On a distribution transformer serving a Nevada C-2E secondary line, the nameplate kVA rating is a rating of:
Transformers
Question · Easy
A single-phase pole-mount transformer serving an overhead secondary is rated 7,200 V primary and 240 V secondary. What is the turns ratio of the windings?
A 25 kVA single-phase transformer is fully loaded on a 240 V secondary that feeds an overhead secondary bus. What is the approximate full-load secondary current?
Question · Medium
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A 75 kVA, 480–208Y/120 V, three-phase transformer supplies a secondary line. What is the approximate full-load secondary line current?
Per NEC 2017 450.11, which item must appear on the nameplate of a transformer rated 25 kVA and larger?
A 150 kVA dry-type transformer is being installed indoors to feed a secondary line. Unless an Article 450 exception applies, NEC 2017 450.21(B) requires the transformer to be:
Unless an Article 450 exception applies, oil-insulated transformers installed indoors shall be installed:
NEC 2017 450.14 requires a transformer disconnecting means to be located in sight of the transformer unless:
NEC 2017 450.13 generally requires transformers to be readily accessible to qualified personnel. A dry-type transformer is permitted in a fire-resistant hollow space of a building, not permanently closed in, only if it is 1000 V or less and does not exceed:
NEC 2017 450.9 requires transformer ventilation to be adequate to dispose of the full-load heat without creating a temperature rise that exceeds the rating. Ventilation openings shall not be located so as to:
A 75 kVA, three-phase, 480 V primary transformer has no secondary overcurrent device. Using NEC 2017 Table 450.3(B) (currents 9 A or more) and Note 1, what is the maximum standard primary overcurrent-device rating?
Question · Hard
The same 75 kVA, 480–208Y/120 V, three-phase transformer has secondary overcurrent protection. Using Table 450.3(B) for secondary current of 9 A or more and Note 1, what is the maximum standard secondary overcurrent-device rating?
A 15 kVA, single-phase, 240 V primary transformer is protected on the primary only. Using Table 450.3(B) (currents 9 A or more) and Note 1, what is the maximum standard primary overcurrent-device rating?
Transformer secondary conductors that leave the tank and land on a single overcurrent device are often installed under the 3 m (10 ft) secondary-conductor rule. That length and protection rule is in which NEC 2017 section?
Two single-phase secondary transformers may be paralleled and switched as a unit under NEC 2017 450.7 only if each tank has overcurrent protection complying with 450.3. In addition, successful paralleling in the field requires the transformers to have:
A resistive load on a 240 V secondary line draws 12 A. What is the load resistance?
Electrical Knowledge
A 240 V single-phase secondary load draws 20 A at unity power factor. What is the true power?
A single-phase distribution transformer is rated 37.5 kVA with a 120/240-volt secondary. What is the full-load secondary current at 240 volts?
In a series circuit on a secondary line, which quantity is the same through every series element?
Power factor of a secondary load is defined as:
A 480 V, three-phase secondary supplies a balanced 30 kW load at unity power factor. What is the line current?
A 480 V, three-phase secondary supplies 60 kW at 0.85 power factor. What is the approximate line current?
A 75 kVA transformer is loaded to its nameplate rating at 0.80 power factor. What true power is being delivered?
NEC 2017 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 recommend limiting voltage drop to about 3 percent on a feeder or branch circuit, and to a total of about what percent to the farthest outlet?
A 240 V, single-phase copper secondary run is 200 ft one-way, carries 80 A, and is 2 AWG (66,360 circular mils). Using VD = 2 × K × I × D / CM with K = 12.9, what is the approximate voltage drop?
A 240 V, single-phase copper secondary must carry 100 A for 250 ft one-way with not more than 3 percent voltage drop. Using CM = 2 × K × I × D / VD, K = 12.9, and Chapter 9 Table 8 areas (1 AWG = 83,690 CM; 1/0 AWG = 105,600 CM; 2/0 AWG = 133,100 CM), what is the smallest listed copper size that meets the drop limit?
Which formula gives the full-load line current of a three-phase transformer?
Which insulation type is identified in NEC 2017 Table 310.104(A) for use in wet locations and is commonly used on secondary conductors in conduit?
Conductors
Type USE cable used on an underground secondary lateral is generally not permitted as interior wiring. That limitation is in NEC 2017:
The grounded conductor of a 120/240 V secondary shall be identified by which means under NEC 2017 200.6?
Per NEC 2017 Table 310.15(B)(16), the 75°C ampacity of 4/0 AWG copper THWN, not more than three current-carrying conductors, 30°C ambient, is:
Per Table 310.15(B)(16), the 75°C ampacity of 3/0 AWG copper THWN, not more than three current-carrying conductors, 30°C ambient, is:
On a 3-phase, 4-wire, wye secondary that supplies a large nonlinear load, the grounded (neutral) conductor shall be counted as a current-carrying conductor for ampacity adjustment under which NEC 2017 rule?
For a 120/240 V, 3-wire, single-phase dwelling service or feeder that supplies the entire load, NEC 2017 310.15(B)(7) permits the conductors to have an ampacity not less than what percent of the service or feeder rating?
Unless the equipment is listed and identified for use at 75°C, NEC 2017 110.14(C)(1) generally requires conductors 14 AWG through 1 AWG, or circuits 100 A or less, to be sized from which temperature column?
A 4/0 AWG copper THWN secondary (75°C ampacity 230 A) is installed in a 40°C ambient. Using Table 310.15(B)(2)(a), the 75°C correction factor is 0.88. What is the corrected ampacity before any bundling adjustment?
Six current-carrying 3/0 AWG copper THWN conductors are installed in one raceway in a 30°C ambient. Using Table 310.15(B)(3)(a), what is the adjusted 75°C ampacity of each conductor?
A 200 A, 120/240 V, 3-wire, single-phase dwelling service supplies the entire dwelling. Using NEC 2017 310.15(B)(7) and Table 310.15(B)(16) 75°C copper, what is the smallest copper size that meets the 83 percent allowance?
NEC 2017 225.6(A)(1) requires open individual overhead conductors 1000 V or less, not supported by a messenger, to be not smaller than which sizes?
Chapter 9, Table 8 of NEC 2017 lists the circular-mil area of 3/0 AWG as:
NEC 2017 230.24(B)(4) requires overhead service conductors 600 V or less to have a minimum vertical clearance of how much over public streets, alleys, roads, and parking areas subject to truck traffic?
Overhead Lines
NEC 2017 230.10 and 225.26 prohibit using which of the following to support overhead service conductors or outside overhead conductors?
Open overhead service conductors shall have a clearance of not less than which distance from windows designed to be opened, doors, porches, balconies, ladders, stairs, fire escapes, or similar locations, per NEC 2017 230.9(A)?
Overhead service conductors not exceeding 300 V to ground shall have a minimum clearance of how much over residential property and driveways, and over commercial areas not subject to truck traffic, per NEC 2017 230.24(B)(2)?
NEC 2017 230.24(B)(1) permits a 10 ft vertical clearance at the service entrance, at the lowest point of the drip loop, and above pedestrian-only areas only when the voltage does not exceed:
Where the voltage to ground exceeds 300 V in the areas otherwise covered by the 12 ft rule, NEC 2017 230.24(B)(3) raises the minimum overhead service-conductor clearance to:
Unless an exception in 230.24(A) applies, NEC 2017 requires overhead service conductors to have a vertical clearance of not less than which distance above a roof surface, maintained for not less than 3 ft in all directions from the roof edge?
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