burried utilities call 811 before driving anything in the ground.

The ground you looking at is for what use?

How far from the load center is this?

You may just pull a ground with the supply.

The ground coming out of the slab may be to ground the internals of the slab to an external load.

There are folks who test these just look up tower builders or telecommunications contractors as they check comm site grounding systems as a part of the construction process so they can test correctly and if local ususlly cheap.

Many need your services to build things so you may gain a customer...

jlevie · Nov 11, 2014

The concrete isn't ideal, as mentioned above. Drive a ground rod in a few inches from the concrete. If that hole missed to power wires the ground rod should also.

stoneaxe · Nov 11, 2014

A few reply's-
I built the post, so I know it is about 36" in the ground, and has a 24" square rebar cage welded around it- 8 upper bars, 8 lower bars, total rebar length 32'. Also, I know the #6 copper wire is fastened to the cage with a rebar rated ground clamp.

Concrete is an excellent conductor, as it retains a lot of water- which is also one of the disadvantages as apparently if it gets enough current the water can turn to steam and blow up the concrete. (from what i have read)

The last locate I had done on this job, the guy missed a phone line 18" deep by an error 32" horizontally. I found it though...

Interestingly, my neighbor is a cell tower electrician- maybe he has earth ground measuring equipment.

The gate is 150 feet from the supply point, the ground I need to put in is apparently for lightning protection and for static protection.

sfriedberg · Nov 11, 2014

Locally, Ufer grounds are becoming expected. When I was getting the electrical permits for my shop (2009), I had to point out to the county planning department that a pole building with a fully insulated slab (not just perimeter) would not be a satisfactory Ufer ground. Fortunately, they have not yet forgotten that the NEC includes driven ground electrodes as well.

There are several potential problems with your gate foundation. The first is that there's a minimum size requirement for a Ufer ground ("concrete encased electrode" in NEC-speak). The second (which may be a deal-breaker, or may be irrelevant, at the whim of your electrical inspector) is that there are specific requirements for how the copper wire is bonded to the rebar and this detail (locally) must be inspected before the concrete is poured.

Third, and most fundamental, a separate grounding electrode that's not bonded to the rest of your system is of limited value. It must not be used for the operator return current, and it must not be used as the effective safety ground. The commentary (2008 NEC) that accompanies Article 250.54 Auxiliary Grounding Electrodes reads in part

Grounding electrodes, such as ground rods, that are connected to equipment are not permitted to be used in lieu of the equipment grounding conductor, ... Sections 250.4(A)(5) and 250.4(B)(4) also specify that the earth not be used as the sole equipment grounding conductor or effective (ground) fault current path.

So, you're going to be running at least three wires to your gate operator to be compliant with the NEC. Line+neutral (120V) or line+line (240V) plus an equipment grounding conductor. And that equipment grounding conductor makes use of the gate foundation as a concrete-encased electrode irrelevant.

Added in edit: The gate foundation would make a fine lightning protection ground. But that's actually a really good reason not to bond it to the powered equipment. A nearby lightning strike can establish a substantial voltage between remote, unbonded grounding electrodes. You don't want that feeding back through the equipment grounding conductor into your house/shop service.

BobRenz · Nov 11, 2014

Around here (MN), you call the location service and they will mark any underground utilities at no cost.

stoneaxe · Nov 11, 2014

sfriedberg said:
Locally, Ufer grounds are becoming expected. When I was getting the electrical permits for my shop (2009), I had to point out to the county planning department that a pole building with a fully insulated slab (not just perimeter) would not be a satisfactory Ufer ground. Fortunately, they have not yet forgotten that the NEC includes driven ground electrodes as well.

There are several potential problems with your gate foundation. The first is that there's a minimum size requirement for a Ufer ground ("concrete encased electrode" in NEC-speak). The second (which may be a deal-breaker, or may be irrelevant, at the whim of your electrical inspector) is that there are specific requirements for how the copper wire is bonded to the rebar and this detail (locally) must be inspected before the concrete is poured.
As far as I have read, 20' of 1/2" rebar is acceptable- I do have photos of the ground clamp set up.

