Grounding electrode conductor options 1

[controltechnician]

I work for an industrial equipment OEM, and in our latest project we are tasked with building portable processing equipment. Our equipment has been built into a semi-trailer. It comes with a power system and filler equipment. It has a 30KVA XFMR to step 480v 3ph down to 120/208 3ph with a neutral for single phase loads. The NEC requires this transformer to have grounding electrodes and a grounding electrode conductor. I found an odd exception in the nec, art. 250.30.a.3exc.2. that seems to be telling me the grounding electrode conductor at the building's main service may be used as the grounding electrode conductor for the seperately derived system, and is not considered a shared grounding electrode conductor (and forced up to a large 3/0 conductor).

How does this exception get applied in the real world, and would that have an application to my portable equipment?

All input is appreciated!

 

[rbulsara]

It would be applied just as it says. NEC is for real world application. I also think that it would apply to your installation. If in doubt, check with your local electrical inspector.



Rafiq Bulsara

http://www.srengineersct.com

 

[controltechnician]

Thanks for the response. Have you heard of the exception i mentioned? I haven't seen any plant I have worked in use the main GEC in place of the seperately derived system's GEC, and wish I knew what exactly this exception allows. Since our equipment is cord connected, I'm trying to find out if this exception will allow me an alternative to driving ground rods and bonding down metal systems wherever our trailer pulls up, or if there is some way I can use the oversized ground wire in our cord to bond down the XFMR and let the sites existing grounding electrode system be used that way.

Thanks.

 

[wareagle]

This installation is no different that a portable generator.
You would be required to install a ground rod and GEC which would be a #6CU + connection to any listed electrodes.
I assume you have a breaker panel in the trailer. You have
3 phase conductors and equipment grounding conductor, EGC,
connected to the transformer per table 250.122. The size of secondary GEC is determined by 250.66. The exception you list does allow the use of the main panel grounding electrode. If the structural components of the trailer are metal and was bonded to the GEC, it could be used for this purpose just as the metal structural beam are used in a building.

 

[controltechnician]

Thanks for the info on the exception. The reason we're looking into this is because the customer was unhappy that there would have to be electrodes driven and outside metal systems bonded together in our original design. He said he already had a couple of other trailers that didn't require that of him. The trailer has 3 panels, two XFMRs (although one is delta/delta) and a transfer sw. The power cord itself is a 100A 3ph with an eq ground (all #2 awg), and we have 100' of flex cord made to the Y-XFMR as a GEC. We wondered if using the ground in the cord (since it is larger than the required electrode grounding conductor) to connect to the electrodes in the facility would be a way to help the customer avoid all the site work. What we're worried about is adding extra resistance to the EGC by increasing its length to who knows how long in these different facilities it will go to. I'm also worried about choking the GEC if the driver doesn't unroll all of the cord when he pulls up. The frame of the trailer is bonded to the onboard service, but I do want to hit earth as soon as possible. I'm still tempted to insist on driving a rod, connecting to the water line at the connecting point for the trailers on-site utilities, and bonding to any metal structures within 12 ft.
Is this the best way?

 

[rbulsara]

Since you have so many questions on a 30 kVA transformer wiring, let a licensed electrician or a professional engineer deal with the installation.

Rafiq Bulsara

http://www.srengineersct.com