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How do you wire up a 75 kVA transformer (going from 480 to

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How do you wire up a 75 kVA transformer (going from 480 to 208) including the grounds and neutrals and any bonding needed?
You are corresponding with JOSEPH - Your "Personal On-Line Electrical Consultant".


Have you done any of this wiring yet? Also, where is the transformer located? Outside or inside? Do you have building steel accessible?
Customer: replied 5 years ago.
none of the wire has been installed yet. We are pulling 1/0 to the transformer and 1/0 out to the gutter where it will be wired to blocks after which it will be wired with #2 down to the 100Amp disconnects. The transformer is located outside and it is right now sitting on building steel(via double strut) and from what I'm told we are putting a ground rod in as well
Ok are you saying that you have a 200amp disconnect on the primary side of this transformer? What is the max voltage rating on this disconnect?
Customer: replied 5 years ago.
sorry I don't have info on the disconnect other than being 200A (non-fused) which is being directly feed from a weatherhead connection above. This service we are doing is temporary but I want to make sure it's done correctly. I have only wired up a transformer with someone right there with me but it's been a long time since that experience and now I'm on my own and I want to be able to do the work on my own but be able to ask smart questions of my boss if I need it.
Ok first problem with this setup is that it will need to be fused on the primary side. And your 200amp non fused disconnect is not going to cut it for this. Not to mention the amperage is too high.

You need to install a 100amp fused 600v disconnect on the primary. On the secondary, you will than have 200amps of 208 available. For the primary, you will only need to install #2 copper wire for the primary, and than you will need to install 3/0 copper for the secondary.

Does this make sense to you?
Now I am sure you will still have more questions, please let me know what you dont understand and what I have not addressed yet. Thanks.
Customer: replied 5 years ago.
Yes I agree with the primary disconnect being fused. I'm thinking I maybe be completely wrong in that the transformer is actually going to be 480 to 480 and that's why my boss was having me pull 1/0 on the primary and secondary sides. But I will get to the bottom of that at work.(because I have not been able to gain access to the final panel which I was assuming was 208V). But my primary question was the wiring of transformer grounds and neutral. From what I remember the ground on the transformer is tied together to building steel, ground from earth which we can directly in this instance ten feet away from a ground rod itself, and to the transformer casing. Then the a single ground wire is tied to the neutral bar. I'm not sure if I have it all down correctly
You will takd and run a ground wire from Xo to the casing of the unit. Than you will also take a #4 wire from there to buildding steel. Building steel has to be either steel posts or the building gurters at the ceiling level.
Customer: replied 5 years ago.
ok for some reason I thought there was a ground post( in the transformer) where you landed the secondary ground, landed the casing ground, and the building steel ground. THEN you would wire land a separate ground wire from the ground post(in the transformer) to XO. And THEN of course you would land your neutral on the XO
You need to install a grounding lug, or in your case a double lug for the grounding. Than Xo to the grounding lug in the transformer. From the double lug out to building steel. You will also have to run a grounding wire and treat everything downstream from the transformer as a subpanel. So separate neutrals and grounds for everything after the transformer.
Customer: replied 5 years ago.

sorry if I seem confused it's been sometime... ok that double lug for the grounding you said is suppose to be installed exactly where? On the casing?? I wish I could see a visual pic so I could get it straight in my head.

You will bolt it anywhere on the bottom of the transformer that is easliy accessible. I usually do it off to the side so the grounding wire is out of the way.
Customer: replied 5 years ago.
ok just so I'm straight on this. Bolt a double lug to the casing. From that double lug land a ground(wrapped in yellow tape) to the XO. Also from that double lug land the #4 from building steel. Then on the XO land the neutral that is going to the seondary side.
No yellow tape, dont know where you got that from.. LOL - Lets start from the double lug.

Wire from double lug to Xo on the secondary and then out to your loads
Wire from double lug out to building steel

Customer: replied 5 years ago.

ok when you say "then out to your loads" are your referring to the neutral wire going out to the secondary load?.... There is no ground wire coming from the transformer because the ground is derived from the neutral at the first subpanel correct?

You will need to run a separate neutral and ground out to your loads from the transformer. All panels on your secondary will be considered to be subpanels.
Customer: replied 5 years ago.
ok I understand that the neutral from the XO is going to the subpanels. So the ground for the subpanels is also landed at the XO correct?
yes that is correct. So from the transformer you will have 5 wires leaving. Three hots, a neutral and a ground.
Customer: replied 5 years ago.
ok i got it! thank you very much. It's a lot clearer now and the yellow stripe on the ground to XO was from a guy I worked with years ago. I have more questions regarding the primary disconnect, and the gutter and disconnects on the secondary side, but I don't know if i'm allowed more questions on what i paid.
Go ahead and ask. Its all part of the original post
Customer: replied 5 years ago.

ok....well from the weatherhead we are connecting we are bringing down a ground to the disconnect. We lug the disconnect and land the ground there. That ground is suppose to go to building steel right?

 

After the ground leaves the transformer it hits a gutter. The ground is landed on the terminal block within the gutter which is connected with conduit to the individual fused disconnects , then from each fused disconnect to the structure's(it's a fire-rated steel storage unit) actual disconnect and finally the subpanel within the structure. The grounding should be straight forward.

 

Now when the neutral hits that gutter I'm a little hazy.... I'm guessing that we will have to put an isolated bar for the neutral to be landed in the gutter so that it can run STRAIGHT through each disconnect and on to the structure subpanel

 

And of course the hots will have to have some sort of isolated bar within that gutter so that each hot can branch out to each phase of the disconnects.

 

 

 

 

 

 

ok....well from the weatherhead we are connecting we are bringing down a ground to the disconnect. We lug the disconnect and land the ground there. That ground is suppose to go to building steel right?

No, you only need to take the transformer to building steel.

After the ground leaves the transformer it hits a gutter. The ground is landed on the terminal block within the gutter which is connected with conduit to the individual fused disconnects , then from each fused disconnect to the structure's(it's a fire-rated steel storage unit) actual disconnect and finally the subpanel within the structure. The grounding should be straight forward.

No, again the only grounding wire going to building steel is from the transformer. All grounding after the transformer will be normal. Grounding bushing, grounding at disconnects and subpanels etc. No need to take anything to builidng from this point.

Now when the neutral hits that gutter I'm a little hazy.... I'm guessing that we will have to put an isolated bar for the neutral to be landed in the gutter so that it can run STRAIGHT through each disconnect and on to the structure subpanel

Just like with the grounding wire there is what is called the tap rule. So you can tap off of each wire and drop it down into each disconnect from the gutter. Think of it as a splice using a split bolt to tap off of the main wire down into each. Does that make sense?

And of course the hots will have to have some sort of isolated bar within that gutter so that each hot can branch out to each phase of the disconnects.

Again tap rule, you can run one set of wires across the gutter, and tap off of them to drop them down into each disconnect. You do not need any terminals in the gutter. Your actually making the project harder than it needs to be by doing that.

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Saturday, October 15, 2011 6:59:05 PM

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