45kva step down transformer

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hbdocm

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Hi,

I was called out on a job to run a new 30a 208v 2 pole circuit in a commercial building only to find the panels full. They are currently running a 600amp main 277/480 to a 100 amp 277/480 sub upstairs and downstairs. They also are running a 100amp from the 277/480 to a Sylvania 45kva stepdown transformer tapped to 208/120 and feeding (2) 100amp subpanels downstairs and I believe one upstairs. They want this job done as cheaply as possible and one of the subs downstairs is a Murray 18 space panel. My question is I was thinking about changing the 18 space to a 24 space to get the 2 spaces I need for the circuit. Do I need to upgrade the feed to the transformer to a 200amp? I have done alot of commercial in the past but usually the basics such as the lighting, pulling circuits etc. and have not had to deal with the transformers much. Is there a limit as to how many amps the transformer can handle? I know it is a 45,000 watt transformer but unsure of the math. Is it 45,000 @ 277, 480? etc. etc.
Thanks in advance!

Martin
 
45 at 08/120 = 125 amps
45 at 480 = 54 amps

present load or load calculations would determine additional permissible loads.
 
brian john said:
45 at 08/120 = 125 amps
45 at 480 = 54 amps

present load or load calculations would determine additional permissible
loads.
Click to expand...

Footnote if I may, nothing adding up totally to over 80% is allowed to be added to any of your existing panels...

Lets say it another way, with adding this new service the sum of all loads can't be over 80% to any panel.

... Please correct me otherwise ...

I love my Job, I just hate the hours !!!
 
hbdocm said:
Is it 45,000 @ 277, 480? etc. etc.
Click to expand...
To answer this directly, it's 45kva at any voltage. It's the current that varies with voltage. The reason we distribute higher voltages is that the conductors can be smaller while still being able to deliver a given amount of power.
 
You mention there are 3-100A panels supplied by the transformer in its existing state.
Now you want to add to the existing load. In order to do so, you will need to determine the existing load.
You may not have much room to play with here.
I am not a betting man, but I would be willing to bet you will be installing a new transformer for this job.
 
If new transformer necessary

If new transformer necessary

If it is currently running off a 100a breaker than the transformer would have to step up to a 90kva? With 1-0 feeders? To go to a 200a? Or is there room to step up to a 125a breaker. Never had to calculate just had to read the blueprints in the past.

Thanks!

Martin
 
hbdocm said:
If it is currently running off a 100a breaker than the transformer would have to step up to a 90kva? With 1-0 feeders? To go to a 200a? Or is there room to step up to a 125a breaker. Never had to calculate just had to read the blueprints in the past.

Thanks!

Martin
Click to expand...

Whether you want to calculate the primary or the secondary amps of the transformer you divide the watts by the volts then divide that by 1.73 (square root of 3) when the system is three phase
 
45kva step down transformer

Section 450.3 permits the primary to be protected 125% without protecting the secondary or to protect the primary to 250% than it does require the secondary to be protected at 125%.
If the transformer primary is protected at 125% the primary breaker is required to be sized not more than 70 amperes.
I = KVA X 1000 / [E X √3]
I = 45 X 1000 / [480 X √3]
I = 45000 / 831
I = 54 A
I = 54 ? 1.25%
I = 67.5 A
Note 1 to table 450.3(B) permits where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.
If the transformer primary is protected at 250%, primary breaker cannot exceed 125 amperes, the secondary breaker could be sized to 175 amperes.
Primary
I = KVA X 1000 / [E X √3]
I = 45 X 1000 / [480 X √3]
I = 45000 / 831
I = 54 A
I = 54 ? 250%
I = 135 A
Section 450.3(B) does not permit the next size up when using the 250% to protect the primary, so the next size down would be 125 ampere breaker.
Secondary:
I = KVA X 1000 / [E X √3]
I = 45 X 1000 / [208 X √3]
I = 45000 / 360
I = 125 A
I = 125 ? 125%
I = 156 A
If the transformer primary is protected at 250% the secondary breaker is permitted to be sized to 125% and can be 170 amperes, per note 1 to table 450.3(B).
As for protection of the secondary conductor section 240.21(C) requires the conductors to be protected there full ampacities, and cannot be more than 25 feet long.
 
if transformer is a 45kva on the primary side it should be pulling around 96 amps this means the transformer you have now will not work for you. on the secondary side its pulling 192 amps. so you will need a bigger kva maybe 100kva. a 100 will give you 208amps on the primary side and 416amps on the secondary side. that is if you have a 200amp service. if you use copper as the feeder use 2/0 if aluminum 4/0
 
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rather than assuming anything, it would be a good idea to do the load calc as was suggested. you might have noncoincidental loads that still leave room for your desired additional load.
 
a got lost a little but here's my take.
45 kva.since its 208Y120, 3 phase
if you don't have the secondary protected at 125% or less then the primary must be at 125% which would be a 70 amp breaker.
If the primary breaker is 70 amps or less, then I see no Code problem with your plan to upsize the panel, but as a number of folks have suggested, your load calculatins probably won't allow this addition without upsizing the transformer.
 
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