How to determine phase amperage/imbalance

[Alfont1120]

Good Afternoon,
I am trying to determine what the load will be per phase for the system we have installed here (not yet energized). We have a 3-phase, 208 volt, 400 amp fusible disconnect with (4) fused disconnects tapped off of it. Each disconnect feeds a panel with the following main breakers: 200A, 150A, 150A and 100A. We'll call them panels 1-4 in that order. These panels are installed inside of mobile office trailers. We've already done load calculations for all 4 trailers so for the sake of this question, let's just assume the calculations are correct and panels 1-3 will draw 102.25 amps, with panel 4 drawing 80.95 amps. If panel 1 his fed with phases AB, panel 2 with BC, panel 3 with AC and panel 4 with AB, what would the total draw be per phase? I am trying to determine how out of balance our system will be and if it will be a potential issue/reason for the inspector to fail us. That being said, is there any standard or mention in the NEC what the allowable max deviation against the average current can be?

Thank you for any help in this matter

 

[kwired]

Good Afternoon,
I am trying to determine what the load will be per phase for the system we have installed here (not yet energized). We have a 3-phase, 208 volt, 400 amp fusible disconnect with (4) fused disconnects tapped off of it. Each disconnect feeds a panel with the following main breakers: 200A, 150A, 150A and 100A. We'll call them panels 1-4 in that order. These panels are installed inside of mobile office trailers. We've already done load calculations for all 4 trailers so for the sake of this question, let's just assume the calculations are correct and panels 1-3 will draw 102.25 amps, with panel 4 drawing 80.95 amps. If panel 1 his fed with phases AB, panel 2 with BC, panel 3 with AC and panel 4 with AB, what would the total draw be per phase? I am trying to determine how out of balance our system will be and if it will be a potential issue/reason for the inspector to fail us. That being said, is there any standard or mention in the NEC what the allowable max deviation against the average current can be?

Thank you for any help in this matter

Loads in such applications do vary and it makes it hard to determine what real world results may be sometimes. Chances are the 200 amp supplied unit has more potential to have higher load though.

That said if your three larger ones had same load level of 102.5 and you balanced those across all three phases - it would draw 177.5 (102.5 x 1.732) amps on all three phases. Add the fourth unit of 80.95 amps and it will increase whichever two phases you connect it to by 80.95 amps.

 

[Alfont1120]

Loads in such applications do vary and it makes it hard to determine what real world results may be sometimes. Chances are the 200 amp supplied unit has more potential to have higher load though.

That said if your three larger ones had same load level of 102.5 and you balanced those across all three phases - it would draw 177.5 (102.5 x 1.732) amps on all three phases. Add the fourth unit of 80.95 amps and it will increase whichever two phases you connect it to by 80.95 amps.
[/QUOTE

Thank you for your response. Just to be clear, if I have the fourth unit on phases A and B, it will increase them individually by 80.95 amps, as in 258.45 amps on A and B and 177.5 on C? Or would the 80.95 be broken up between A and B, around 217 amps?

 

[GoldDigger]

The full current will be added to each of the two ungrounded conductors. Since it is unbalanced you should use vector addition, but just adding the magnitudes will give a conservative number which is fine for your situation.

Sent from my XT1585 using Tapatalk

 

[Hv&Lv]

Good Afternoon,
I am trying to determine what the load will be per phase for the system we have installed here (not yet energized). We have a 3-phase, 208 volt, 400 amp fusible disconnect with (4) fused disconnects tapped off of it. Each disconnect feeds a panel with the following main breakers: 200A, 150A, 150A and 100A. We'll call them panels 1-4 in that order. These panels are installed inside of mobile office trailers. We've already done load calculations for all 4 trailers so for the sake of this question, let's just assume the calculations are correct and panels 1-3 will draw 102.25 amps, with panel 4 drawing 80.95 amps. If panel 1 his fed with phases AB, panel 2 with BC, panel 3 with AC and panel 4 with AB, what would the total draw be per phase? I am trying to determine how out of balance our system will be and if it will be a potential issue/reason for the inspector to fail us. That being said, is there any standard or mention in the NEC what the allowable max deviation against the average current can be?

Thank you for any help in this matter

250A, 250A,177A

 

[troque]

Assuming the load current is the same on each leg,

	L1	L2	L3
Panel 1	102.25	102.25
Panel 2		102.25	102.25
Panel 3	102.25		102.25
Panel 4	80.95	80.95
Sub-Total	285.45	285.45	204.5	Amps