2.
When installing NM type cable in a residential construction project, you find that you are compelled to use a shallow square metal box, 4" x 1.5" deep. If you are to use this box as a junction box for wiring in a finished area wall, how many cubic inches will the plaster ring have to be, minimum, if you have five (5) number twelve two conductor cables with ground wires (#12/2 +gnd) coming into this box?
a. 12.75 cu. in.
b. 3.75 cu. in.
c. 1 cu. in.
d. 1.5 cu. in.
You have 10 current carrying conductors and 1 equipment grounding conductor to count in your conductor fill volume allowance, 314.16 (B)(1),(5). Table 314.16(B) shows an allowance of 2.25 cu. in./ #12. So, 11 conductors multiplied by 2.25 equals 24.75. Table 314.16(A) shows the box having a capacity of 21 cu. in. With 24.75 cu. in. required, subtracting the available 21 cu. in. leaves 3.75 cu. in. minimum to be accommodated with the plaster ring.

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8.
You are preparing to pull in the wires from service equipment to a sub-panel through parallel conduits that you have already installed. It is a 400 ampere three phase four wire feeder, plus the equipment grounding conductor. Due to the distance, to accommodate voltage drop you have to increase your ungrounded conductors from 3/0 cu to #250 kcmil. What minimum size will your copper (cu) equipment grounding conductor need to be?
a. #1 al
b. #3 cu
c. #2/0 cu
d. #1 cu
250.122(B) requires the EGC to be increased proportionately to the ungrounded conductors. Table 250.122 gives the EGC size for a 400a feed to be a #3 cu wire. Ch. 9 Table 8 gives the conductor circular mil area, for a #3/0 - 167,800, a #3 - 52,620. To find the ratio of increase in conductor size: 250,000 kcmil / 167,800 = 1.49. Apply the ratio to the #3 area: 52,620 x 1.49 = 78,404. Ch.9 Table 8 - 78,404 is larger than a #2, but smaller than a #1.

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13.
In the circumstances described in the previous question, the breaker feeding the workshop is a 150A 240V 2 pole device. What is the minimum required size for the copper Equipment Grounding Conductor in THWN insulation?
a. #8 THWN
b. #6 THWN
c. #3 THWN
d. #1/0 THWN
Table 250.122 shows a #6 required; 250.122(B) states that if the ungrounded conductors are increased in size, the ECG must be increased proportionately. Per Table 310.15(B)(16), #1/0 would be sufficient for 150A, that was increased to #350 kcmil. Ch. 9 Table 8 gives the circular mil area of #1/0 to be 105,600. To determine ratio: 350,000 ÷ 105,600 = 3.314. Circular mil area of #6 is 26,240, so applying the ratio = 26,240 x 3.314 = 86,959 circ. mils = #1/0.

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16.
You have 132 #14 THWN copper wires used for signaling in a Class 1 system, and 5 #12 THWN spare copper conductors. To get the wiring from the PLC to the machinery, you will use EMT conduit. What is the minimum size that the EMT will be required to be?
a. 2.5"
b. 2"
c. 4"
d. 3"
725.51(A) refers you to 300.17, which directs you to refer to the appropriate wiring method section of the NEC. In this case, you would use 358.22. Here it refers you to the appropriate Tables in Ch. 9, you will need to use Tables 1, 4, and 5 for the following numbers: #14 = 0.0097 circ. mil, #12 = 0.0133 circ. mil: 0.0097 x 132 = 1.2804, 0.0133 x 5 = 0.0665, 1.2804 + 0.0665 = 1.3469"^{2} . Table 4 shows 2.5" EMT to be the smallest available with sufficient area.

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18.
In the interior of a finished commercial facility, you are asked to add a new 100 ampere subpanel, fed with a 100A 3 phase 4 wires plus ground feeder from the main service equipment, which is sitting 50' away. Both the new subpanel and the existing main are surface mount units, and you can install surface mounted EMT conduit. You are given no details of what the load will be. Without applying any exceptions or special conditions requirements, what is the minimum THWN copper wire size that you can use for the current carrying conductors?
a. #4 THWN
b. #2 THWN
c. #3 THWN
d. #1 THWN
310.15(B)(3)(a) requires an ampacity adjustment for more than 3 current carrying conductors, and since you do not know what the loads are, you must count the neutral. Table 310.15(B)(3)(a) shows an 80% adjustment factor needing to be applied, Table 310.15(B)(16) shows #2 THWN having an ampacity of 115A: 115A x 80% = 92A. 240.4(B) allows you to use the next size up if ampacity is not equal to a standard size, therefore this wire can be connected to a 100 breaker.

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