General Knowledge Flashcards
Operation at a substantially constant load for an indefinitely longtime.
Continuous duty
Voltage drop in a series circuit
3 100W @ 120V light bulbs are connected in series. What is the voltage drop on each lamp?
R = 120V x 120V / 100W = 144ohm
Rt = 144 + 144 +144 = 432 ohm
I = 120V / 432ohm = .2777
R1 = .2777 x 144 = 40V
R2 = .2777 x 288 = 79.97V
R3 = .2777 x 432 = 119.96V
- find total resistance (series) Rt =
1. Find resistance R = E x E / W
A clamp ammeter is used to measure the current flowing in each ungrounded leg of 120/240V, 3wire service. The current flowing on leg A is 27A and the current flowing on leg B is 19A. What is the current flowing on the nuetral conductor?
I n = Ia - Ib
In = 27-19A = 8A
An apartment building is supplied power from a 3 phase, 4wire 208/120V electrical system with a 3wire, 208/120V feeder with two ungrounded conductors and a neutral conductor to each living unit. Only 120V loads are operating and the loads are balanced. The current on each ungrounded conductor is measured at 42A. What is the current flowing on the neutral?
In a 3 phase system, the current will never be smaller than the smallest phase current or larger than the largest phase current. A good estimate is the average of the two currents which, in this case is 42A.
An electrical conductor is made up of 19 strands each with a diameter of 0.0837 inches. What is the approximate circular mil area of the conductor?
- Convert inches to mils to get circular mils
- That gives the area of one strand
- Multiply that number by 19 strands
1inch = 1/1000 of a mil
0.0837 = 83.7 mil
cmil = mil squared = 1 strand
83.7mils x 83.7mils = 7005.7 cmils /per strand
19strands x 7005.7cmils = 133,108 cmil
An electrical conductor is made up of 19 strands each with a diameter of 0.0837 inches. What is the approximate circular mil area of the conductor?
- Convert inches to mils to get circular mils
- That gives the area of one strand
- Multiply that number by 19 strands
1inch = 1/1000 of a mil
0.0837 = 83.7 mil
cmil = mil squared = 1 strand
83.7mils x 83.7mils = 7005.7 cmils /per strand
19strands x 7005.7cmils = 133,108 cmil
An electrician installs two 20 ampere, 120 volt circuits to supply 16 fluorescent lighting
fixtures. Each fixture draws 1.4 amperes. In addition there are two 120 volt fans on
individual circuits each drawing 5.6 amperes. If the building is served by a 120/240 volt,
single-phase, 3-wire service, the minimum neutral current to serve these loads is:
A. 0 amperes. C. 5.6 amperes. E. 33.6 amperes.
B. 2.8 amperes. D. 16.8 amperes.
A 0 amperes
It is expected the electrician will attempt to balance the loads on the ungrounded
conductors and minimize the current on the neutral conductor. In this case, it is possible
to balance the loads by placing 8 lighting fixtures on one circuit and 8 fixtures on the other
circuit as follows:
PH A = 1.4 x 8 =
+ 5.6 A fan
PH B = 1.4 x 8 =
+ 5.6 A fan
A 3-phase 480 volt capacitor bank rated 90 kVARs is connected near the main service of a
building to correct the power factor. The minimum permitted ampere rating of the
conductors to the capacitor bank is:
A. 115 amperes. C. 146 amperes. E. 260 amperes.
B. 130 amperes. D. 198 amperes.
C 146 Amperes
First the current must be determined that will flow to the capacitor bank when it is
energized. The same formula is used as would be used to determine the full load current
of a transformer except in this case kVAR is used instead of kVA.
Icap = kVAR × 1000 / 1.73 × V
Icap = 90 x 1000 / 1.73 x 480 = 108.4A
The question is what is the minimum ampere rating of the conductors to the capacitor
bank, therefore, follow the rule in 460.8(A). Multiply the capacitor current by 1.35.
I CONDUCTOR = 108.4 A × 1.35 = 146 Amperes
