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Old 10-08-2019, 02:56 PM   #1
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Default Grounding electrode to Transformer

I will be installing a 75kva transformer in a concrete tilt up building and all of the steel columns are covered with drywall so it might be difficult to run my GEC. I think the closest column is about 25 ft from where the transformer will be location. My thought is to install an access door in the drywall so that the connection is accessible. Any thoughts on this? Should I just drive a ground rod or is that even an acceptable alternative ? I think the columns are round post and I'm not even sure if it's appropriate to drill and tap my connection point or if it might be require to cad weld the connection?
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Old 10-08-2019, 04:06 PM   #2
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I think the columns are round post and I'm not even sure if it's appropriate to drill and tap my connection point or if it might be require to cad weld the connection?
I think drilling and tapping works as a means of connecting a lug to the steel.
I have drilled and tapped beam flange rather than use a bonding beam clamp and seen others.

However the way the rule is written, I think it might not fly:

Quote:
Originally Posted by NEC 2017
250.68(C) Grounding Electrode Conductor Connections.

(2) The metal structural frame of a building shall be permitted to be used as a conductor to interconnect electrodes that are part of the grounding electrode system, or as a grounding electrode conductor. Hold-down bolts securing the structural steel column that are connected to a concrete-encased electrode that complies with 250.52(A)(3) and is located in the support footing or foundation shall be permitted to connect the metal structural frame of a building or structure to the concrete encased grounding electrode. The hold-down bolts shall be connected to the concrete-encased electrode by welding, exothermic welding, the usual steel tie wires, or other approved means.
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Old 10-08-2019, 06:10 PM   #3
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Thanks Splatz .. I was mostly curious about how the GEC would typically tie to building steel and if that was not an option, what other methods might be used, such as a Ground Rod. The EE might spec it out in his drawings and I will have a better idea. The columns support wood trusses and who knows how they are connected to the concrete slab at their base. I have never seen a tilt up during construction so I really have no Idea. Most of the buildings I have worked in are steel buildings and all the columns are tied together with catholic grounds and cad welded.
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Old 10-08-2019, 10:37 PM   #4
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Defiantly do not drive a remote ground rod. This will create problems as this ground rod would be at a different potential than other grounding and bonding.

Either access the column and attach your GEC or run the GEC up to the overhead steel building structure. At either location, lugs and drilling and tapping (or bolting) would be the way to go.
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Old 10-08-2019, 10:45 PM   #5
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Having installed just such XFMRs and in just such tilt-ups -- our standard scheme was:

Run the GEC -- of the new transformer -- back all the way to the Service -- yeah -- the Big Box.

A GEC will run a size larger -- or more if distance is an issue -- than a strictly bonding conductor.

You'll have to use bonding bushings across the ENTIRE run of raceway. ( PVC excepted )

By tying in the the Main power buss -- and because it's already perfectly bonded, etc.

This solution is the cheapest way to go.

1) A bump in copper -- say #4 instead of #6...
2) And some bonding bushings....

Pull your GEC conductor right along with your primary hots.

Done.

Don't get caught jumping all over the building trying to bond this than and the other.
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Old 10-09-2019, 12:47 PM   #6
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Quote:
Originally Posted by telsa View Post
Having installed just such XFMRs and in just such tilt-ups -- our standard scheme was:

Run the GEC -- of the new transformer -- back all the way to the Service -- yeah -- the Big Box.

A GEC will run a size larger -- or more if distance is an issue -- than a strictly bonding conductor.

You'll have to use bonding bushings across the ENTIRE run of raceway. ( PVC excepted )

By tying in the the Main power buss -- and because it's already perfectly bonded, etc.

This solution is the cheapest way to go.

1) A bump in copper -- say #4 instead of #6...
2) And some bonding bushings....

Pull your GEC conductor right along with your primary hots.

Done.

Don't get caught jumping all over the building trying to bond this than and the other.
Great Advice! I was contemplating doing this as it makes the most sense. As you stated bonding everything would be more difficult. Thankfully I'm not restricted by cost as the customer is actually my boss. The transformer will be about 200 ft from the main service so I'ts not so bad. If I end up with more than 360 deg of bends I could just install a conduit body rather than a pull box and save on the cost of 2 grounding bushings. Thanks again ..
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Old 10-09-2019, 01:02 PM   #7
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I guess I didn't read your post that well. You said to install the GEC in the same conduit as my feeders. I was thinking on running the GEC in a separate 3/4" and run my EGC along with the feeders. I guess either way it works and doing it the way you suggested is probably the preferred method.
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Old 10-09-2019, 01:42 PM   #8
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The GEC, when run this way, doubles as an EGC, so:

Two raceways become just one. Two wire pulls become just one.

At 200 feet, you're good to go.

If the transformer figures to be fully loaded -- doubtful -- then you'd want to up-size your primaries.

When the GEC gets to the transformer, bond it every which way -- see NEC Handbook.

You'll need bonding bushings even as the secondaries leave the transformer ( always true ) until the secondaries meet an OCPD -- the Main breaker in your new panel.

I pull the head as the GEC -- with a L O N G advance -- followed 15 feet back by the three primaries. By the time they pop out I'll have enough GEC to loop it through the XFMR and onto the new panel where it will be bonded to everything, too.

Don't forget that the 208Y120 system will have its phasing 30 degrees behind the 480Y277 system. See the foot note in Uglys
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Old 10-09-2019, 01:58 PM   #9
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This all sounds good. I can't say how the panels will be configured because most of the equipment will be 480 volt so I may still have to run the EGC in a separate run. I don't have any drawing from the EE yet. I will probably have at least 2 - 480 volt runs feeding 2 - 200 amp 480 volt panels. One panel would be loaded up and the other not so much. They will both be next to the transformer so I was going to supply the transformer from the 2nd panel then to a 120/208 panel with a main breaker. That way I have over-current protection on the primary and secondary.
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Old 10-09-2019, 02:05 PM   #10
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I meant to say the GEC in a separate run not the EGC. By doing it this way I would also have a way to connect another transformer if it was required in the future.
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Old 10-09-2019, 02:48 PM   #11
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Something like this. I really doubt I will need anywhere near a 200 amp panel for the 120 volt equipment.
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Old 10-09-2019, 03:57 PM   #12
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That's workman-like, clean.

Just be sure that your GEC gets the bonding bushings all the way.

This is where the AHJ will jump on you.

Your diagram shows that you'll need four.

&&&

Another consideration: noise from XFMR vibration. The factory setting may require field adjustment. If so, kill the power, lift the top lid, and make adjustments.
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Old 10-09-2019, 05:08 PM   #13
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Thanks Telsa. I'm learning good stuff here. Bonding Bushing, {check} Proper secondary voltage {check} ... As for noise I will be installing rubber pads when mounting. I could run conduit in and out of the transformer without flex but that might transfer vibration and cause noise ? So probably run some short seal tight. I definitely won't run my flex like shown in this picture. I can envision someone climbing on it and breaking it.
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Old 10-09-2019, 05:12 PM   #14
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This might be a better way of running flex to a transformer. It's supported and looks clean.
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Old 10-09-2019, 08:54 PM   #15
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The EGC can also be the GEC if it is sized right and is unbroken all the way to the service.
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Old 10-09-2019, 09:37 PM   #16
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Thanks .. With the cost of copper you "save more if you buy more" especially if you come up short on a run. I try to buy in full rolls.
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Old 10-09-2019, 09:55 PM   #17
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The EGC is very important. You guys explained it in a very clear way. Thanks again
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