I work for a company "On-Call" (that is they call me when the need for work arises). They have a 60kW generator that they use for large gigs (its a sound and lighting company) whether the gigs be inside or out. In any case its always temporary and never intended to be a permanent setup. My employer has bought a few 8 foot ground rods and at the end of a gig is always so determined to remove the 8 foot ground rod (which he never gets in all the way) just to save 15 bucks on a ground rod. I keep telling him to just charge the customer the 15 bucks for the ground rod and be done with it. Now my question is, I have heard that you can take 3 ground rods placed in the ground at a depth of 3-4 feet per rod and tie them together in a triangle pattern with 6 feet between any grounding rod. Making sure to bond all three ground rods together and back to the generator of course. Also, is 6AWG ground suitable for a 60kW generator on a temporary basis?

What type of loads are being supplied by the generator? If this is a portable generator, and the loads are cord and plug connected to the generator receptacle outlets, you don't need ground rods at all.

But if you are required to install a grounding electrode, I have never heard of a system of shallow driven rods as you described, being acceptable. The conductor attached to your rod or rods is not required to be larger than a #6 copper wire.

 

Ground rods do NOT ground anything. They are pretty much useless for your application.

 

What does this mean a ground rod does not ground anything?

 

In the event it is inspected on-site, ground rods are required. Most times its never inspected, however I witnessed on one such occasion that two generators were in use outside a school auditorium (old school that had in-adequate power for the setup) the generator we were running controlled a dimmer rack, and the other generator was setup for several tv trucks and equipment for a televised debate. The electrician wired both generators up to the same ground field, which was 3 rods in a triangle all tied together. My guess is he was with the TV station, as we didn't have him show up.

Most times the generator supplies a dimmer rack, which consists of 36 1200 watt dimmers (most often its a load of 750 watts per dimmer, sometimes more), other times it supplies a power distro for large sound applications and sometimes this includes portable dimmer packs (plug-in variety). There are 3 50amp outputs for spider boxes, plus a hardwire connection as well. This is a tow behind generator.

What I don't understand is why the one electrician at the school would have wired up three ground rods and shared them between the two generators and how that is acceptable. In this particular application, there was nothing but lighting on the one generator we supplied, so tying the grounds together to prevent ground loops would have no effect. I hope this provides enough addition information, let me know if you need anything further.

 

When people don't understand what they're doing, they often put ground rods where they don't need them. I don't see any need for it on this application. I know of an inspector that requires a driven rod attached to the #8 copper wire used for the pool bonding grid...Completely worthless but it makes him feel warm and fuzzy because he doesn't understand what the reason for having/not having it is.

 

Anyone else have anything to add? I mean I know that it is a requirement to have the ground rod on the generator, especially for outdoor events where power is run to multiple locations. I'd just like to know if this 3 shallow grounds is considered "acceptable". Does the NEC code book cover temporary generator codes for like outdoor events? I just bought a 2008 NEC Code book but it hasn't shown up yet, and figured talking to a few electricians on here would be easier to find the answer.

 

Not meaning to insult you, but you have found the answers from a few electricians on here. You just don't wish to accept them. And that is fine.

But your 3 "shallow" ground rods are not an acceptable grounding electrode system when a grounding electrode is required. When you get your 2008 code book check out 250.53 A and G.

250.34 covers generators in regard to grounding electrodes. And 445 covers generators. You may also face state or local codes.

 

Just got the 2008 Code book today, didn't quite expect it to be this big, haha. Anyway, I found the code references you mentioned regarding the grounding electrodes, and it looks like other than the shallow depth, code 250.53 C indicates that more than one rod or plate can be connected together and spaced no closer than 6 feet apart with proper wire size and connected to the electrode properly. I was just thinking about how the three shallow rods were installed and now remember that the generator we were using was at that time rented and came with the three short ground rods and were already tied together that way. So I assume the electrician for the TV crews generator must have driven the rods into the ground and just attached the ground to the same electrode system (in all honesty I can't be sure that he was an electrician). Any idea where I would be able to look locally to see if thats something allowed in my area? I would have to imagine it is somewhat acceptable if the rental place supplied the ground as 3 short rods tied together, unless the code was interpreted as 8 feet of total depth. But the code specifically says to a depth of not less than 8 feet (250.53 G).

Btw jrclen, I am just trying to figure out why it was done that way or how it is considered an "acceptable" practice to use 3 shallow grounding conductors.

 

Really .,, 3 ground rods it just don't really do anything very well at all.

I done quite few portables units and majotry of the time it will be either one* or NONE ground rod depending on the type of load and set up.

and with what Jclen saying he is right on the target there.

