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34
Ohm’s Law
Ohms law states that current in a resistance varies in direct
proportion to voltage applied and inversely proportional to
resistance.
I=V/R
V= voltage in volts (V)
I= current in amps (A)
R= resistance in ohms (R)
The 3 most common mathematical expressions are:
V=I*R I=V/R R= V/I
An unknown electrical value can be obtained by using any
two known values in one of the variations of Ohm’s Law shown
in the circle.
Common Conversion Factors
Typical 3-Phase Wiring Diagrams & Equations for Resistive Heaters
VP= Phase Voltage
VL= Line Voltage
IP= Phase Current
IL= Line Current
R=R1+R2=R3= Resistance of each branch
W= Wattage
Percentage of Rated Wattage for Different Applied Voltages
1 =I
V =V /1.73
W =V /R= 3(V )/R
W = 1.73VI
1 =I /1.73
V =V
W =3(V )/R
W = 1.73VI
P
P
P
P
P
WYE
DELTA
WYE
WYE
L
L
L
L
L
L
L
L
L
L
2
2
2
Applied
Voltage
120V
100%
100%
100%
75%
25% 12%
12%
19%33%
120V
208V
208V
240V
240V
277V
Rated Voltage
277V
347V
347V
480V
480V
600V
600V
56%
75%
100%
36%
25%48%
64%
64%
33% 21%
33%100% 52%
100%
100%
%57%57
6% 4%
19%
16%
To Convert
Kilowatt/hours (kWh)
Kilowatt (kW)
Meter (m)
Meter (m)
Square feet (ft )
Square meters (m )
Inches (in.)
Inches (in.)
3,412
3,412
3.281
39.37
0.0929
10.76
2.54
25.4
British ther. units/hr (Btuh)
British thermal units (Btu)
Feet (ft)
Inches (in.)
Square meters (m )
Square feet (ft )
Centimeters (cm)
Millimeters (mm)
Multiply By
To Obtain
2
2
2
2
C
F
o
o
-30
-22
-25
-13
-20
-4
-15
-5
-10
14
-5
23
0
32
5
41
10
50
15
59
20
68
25
77
30
86
35
95
40
104
45
113
50
122
60
140
55
131
Fahrenheit to Celsius F=1.8 x C + 32
Celsius to Fahrenheit C = 5/9 x ( F-32)
o
o
o
o
Cable spacing (inch) =
Area (sq.ft.) x 12
Cable length (ft)
Equations for Wye Only
3-Phase Wye (Balanced Load)
3-Phase Delta (Balanced Load)
Equations for Delta Only
2
V
V
V
V
2
V
V
R
R
R
2
A
A
A
A
2
A
A
W
W
W
W
W
W
W
V
R
A
R
R
R
WATTS
VOLTS
OHMS
AMPS
Technical Information