Can I add a transformer to derive a neutral?

[Tjh4405]

I’m working on a commercial job where they put a sign but never ran power to it. They want me to run power to it now. The nearest power is at a light pole which seams like a fine place to tie in. The sign load is 11 amps. The power is 3 phases 208v and a ground. I am not sure yet if the load is balanced. They also want to add an outlet at the sign for seasonal lighting. Can I just add a transformer to get my neutral and at 120 volts? This is new for me so I just want to make sure I’m on the right track with my plan.

 

[LarryFine]

Welcome to the forum.

You need to know what's at the pole now, and what load is already on it.

If the pole is supplied with conduit, look into adding a new circuit/wires to it.

 

[Tjh4405]

Welcome to the forum.

You need to know what's at the pole now, and what load is already on it.

If the pole is supplied with conduit, look into adding a new circuit/wires to it.

Yeah. They upgraded the entire parking lot to LED and I haven’t actually taken amp readings yet. You’re right though. I should have done that by now.
No conduit either. All direct burial. There are about thirty lights and split between 3 phases. They ran 2 #6’s for each phase. I do have the option of going back one mor pole and picking any phase to add the load.

 

[LarryFine]

Then find out whether you have L-L or L-N voltage; i.e., is there a grounded conductor in the pole?

If you only have L-L, and you want 120v at the sign then yes, you'll need a 208-240/120v transformer and a GES.

What voltage does the sign require?

Another thought: With LED lights, look into re-wiring each branch, or jut the one you need, to L-N.

 

[Tjh4405]

Then find out whether you have L-L or L-N voltage.

It’s L-L because I got 120 from L-G on each phase but 208v L-L

 

[LarryFine]

It’s L-L because I got 120 from L-G on each phase but 208v L-L

Then we're back to the options I listed above. What does the sign require, and how important is the receptacle?

 

[Tjh4405]

Then we're back to the options I listed above. What does the sign require, and how important is the receptacle?

They said it’s not too important to have a receptacle but they would easily spend 2-$300 for it. The sign has fluorescent tubes and can take 90-277 if I remember correct.

 

[LarryFine]

Then you're good on the sign, and only the receptacle matters, so look for a 208-120v transformer; 2.5kva will give you 20a @ 120v.

Don't forget to ground one leg of the secondary, and you should use a GFCI receptacle for that reason.

 

[paulengr]

A second but rare option is a zig zag transformer. It has 4 leads, 3 phase and a neutral. It creates a neutral from a delta source.

 

[LarryFine]

A second but rare option is a zig zag transformer. It has 4 leads, 3 phase and a neutral. It creates a neutral from a delta source.

His source is only 208v 1ph.

 

[Tjh4405]

Then you're good on the sign, and only the receptacle matters, so look for a 208-120v transformer; 2.5kva will give you 20a @ 120v.

Don't forget to ground one leg of the secondary, and you should use a GFCI receptacle for that reason.

Awesome. That is the direction I was going. Thank you. I’ve never added a transformer in the middle of a load so I was unsure of if I could. Would have been nice if they had run the neutral from the lighting transformer.

 

[Tjh4405]

Awesome. That is the direction I was going. Thank you. I’ve never added a transformer in the middle of a load so I was unsure of if I could. Would have been nice if they had run the neutral from the lighting transformer.

His source is only 208v 1ph.

Correct, that was part of what confused me. I’m used to a delta-wye setup. That’s all we get at the underground coal mine where I also work. But doing it with only one phase was different.

 

[LarryFine]

You have two legs of a wye secondary, without the neutral. Wiring the sign for that relieves the transformer of its current.

 

[Tjh4405]

You have two legs of a wye secondary, without the neutral. Wiring the sign for that relieves the transformer of its current.

That is a better way of saying it. And I’m glad you mentioned not putting the sign load on the added transformer because that should make it even less expensive.

 

[winnie]

A second but rare option is a zig zag transformer. It has 4 leads, 3 phase and a neutral. It creates a neutral from a delta source.

Not a good solution here. The OP certainly has a 208/120 grounded supply, simply a circuit/feeder without the neutral.

A zig-zag transformer would re-derive the neutral, which you couldn't ground, since you already have a grounded system. So you have an ungrounded conductor at almost ground potential that you want to use as a grounded conductor.

A single phase 208:120 transformer is the way to go.

Jon

 

[GoldDigger]

...
...
A single phase 208:120 transformer is the way to go.

Jon

Which would have to be connected as an SDS, since there is no neutral available. One side of the secondary would have to be grounded, and that might require an additional local ground electrode if the existing EGC does not qualify as a GEC.

 

[oldsparky52]

Which would have to be connected as an SDS, since there is no neutral available. One side of the secondary would have to be grounded, and that might require an additional local ground electrode if the existing EGC does not qualify as a GEC.

Just part of the job. I assumed ground rods would be required. Also, OCP would have to be provided for the receptacle.

 

[hillbilly1]

I would check to see if the sign is multi volt, if it is, all you could use the 208, and just fuse accordingly.

 

[retirede]

I would check to see if the sign is multi volt, if it is, all you could use the 208, and just fuse accordingly.

He states “the sign is good for 90-277”.

He wants to add a 120v convenience receptacle.

 

[petersonra]

I think it will cost more than $2-300 just in labor to add the 120 V xfmr for the receptacle, unless your labor is free.