3 Phase load study question.

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Euxkan

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Manager
Hello,
Urgent Help!
Our warehouse has a 3 phase 277/480V 4 wires panel with a 1000A breaker.
We need to add 75-KVA step-down transformer to supply the new 200A 120/208V 3 phase 4 wire PNL and need help to confirm if the below is correct.
My electrician just completed 30-day load log and reported that,
Phase A: has 280A as a peak with an average of 112A,
Phase B: has 276A as a peak with an average of 103A, and
Phase C: has 281A as a peak with an average of 107A.
He states that 1000V*0.8 - 279V(avg. of peaks) which is 521A(equivalent to 568KVA) is available.
But my maintenance department says we can not add any more because sums of 3 phases current already exceeded 80% of 1000V and there is no room.
Who is right? And Can we add a transformer to feed 200A 120/208V PNL?
If the electrician is right, what is the maximum amount available? Can we put a larger size PNL?
Thank you for help.
 
Your electrician is correct.
Add with no concern

(It is by no means the proper method but using your maintenance dept's reasoning of adding phases they would need to use 3000 as the available number :) )
 
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Euxkan said:
Phase A: has 280A as a peak with an average of 112A,
Phase B: has 276A as a peak with an average of 103A, and
Phase C: has 281A as a peak with an average of 107A.
He states that 1000V*0.8 - 279V(avg. of peaks) which is 521A(equivalent to 568KVA) is available.
But my maintenance department says we can not add any more because sums of 3 phases current already exceeded 80% of 1000V and there is no room.
Who is right? And Can we add a transformer to feed 200A 120/208V PNL?
Click to expand...
The math is fuzzy. Your maximum current on an one phase is 281 amps. Your panel is rated for 1000 amps. Unless I'm missing something that leaves up to 719 amps not being utilized.
 
So then can I understand that the Electrician is correct? Does it mean that we can have a total of 3 x 200A PNLs by increasing the size of the transformer since only utilized at 281A out of 800A?
 
Euxkan said:
So then can I understand that the Electrician is correct? Does it mean that we can have a total of 3 x 200A PNLs by increasing the size of the transformer since only utilized at 281A out of 800A?
Click to expand...
Short answer yes. Lets say that the available load on the panel is 700 amps that's at 480 volts. A rough conversion via a tranformer to 208Y/120 volts is about 1600 amps.
 
augie47 said:
You are adding less than 100 amps per 200 amp panel....
Click to expand...
Correct. The rough ratio of high to low is 2.3 amps (480/208=2.3). The 200 amps of load on the 208 volt panel would translate to about 87 amps at 480 volts.

Score one for the OP's electrician. (y)
 
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