Transformer size selection

Status
Not open for further replies.
C

curious101

Senior Member
Location
Atlanta, GA, USA
Good morning,
I have a proto food/retail commercial job, 120/208V, 3ph, 4w: 1200 service disconnect switch, 1000A MLO MDP and panels fed from here. Connected kVA load on MDP is 460 kVA. At a certain site suddenly only 480V is available as service. Do I have to use 500kVA 480 to 208V transformer ahead of all the described above, or can I use a 300 kVA transformer?
Thanks, and have a great day!
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
If the load is 460kVA, how can a 300kVA transformer handle that without overloading and/or the required protection tripping?
The transformer needs to be sized for the load.

You can't size the transformer like the utility can, where they use historical trends of similar installations to guess at the actual demand load and count on short term overloads, where they can oversize the protection on their side.
 
C

curious101

Senior Member
Location
Atlanta, GA, USA
ron said:
If the load is 460kVA, how can a 300kVA transformer handle that without overloading and/or the required protection tripping?
The transformer needs to be sized for the load.

You can't size the transformer like the utility can, where they use historical trends of similar installations to guess at the actual demand load and count on short term overloads, where they can oversize the protection on their side.
Click to expand...
That's what I thought and was pushing for, but just wanted to get someone else's opinion on it too. Thank you so much! Have a great weekend!
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
ron said:
If the load is 460kVA, how can a 300kVA transformer handle that without overloading and/or the required protection tripping?
The transformer needs to be sized for the load.
Click to expand...
Never mind the transformer...how is the 1000A MDP going to handle the 460kVA load?
 
D

d0nut

Senior Member
Location
Omaha, NE
If it doesn't mess up a brand's standard purchasing agreements, I would see how much of the mechanical equipment and other larger loads you could get at 480V. It could save you some transformer size.
 
C

curious101

Senior Member
Location
Atlanta, GA, USA
david luchini said:
Never mind the transformer...how is the 1000A MDP going to handle the 460kVA load?
Click to expand...
Yes, that's what I was thinking about too. I am just helping out on this job. What I am being told is that the load that they are seeing is probably 800A at 208V, 3ph (288kVA)...yes, I know the connected is 460kVA.... They want to keep the prototypical setup, which they usually use (1000A MLO, MDP). So, I was thinking if they use 800A/3P fused service disconnect switch on the primary side of the 500kVA transformer, the secondary of the transformer would feed the 1000A MLO MDP. The transformer would be protected on primary only per 450.3(B).
 
C

curious101

Senior Member
Location
Atlanta, GA, USA
d0nut said:
If it doesn't mess up a brand's standard purchasing agreements, I would see how much of the mechanical equipment and other larger loads you could get at 480V. It could save you some transformer size.
Click to expand...
They don't want to mess up the prototypical setup.... My original idea was like yours...
 
david luchini

david luchini

Moderator
Staff member
Location
Connecticut
Occupation
Engineer
curious101 said:
Yes, that's what I was thinking about too. I am just helping out on this job. What I am being told is that the load that they are seeing is probably 800A at 208V, 3ph (288kVA)...yes, I know the connected is 460kVA.... They want to keep the prototypical setup, which they usually use (1000A MLO, MDP). So, I was thinking if they use 800A/3P fused service disconnect switch on the primary side of the 500kVA transformer, the secondary of the transformer would feed the 1000A MLO MDP. The transformer would be protected on primary only per 450.3(B).
Click to expand...
For a 288kVA load, you could use the 300kVA transformer. You will need an OCPD on the transformer secondary, so an MCB MDP would be the obvious way.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
There's a difference between the total connected load and the calculated load, which takes into account non-coincidental loads like heating and air-conditioning, that won't run simultaneously; only the larger of the two need be considered.
 
C

curious101

Senior Member
Location
Atlanta, GA, USA
LarryFine said:
There's a difference between the total connected load and the calculated load, which takes into account non-coincidental loads like heating and air-conditioning, that won't run simultaneously; only the larger of the two need be considered.
Click to expand...
Actually, I am just getting more familiar with the drawings... sorry..... calculated demand load is 323 kVA...
 
Status
Not open for further replies.
Top