 
TM 9254
73.
Alternating Current  Continued
c.
AC Power. The power expended in an AC circuit is calculated by the formulas:
NOTE
The phase angle q is the difference in degrees that the current leads or lags the voltage in a
reactive circuit. The phase angle is determined by the formula:
q = arctan
X
R
Where:
X is the inductive or capacitive reactance in ohms.
R is the resistance in ohms.
P = I^{2}Z cos q
P = IE cos q
P = E^{2} cos q
Z
Where:
P is the power in watts.
E is the voltage in volts.
I is the current in amperes.
Z is the impedance in ohms.
q is the phase angle in degrees.
Table 72 shows the basic voltage, current, and power relationships for AC circuits.
Table 72. AC Relationships
Known
Formulas for determining unknown values of
values
I
R
E
P
I & Z


E = IZ
P = I^{2}Z cos q
I & E

Z = ^{E}
I

P = IE cos q
I & P

Z =
P
I^{2} cos q
E =
P
I cos q

Z & E
I =^{E}
Z


P = E^{2} cos q
Z
Z & P
I =
P
Z cos q

E =
PZ
cos q
E & P
I =
P
E cos q
Z = E^{2} cos q
P


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