Weird voltages, step-down transformer, 277VAC-120VAC

[Cederbee]

Hello,

I have somewhat of a weird scenario, I'm hoping someone can shed some light on my issue.

I am on a job where I only have 1 EM circuit, 277VAC. It is shared between multiple floors.

In my unit I have installed lights for both 277VAC EM lighting, and 120VAC EM lighting. I have nLight lighting controls for my lights.

My lights on the 277VAC EM are fine, controls work, et cetera. This is feeding straight from the breaker, on another floor.

Since this is my only EM circuit and I have 120VAC EM lights, I have to step-down the voltage, obviously with a step-down transformer.

I have now gone through 3 different step-down transformers, all resulting with the same issues.

1) I hook up L1 on H1, L2 on H2 (277VAC)
2) X1,X3 together, X2,X4 together, no other leads coming off the step-down to hook-up
3) testing voltage off the leads gives me hot on X1,X3, neutral on X2,X4
4) test voltage, 120VAC

So, I have what I need right off the leads. From my leads I have a 12/2 MC carrying the 120VAC to a J-Box. With nothing connected to my 12/2 whip in the J-Box, both hot and neutral carry voltage. 120VAC between hot and neutral, 12VAC to ground on hot and neutral.

My loads I need to connect to this are:
1) 120VAC EM lights, being feed through an nLight EM power-pack (this power-pack works, I have temped in from another local 120VAC power source to verify devices are working correctly, they are when not connected to the secondary of the step-down)
2) 120VAC fire-dampers

When I hook-up the fire-dampers, only, I get 60VAC on both neutral and hot.
When I hook-up the power-pack, only, I get back feed only the neutral, 120VAC.
When I hook-up both loads, I get voltage only on the neutral.

I have set the transformer up in every configuration, I still get the same issue.
Whether I go parallel, X1-X3, X2-X4; series and pull neutral off X2-X3; or even isolate coils; I still get the same weird voltages.

At this point I'm wondering if a bonding jumper would help to create a new neutral, or maybe the service has an issue with their neutral installation; but I'm not sure if I would want to bond that on the load side. Anyway, my guess is there is something up with the system neutral, any help is greatly appreciated! Thank you!

 

[hbiss]

12VAC to ground on hot and neutral.

Think about it. Your 120v secondary is completely isolated so how would you expect to see anything to ground? I would expect to see nothing but you are probably using a high impedance meter and the 12v is a phantom voltage. Use a Wiggy or just put an incandescent light bulb across your test leads and it will go away.

Now, think about how your secondary neutral needs to be bonded to the primary neutral.

-Hal

 

[zbang]

I'm not sure if I would want to bond that on the load side.

You don't have much choice- read up on Separately Derived Systems.

 

[LarryFine]

You're so used to working with utility supplies, you don't realize that having a grounded source requires an intentional action.

You need to intentionally ground one of your secondary conductors. Follow Zbang's advice, both here (the forum) and the NEC.

 

[SSDriver]

You have to ground it(secondary) or comply with the rules for an ungrounded system. Check out 250.30 (A).

 

[Jerramundi]

Just to clarify a bit to the previous three posts (posts #3-5), that's only if it's a SDS such as an isolation transformer. The OP never stated what it was. It could be an autotransformer.

Although he did at one point mention the word "secondary" and said he tried "isolating the coils." But I think we've all been engaged in that debate at one point or another (i.e. that application of the word "secondary" to either an isolation transformer or an autotransformer).

 

[Cederbee]

Thank you everyone who replied.

I realize where my error was, thanks for shedding some light on my problem.

I did connect X2-X4, bonded to ground on the secondary side.

All voltages have become balanced and issue is resolved!

W00t!