Merci,Marc


{ * the ground rod size is ½ inch size min with 3.6 meter rod [ 12 feet ]}

 

Well actually according to the NEC code book, min size is 1/2" but should be 5/8" and it only specifies 8feet long. (250.52 5) Usually the loads are either pretty high (lighting) or fairly low (sound only). So id imagine in a low power consumption, a ground rod probably wouldn't be required where as a higher load would maybe be a good idea. We have never used a ground rod on any "portable" generator, that is to say not attached to a vehicle or a tow behind generator. Does anyone have any recommendations on when to use the ground rod and when not too? On occasion we are just using the 50 amp spider boxes usually just two at a time, others we are using the dimmer rack and direct wiring it into the generator, and still others we use a 100 amp tap tied into the generator terminals. Usually the three most common setups, on occasion there is maybe 1 or 2 of the 20 amp outlets used on the front panel, but those are GFCI outlets and on their own circuits (total of 3). Its been said on here that a ground rod isn't really doing anything with a generator, but I am sure its required if it ever gets inspected on-site (usually with larger events with multiple generators and vendors at the event).

 

Take a look at 250.34. That tells you when a ground rod is not required on any portable generator. To sum it up, if the loads are cord and plug connected you don't need a grounding electrode. The size of the loads are not relevant.

If the loads are other than cord and plug, you will need the grounding electrode. This is the national code. Local code may be different and inspectors might have other ideas. That happens.

Look at 250.53. (A) requires the electrode to be embedded below the permanent moisture level. A short rod might not accomplish that. (G) requires 8 feet of length of the electrode to be in contact with the soil. If an electrode needs to be exposed above ground, a longer rod would be required so that the 8 foot rule is complied with. Close reading will make it clear the intent is for "each" electrode to comply. 16 6" rods will not pass inspection.

250.56 states a supplemental electrode may be required if the first does not have a resistance of 25 ohms or less. You can put in as many rods as you wish. But at least one would need to meet all the requirements of 250.53.

Have you ever seen one of those devises for pulling metal fence posts out of the ground? It's just a tee type outfit that uses leverage to pull the post. I bet you could come up with something similar to pull a 1/2" rod out of the ground after the concert. Just getting back to your original problem.

 

Yeah I was trying to think of some possibilities to removing the rod after the event, there were 4 of us on one particular event just recently trying to pull a 5/8" rod out of the ground (that was only buried 5ft) we finally got it but it wasn't easy.

 

I remembered that was your original purpose. They can be difficult to say the least. Here is the tool I was referring to.

http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_13224_13224

 

Yeah the idea of that tool is good, however with it being 3 feet off the ground, it would be difficult to be able to start pulling the ground rod out of the ground (saying it goes in the required 8feet). I had thought about drilling a hole in the top of the ground rod to facilitate putting a hook of some sort on the end to have a point where something could be tied to it and use some leverage (like what you were showing in that link) to remove it. Other idea I had was to build something out of angel iron, basically a square bottom corner with a short bottom leg parallel with the ground where in the very end drill a 5/8" hole into and use that as leverage. The idea being that it would rely on the rod being pinched in the hole and would be able to move it a couple inches, drop the tool and do it again. Only this would allow for it to be a lot lower to the ground.

 

I mean a ground rod alone does NOT ground anything in so far as a safety ground that trips breakers under fault current. It does NOT "create" a ground for a circuit.

If you stick a ground rod in the ground, and touch 120v to it, all you will do is electrify the ground and bring up worms.

IF that rod is bonded to the service the breaker will trip.

 

Portable generators should NOT have ground rods connected to them.

This is what I recently read to clear things up in my mind...

http://www.osha.gov/OshDoc/data_Hurricane_Facts/grounding_port_generator.pdf

Ground rods are for high voltage events...ain't nothin like that in your case.

- pete

 

Ok so I spoke with my employer regarding the ground rods and proper grounding of the generator, and informed him that the 3 shallow rods bonded together is not allowable by the NEC code. Told him that a ground rod is not required for cord-and-plug connected equipment (which is mostly what is being used with this generator). I'd assume that spider boxes that connect to the generator via 50amp multi-pin twistlock cables is still considered "cord-and-plug" connected.

He then told me that he talked to a local electrician/inspector who said that 3 rods in a triangle pattern at a depth of 4 feet each was acceptable, actually it sounded like it was preferred. Since driving an 8 foot rod into the ground is pretty difficult to remove, most people leave it in place and the inspector would rather see the shallow rods rather than rods being left in place. Which makes sense in that regard, especially with this being a temporary situation. This is apparently local to our area; which would explain the ground rod setup I originally posted about in the first place.

Just as an informational bit, this is a tow-behind 60kW generator that has 3 50amp twistlock connectors (for use with the spider boxes) and 3 20amp gfi outlets mounted on the front panel, and on the side is a high current tap connection. The generator is made by Baldor.

Thanks for all your input to this topic!

 

I'd assume that spider boxes that connect to the generator via 50amp multi-pin twistlock cables is still considered "cord-and-plug" connected.

