XFRMR PRIMARY AND SECONDARY BOTH FEEDING 208/480V

mstrlucky74

Senior Member
Location
NJ
Have you ever seem this? It's not stepping the voltage up or down. I double checked the xfrmr schedule and panel schedules to see if there was a mistake and I don't think so according to the drawings. The xfrmr schedule reads as follows:

KVA= 75
PRIMARY AMPS= 90
SEC AMPS= 90
480V OVERCURRENT= 100A/3P
208V OVERCURRENT= 480V. 100A/3P

So it sure looks like the engineer knows its feeding 208/480 on both sides.
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
I don't know how you would get 480v & 208v on either, let alone both sides.

My guess would be its intended to be a 480V - 480/277V transformer, to create a 4-wire system from a 3-wire system.
 

mstrlucky74

Senior Member
Location
NJ
I don't know how you would get 480v & 208v on either, let alone both sides.

My guess would be its intended to be a 480V - 480/277V transformer, to create a 4-wire system from a 3-wire system.
Your right and that is what it says....480v-to 480/277 but I don't understand that. Is that stepping it down? I thought a transformer always had to go from 120/208 to 277/480 or the other way around? Both panels say 277/480:?. What are they intending to do here.

Thanks
 

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
Your right and that is what it says....480v-to 480/277 but I don't understand that. Is that stepping it down? I thought a transformer always had to go from 120/208 to 277/480 or the other way around? Both panels say 277/480:?. What are they intending to do here.

Thanks
I would guess, they have a 480V 3-wire system (no neutral). They have some 277V loads that they want to supply. They are creating a 480/277V separately derived system with a 480-480/277V Delta-Wye transformer.

There is no stepping up or down, just transforming from 3-wire (A,B,C) to 4-wire (A,B,C,N).
 

mstrlucky74

Senior Member
Location
NJ
I would guess, they have a 480V 3-wire system (no neutral). They have some 277V loads that they want to supply. They are creating a 480/277V separately derived system with a 480-480/277V Delta-Wye transformer.

There is no stepping up or down, just transforming from 3-wire (A,B,C) to 4-wire (A,B,C,N).
That is exactly right, that is what they show.
 

mstrlucky74

Senior Member
Location
NJ
I would guess, they have a 480V 3-wire system (no neutral). They have some 277V loads that they want to supply. They are creating a 480/277V separately derived system with a 480-480/277V Delta-Wye transformer.

There is no stepping up or down, just transforming from 3-wire (A,B,C) to 4-wire (A,B,C,N).
Oh and thanks. I guess that can't be done(supply some 277 loads) without the xfrmr??? They are 277/480v panels.
 

Smart $

Esteemed Member
Location
Ohio
...They are 277/480v panels.
Is the transformer primary fed from a 480/277 panel? If so, is either the primary or secondary system an ungrounded or high-impedance grounded neutral system??? If so, the transforme is to provide isolation between the two types of systems.
 

mstrlucky74

Senior Member
Location
NJ
Is the transformer primary fed from a 480/277 panel? If so, is either the primary or secondary system an ungrounded or high-impedance grounded neutral system??? If so, the transforme is to provide isolation between the two types of systems.
Yes it is primary fed from 277/480. How do I tell if the secondary or primary is either ungrounded or high-impedance grounded neutral system? Teach me:dunce:

Thanks a lot Smart!!
 

Smart $

Esteemed Member
Location
Ohio
Yes it is primary fed from 277/480. How do I tell if the secondary or primary is either ungrounded or high-impedance grounded neutral system (HRG)? Teach me:dunce:

Thanks a lot Smart!!
First place I'd check is the supplying transformer symbols on the one-line diagram. Typically for HRG, the symbology will have a rectangle (the resistor) in the neutral to ground connecting line. An ungrounded system should have no neutral to ground connecting line... but you can't rely on this alone.. as some dingalings don't show a N-G line for grounded systems. So look for another symbol or a note marker indicating a ground detector.


If I get a chance later, I'll try to find a one-line of an ungrounded system (unless someone else does so first).

For HRG symbology, look at the transformers (and emergency generators) in Figure 1 and 2 of this document:

http://static.schneider-electric.us/docs/Electrical Distribution/Low Voltage Switchgear/0100SM0501.pdf
 

Smart $

Esteemed Member
Location
Ohio
...

If I get a chance later, I'll try to find a one-line of an ungrounded system (unless someone else does so first).

...
Went looking, couldn't find a decent one. 480/277 systems are typically grounded (HRG or solid). Ungrounded 480V 3? is usually 3W (or delta).
 

mstrlucky74

Senior Member
Location
NJ
First place I'd check is the supplying transformer symbols on the one-line diagram. Typically for HRG, the symbology will have a rectangle (the resistor) in the neutral to ground connecting line. An ungrounded system should have no neutral to ground connecting line... but you can't rely on this alone.. as some dingalings don't show a N-G line for grounded systems. So look for another symbol or a note marker indicating a ground detector.


If I get a chance later, I'll try to find a one-line of an ungrounded system (unless someone else does so first).

For HRG symbology, look at the transformers (and emergency generators) in Figure 1 and 2 of this document:

http://static.schneider-electric.us/docs/Electrical Distribution/Low Voltage Switchgear/0100SM0501.pdf
THank a lot Smart..your a BIG help. THere was not symbol like you describe on the drawings. Thanks for the link....helpful
 

Smart $

Esteemed Member
Location
Ohio
THank a lot Smart..your a BIG help. THere was not symbol like you describe on the drawings. Thanks for the link....helpful
Glad to help... but it doesn't sound like the mystery is solved.

There is no certainty that everyone uses the same HRG symbology... or any at all for that matter. Also, forgot to mention earlier that sometimes the resistor isn't located at the transformer... so it may be shown in the switchgear. See page 14 and 15 in the doc I linked to earlier... there's a rectangle with an offset (ziggy) line through it, between an upside down ground symbol and the bus line... but I'm not certain that's what it indicates. I'd have to dig out my symbol references.

If the transformer is fed by a 480/277 system, and it feeds a 480/277 system, there has to be some reason... otherwise it's just a waste.
 

SG-1

Senior Member
Glad to help... but it doesn't sound like the mystery is solved.

There is no certainty that everyone uses the same HRG symbology... or any at all for that matter. Also, forgot to mention earlier that sometimes the resistor isn't located at the transformer... so it may be shown in the switchgear. See page 14 and 15 in the doc I linked to earlier... there's a rectangle with an offset (ziggy) line through it, between an upside down ground symbol and the bus line... but I'm not certain that's what it indicates. I'd have to dig out my symbol references.

If the transformer is fed by a 480/277 system, and it feeds a 480/277 system, there has to be some reason... otherwise it's just a waste.
The symbol is a surge suppressor. Zork is one brand.
 

dicklaxt

Senior Member
I'm not an expert on this by any means but this scenario is sometimes used in drive isolation to reduce transient generation and act as a buffer for SCR current surges.

That statement kind makes me sound like a smart guy but I looked it up in a transformer listing LOL. I'm the same dumb old man passing the time away. Do you by chance have any DC drives on the load side?

dick
 
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