Electrical FORMULAS
CONVERSION Formulas
Area of Circle = πr2
Breakeven Dollars = Overhead Cost $/Gross
Profit %
Busbar Ampacity AL = 700A Sq. in. and CU =
1000A Sq. in.
Centimeters = Inches x 2.54
Inch = 0.0254 Meters
Inch = 2.54 Centimeters
Inch = 25.4 Millimeters
Kilometer = 0.6213 Miles
Length of Coiled Wire = Diameter of Coil
(average) x Number of Coils x π
Lightning Distance in Miles = Seconds between
flash and thunder/4.68
Meter = 39.37 Inches
Mile = 5280 ft, 1760 yards, 1609 meters, 1.609
km
Millimeter = 0.03937 Inch
Selling Price = Estimated Cost $/(1 - Gross
Profit %)
Speed of Sound (Sea Level) = 1128 fps or 769
mph
Temp C = (Temp F - 32)/1.8
Temp F = (Temp C x 1.8) + 32
Yard = 0.9144 Meters
Electrical Formulas Based on 60 Hz
Capacitive Reactance (XC) in Ohms = 1/(2π f C)
Effective (RMS) AC Amperes = Peak Amperes x
0.707
Effective (RMS) AC Volts = Peak Volts x 0.707
Efficiency (percent) = Output/Input x 100
Efficiency = Output/Input
Horsepower = Output Watts/746
Inductive Reactance (XL) in Ohms = 2 π f L
Input = Output/Efficiency
A2 + B 2 + C 2 − ( AB + BC + AC)
Neutral Current (Wye) =
Output = Input x Efficiency
Peak AC Volts = Effective (RMS) AC Volts x 2
Peak Amperes = Effective (RMS) Amperes x 2
Power Factor (PF) = Watts/VA
VA (apparent power) = Volts x Ampere or
Watts/Power Factor
VA 1-Phase = Volts x Amperes
VA 3-Phase = Volts x Amperes x 3
Watts (real power) Single-Phase = Volts x
Amperes x Power Factor
Watts (real power) Three-Phase = Volts x
Amperes x Power Factor x 3
Parallel
Circuits
Note 1: Total resistance is always less than
the smallest resistor
RT = 1/(1/R1 + 1/R2 + 1/R3 +...)
Note 2: Total current is equal to the sum of
the currents of all parallel resistors
Note 3: Total power is equal to the sum of
power of all parallel resistors
Note 4: Voltage is the same across each of the
parallel resistors
Series Circuits
Note 1: Total resistance is equal to the sum
of all the resistors
Note 2: Current in the circuit remains the
same through all the resistors
Note 3: Voltage source is equal to the sum of
voltage drops of all resistors
Note 4: Power of the circuit is equal to the
sum of the power of all resistors
Transformer Amperes
Secondary Amperes 1-Phase = VA/Volts
Secondary Amperes 3-Phase = VA/Volts x 3
Secondary Available Fault 1-Phase = VA/ (Volts
x %impedance)
Secondary Available Fault 3-Phase = VA/ (Volts
x 3 x %Impedance)
Delta 4-Wire: Line Amperes = Phase (one
winding) Amperes x 3
Delta 4-Wire: Line Volts = Phase (one Winding)
Volts
Delta 4-Wire: High-Leg Voltage (L-to-G) =
Phase (one winding) Volts x 0.5 x 3
Wye: Line Volts = Phase (one winding) Volts x
3
Wye: Line Amperes = Phase (one winding)
Amperes Voltage Drop
VD (1-Phase) = 2 KID/CM
VD (3-Phase) = 3 KID/CM
CM (1-Phase) = 2 KID/VD
CM (3-Phase) = 3 KID/VD
Code Rules
Breaker/Fuse Ratings – 240.6(A)
Conductor Ampacity – 310.15 and Table 310.16
Equipment Grounding Conductor – 250.122
Grounding Electrode Conductor – 250.66
Motor Conductor Size – 430.22 (Single) 430.24
(Multiple)
Motor Short-Circuit Protection – 430.52
Transformer Overcurrent Protection – 450.3
π (Pi) = (3.142 approximately), 2 = 1.414
(approximately), 3 = 1.732
(approximately), f = Frequency, r = radius, d
= diameter, C = Capacitance (farads),
L = Inductance (henrys), CM = Circular Mils
(Chapter 9, Table 8), VD = Volts Drop,
K75°C = (12.9 ohms CU) (21.2 ohms AL), I =
Amperes of load, D = Distance one way
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