Transformer hookup and voltage drop? - Electrician Talk - Professional Electrical Contractors Forum

myles · 08-13-2009, 04:16 PM

Alright so i'm having some issues with an existing installation on one of my projects. Here's the situation...

We have a 120v service that is about half a mile away from our cabinet that receives the power. The original contractor installed (2) step-down dry transformers, just reverse feeding one of them in order to step 120 up to 480. Then at the cabinet stepping down 480 to 120.

The application:
Convert 120v up to 240v on two legs. Run the two legs and ground half a mile to the other transformer then step it down from 480 to 120.

The problem:
The problem that we're having is that when i test the power at the transformer on H1 i'm getting 220v and on H4 i'm getting 260v, when i test the power at the first pull box i'm getting like a 60v drop on the 260v line. I switched the wires and on the same wire i'm getting a voltage drop.

My first assumption is that the wire is damaged and somehow draining some of the voltage without tripping the breaker. Thing is...per plans there are not any more pull boxes between where i tested the wire and the transformer.

I just want to make sure that we have this transformer hooked up correctly before i have some guys try and find another pull box (could be a lot of sod damage).

Attached is picture of the front of our tranformer.

On the power side where the power comes in at 120v we have it hooked up as follows:

X4 & X2 - connected together and connected with line feed 120v
X3 & X1 - connected together and connected with line neutral

H2 & H3 - connected together and capped off
H1 - connected to one leg that goes to cabinet
H4 - connected to the other leg that goes to cabinet

In theory this should be correct; however, whenever i test the voltage on H2 & H3 i'm getting like 60v, when i test H1 to H4 i get 480, H1 to ground i get 220v, H4 to gorund i get 260v.

But as i said before when i test the wires at the first pull box i'm getting like a 60v drop on one of the legs.

What do you guys think? Anybody got any kind of suggestions?

I was thinking, if i can't figure out the problem i could just use the one leg that doesn't have a voltage drop and then just convert 240 down to 120 at the cabinet.

Any suggestions would be greatly appreciated. If anybody has any links to or any pictures that explains transformer wiring that would be great too.

Thanks for the help!

Darkjim · 08-13-2009, 07:09 PM

I suggest that you contact Siemens to find out if that Xfmr can be used in reverse applications. Alot of smaller xfmrs are wound with secondary windings inside of the primary windings. When you use them in reverse of what they are intended, they have induction issues. Assuming that you are using a DMM to measure voltage, you are probably picking up ghost voltages at the transformer(because of induction issues)that you do not get further away.

Jim

InPhase277 · 08-13-2009, 08:05 PM

The transformer doesn't know which direction the feed is coming from. I think our problem is that you are trying to get a voltage reading from an ungrounded secondary to ground. In other words, 120 V is being stepped up to 480, but the 480 side has no electrical connection to ground, therefore any reading you take between it and ground is a phantom, because there is no real reference.

On the supply end, the center tap, H2-H3, of the 480 should be tied to the 120 V circuit ground. You should probably put some fuses in the 480 lines and fuse the 120 V secondary as well. On the receiving end, one leg of the 120 V secondary should tied to ground. Like this:

Richard Rowe · 08-14-2009, 04:12 PM

Don't mean to be smart about this but did it ever work? I know you said someone else put it in and now your working on it. When you test it at the Boost trans have you put some type load on it or maybe a wiggy to see if the power reading is really there? If you switched the wires and only had a drop on the same wire..... shovel time!

Richard Rowe · 08-14-2009, 04:14 PM

However don't go a diggin just cause I said so.

InPhase277 · 08-14-2009, 05:36 PM

Quote:

Originally Posted by Richard Rowe

Don't mean to be smart about this but did it ever work? I know you said someone else put it in and now your working on it. When you test it at the Boost trans have you put some type load on it or maybe a wiggy to see if the power reading is really there? If you switched the wires and only had a drop on the same wire..... shovel time!

I'm willing to bet he put a digital meter between an ungrounded secondary and ground, and got his strange reading. The conductors between the transformers probably had no connection anywhere to ground, and therefore were "floating", with no real ground reference. The voltages you will read in that case are purely fictional, generated by the capacitance between the wires and whatever you are measuring against.

myles · 08-14-2009, 07:54 PM

Quote:

Originally Posted by InPhase277

I'm willing to bet he put a digital meter between an ungrounded secondary and ground, and got his strange reading. The conductors between the transformers probably had no connection anywhere to ground, and therefore were "floating", with no real ground reference. The voltages you will read in that case are purely fictional, generated by the capacitance between the wires and whatever you are measuring against.

You would be correct. I wasn't the one who actually did all the testing i had a technician go out there and obviously he knows nothing about electrical. After talking with Siemens today and going out there myself i found out the "actual" problem.

Turns out the transformers were perfectly fine. When i measured the reading between the two hots i had a perfect 480 all the way to the second pull box. After that pull box i was losing about 100 volts on the line.

The technician was testing it line by line which since he is only used to residential i wont hold too much against him.

Anyways, turns out the real problem...somehow inside the conduit in the very middle one of the wires split (how...i have NO idea, it looks like maybe there was a nick in the wire and eventually it corroded away the copper?) The entire conduit is full of water and going to be so much fun on Monday morning.

Anyways, i appreciate all the input guys, i just wanted to make sure the transformer was hooked up correctly before we tried doing anymore troubleshooting. Especially since when i was an electrician i was told to NEVER reverse wire a transformer...

micromind · 08-14-2009, 07:55 PM

Deleted, OP found problem.

Rob

Richard Rowe · 08-15-2009, 10:40 PM

InPhase277 good call you were right on top of that one. I have noticed you are pretty good on trouble shooting posts. I have a habit of overlooking the basic and trying to find complex things that don't exist. Good job.

InPhase277 · 08-16-2009, 12:02 PM

Quote:

Originally Posted by Richard Rowe

InPhase277 good call you were right on top of that one. I have noticed you are pretty good on trouble shooting posts. I have a habit of overlooking the basic and trying to find complex things that don't exist. Good job.

Thanks. I have had my days where I search high and low to figure out why a light doesn't work, and it turns out it was the bulb

A couple of those and you start looking at the basics first.