Basic Formulas
Ohms Law | Capacitance | Inductance | Legend
The following pages of formulas, some derivations and discussions are intended to assist in
designing and applying capacitor banks properly.
1. Ohm’s Law
R
I = V (1)
or V = I Z
(2)
(3)
Then Z =
(4)
and I =
(5)
P=I
From (4)
This becomes
V=
(6)
Let P = VAR
Then Z =
or Z =
Or Z =
Letting Z = XC Or XL
We have XC (Or XL) =
(7)
(8)
(9)
(10)
(11)
This will allow you to determine the reactance of a capacitor or reactor when voltage and
KVAR is given.
Example:
A three-phase 600KVAR capacitor bank at 12,470 will yield
Xc =
=
= 259.168 Ohms
A three-phase shunt reactor of 150KVAR, at 13.8KV will yield
XL =
=
= 1269.6 Ohms
2. Capacitance
(12)
XC =
But, as stated VAR =
(6)
(13)
VAR =
(14)
Or =
(15)
Then VAR =
(16)
KVAR =
Example:
From above 600KVAR at 12.47KV equals 259.168 Ohms and from (12)
XC =
. Then C =
Let f = 60 Hertz
Then C = 0.000010235 farads or 10.235 microfarads.
From (16) KVAR =
KVAR = 600
3. Inductance
(17)
VAR =
(18)
from (6) =
(19)
KVAR =
From above 150KVAR at 13.8KV equals 1269.6 Ohms and from (17)
. Let f = 60 Hertz
Then L =
=
= 3.36773 Henries
Or 3367.73 millihenries.
From (19) KVAR =
=
KVAR = 150
R
I
V
Z
P
VAR
K
Xc
XL
M
LEGEND
Resistance
Amps
Volts
Impedance
Power (in Watts and/or Vars)
Reactive Power
Kilo (103)
Capacitive Reactance
Inductive Reactance
Mega (106)