That is correct

He then told me that he talked to a local electrician/inspector who said that 3 rods in a triangle pattern at a depth of 4 feet each was acceptable

Well, seeing as you don't need ANY rods, I see no harm in doing what the "electrician/inspector" and your boss considers acceptable. Sometimes keeping everyone happy, without doing any harm, is the easiest way to go. Just be careful about the tripping hazard with the rods. Falling down tripping, not breaker tripping. :thumbup:

 

It is hard to understand who knows what is correct about anything anymore.
All standby generators should be grounded using #6 solid copper wire and an 8-foot ground rod, which must be properly bonded to the electrical grounding system with approved clamp/connections to assure good electrical contact. Cover all generator openings with wire mesh to exclude rodent entry and potential damage.
Source http://www.aecc.com/ ( Arkansas Electric Cooperatives, Inc.)

 

It is hard to understand who knows what is correct about anything anymore.
All standby generators should be grounded using #6 solid copper wire and an 8-foot ground rod, which must be properly bonded to the electrical grounding system with approved clamp/connections to assure good electrical contact. Cover all generator openings with wire mesh to exclude rodent entry and potential damage.
Source http://www.aecc.com/ ( Arkansas Electric Cooperatives, Inc.)

Well, this is Arkansas!

No seriously Billy, I think that the distinction is in the second word, standby. My inlaws are from Arkansas and a lot of houses down there have supplemental, permanently-installed generators. In this instance, the generator should be grounded as it can become the electrical source for the dwelling and in this case, ground fault energy would want to return to the generator when it is on-line. Another big difference is the fact that these applications are NOT serving "cord and plug connected" loads.

 

Just push your boss for plug and cord, instead of electrode design and or removal and be done with it. I am aware that cables and plugs are not cheap, but I sure would rather make the investment and never use electrodes again.

 

Haha, one of the biggest loads we use on the hard-wired lugs is a 36 channel dimmer rack, that would be rather hard to convert to a "cord-and-plug" connected load.

 

With hard wired lugs I believe you do need a grounding electrode system. What amperage and voltage is that load? Just curious.

 

Each channel can handle 1200 watts each, though its typically only loaded at 600-750 watts per channel. So on the low side with 36 channels at 600 watts each its 180 amps total (90 amps per leg). At 750 watts per channel, its 225 amps total (112.5 amps per leg). According to the generator spec sheet, it has a capacity of 179 amps per leg in 120/240 mode. The generator automatically shuts down if the load is out of balance by more than 25%.

I believe it is the Baldor TS60T: http://www.baldor.com/products/generators/ts.asp

 

In the instruction manuals for the Baldor rigs, it says: "WARNING: Be sure the system is properly grounded before applying power. Do not apply AC power
before you ensure that grounds are connected. Electrical shock can cause serious or fatal
injury. NEC requires that the frame and exposed conductive surfaces (metal parts) be connected to an approved earth ground. Local codes may also require proper grounding of generator systems."
I guess it does need to grounded then?
Bob

 

http://www.nfpa.org/freecodes/free_access_agreement.asp?id=7008SB

 

Well the Dimmer rack has Cams on the back for the input, however the generator does not have cams, its only a hardwired high current output. I believe the wire is 2/0 stranded copper for each of the 4 conductors to the dimmer rack. My guess is that if the generator had cams that would still not qualify as "plug and cord" with 4 separate connections? For a load to be considered "plug-and-cord" does it just need to have all of the wires contained in one plug? Haha, anyone know where to find a 4 pin plug capable of say 250 amps @240 volts and capable of accepting 2/0 stranded copper?

 

Just thought I would post an update/end to this, in talking with my employer the other day about always being required to drive a grounding rod, he mentioned the section of code I told him to talk to the inspector about the next time it was inspected. One inspector apparently just looked at my boss and kinda walked away (like I don't want to talk to you). Another inspector said something about it being state code that a ground rod is required. It really makes no sense at all, and I have explained it to my employer and he seems to understand what it is I am talking about. Anyway, its just a local state code to drive a ground rod for generators (I wonder if they would require a ground rod on a small honda generator with that single outlet on it?)

 

Well, with the hardwired output, the NEC requires the grounding system also. So you seem to be stuck with the ground rods. Generators (like the Honda) having only cord and plug connected loads, would not need the ground rod.

So are you going to use the three short rods?

 

Well it seems that the 3 short rods is considered acceptable in this area, at least according to one inspector anyway. I am usually not the one thats doing the ground hook-up and I have not been there when the inspector comes by to check it out. I usually do the hardwire tap later or connect the loads to the generator when I am on that particular job. This generator does have the ability for a hardwire tap, but most times we only use the "turtles" which my understanding is that these are classified under the "cord and plug" devices since they have the 50 amp twist lock right on the generator front panel. It really seems stupid to have a ground rod required when all we are doing is plugging in a couple cords to some turtles (and on occasion the 3 20 amp gfi's on the front panel).