Electrical Masters

The line current of a three phase transformer can be calculated with the formula:

I = VA / ( E x 1.732 )

The line current of a single phase transformer can be calculated with the formula:

I = VA / E

Mutual Induction

The process of inducing a current flow through a secondary coil, by passing a varying current on the primary coil

Eddy Currents

Loops of electrical current induced within conductors by a changing magnetic field in the conductor due to Faraday's law of induction.

If the secondary voltage of a delta-delta transformer is 120/240V, the high-leg voltage to ground (or neutral) is approximately ___________. a. 191V b. 196V c. 202V d. 208V

d. 208V High Leg Voltage = Voltage to Ground X 1.732 120V X 1.732 = 207.84V

What's the voltage ratio for a single-phase 480V to 240V transformer? a. 4:1 b. 1:4 c. 1:2 d. 2:1

d. 2:1

The turns ratio is defined as:

number of secondary windings divided by primary windings

A 45kVA, 480V primary to 120/208V secondary, three phase transformer with a 175A secondary overcurrent device will require at least ____________ secondary conductors. (Conductors are 10') a. 2/0 b. 4/0 c. 350 kcmil d. 400 kcmil

a. 2/0 45,000 / 360 = 125 X 125% = 156.25 2/0 is good for 175A and is not less than the overcurrent device rating of 175A

What's the primary line current for a fully loaded 45kVA, 480V to 240V, three phase transformer? a. 43A b. 54A c. 108A d. 124A

b. 54A I = VA / (E X 1.732) VA = 45,000 E = 480V 480V X 1.732 = 831 45,000 / 831 = 54.15

In a delta-configured transformer, the ratio of the primary-to-secondary line current is directly proportional (the same ratio) to the voltage turns ratio. a. True b. False

b. False

______________ voltage is the voltage measured between any two ungrounded (hot) conductors. a. Load b. Line c. Grounded d. and of these

b. Line

The primary overcurrent protection for a transformer rated 600V, nominal or less, having a primary current of over 9A or more must be set at not more than _________ percent of the primary current when using primary only protection. a. 125 b. 167 c. 200 d. 300

a. 125

Faraday's Law

Is a basic law of electromagnetism predicting how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF)—a phenomenon called electromagnetic induction. It is the fundamental operating principle of transformers, inductors, and many types of electrical motors, generators and solenoids.

Secondary Induced Voltage

Voltage induced in the secondary winding of a transformer is equal to the sum of the voltages induced in each loop of the secondary winding.

What is the turns ratio for a 480V to 120/208V, three phase delta/wye-configured transformer? a. 4:1 b. 1:4 c. 1:2 d. 2:1

a. 4:1

Delta-Connected Transformer

Delta-Connected Transformers have three transformer windings connected end to end with each other. The line conductors are connected to each point where two windings meet.

Transformer kVA ratings

Transformers are rated in kilovolt-ampere (kVA), where 1 kilovolt-ampere = 1000 volt-amperes (VA).

Wye-Connected Transformer

Wye-Connected Transformers have one lead from each of the three windings connected to a common point. The other leads from each of the windings are connected to the line conductors.

The size of the grounding electrode conductor for a single separately derived system must not be smaller than that identified in ____________, based on the area of the largest derived ungrounded supply conductor. a. 250.66 b. 250.122 c. Table 310.15(B)(16) d. none of these

a. 250.66 [250.30(A)(5)]

What's the maximum primary and secondary full load line current for a 480V to 240V, 25kVA single phase transformer? a. 52/104 b. 104/52 c. 12/24 d. none of the above

a. 52/104 I = VA / E 25,000 / 480 = 52 25,000 / 240 = 104

What 4-wire system is called the high-leg system because the voltage from one conductor to ground is between 190 volts and 208 volts to ground? a. Delta b. Wye

a. Delta

In a delta-configured transformer, the line voltage is equal to the phase voltage. a. True b. False

a. True

Overcurrent protection for the secondary of a transformer rated 600V or less having a secondary current rating of 9A or more must be set at not more than ________ percent of the secondary current rating. a. 100 b. 125 c. 150 d. 170

b. 125

The turns ratio is the relationship between the number of primary windings compared to the number of secondary windings. This is the same as the turns ratio between the primary phase voltage and the secondary phase voltage. For under 600V, a typical delta/delta system phase voltage ratio is ______ and a typical delta/wye system phase voltage ratio is ______. a. 1:2, 1:4 b. 2:1, 4:1 c. 4:1, 2:1 d. 10:1, 20:1

b. 2:1, 4:1

A 45kVA, 480V, primary to 120/208V secondary, three phase transformer with a 70A primary overcurrent device will require at least ___________ primary conductors. a. 6 AWG b. 4 AWG c. 2 AWG d. 1 AWG