Third, and most fundamental, a separate grounding electrode that's not bonded to the rest of your system is of limited value. It must not be used for the operator return current, and it must not be used as the effective safety ground. The commentary (2008 NEC) that accompanies Article 250.54 Auxiliary Grounding Electrodes reads in part this will not be a return path for current or a safety ground

So, you're going to be running at least three wires to your gate operator to be compliant with the NEC. Line+neutral (120V) or line+line (240V) plus an equipment grounding conductor. And that equipment grounding conductor makes use of the gate foundation as a concrete-encased electrode irrelevant. the operator manufacturer wants its , I don't know why- maybe as a static or lightning ground which would be why they want it close to the operator.

Added in edit: The gate foundation would make a fine lightning protection ground. But that's actually a really good reason not to bond it to the powered equipment. A nearby lightning strike can establish a substantial voltage between remote, unbonded grounding electrodes. You don't want that feeding back through the equipment grounding conductor into your house/shop service.

can you expand on this a little? I may be getting over my head.

Bill D · Nov 11, 2014

The NEC does not want multiple ground points on a system. If you tie that rebar to a ground rod that is a hundred feet away you may set up a battery effect and either cause low voltage signal problems or electrolytic erosion of import metal anywhere in the system. My old friend an electrical engineer told me about a power plant that had control signal problems causing shut downs for no reason. He found a bad ground on a steel gangway connecting the control building to the generator plant. There was over fifty feet of height between the two so it set up a imposed current on the wiring.
This is why medical equipment must be located on the same floor as the patient if it is up one floor in a observation balcony the voltage difference from the height can override the biological voltages, millivolts, being measured.
Bill D.

EPAIII · Nov 11, 2014

Your statement that concrete is an excellent conductor is immediately contradicted by the one saying that it can explode from the water in it turning to steam. In order to heat the water, the current flowing through the wet concrete must be turned into heat and the equation for that is I^2 R. The R is the resistance of the circuit. An excellent conductor has a low resistance and, hence, very little heat is produced in it. So, if the wet concrete was an excellent conductor, it would not explode because little or no heat would be produced.

If you are trying to dissipate a lightning strike, you will need a better ground. And I would not recommend running it back through the power circuit to the breaker panel. If there is a need to protect against lightning strikes, you really need an actual ground rod at that point.

stoneaxe said:
A few reply's-

...<snip>...

Concrete is an excellent conductor, as it retains a lot of water- which is also one of the disadvantages as apparently if it gets enough current the water can turn to steam and blow up the concrete. (from what i have read)

...<snip>...

The gate is 150 feet from the supply point, the ground I need to put in is apparently for lightning protection and for static protection.

JST · Nov 12, 2014

The problem with the pundits here on lightning is that you are SUPPOSED TO bond the electrical ground to the lightning ground.... That brings it all to the lightning potential, WHICH IS WHAT YOU WANT.

You DO NOT want the lightning ground arcing to the electrical ground, you want everything at the same potential as much as you can get it. Earth, structure, etc. Any other setup WILL get side arcs into the structure and cause fires etc. Bad nojo. You still may get that, but at least it won't be as bad as it could be.

That poses a problem at a remote location such as the gate. About all you are allowed to do by many jurisdictions is to bury conduit from main service to the gate, and then that is the first line of lightning defense. Your service ground is far away.

Lakeside53 · Nov 12, 2014

A powered gate isn't much different to remote buildings with sub panels, which require their own ground rod and for the last few years, a separate ground wire tying the two together. As for a comment above (BillD) about NEC only wanting one ground... Metal framed buildings are required to have the structure bonded to panel ground and it's common to see multiple distribution transformers and panels throughout a large building with their own ground rods (and a ground wire tie). The steel structure alone creates many separate ground points though the foundation bolting.

