Multiple Transformer Load Calculation

[RockoD]

Hello,

I have a 600V 3phase feeding a control cabinet that will include:

1x 1.5HP 3 phase motor FLA 2.4A
1x 3HP 3 phase motor FLA 2.94A
1x 1 phase 600/240 5Kva transformer. 8.3A/20.8A
1x 1Phase 600/120 3Kva transformer 5.0A/25.0A

The 5 Kva Tsmr will be connected to Line 1 and Line 2
The 3Kva Tsmr will be connected to Line 2 and Line 3

My Question is about calculating the 600V Load for the two 1 phase transformers and the motor loads.
I'm not sure if I can sure simple arithmetic to calculate the Load on each line.
If so, I would have:

Line 1 Line2 Line3
2.4 2.4 2.4
2.94 2.94 2.4
8.3 8.3
5.0 5.0

Line 1=13.6 Amp
Line 2=18.6 Amp
Line 3=10.3 Amp

Is there another calculation that I should be considering for this application?

Any help would be appreciated.

Thank you.

 

[bob]

Assuming that all the load is on at the same time, your answer looks correct.

 

[iggy2]

Math looks correct, but FLA for 3 HP motors looks low...

 

[mull982]

Assuming that all the load is on at the same time, your answer looks correct.

Dont you need to add these vectorally to account for the L-L currents connected between the three phases.

Phase A simply wont be the addition of the currents listed for phase A but will be the vector addition of all the AC + AB currents connected to phase A.

 

[RockoD]

Dont you need to add these vectorally to account for the L-L currents connected between the three phases.

Phase A simply wont be the addition of the currents listed for phase A but will be the vector addition of all the AC + AB currents connected to phase A.

That's the position a colleague is taking. I am having trouble finding any similar scenario online to answer it one way or the other.

 

[bob]

Mull982 is correct. This is a more accurate method to determine the phase current. This method gives you a slightly lower answer than the addition of the currents.

 

[mike_kilroy]

assuming ur 3ph motor FLA numbers are accurate (which seems fine if the 1.5hp motor is like 1200rpm base and the 3hp is like 1800rpm base maybe - I did not look up typical currents but look ok).

so:

2.4a*1.73*600=2.491kva
2.94*1.73*600=3.052kva
+5kva
+3kva
------------------
13.5kva total

Not sure what u wud do with the misc currents per phase, but this is the kva load for all that stuff. so get a 15kva xfmr and u should be good to go -- unless u subscribe to the notion that u shud oversize a xfmr 20%+ "for future growth." then go bigger yet.

keep in mind that a 3 or 5 leg xfmr will try to average out the different currents in each phase (the magnetic fields in each are common across the top and bottom steel of the core) so that is why I say 13.5kva is the right answer, not phase currents. but this is just my opinion.

 

[RockoD]

It's more about calculating what the OCP should be on that system.