Utility Transformer feeding power distribution box, feeding three transformers. Are ground rods needed at each transformer

[Greenautomator]

This is for an EV site, a utility transformer is feeding a 2000 amp discount.
Main bond jumper installed here tied to 2 ground rods.
The 2000 amp disconnect feeds 8 450A disconnects.
Each feed to a 480V 400A fused disconnect.
Each 400A fused disconnect feeds a 167KVA transformer 480V single phase primary to 240/120V single phase secondary.
Each Transformer feeds 2 120/240 branch 400A branch panels, each having 4 2 pole 100A breakers for EV chargers.
EV's together are limited to 320A

Question is if each of these transformers need its own ground rod?

 

[Elect117]

They require a grounding electrode. Whether it is two rods or the system's electrode like building steel or water pipe is based on the types present. See 250.50. The short answer is two rods at each transformer, 6' apart and 8ft deep. Or tie all of them into a single grounding electrode bus/conductor. See 250.30(A)(6). I guess that kind of depends on the layout.

Don't forget protection is based on both 450.3 and 240.21(C). You will need a low side OCPD for the conductors coming off the transformer rated at the conductor ampacity. A MCB in each 400A board and parallel runs of 3/0 will do that. Taps are not permitted on the secondary side of the transformer before the first OCPD. So each set feeding the down stream boards must originate at the transformer's point of connection.

 

[jaggedben]

If the transformers are considered to be at separate structures (e.g. they are pad mounted and spread around a grade level parking lot) then each would have its own electrode(s). See 250 32. If all the equipment is in the same building or structure (e.g. different stories of a built parking garage) then they are all supposed to be connected to the *same* grounding electrode(s.)

IOW all the transformers require a connection to the grounding electrode system at their respective building or structure.

 

[Elect117]

If the transformers are considered to be at separate structures (e.g. they are pad mounted and spread around a grade level parking lot) then each would have its own electrode(s). See 250 32. If all the equipment is in the same building or structure (e.g. different stories of a built parking garage) then they are all supposed to be connected to the *same* grounding electrode(s.)

IOW all the transformers require a connection to the grounding electrode system at their respective building or structure.

I was under the impression you could do either if it was outdoors. As in, create a single bus grounding electrode conductor near the main and run the transformer's electrode conductors to that along with the service main.

And if it was easier, than drive the rods at the transformers.


** Edit - reading it back, it sounds like you are elaborating.

 

[jaggedben]

I was under the impression you could do either if it was outdoors. As in, create a single bus grounding electrode conductor near the main and run the transformer's electrode conductors to that along with the service main.

And if it was easier, than drive the rods at the transformers.


** Edit - reading it back, it sounds like you are elaborating.

Yes, I don't disagree. Note I said if the transformers are 'considered' to be (or be at/on/in) separate structures. If you have a group of pad mounted transformers on the ground outside the code definitions give no clear guidance on how close to or far away from each other they have to be in order to be considered the same or different structures. So within reason I'd say you can make the call either way.

 

[Greenautomator]

Thank you for the replies. Just thinking of a worst case event - There are two grounding electrodes in the main 2000 amp service and the bond jumper is installed there. You still would need to tie each grounded conductor to the grounding conductor for each transformer in the run.
Going with the option of no individual ground rods in each of the transformers - which are all fed from the same system. If there as a ground fault in one of the sub panels, it has to go back to the 2000 amp system to travel back to the source - correct?
If you had an individual ground rod in each transformer, it would go straight to the source.
I was told the issue is circulating fault current if each transformer had its own grounding rod. Too me, each one is a separately derived system and requires the ground rod as stated above. Am I thinking correctly?

 

[Elect117]

Yes and no. The source is the transformer if you are considering faults on the secondary side. You are correct about separately derived systems not being electrically connected to their primary. They are magnetically coupled.

