I have a 600 amp 3 ph 4 wire 480 volt I-Line panel with a main breaker being fed from utiliy power 2 runs of 350 mcm copper. out of the I-line it feeds a 75 kva 480/120/208 3 ph 4 wire transformer for a lighting panel in the same room as the I-line, 480 side is in pvc with # 2 copper secondary side is pvc with 3/0 copper, bonding to be done in transformer. Also out of the I-line we are feeding a 6' hoffman enclosure used as a motor control panel, within the hoffman is a 400 amp 3 ph breaker used as a main for all motor starters,they are all individually fused within the box, also pvc from the I-line with 500 mcm no neutral. Then last a 5kva transformer 1ph 480/120/240 being used for 120v control power on one leg and computer power on the other leg. This transformer is on the outside of the hoffman box with metal sealtight, being fed off the 400 this is so that when the hoffman is locked out, the control and computer are also dead. please help with grounding and bonding.
I am not sure I am 100% on board with what we are talking about but here is my take on it
You have 700 Kcmil of service conductor, that gets you a 2/0 grounding electrode conductor. in 250.66 Then you have a 600a service disconnect That gets you a #1 EGC up to the point you have another breaker. I assume these are all taps. Then you have a 75kva 3p 208 transformer with what looks like it will be a 200a breaker on the secondary. On the load side of that your EGC is #6. The motor controller panel is protected at 400a, the load side of that will have a #3 EGC max or whatever you get from the individual circuit OCPs in 250.122. (again you didn't say but it is worth looking into) The 5kva looks like you will just have a couple of 20a breakers and you will be running all #12 from that. You are going to need a breaker or fuses from the Hoffman to get to the primary because you are >10x any conductor you would be likely to use.
I am willing to be wrong here but that is my first swing at it.
Each transformer is considered an SDS. (Separately Derived Service)
That includes even the dinky 5 kVA center-tapped one-phase job.
So you'll need a #8 GEC for it -- I usually bond it inside the transformer. (Xo) [250.66]
The 75kVA unit will typically require #4 GEC, also best bonded on the Xo lug.
It's common practice to route these GECs back up the line -- bonding them with the larger Service GEC. This ensures an equipotental plane without having to jump out from each transformer to bond with the building's steel, gas and water and whatnot.
Look up SDS in the NEC Handbook for drawings, reasoning, and all the rest.
Additionally, these GEC runs require bonding bushings at every step of the way -- so as to avoid choking conditions during a transient fault.
Fault energies are addressed in Ugly's... in the transformer pages.
Bonding bushings are not required if the conductors are routed via PVC (underslab?) -- as it's non-ferrous.
Table 250.122 is used for all EGCs loadside of the OCPD. (the breaker/fuses) The GEC, sized per 250.66, has to reach all the way into the can that holds the OCPD. (bonding it)
One rarely sees this type of layout run under the slab. Hitting the 'windows' is such a chore/ risky. This goes double for most dry type transformers. Their bottoms/ points of entry are not that consistent. Routing the conductors from underslab is sure to be a real chore. Don't be surprised if the clearance is lousy or that the terminal bending radius in the secondaries is brutal. The tap rail is usually close above the bottom chassis support -- which must always be massive.
The usual drill is Greenfield and flex-ninety connectors with bonding bushings. This assembly is best performed before the transformer is shoved back and anchored down. The result is a clean look -- with the secondaries routed from behind and around one side, down low, inside the transformer body.
Nothing will go quickly, and many steps are two-man jobs. Aluminum secondaries will be much easier to bend -- if the contract permits them. If copper is required, bump up the Greenfield to the next size, you'll be glad you did.
Expect to feed the 208Y120 daughter panel from the bottom. Routing secondaries up to the top is a waste in every sense.
Stock up on multi-barrel Xo lugs.
If you can do so: check out previously installed dry type transformers. The first time out the door, most journey-men screw up... on just these issues.
I can't recommend the NEC Handbook too highly. You'll get your investment back on just this one job.
Thanks Tesla, I knew I was forgetting something. Don't post after midnight
The reality is if this is a tap, 250.122 would get you a bigger conductor than 250.66 sized to the transformer except for the tiny one. He can still land them on the bus in the service disconnect enclosure.
So for the gec if I run a 2/0 from the 3" copper water pipe to the main breaker neutral bar a #6 from ground rods to the same. For the gec to the 75kva trans can I take a #4 from the same neutral bar to the 75kva trans or do I have to run the #4 back to the water pipe. I thought the taps had to be made outside the main panel.
The "ground" discussed here has nothing to do with connecting the house to Mother Earth. Rather, we are concerned with BONDING. That is, making sure that anything electric has a good, solid, low-resistance path for any 'lost' electricity to find its' way back 'home.'
In this case, 'home' is the transformer that made the electricity, and the path is through the ground wire to the service connection with the service neutral. Ground rods are irrelevant to this issue.
Somehow you're getting power to the pump. That's where you connect your green wire to the motor's ground lug.
Now, it IS possible your pump is also used on some pool equipment, so it might have a ground lug outside the wiring compartment. This is for also bonding the motor to the equipotential grid .... or, actually, for connecting the grid to the house grounding network. That exterior lug is already connected to the motor's frame. You can also use this lug to bond any metal frame, tub, etc., to the bonding network- those items ought to have lugs of their own, and you simply connect together them with a bit of wire.
We seemed to have drifted from SDS to falling into the hot tub thread. As for the last question, since all of this equipment seems to be grouped, I see no problem with bonding everything to the neutral bus in the service disconnect enclosure. The handbook makes lots of references to building steel but that is not our electrode here. For the example of a water pipe electrode they show a GEC snaking through the building that all the transformers are tapped to. (exhibit 250.25) In our case, simply tapping it at the grounding bus seems to be a far cleaner installation and meets the intent of the code. Everything will be grounded to a single point.
Where on earth did I get the idea this was another 'hot tub' thread? I must be getting senile, or at least posting answer "a" to thread "b."
Transformers are only SDS when there's no DIRECT connection between the line and load "neutrals." While this covers "ordinary" transformers, it's a point worth considering.
"Bonding" is continuous, with both sides bonded together within the transformer enclosure. In addition, you're required to 'ground' most power transformers to a grounding electrode. That's one notable feature of the 2014 NEC: it recognizes building steel as a grounding electrode for ALL uses; previous editions only specifically mentioned it for transformer installation. Go figure.
Even transformers without neutrals are bonded to the building's grounding electrode in some way, and their cases should have equipment bonding conductors bonded to them.
The model I used above still applies; "lost" electricity still needs a quick path "home," with home being the transformer that created it. In a plant with both 480v 3-phase power and 120v 1-phase power, each flavor needs to be able to return to the specific transformer that it came from.
This means that there are many installations where the "service" - the place where the neutral is bonded to the ground - is right after each and every transformer. These bonds are in addition to any bond there might be at the actual power company service gear.