Conductors Flashcards
A run of rigid metal conduit contains 6 size 4/0AWG copper THW wires and three size 1AWG copper THWN wires. The minimum trade size rigid metal conduit permitted for these is:
- Determine area of conductor Ch 9, Table 5
THWN* 1AWG - 0.1562 in SQ x 3 = 0.4686
RHW* 4/0AWG- 0.3718 in SQ x 6 = 2.2308
Total conductor size = 2.6994 in sq
- Max RMC fill is 40% (Table 4 , CH 9) Art. 344
- GO to Table 4 RMC, 40% column-
- 2.6994 or larger =** 3in **
Single conductor cables size 4/0 AWG are laid in a singular layer in ALuminumm ladder type cable tray. The cable tray is not permitted too have a rung spacing greater than:
9in
392.10(B)(1)(a) Single Conductor Cables
Conductor Resistance
Conductor resistance for 1000 ft lengths is given in Table 8, Chapter 9
A circuit consisting of three copper THHN wires is run in electrical metallic tubing and
serves a continuous load of 44 amperes. There are six other wires with THHN insulation in
the same run of EMT for a total of nine current-carrying wires as shown in Figure 14. In
route to the room where the loads are located, the raceway with the nine conductors,
passes through an area with a 120EF ambient temperature. If a 60 ampere overcurrent
device protects the circuit, and all terminations are 75EC rated, the minimum size wire
permitted for the circuit is:
A. 10 AWG. C. 6 AWG. E. 3 AWG.
B. 8 AWG. D. 4 AWG.
D 4 AWG
This is the same branch circuit as in the previous question except this time there are six
extra conductors in the raceway so the adjustment factors of Table 310.15(B)(2)(a) will
apply, and the raceway also passes through an area where the ambient temperature is
120degrees F. The temperature correction factors at the bottom of Table 310.16 will also apply.
With nine conductors in the raceway, the values of ampacity in Table 310.16 must be
reduced to 70% of their listed value for this situation. There are no 75degree C terminations in the raceway, and the conductor insulation is rated 90degree C, therefore, the 90degree C column of Table 310.16 can be used for the purpose of applying the adjustment factors, which for a size 6 AWG copper conductor is 75 ampere. Because the 90degree column of Table 310.16 was used, the temperature correction factor Table 315(B)(1)(1) in the 90degree C column of 0.82, corresponding to(120degree F ambient temperature) will apply.
75 A × 0.7 × 0.82 = 43 A
The requirement in 210.19(A) that the conductor ampacity was required to be 125% of the
continuous load only applied when determining the minimum conductor size before the
adjustment factor was applied. *Therefore, compare the adjusted and corrected ampacity
of 43 amperes for the size 6 AWG copper conductor with the load 44 amperes.
.* Note in this case the adjusted ampacity is less than the load of 44 amperes, therefore, a larger wire size must be used. Apply the adjustment and correction factors to a size 4 AWG copper conductor to get 55 amperes.
95 A × 0.7 × 0.82 = 55 A
A building served with a 208/120 volt, 3-phase electrical system has a demand load of 750
amperes. The service consists of a single 1000 ampere main circuit breaker supplied with
three parallel sets of service conductors each in a separate rigid metal conduit. The
minimum size copper THW wire permitted for this service is:
A. 250 kcmil. C. 350 kcmil. E. 500 kcmil.
B. 300 kcmil. D. 400 kcmil.
D 400 kcmil
Section 240.4(C) requires the conductors to have an ampacity not less than the rating of
the overcurrent device when the overcurrent device is rated in excess of 800 amperes.
There are three sets of conductors each in separate raceway and each conductor must
have an ampacity of not less than 333 amperes.
Each conductor ampacity = 1000A / 3 sets = 333A
Go to TAble 310.16 = 335A - 400kcmil
If the resistance of a copper conductor is 0.410 ohms/k ft, the total resistance of the circuit conductors for a single-phase load 125 ft from the current supply is:
A. 0.0514 ohms. B. 0.1025 ohms. C. 0.1135 ohms. D. 0.5125 ohms. E. 1.0250 ohms.
B 0.1025 ohms
There are two wires in the 125 ft run so the total length of wire is 250 ft. 250 ft is 25% of 1000 ft so the resistance will be 0.1025 ohm.
(0.25 × 0.410 ohm = 0.1025 ohm)
A size 3/0 AWG, THWN, copper conductor if run in a conduit where there are a total of 8 current carrying conductors has an allowable ampacity of:
A. 100 amperes. B. 125 amperes. C. 140 amperes. D. 160 amperes. E. 200 amperes.
**C 140 **amperes The allowable ampacity values given in Table 310.16 are only valid if there are no more than three current carrying conductors in a raceway or cable and if the ambient temperature does not exceed 30C. There are ambient temperature correction factors at the bottom of the table and adjustment factors in Table 310.15(B)(2)(a) when there are more than three current carrying conductors in the raceway or cable. In this case, the adjustment factor from Table 310-15(B)(2)(a) is 0.7.
Multiply the ampacity of a size 3/0 AWG, copper THWN conductor by 0.7 to get the adjusted ampacity of the conductor which is 140 amperes. 200 A × 0.7 = 140 A MRC — E3605.3, Table E3605.3, and Table 3605.1
Assume a feeder consists of size 3 AWG copper conductors run in rigid
nonmetallic conduit and protected with a 100 ampere circuit breaker. The length of run is
long, and to prevent excessive voltage drop, the circuit conductor is increased to size 2
AWG rather tan using size 3 AWG which is the minimum permitted. If a copper equipment
grounding conductor is in the rigid nonmetallic conduit, the minimum size permitted is:
A. 8 AWG. C. 4 AWG. E. 2 AWG.
B. 6 AWG. D. 3 AWG.
B. 6 AWG
Look up the circular mil area of the conductors from Table 8 and divide the
largest by the smallest. (size 3 AWG is 52,620 circular mils and size 2 AWG is 66,360
cmil) Divide 66,360 by 52,620 to get 1.26. Next look up the minimum size of equipment
grounding conductor for the circuit. Assuming copper, the minimum is size 8 AWG for a
100 ampere circuit which from Table 8 has an area of 16,510 cmil. Now multiply 16,510 by
1.26 to get 20,803 cmil. Finally look up the minimum size which is a 6 AWG with an area
of 26,240 cmil.
Electrical conductors run within rigid metal conduit and installed under a parking lot with a cover equivalent to a 4 in. thick slab of concrete are to be at a depth from the finished grade level to the top of the conduit of not less than:
A. B. 4 in. 6 in. C. 12 in. D. 18 in. E. 24 in.
24in
Table 300.5 required a depth of burial of 24 in. under a parking lot no matter if there is a concrete cover or not.
In the case of nonmetallic single-gang device boxes installed in walls and ceilings, clamping of type NM-B cables to the box is not required if the cable is supported as measured along the cable a distance of not more than:
A. 18 in. of the box. B. 14 in. of the box. C. 12 in. of the box. D. 10 in. of the box. E. 8 in. of the box.
8 in of the box
314.17(B)(2) Exception
