3-Single Phase UPS's on a 3 Phase System

Status
Not open for further replies.
B

bnbowen@uiuc.edu

Member
I need to pull a 3 phase, 4 wire, 120/208 volt service to a panel that will power three single phase UPS's and a few smaller single phase non linear loads. Each UPS will feed a single phase panel. All UPS loads will be non linear either 120, 208 or 240 single phase. The UPS inputs will be balanced across all three phases (AB, BC, CA). All MWBC's will have their own neutral.
Do I have to double the neutral on the 3 phase service and on the single phase UPS output panels? Thanks for your help and HAPPY NEW YEAR.
 
How are the single phase UPSs powered, and how do they produce the required output voltages.

Are the UPSs supplied with 208V and then internally produce the required 120, 208, and 240?

Or are the UPSs supplied with 120/208V, connected to two lines and the neutral, and then supply 120/208V to the loads. In this case, where does the 240V come from?

-Jon
 
The three UPS's are APC Symmetra with 120/208 volt single phase input and 120/208 volt single phase output. They do require a neutral on the input.
 
bnbowen@uiuc.edu said:
The three UPS's are APC Symmetra with 120/208 volt single phase input and 120/208 volt single phase output. They do require a neutral on the input.
Click to expand...

After reading the electrical requirements in the Owner Manual, the Symmetra UPS's are connected Line-to-Line only and do not require a neutral on the input. Cord-connected models use the L6-30R twist lock configuration. Since this connection is only a three-blade configuration, it indicates two lines (208V, in your case) and an EGC only.... no neutral. With no neutral on the UPS's input, there is no justification for a double-size, panel-feeder neutral.

In fact, the panel-feeder would likely be fine with even a reduced-size neutral... but I do not have enough information to make that determination with any degree of certainty. You did not mention whether the other smaller loads were L-L or L-N. I'd need to know all the loads. The picture I'm getting now is that the max unbalanced neutral load is only a small percentage of the Line loads.
 
Smart $ said:
The picture I'm getting now is that the max unbalanced neutral load is only a small percentage of the Line loads.
Click to expand...
For the input side of the UPS, you need also to consider harmonic currents. As the units are all single-phase, there is likely to be considerable third harmonic content and, as I'm sure you know, these will add in the supply neutral.
 
Besoeker said:
For the input side of the UPS, you need also to consider harmonic currents. As the units are all single-phase, there is likely to be considerable third harmonic content and, as I'm sure you know, these will add in the supply neutral.
Click to expand...


After reading the electrical requirements in the Owner Manual, the Symmetra UPS's are connected Line-to-Line only and do not require a neutral on the input.
Click to expand...

How do we consider harmonic currents on a non existent conductor?
 
Besoeker said:
Post #4 states that the units require a neutral.
Click to expand...

I undestand that, but that seems to be an error.

After reading the electrical requirements in the Owner Manual, the Symmetra UPS's are connected Line-to-Line only and do not require a neutral on the input. Cord-connected models use the L6-30R twist lock configuration. Since this connection is only a three-blade configuration, it indicates two lines (208V, in your case) and an EGC only.... no neutral. With no neutral on the UPS's input, there is no justification for a double-size, panel-feeder neutral.
Click to expand...
 
iwire said:
I undestand that, but that seems to be an error.
Click to expand...

Here's the hardwiring configuration...

input_wiring.gif
 
Besoeker said:
Given that the second terminal is matked L2/N, does that mean that the unit could be connected L-L or L-N?
Otherwise what would be the point of the "N" marking?
Just asking.
Click to expand...

I imagine it's a generic block for all models, perhaps inclusive of international models where +200V connection could be L1 + N. The equipment itself don't care whether the voltage is L-L or L-N.

In the OP's case, a 1? 208V connection to a 208Y/120 3? 4W panel would be Line-to-Line... no neutral.
 
Last edited:
Smart $ said:
I imagine it's a generic block for all models, perhaps inclusive of international models where +200V connection could be L1 + N. The equipment itself don't care whether the voltage is L-L or L-N.
Click to expand...
Yes, true. That said, I don't know anywhere that uses 200Vac L-N. More common around that level is 220/230Vac 50Hz.

Smart $ said:
In the OP's case, a 1? 208V connection to a 208Y/120 3? 4W panel would be Line-to-Line... no neutral.
Click to expand...
I agree. I just made the rather rash assumption that, when it was stated in post #4 that a neutral was required, that a neutral was required.
Obviously, if post #4 is incorrect and no neutral is required, there will be no neutral current.
 
Besoeker said:
Yes, true. That said, I don't know anywhere that uses 200Vac L-N. More common around that level is 220/230Vac 50Hz.
Click to expand...

I think you missed my plus sign (+) in front of the "200V". Perhaps I should have used greater than sign (>).
 
Last edited:
Smart $ said:
I think you missed my plus sign (+) in front of the "200V". Perhaps I should have used greater than sign (>).
Click to expand...
I didn't miss it, Smart.
A positive or negative sign related to voltage would, for me, imply DC.
The > sign would have avoided that ambiguity.
No big deal.
 
It is possible (without pulling the manual) that these UPSs can do 208 in, 120 out. If they can and are so configured then there is no neutral connection so no harmonic current issues. However, they also cant do bypass as the input voltage is different to the output voltage.

If they are wired normally (ie 120V in, 120V out, with the source being L to N) then given that most UPSs are non-linear loads then an uprated neutral would be an excellent idea.
 
Status
Not open for further replies.
Top