Multiple Grounding Conductors

[robertcsimplex]

My company manufactures equipment requiring the customer to connect multiple grounding conductors, either in a single conduit, or in free air. The equipment is either stationary or portable. For Example: a portable unit requiring (7) 4/0 cables per phase and (3) 4/0 cables for the ground connection. The customer hooks up these cables themselves prior to energizing the equipment. Is this ok per the NEC?

 

[petersonra]

Re: Multiple Grounding Conductors

This sounds kind of screwy to me. normally, such an installation detail is not within the purview of the equipment manufacturer, since he cannot tell with any certainty what size wires would be needed.

i'm not sure why you need 3 grounding conductors, that is especially screwy.

Maybe you could give us additional info.

 

[stud696981]

Re: Multiple Grounding Conductors

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For Example: a portable unit requiring (7) 4/0 cables per phase and (3) 4/0 cables for the ground connection.
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Seven 4/0 cables per phase? This must be one heck of a load. It sounds like your equipment requires its on nuclear power plant. I wouldn't want to see the electric bill from this.

 

[dereckbc]

Re: Multiple Grounding Conductors

Need more details to answer. There are very few circumstances where you can run a ground conductor in free air (GEC and DC frame ground).

 

[robertcsimplex]

Re: Multiple Grounding Conductors

OK, the equipment in question are Generator Load Banks, artifical load units to test electrical genets. The example unit above is a 2000KW bank at 480Vac. This translates to 2405A. From NEC table 250-95, and using a 125% factor for the circuit breaker, that means a 3000A breaker, requiring a 400MCM ground cable. We use (3) 4/0 cables instead. The load bank has (3) individual cable connections on it to which the customer connects (3) individual cables, supplied by us. As it is a portable unit, the cables are in free air.

 

[petersonra]

Re: Multiple Grounding Conductors

I think the issue of whather it is permissable to use parallel cables for your grounding conductor has been discussed here before and we have pretty much agreed that you must use the actual size from the table. You cannot use multiple smaller conductors in parallel in lieu of the size in the table like you can for conductors that are actually carrying a load.

I don't know that you can count on the genset having a 3000A max CB. What if the user wants to test a 4MW genset at half load on your unit and it has a 6000A CB on it?

What if they have to run several hundred yards of cable? They might end up upsizing the circuit conductors which means they would have to upsize the ground wire as well.

That's why this kind of thing is not an equipment issue but an installation issue.

<added> Just out of curiousity, how did you come up with (7) 4/0 cables being good for 2405 amps off a 3000A CB?

[ February 23, 2005, 06:42 PM: Message edited by: petersonra ]

 

[petersonra]

Re: Multiple Grounding Conductors

Originally posted by stud696981:
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For Example: a portable unit requiring (7) 4/0 cables per phase and (3) 4/0 cables for the ground connection.
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Seven 4/0 cables per phase? This must be one heck of a load. It sounds like your equipment requires its on nuclear power plant. I wouldn't want to see the electric bill from this.

I also liked the part about putting the (24) 4/0 wires in a single conduit.

 

[robertcsimplex]

Re: Multiple Grounding Conductors

OK, lets see...

From tabel 310-17, using 90-deg C 4/0 cable in free air, each cable has a capacity of 405A. 2406A divided by 405 gives me 5.94 = 6 cables. We add the 7th cable as a safety margin.

We dont expect to have 24 4/0 cables in a conduit. Most gensets are located outside, and our portable unit is also located outside. (you need a lot of airflow to cool 2000kw) The 24 cables would be in free air.

I'm new to the forum, so have no reference to the old discussions. Can you direct me to where they might be located?

 

[petersonra]

Re: Multiple Grounding Conductors

http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=7;t=001140

Although you now say you don't expect them to put the wires in a single conduit, your original post did say "either in a single conduit, or in free air".

<added> If the conductors are not installed in conduit, it probably would be considered a temporary installation, especially based on your description of what is being done. If it is temporary, it would have to comply with article 527. It would be interesting to see if such an setup could even be made compliant with that article.

It has also occured to me that this might be considered an outside branch circuit. I seem to recall additional requirements for that. IIRC, you can run individual conductors outisde a raceway in such a case, but they have to be done in certain ways.

