1. Senior Member
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## Voltage Dip Estimation

Downloaded attached formula around one year ago. Did not keep a copy or recall source. Tried finding again with no luck.

Question, is this a valid calculation method for Voltage Dip/Sag ?

2. that will work
I find this easier
assume 1 mva 480/3 5% puZ

ifault = 1000000/(1.732 480 0.05) = 24056
xfmr Z in Ohms = 480/24056 = 0.02

assume feeder is 0.01 Ohm (value from nec table x 1.732 x length/1000)
total Z = 0.03
motor starting i = fla x 6 or calculated from mtot code)
assume 100 x 6 = 600 A

vdrop = 0.03 x 600 = 18 vac or 3.75%

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Ingenieur...Thank you

Using your method on PDF problem yields 11.34%.
Assume this method is more accurate ?

Max VD at MOTOR of 10%...acceptable target value ?

Max VD at Service Equipment (for a given motor start) value ?

4. imo theirs is wrong

should be 618.55/5454.46 x 100 = 11.34%
should not add the motor lr kva to the fault kva in the numerator

5. Originally Posted by MyCleveland
Assume this method is more accurate ?

Max VD at MOTOR of 10%...acceptable target value ?

Max VD at Service Equipment (for a given motor start) value ?
It is just a different way of going about it.

10% depends on the target reasoning. Are you concerned about lights flickering?

Close to 25-30% could be acceptable if you were just confirming the motor controller and/or BMS wouldnâ€™t drop out.

6. Originally Posted by Ingenieur
imo theirs is wrong

should be 618.55/5454.46 x 100 = 11.34%
should not add the motor lr kva to the fault kva in the numerator
Faults are fed by all sources, both utility and motor contribution.

7. Originally Posted by publicgood
Faults are fed by all sources, both utility and motor contribution.
true
but this not a fault calculation
this is a voltage drop calc due to motor inrush
the xfmr can't supply more than it's sc fault current
the motor contributes nothing, it is a load in this case

my calc uses xfmr internal Z plus conductor Z x starting current to give V at the motor terminals
very simple, like any v drop calc
matches the op formula if the motor inrush is not added to xfmr sc

8. Originally Posted by Ingenieur
true
but this not a fault calculation
this is a voltage drop calc due to motor inrush
the xfmr can't supply more than it's sc fault current
the motor contributes nothing, it is a load in this case

my calc uses xfmr internal Z plus conductor Z x starting current to give V at the motor terminals
very simple, like any v drop calc
matches the op formula if the motor inrush is not added to xfmr sc
True I retract my statement

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What is the consensus on MAX target values ?

Max VD at MOTOR during start...acceptable target value ? Max %VD ?

Max VD at Service Equipment (for a given motor start) value ? Max %VD to avoid flicker?

10. Senior Member
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Anyone.....?
before this slips off the page.

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