jeff48356
Senior Member
- Location
- Livonia, Michigan
Please let me know how I did on this problem, or if you would have answered any of these differently. Thanks.
QUESTION:
A person buys a house without a garage, and later decides to have a 2-car detached garage built. The owner?s electrical needs are as follows: Garage is to be powered with a 120/240V subpanel installed in the garage, using a 40-amp underground feeder from the main panel in the basement of the house. The owner also wants to be able to control the outdoor garage lights from inside of the garage as well as inside the house. The lighting circuit is to be rated at 15 amps with no other non-lighting outlets connected to it. Receptacle outlets are to be fed by two 20-amp circuits.
1. What is the minimum allowable size for each of the ungrounded (hot), grounded (neutral), and equipment grounding (green) conductors if THHN copper conductors are to be used?
2. If the minimum size conductors are used, what is the minimum allowable size PVC conduit that can be used?
3. What is the minimum allowable size metal junction boxes to be used in the basement and garage to contain all conductors in Question 1?
ANSWERS:
1. Based on Table 310.16: #8 THHN has a rating of 55 amps, and since there are 6 current-carrying conductors in the raceway, (Table 310.15 B2a), 55 * 0.8 = 44 amps. So use #8 ungrounded (red/black) and #8 white (neutral). Based on Table 250.122, use #10 green grounding wire. For the 3-way common and traveler wires, use #14 black, red, and blue.
2. Since the maximum fill of conduit is 40% (Chapter 9, Table 1), and the sums of the cross-sectional areas of the conductors are as follows: 3@0.0366, 3@0.0097, 1@0.0211, for a total of 0.16 square inches. The area of a 3/4? conduit is 0.442 square inches. 40% is 0.177. So a ?? barely meets the requirement. I would personally use 1? to make the wires easier to pull.
3. Each junction box would contain all 7 conductors in Question 1, as well as a #14-3 NM-B cable and a #8-3 NM-B cable. All grounding conductors count as one. The remaining 6 are paired with a conductor from the cables, giving a total of six #8 and 6 #14, and one #10 grounding. For each #14, 2 cubic inches are needed. For each #8, 3 cubic inches. For a #10, 2.5 cubic inches. Total cubic inches needed is 17.5. Therefore, a 4? square by 1.25? deep barely passes with 18 cubic inches. I would personally use a 4-11/16? square X 1.25? because it would provide more freedom, and a 4? box is not likely to have 1? knockout openings, where a 4-11/16? would.
QUESTION:
A person buys a house without a garage, and later decides to have a 2-car detached garage built. The owner?s electrical needs are as follows: Garage is to be powered with a 120/240V subpanel installed in the garage, using a 40-amp underground feeder from the main panel in the basement of the house. The owner also wants to be able to control the outdoor garage lights from inside of the garage as well as inside the house. The lighting circuit is to be rated at 15 amps with no other non-lighting outlets connected to it. Receptacle outlets are to be fed by two 20-amp circuits.
1. What is the minimum allowable size for each of the ungrounded (hot), grounded (neutral), and equipment grounding (green) conductors if THHN copper conductors are to be used?
2. If the minimum size conductors are used, what is the minimum allowable size PVC conduit that can be used?
3. What is the minimum allowable size metal junction boxes to be used in the basement and garage to contain all conductors in Question 1?
ANSWERS:
1. Based on Table 310.16: #8 THHN has a rating of 55 amps, and since there are 6 current-carrying conductors in the raceway, (Table 310.15 B2a), 55 * 0.8 = 44 amps. So use #8 ungrounded (red/black) and #8 white (neutral). Based on Table 250.122, use #10 green grounding wire. For the 3-way common and traveler wires, use #14 black, red, and blue.
2. Since the maximum fill of conduit is 40% (Chapter 9, Table 1), and the sums of the cross-sectional areas of the conductors are as follows: 3@0.0366, 3@0.0097, 1@0.0211, for a total of 0.16 square inches. The area of a 3/4? conduit is 0.442 square inches. 40% is 0.177. So a ?? barely meets the requirement. I would personally use 1? to make the wires easier to pull.
3. Each junction box would contain all 7 conductors in Question 1, as well as a #14-3 NM-B cable and a #8-3 NM-B cable. All grounding conductors count as one. The remaining 6 are paired with a conductor from the cables, giving a total of six #8 and 6 #14, and one #10 grounding. For each #14, 2 cubic inches are needed. For each #8, 3 cubic inches. For a #10, 2.5 cubic inches. Total cubic inches needed is 17.5. Therefore, a 4? square by 1.25? deep barely passes with 18 cubic inches. I would personally use a 4-11/16? square X 1.25? because it would provide more freedom, and a 4? box is not likely to have 1? knockout openings, where a 4-11/16? would.