I've install a number of gate operators. In general they are 120v 15amp, UL listed and the manf clearly states a ground rod is required with ground bonding to the fixed gate structure gate and any operator console. A couple were inspected, and the ground rod and bonding checked. In many installs lights are part of the metal gate structure. That requires a ground on the structure no matter.

If a slide operator, in most cases the gate operator ground via incoming power only makes a lousy connection to the moving portion via the chain drive and chain; the moving portion is guided on polyurethane rollers and makes little to no contact with the fixed portion. If you bond the power ground to the fixed gate structure or operator console, you may OR may not ALSO have a good separate ground - depends on the sub ground structure and coatings, but installing a ground rod guarantees that.

stoneaxe · Nov 12, 2014

As far as I can tell, the concrete post base with rebar would be OK- but my #6 wire is undersize for concrete rebar connection per code.

So I am going to use a plate type ground electrode and bury it 30" deep so there is no risk of whacking that primary line with a rod. This will be bonded to the operator.
Should I attach the wire from the rebar to the plate as well? It will be three feet away.

Modelman · Nov 12, 2014

Unless your utility locate was done within the last two weeks, call for another locate... they're FREE for Pete sakes. Your 30" depth for the plate is well deep enough to hit a line.

Dennis

stoneaxe · Nov 12, 2014

About that rocket science- the last locate I had done, for this job, was for digging the post holes.
The locate guy was off center 32" on a 18" deep line. Whats that, a 200% error? Yes, I "found" it. Fortunately it was a phone line.
So my faith in locating was shaken a bit. What is a primary line to a transformer? ? 2500 volts? 5000 volts?

Now there is a layer of hardpan 36" down, not packed clay or the other stuff people erroneously refer to as hardpan- This is a substance that is about the consistency of sandstone, where all the tree roots are forced horizontal. With mixed stones and gravel. Makes the shovel ring like a bell. No digging through it, you break it into pieces with a bar or pick, then scrape out the chunks and repeat. No guarantee, even if I can drive it, that a rod will go straight down. Now with Murphy looking over my shoulder, and that high voltage line down there about four or five or six feet,
tell me again why a $30 ground plate such a bad idea? There is already a 24" deep trench, I can carefully scrape out the last 6" to get to depth. Just seems slightly more expensive but with radically reduced risk.

Lakeside53 · Nov 12, 2014

If it's approved in your area, it's not a bad idea.

It's unusual for a power locate to be off more then a few inches on center at shallow depth, maybe a foot at 4-5 feet. If you are concerned, be there when the locate is done. They might even tell you the depth. My underground feeders to the distribution transformers are 7200 volts; yep... we took great care. Those we dug with a vacuum truck ($$); they were exactly where they said they would be but we still felt safer.

Tony Quiring · Nov 12, 2014

A couple things...

First ask your building department what is required as it is their job to know and provide that information.

Call 811 and make a ticket.

Place white flags where you wish to dig as well as the working space.

IT DOES NOT MATTER IF THEY MARK IT WRONG...IT MATTERS THAT THEY MARKED IT.

The utilities have law departments specific to collecting damages from this and if you happen to be along the side of the road and cut a traffic carrying fiber they waste no time or money fixing it and you are billed for that cost plus penalty for customer impact.

If you called and they mark it wrong then it falls under "oops"...

Some times you must hand dig under their watch as well.

Chip Chester · Nov 12, 2014

Heh. Here in Ohio, it's actually called OUPS. Not only pronounced "oops", but the joke was made in their early ads.

Had them come out and mark the utilities to the house (gas and water -- all in the front yard) when digging trench to bury electric/phone/cable lines in the back yard. They marked the gas line 50' from where it actually was -- and on the opposite end of the house from the clearly-visible meter. Plus they said they don't mark water, so good luck. Oops is right.

Chip

electrical question- use a concrete encased electrode for a gate operator ground?

Stainless

Hot Rolled

Stainless

Titanium

Cast Iron

Stainless

Diamond

Stainless

Stainless

Diamond

Diamond

Diamond

Stainless

Stainless

Titanium

Stainless

Stainless

Titanium

Diamond

Similar threads