3 phase current draw from 3 xformers

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dm9289

Industrial Maintenance Electrician
Location
Pennsylvania
Occupation
Industrial process repair/ maintenance Electrician
Where I work we are rerouting and dividing some cabling and I came across a 3 phase calculation that stumped me let me build it below

Low voltage switch gear 480V Delta ungrounded

Phase A B C

3 Transformers fed by

AB BC AC

Output to 3 fixed loads 600a 600a 600a
240volt 2 wire each


My question is given I know when the heating grids are fed with 240v I will get 600a current draw how much current will each phase see on the 480 delta ?


I wanted to simply amp clamp this but this could not be done due to company safety regulations plus I could not remember how to calculate this.


Thanks
Dave
 
Correct 600a 240 volt single phase on the secondary side what I am trying to calculate is primary current on 480 3 phase side. There are 3 heating grids each draw 600a at 240v
 
So I should suspect the clamp on amp meter to read 1038 amps on each phase 480v side and size conductors accordingly
 
dm9289 said:
Correct 600a 240 volt single phase on the secondary side what I am trying to calculate is primary current on 480 3 phase side. There are 3 heating grids each draw 600a at 240v
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dm9289 said:
So I should suspect the clamp on amp meter to read 1038 amps on each phase 480v side and size conductors accordingly
Click to expand...

texie said:
If I understand the question correctly: 520 amps on the primary side.
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Primary of each transformer 300A 480V 1? and where lines are separate from other transformers. On the line (supply) side of where/if joined, it will be 520A.
 
dm9289 said:
Correct 600a 240 volt single phase on the secondary side what I am trying to calculate is primary current on 480 3 phase side. There are 3 heating grids each draw 600a at 240v
Click to expand...
Each individual primary circuit will draw 200 amps. If all three of those circuits are balanced across a three phase system the system will see 520 amps per phase at what ever point each load becomes common (or starts to share conductors) to one another and from there back to the source. Phase A is seeing some current from B and some from C as well as the contribution to the load coming from A. If everything is balanced that 1.732 is the factor you use to determine what value is measured on A, and you don't even need to fully understand the phase angles and how they play a part in this. Start unbalancing things or throwing in non linear loads and it gets more complex. Same applies to the other two phases.
 
would it depend on if all three grids are on at the same time, not sure how the grids are controlled. wouldn't the answer be worst case 600 and the answer somewhere between 520 and 600 amps

sorry I didn't see post# 12 when I posted. already answered
 
kwired said:
Each individual primary circuit will draw 200 amps. If all three of those circuits are balanced across a three phase system the system will see 520 amps per phase at what ever point each load becomes common (or starts to share conductors) to one another and from there back to the source. Phase A is seeing some current from B and some from C as well as the contribution to the load coming from A. If everything is balanced that 1.732 is the factor you use to determine what value is measured on A, and you don't even need to fully understand the phase angles and how they play a part in this. Start unbalancing things or throwing in non linear loads and it gets more complex. Same applies to the other two phases.
Click to expand...

600 amps @ 240 volts is 144kw.
144kw/480=300 amps single phase on the primary side of the transformer.

let say we have a three phase delta to delta transformer and the load is wired in delta:

3x144kw is 432kw/240/1.732= 1039.2 amps secondary load.
And 432kw/480/1.732= 519.6 amps primary load.
 
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