Conductors

Question 1

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:

Answer 1

1. 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

2. Max RMC fill is 40% (Table 4 , CH 9) Art. 344
3. GO to Table 4 RMC, 40% column-
4. 2.6994 or larger =** 3in **

Question 2

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:

Answer 2

9in

392.10(B)(1)(a) Single Conductor Cables

Question 3

Conductor Resistance

Answer 3

Conductor resistance for 1000 ft lengths is given in Table 8, Chapter 9

Question 4

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.

Answer 4

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
120EF. 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 75EC terminations in
the raceway, and the conductor insulation is rated 90EC, therefore, the 90EC 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 90E column of Table 310.16
was used, the temperature correction factor in the 90EC column of 0.82, corresponding to
120EF 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

Question 5

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.

Answer 5

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