# ENGINE INSTALLATION & SERVICE HANDBOOK Caterpillar

Electrical Fundamentals*

## 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.

## 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 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

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