CMS Electrical Formulas

What is the equation for calculating the cross-sectional area of a round conductor?

Diameter squared

Write the Ohm's Law equation used to solve for: a) voltage, B) current, And c) resistance.

(a) E = I x R (b) I = E/R (c) R = E/I

One circular mil is equal to_______square mil(s).

.785

There are two light bulbs in a series circuit. Bulb A has a resistance of 10 ohms and bulb B has a resistance of 20ohms. If the current in bulb A is 1.5A, what is the current in bulb B?

1.5Amps, Components that are connected in series with each other will always have the same current flow.

When calculating the total current in a balanced three-phase circuit, you must use a constant factor of

1.73

What is the total resistance of the circuit if the same three resistors are Connected in parallel?

10 ohms

An electric circuit has two 25 Ohm resistors and one 50 ohm resistor Connected in series. What is the total resistance of the circuit?

100 ohms Rt = R1 + R2 + R3 .....etc

A Current of 2 A flows through a lamp that has a resistance of 25 ohms. Determine the power consumption

100 watts

What is the total resistance of a circuit in which a 20-Ohm resistor, a 40-Ohm resistor, and an 80-Ohm resistor are connected in parallel?

11.43 ohms

The current in a simple circuit is 2 A, with a 24-V potential. What is the Resistance?

12 Ohms ( R = E/I )

What is the total resistance of a circuit in which two 25-Ohm resistors are connected in parallel?

12.5 Ohms (Two resistors of equal value connected in parallel always equals ½ the value of one of the resistors.

What is the total resistance of a circuit in which one 40-Ohm resistor and one 20-Ohm resistor are connected in parallel?

13.33 Ohms ( 40 x 20 / 40 + 20 )

A transformer has 500 turns on the primary and 1,500 turns on the secondary If a voltage of 45 V is applied to the primary, what is the voltage developed across the secondary? (Assume no losses.)

135 volts (Formule is Ep/Es = Np/Ns), cross multiply and the equation becomes (Ep x Ns = Es x Np) now substitute 45 x 1500 = Es x 500 next step, simplify 67500 = Es x 500, next step, divide both sides by 500 to get the answer, 135 = Es or a simple way. This is a 1 : 3 ratio transformer. Any voltage applied to the primary will be 3 times that applied voltage

An 8-gauge wire has a diameter of 128 mils. Calculate its area, first in circular mils, and then in square mils.

16,384 CMA, 12,861 SMA 128 sq then X .785

Calculate the capacitive reactance of a 500 mf capacitor to a 120 Hz current.

2.6525761 ohms (500 mf = .000500 f)

Convert 2,850 watts to kilowatts.

2.85 Kilowatts

If two 400 uF capacitors are connected in series, what is their total capacitance?

200 uF

If two 400-uF capacitors are connected in series, what is their total capacitance?

200 uF

Calculate the impedance of a coil that has a dc resistance of 40 ohms a capacitive reactance of 400 ohms, and an inductive reactance of 200 ohms.

203.96

If 5,000 watts of power are used for 48 hours, solve for kilowatt hours.

240 kWh (KW x hours = KWH)

If two capacitors rated at 125 V are connected in series, what is their total voltage?

250 V

If two capacitors rated at 125 V are connected in series, what is their total voltage?

250V

A transformer has a turn ratio of 7:1. If a voltage of 5 V is developed across the secondary, what is the voltage applied to the primary?

35 volts

If the total voltage in a simple circuit is 120V and the total resistance is 40 ohms, solve for power.

360 watts (P = E2 / R)

Use Ohm's Law to calculate the voltage across a lamp with a resistance Of 20 ohms and a current of 2 A.

40 volts ( E = I x R )

What is the total capacitance of a circuit in which a 30 uF capacitor, a 20 uF capacitor, and a 10 uf capacitor are connected in series?

5.46 uF

Determine the power used by a lamp that has a resistance of 20 ohms and a 12 V source.

7.2 watts

A parallel circuit has three branches, all connected to a 20 V source. Branch A includes a lamp with a resistance of 5 ohms. Branch B includes A lamp with a resistance of 10 ohms. Branch C includes a lamp with a Resistance of 20 ohms. Determine the voltage and current in each branch and the total current.

A = 20v/4amps B = 20v/2amps C = 20v/1amp Total current = 7amps

What is the total capacitance of a circuit in which a 5 uf capacitor, a 13 uf capacitor, and a 12 uf capacitor are connected in parallel?

C = 5 + 13 + 12 = 30 uF

Using E for voltage, I for current, and R for resistance, complete the Following three equations derived from Ohm's Law:

E = I * R I = E / R R = E / I

Write the three most common expressions of Ohm's Law (for voltage, current, and resistance).

E = I x R ; I = E/R; R = E/I

The input power to a transformer is 1,000 W and the output power is 500 W. What is the efficiency of the transformer?

Efficiency = Pout/Pin X 100 ((500/1000)X100) = 50%

A transformer has 60 V applied to the primary and 420 V developed across the secondary. If there are 800 turns on the primary, what is the number of turns on the secondary?

Figure out the transformers ratio of turns by using the input and output voltages. 60 : 420 is 1 : 7 if the turns in the primary is 800, the turns in the secondary must be 7 times the primary or 5600 turns

When capacitors are connected in series, the total capacitance of the circuit is always ________ than the ________ capacitance of any of the individual capacitors.

Less than the smallest

How can you determine the synchronous speed of a motor?

Multiply the frequency of the power supply by 120, and divide the result by the number of poles the motor has.

A transformer has a turns ratio of 5:1 and a current of 5 A in the secondary. What is the current in the primary? (Assume no losses.)

One Amp (A transformer that steps down voltage, steps up current.

Write the equation for calculating power when the current and the voltage are known.

P = I x E

Write the equation used to determine power in a simple resistive circuit when only current and voltage are known.

P = I x E

Write the equation for calculating power in a dc circuit if current and voltage are known.

P=I x E

Calculate the power factor of a load from these instrument readings: Voltage = 240 V, current = 20 A, actual power = 4,000 W

PF = (Actual Power/Apparent Power) x 100 (4,000/240 x 20) x 100 = (4000/4800) x 100 = (.8333) x 100 = 83.33 %

What is the equation for calculating electric charge?

Q = It (Q = Electrical charge in coulombs, I = current in amperes, t = time is seconds)

Using the equation in the previous question, calculate the quantity of Electric charge that flows through a circuit in 60 seconds if the current in the circuit is 5 amperes.

Q = It Q = 5 x 60 Q = 300

Calculate the percent of slip for a motor with a synchronous speed of 3,600 rpm and a rotor speed of 3,400 rpm.

S = ((Ss - Sr)/Ss) X 100 (3600 - 3400/3600) X 100 = 200/3600 X 100 = 5.55%

State Kirchhoff's Law of voltages that pertains to a series circuit.

The sum of the voltages in a series circuit is equal to the source voltage.

If the average current through the windings of a three-phase motor is 25 A, and the current measured through one of the individual windings is 28 A, is the maximum allowable current imbalance being exceeded?

Yes. The deviation from the average current is 3 A. When divided by the average current of 25 A and multiplied by 100, this amounts to a 12% current imbalance, well over the 2% limit.

What is the synchronous speec of a three phase motor that has six poles and is operated on 60-Hz alternating current?

1,200 rpm

If a relay with a VA rating of 20 VA has a control voltage of 24 V applied to its coil, what current does it draw when the armature is pulled all the way down?

0.833 Amps