A fault, when it occurs on electrical equipment, would travel through the system bonding jumper. That connects the neutral to the electrical equipment at either the transformer or the first OCPD. That is also typically where the GEC gets connected to the neutral.

If you have a fault on a water pipe, then you need that water pipe connected to your grounding electrode system in order for it to have a "low resistance path".

Electrodes are also used to reduce voltage surges and reduce lightning hazards to equipment by earthing.

The reasons we bond and ground vary and there can be more than one reason we need to do it a particular way.

Like Jaggedben and I stated, you can do either.

Lets say you are in a parking lot and this service is not even near any kind of structure. Like a tesla super charger station. You would have your service grounded with 2 rods and maybe a ufer depending on the cement and rebar availability. Lets say your first transformer is 6 ft away. Well, you could run a extra pvc pipe to the switchgear and bond it directly to the same point as the other electrodes. The GEC not to be run with your EGC and feeder conductors. There is no rule against it, I just don't like it. It can create issues where ferrous metal raceways and bonding and ground requirements of 250.97 come in.

Now lets say your transformers are 100s of feet apart. Well then you can drive two rods at the location of the transformer and that is your GEC for it.

Now lets say you are in a building or structure, well then you need to combine all possible electrodes and any metal piping that might become energized by the separately derived system.

 

[tortuga]

The 2000 amp disconnect feeds 8 450A disconnects.
Each feed to a 480V 400A fused disconnect.
Each 400A fused disconnect feeds a 167KVA transformer 480V single phase

Just curious why use single phase transformers? With limiting each secondary load to (320X2) 640A @ 240V = 153kva looks like it will appear on the 480 side as 8 320A L-L single phase connected loads which wont balance on the 3 phase side, not that it matters, but one phase will have 1920A while the other two will have 1600.
Plus you have the 27/7 losses of 8 167 kva transformers.
If the chargers just need 240V i'd just use a 416Y240 system.

 

[jaggedben]

Just to add on again...

Going with the option of no individual ground rods in each of the transformers - which are all fed from the same system. If there as a ground fault in one of the sub panels, it has to go back to the 2000 amp system to travel back to the source - correct?

That's incorrect. It has to make it's way back to another terminal of the same transformer supplying the subpanel. For what we mean by a 'ground fault', that other terminal should be the grounded conductor terminal (i.e neutral, terminal XO, almost always). For a ground fault to bonded metal raceways or equipment, the EGC carries the fault current, and the grounding electrode plays no role in this. For a fault to earth itself, the earth and the grounding electrode and GEC may carry current, but it is not so likely to be a low-impedance path that will open a breaker. So your question about how the NEC requires/permits the transformers to be grounded (i.e. connected to electrodes) most likely has nothing to do with fault current. And fault current has not much of anything to do with which electrode(s) the NEC requires you to use, or their locations.

In my experience the importance of NEC grounding electrodes is vastly overestimated by most people in the industry (at least if we are talking 600V or less) and the importance of the EGC is frequently underestimated.

If you had an individual ground rod in each transformer, it would go straight to the source.

Which source? Regardless, whether each transformer has individual electrodes or they all go to the same ones does not change that you are connecting each transformer XO to the earth. There is only one earth.

I was told the issue is circulating fault current if each transformer had its own grounding rod. Too me, each one is a separately derived system and requires the ground rod as stated above. Am I thinking correctly?

It is correct that each separately derived system requires a connection to earth, although they can all share one. I suspect that the person who mentioned circulating fault current doesn't know what they are talking about, but even if they do there are still typically NEC requirements to ground each SDS.

 

[tortuga]

Just to add if I figure the just no-load losses of a 167 kva at ~212 watts thats ~ 2382 kwh annually so X 8 thats 19061 kwh per year down the drain.
Even if your getting wholesale electric rates that's $2k per year of losses plus the cost of the 8 transformers and all this grounding.
A 416/240V system can use standard 480/277 breakers so I'd imagine even with the cost of a less than standard 416/240 transformer you come out ahead.