[ February 24, 2005, 12:35 PM: Message edited by: petersonra ]

 

[jtester]

Re: Multiple Grounding Conductors

How long is this wiring hooked up? I wonder if the NEC is applicable at all. If this is some kind of test equipment connection which it seems to be, I would hesitate to turn to the NEC, however I don't know of another standard.
Jim T

 

[petersonra]

Re: Multiple Grounding Conductors

A point I had not considered. Since the NEC only applies to "installation", a good argument could be made it does not really apply in this case, since there is no real installation.

How would you inspectors feel about that line of reasoning? It would be hard to argue that something that is only connected for a very short period of time in the manner described is actually an installation.


90.2 Scope.
(A) Covered. This Code covers the installation of electric conductors, electric equipment, ...

 

[iwire]

Re: Multiple Grounding Conductors

Temporary wiring is covered by the NEC.

ARTICLE 527 Temporary Installations
527.1 Scope.
The provisions of this article apply to temporary electrical power and lighting installations.

IMO if connections have to be made (excluding plugs and receptacles) it is 'an installation'.

 

[boptrop]

Re: Multiple Grounding Conductors

It's often extremely difficult to comply 527. It seems to me that this equiptment will be connected and operated by qualified personnel. ISTM that as long as you don't get too close and mess with anything while testing the gen then you should be ok.

P.S. don't let any children play near the equiptment while testing!

 

[robertcsimplex]

Re: Multiple Grounding Conductors

Oops! My mind must have been wandering when I made the reference to conduit. Sorry about that.

I'm consulting a 2005 NEC, and can't find an article 527. What am I missing?

Returning to the original question, are the (3) grounding conductors permissable? Where can I find a definitive answer to that?

 

[bphgravity]

Re: Multiple Grounding Conductors

Article 527 has been moved to Article 590 in 2005 NEC.

 

[petersonra]

Re: Multiple Grounding Conductors

Originally posted by robertcsimplex:
question, are the (3) grounding conductors permissable? Where can I find a definitive answer to that?

There is no provision that anyone here was able to find that allows multiple smaller wires to be used in parallel to make up the required size euqipment grounding conductor (EGC).

In fact if you look at the code sections that allow parallel conductors, it specifically requires full size EGCs with each set of parallel wires.

I just don't see any way you can get around having a single full size EGC.

Whats the problem with having a full size EGC anyway?

 

[petersonra]

Re: Multiple Grounding Conductors

Originally posted by boptrop:
It's often extremely difficult to comply 527. It seems to me that this equiptment will be connected and operated by qualified personnel. ISTM that as long as you don't get too close and mess with anything while testing the gen then you should be ok.

What code section allows exceptions to the rules for equipment connected and operated by qualified personnel?.

I think that OSHA would love to hear about such an installation.

The thing is if someone gets hurt, for the most part the employer is off the hook, but the manufacturer of the equipment can be sued by everyone, including the employer. If I were the manufacturer, I would not be accepting such dubious advice.

Best bet would be to go to the AHJ and get some kind of ruling. IMO, that would have to be done by the end user.

 

[robertcsimplex]

Re: Multiple Grounding Conductors

The problem with the full size Equipment Grounding Conductor (ECG) is that my company doesn't want to have to stock every size wire made. Also, these portable units use cam-lock type devices for cable connection, and they aren't available to fit any size wire.

So what's an AHJ?

 

[crossman]

Re: Multiple Grounding Conductors

...very interesting topic. This generator load bank is really "test equipment" isn't it? I wonder if the code would even apply.

They hook it up, run the test, and disconnect it.

Would the AHJ require a permit? Inspection?

What about when you connect your DMM into a circuit to check voltage? Covered by Code? Okay, maybe I am stretching it...

A major telecommunications company had a load bank they used to test their UPS system a couple of times a year. They would have us come in to make the connections and hang around during the test, then disconnect it afterwards. I can tell you that the cables were only about half of what is required by Art 310 based on amperage. Max amps tested was around 450. For a 1 hour test, the cables would get a little warm, but the telephone company engineers were cool with it, they were the ones who figured the size they needed, not us.

 

[rbalex]

Re: Multiple Grounding Conductors

As described, I believe the installation is compliant with [2005] 590.3(C), and 590.4(B) Exception . This would include grouping paralleled insulated conductors to achieve the equivalent cross sectional area of a specified equipment grounding conductor.

?Emergencies and Tests? are to be installations so transitory in nature that the primary safety method is to be ?make it big and keep everybody away from it for the duration.?