120/208v 3 ph to 120/240v 1 phase

[bcm]

Happy Friday, Sparky Friends!

I have a client who wants to install a panelboard to feed some antenna rectifiers in a hotel building. The building has a 120/208V, 3 phase, 4000A service. The client refuses to tell me (or doesn't know?) if the rectifier equipment can accept 208V (I've been asking for over a month), so I can only assume it won't accept 208v.
They want to install a 120/240V, 200A, 1 phase panelboard with interlocked ATS (connected to a natural gas gen) to serve the rectifiers which require 2P breakers for each rectifier.
I would put in a 200A, 120/208V, 3ph breaker in the MDP, route the 5 wires (L-L-L-N-G) in conduit up to the roof level, and use a fused disconnect mounted near the panel.
At this point, I am trying to figure out the best way to handle the voltage change. I thought about using buck/boost, but going from 3 phase to 1 phase is going to leave one leg unused and thus potential for load balance issues, right? That's going to happen no matter what since they seem hell-bent on the 200A, 120/240V, 1 phase panel (probably because they already have it). I haven't dealt with this scenario before. Would I use a buck/boost transformer on each of the 2 legs going into the panel or is there a single transformer that could handle it?
Thanks for your input!

 

[don_resqcapt19]

How much total load?

 

[bcm]

How much total load?

Around 135 amps at 120/240V, but they want some room to maybe add a couple more rectifiers in the future.

 

[LarryFine]

There's no difference between a panel for 120/240v 1ph and one for 120/208v 1ph.

Unless the equipment will work on 208v, I suggest a 1ph 208-to-120/240 transformer.

Don't forget about properly bonding/grounding the newly-derived neutral.

 

[d0nut]

Since you will be feeding a panel, don't use a buck/boost transformer. You won't get a usable neutral with a buck/boost transformer scheme. Also, you are bringing too many conductors up to your transformer.

 

[bcm]

There's no difference between a panel for 120/240v 1ph and one for 120/208v 1ph.

Unless the equipment will work on 208v, I suggest a 1ph 208-to-120/240 transformer.

Don't forget about properly bonding/grounding the newly-derived neutral.

I would calculate it as a single phase transformer using the secondary voltage though, correct? 240v * 200A = 48kVA -> 50kVA?

 

[augie47]

agree with others. bring single phase 208 to a 50 kva 208-240/120 transformer.

 

[bcm]

Since you will be feeding a panel, don't use a buck/boost transformer. You won't get a usable neutral with a buck/boost transformer scheme. Also, you are bringing too many conductors up to your transformer.

Oh, that's right... I forgot there's no neutral with a buck/boost. I was thinking I should bring all the conductors to the roof level, but if I understand correctly, that's just a waste of copper. I only need (L-L-G) to the transformer which will then provide the neutral and give me (L-L-N-G) downstream of it, right? The third L isn't going to be used, so there's no reason to have it.

 

[LarryFine]

I would calculate it as a single phase transformer using the secondary voltage though, correct? 240v * 200A = 48kVA -> 50kVA?

Yes, on the secondary side. Remember that the same 48kva will require greater current (~230a) at 208v.

 

[don_resqcapt19]

If there are no 120 volt loads, I would look at a 208, 240Y/138 volt transformer. Often used as drive isolation transformers.

 

[bcm]

If there are no 120 volt loads, I would look at a 208, 240Y/138 volt transformer. Often used as drive isolation transformers.

They want some convenience receptacles coming from this panel too, so there would be some 120V loads. Thanks!

 

[tortuga]

I have a client who wants to install a panelboard to feed some antenna rectifiers in a hotel building. The building has a 120/208V, 3 phase, 4000A service. The client refuses to tell me (or doesn't know?) if the rectifier equipment can accept 208V

Are you talking about telcom stuff like a micro cell site? I have never seen one that does not accept 208, some even accept 100 - 300 volts input.
I have seen quite a few that wont accept 277 oddly, so its generally 240V max. in their specs.

 

[petersonra]

Are you talking about telcom stuff like a micro cell site? I have never seen one that does not accept 208, some even accept 100 - 300 volts input.
I have seen quite a few that wont accept 277 oddly, so its generally 240V max. in their specs.

View attachment 2574396

Hmm. Can't spell switch right but it's good enough to use in a cell tower

 

[Elect117]

I have seen quite a few that wont accept 277 oddly

line to neutral. They probably require phase to phase to ground (where the ground is reference).

 

[bcm]

Are you talking about telcom stuff like a micro cell site? I have never seen one that does not accept 208, some even accept 100 - 300 volts input.
I have seen quite a few that wont accept 277 oddly, so its generally 240V max. in their specs.

View attachment 2574396

No, it's not microcell. I've done some of that work before. They won't give me any specs on the rectifiers. Top secret, I guess.