b. 4 AWG 45,000 / 831 = 54.15 x 125% = 67.68A #4 AWG at 75°C is good for 85A

The term "_________" is used to identify the ungrounded (hot) conductors in an electrical system. a. load b. line c. system d. grounded

b. line

What's the secondary line current for a fully loaded 45kVA, 480V to 240V, three phase transformer? a. 43A b. 54A c. 108A d. 124A

c. 108A I = VA / (E X 1.732) VA = 45,000 E = 240V 240V X 1.732 = 415.68 45,000 / 415.68 = 108.25A

A 75kVA, 480V primary to 120/208V secondary, three phase transformer with a 300A secondary overcurrent device will require at least ____________ secondary conductors. (Conductors are 10') a. 2/0 b. 4/0 c. 350 kcmil d. 400 kcmil

c. 350 kcmil 75,000 x 360 = 208.33 x 125% = 260.4 350 kcmil is good for 310A at 75°C and is not less than the 300A secondary overcurrent device.

The Delta High-Leg conductor is required to be terminated on the _______ phase position in a meter socket enclosure? a. A phase b. B phase c. C phase d. Doesn't matter

c. C phase

What's the primary voltage a transformer if the turns ratio is 10:1 and the secondary voltage is 12V? a. 24V b. 1.2V c. 120V d. 12V

c. 120V Primary Voltage = Secondary Voltage times 10

The connection of the system bonding jumper for a separately derived system must be made ________ on the separately derived system from the source to the first system disconnecting means or overcurrent device. a. in at least two locations b. in every location that the grounded conductor is present c. at any single point d. none of these

c. at any single point [250.30(A)(1)]

In an ac system, the size of the grounding electrode conductor to a concrete-encased electrode isn't required to be larger than a(n) __________ copper conductor. a. 10 AWG b. 8 AWG c. 6 AWG d. 4 AWG

d. 4 AWG [250.66(B)]

What's the secondary line current for a fully loaded 22kVA, 480V to 120/208V, three-phase transformer? a. 22A b. 27A c. 54A d. 61A

d. 61A I = VA / (E X 1.732) VA = 22,000 E = 208V 208V X 1.732 = 360 22,000 / 360 = 61.11A

In an area served by a separately derived system, the grounding electrode must be as near as practicable and must be the nearest _____________. a. structural metal grounding electrode b. metal water pipe electrode c. metal building skin d. a or b

d. a or b [250.30(A)(4)]

On a 4-wire, delta-connected system where the midpoint of one phase winding is grounded, the conductor having the higher phase voltage to ground must be duribly and permanently marked by an outer finish that is ___________ in color. a. black b. red c. blue d. orange

d. orange

The line current for both delta- and wye-configured three phase transformers can be calculated by the formula: I = VA / (E X 1.732) a. True b. False

a. True

When a load is connected to the secondary windings of a transformer, current flows through the secondary winding. The current flowing in the secondary creates an electromagnetic field that opposes the primary electromagnetic field. a. True b. False

a. True

The grounding electrode conductor for a single separately derived system is used to connect the grounded conductor of the derived system to the supply-side bonding jumper. a. True b. False

a. True [250.30(A)(1)(a)]

Delta configured means that the windings of three single phase transformers are connected in __________ with each other. A delta-connected transformer is represented by the Greek letter delta. a. series b. paralell c. series-paralell d. a and b

a. series

Article 450 is for the protection of the ____________________. a. transformer windings b. transformer primary conductors c. transformer secondary conductors d. none of the above

a. transformer windings

What's the primary line current for a fully loaded 22kVA, 480V to 120/208V, three-phase transformer? a. 22A b. 26A c. 54A d. 61A

b. 26A I = VA / (E X 1.732) VA = 22,000 E = 480V 480V X 1.732 = 831 22,000 / 831 = 26.47A

If the primary current of a transformer is less than 9A, and doesn't correspond to a standard fuse or nonadjustable circuit breaker, can you use the next higher rating of overcurrent device? a. Yes b. No

b. No

"_________ - connected" means a connection of three single phase transformer windings to a common point (neutral), with the other end of each winding connected to the line conductors. a. Delta b. Wye c. a or b d. none of these

b. Wye

A 75kVA, 480V primary to 120/208V secondary, three phase transformer with a 125A primary overcurrent device will require at least ____________ primary conductors. a. 6AWG b. 4AWG c. 2AWG d. 1AWG

c. 2AWG 75,000 / 831 = 90.25 X 125% = 112.8 # 2 AWG at 75°C is good for 115A

What's the turn ratio of a Delta/Delta transformer if the primary phase voltage is 480V and the secondary is 240V? a. 4:1 b. 10:1 c. 1:1 d. 2:1

d. 2:1