Ohm's Law
E = IR
where E = voltage in volts
I = current in amperes
R = resistance in ohms
By simple algebra this equation may be written:
Power
P = IE
where P = power in watts
I = current in amperes
E = voltage in volts
This equation for power may also be transposed to:
From Ohm's law it is known that E = IR. If this expression for voltage is substituted in the power law, we can derive the additional equation: P = I2R
If we use the equation for current from Ohm's law, I = E/R, the equation for power becomes:
*See "Ugly's Electrical Reference" (SEBD0983) for additional information.
Resistance
Series Circuits RT = R1 + R2 + R3 + ... RN
where RN = resistance in the individual resistors
RT = total resistance in circuit
Reactance
XL = 2 π f L
where XL = inductive reactance in ohms
f = frequency in hertz
L = inductance in henries
π = 3.1416
where XC = capacitive reactance in ohms
f = frequency in hertz
C = capacitance in farads
π = 3.1416
Impedance
where Z = impedance in ohms
R = resistance in ohms
XL = inductive reactance in ohms
XC = capacitive reactance in ohms
Note that the impendance will vary with frequency, since both XC and XL are frequency dependent. In practical AC power circuits, XC is often small and can be neglected. In that case, the formula above simplifies to:
Transformer Voltage Conversion
where VS = secondary voltage
VP = primary voltage
NS = number of secondary turns
NP = number of primary turns
Power Factor
In mathematical terms, the power factor is equal to the cosine of the angle by which the current leads or lags the voltage. If the current lags the voltage in an inductive circuit by 60 degrees, the power factor will be 0.5, the value of the cosine function at 60 degrees. If the phase of the current in a load leads the phase of the voltage, the load is said to have a leading power factor; if it lags, a lagging power factor. If the voltage and current are in phase, the circuit has a unity power factor.
Equation Summary Diagram
Three Phase Connection Systems:
Electrical Enclosure Protection = IEC
The degrees of protection provided within an electrical enclosure is expressed in terms of the letters IP followed by two numerals. Mechanical protection against impact damage is defined by an optional third numeral.
Example: An IP55 enclosure protects its contents against dust and spray from water jets.
Reference: DIN 40050 of July 1980, IEC 144 of 1963, IEC 529 of 1976, NF C 20-010 of April 1977
Electrical Enclosure Protection - NEMA
Electrical Tables
Table 1 Electrical Formulae
Table 2 KV·A of AC Circuits
Table 3 Copper Wire Characteristics
Table 4 Single-Phase AC Motors Full Load Currents in Amperes
Table 5 Three-Phase AC Motors - 80% Power Factor Full Load Current in Amperes - Induction-Type, Squirrel Cage and Wound Rotor
Table 6 Direct Current Motors Full Load Current in Amperes
Table 7 Conduit Sizes for Conductors
Table 8 Allowable Current-Carrying Capacities of Insulated Copper Conductors
Table 9 Code Letters Usually Applied to Ratings of Motors Normally Started on Full Voltage
Table 10 Identifying Code Letters on AC Motors
Table 11 Conversion - Heat and Energy
Table 12 Approximate Efficiencies - Squirrel Cage Induction Motor
Table 13 - Approximate Electric Motor Efficiency to Use in Calculating Input
Table 14 Reduced Voltage Starters
