NEC Questions and Answers
June Part 1 of 2
Here is the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent yesterday so you can see how you did.
By Mike Holt for EC&M Magazine
Q1. What is the minimum depth for PVC schedule 40 conduit below a concrete slab?
A1. If it is under a building, there is no depth requirement. If it is outside under a concrete slab, then the minimum cover is 4 in. [Table 300.5]
Q2. I want to come into a power distribution block with one feeder fused at 400A and feed three 150A panels. How do I size the tap conductors for each panel if they don’t exceed 10 ft in length?
A2. The tap conductors must be sized not smaller than the calculated load in accordance with Article 220, and not smaller than the rating of the overcurrent protective device at the termination of the tap conductors. Also, the tap conductors must have an ampacity not less than 10 percent of the ampacity of the overcurrent protection device that protects the feeder [240.21(B)(1)].
Example: What size tap conductor would be required from a 400A overcurrent protection device supplying a 150A circuit breaker if the calculated continuous load was 100A?
(a) 3 AWG, rated 100A (b) 2 AWG, rated 115A
(c) 1 AWG, rated 130A (d) 1/0 AWG, rated 150A
Answer: (d) ‑1/0 AWG tap conductors would be required to supply the circuit breaker. A 1 AWG “feeder” can be protected by a 150A overcurrent protection device because of the “next size up” rule [240.4(B)], but this rule does not apply to feeder “taps.”
Q3. Is there a limit to the number of NM cables that can be run through the same drilled hole in wooden floor joists?
A3. No. However, if you bundle them continuously for more than 24 in., they must have their ampacity adjusted in accordance with 310.15(B)(2)(a).
Q4. How far away from a diesel fuel dispensing tank must the nonexplosion-proof equipment be located?
A4. Diesel fuel isn’t a “combustible” liquid. Therefore, diesel dispensing equipment and associated wiring isn’t required to comply with the hazardous locations requirements of Article 514 [514.3(A)].
Q5. When a conduit leaves a Class 1, Division 1 classified paint spray room and goes under the concrete floor into a cabinet in the motor control room which is non-classified, is one sealing fitting in the paint mix room sufficient, or is an additional sealing fitting required in the non-classified area as well?
A5. A single conduit seal in the spray area is OK, as long as it is within 10 ft of the Class I, Division 1 location boundary. There must be no fitting, except a listed explosionproof reducer, between the seal fitting and the point at which the conduit leaves the Class I, Division 1 location [516.4(A) and 501.15(A)(4)].
Q6. A conductor passing through a box unbroken counts as one conductor, so if I loop a conductor through the box unbroken and strip off enough insulation to terminate on a receptacle without cutting the wire, does the conductor count as one or two conductors for box sizing?
A6. It counts as two conductors. Each conductor that runs through a box and does not have 6 in. of free conductor for splices or terminations in accordance 300.14, and each conductor that terminates in a box is counted as a single conductor volume in accordance with Table 310.16(B). If the conductor includes a loop long enough to terminate on a device legally (at least 12 in.), it must be counted as two conductors [314.16(B)(1)].
Q7. Can I use NM cable to install lay-in fluorescent fixtures in a drop ceiling or cove lighting in a dwelling unit?
A7. Yes, type NM cable can be used in any ceiling in a dwelling unit [334.10(1)]. It cannot, however, be used exposed in dropped or suspended ceilings in other than one- and two-family and multifamily dwellings [334.12(2)].
Q8. If structural steel is used as a grounding electrode for the electrical service, is it also necessary to supplement the steel electrode with an additional electrode like a ground rod?
A8. Structural metal is only an electrode when it meets one of the following [250.52(A)(2)]:
(a) 10 ft or more of a single structural metal member is in direct contact with the earth or encased in concrete that is in direct contact with the earth.
(b) The structural metal is bonded to an electrode as defined in 250.52(A)(1), (3), or (4).
(c) The structural metal is bonded to two ground rods if the ground resistance of a single ground rod exceeds 25 ohms [250.52(A)(5) and 250.56].
(d) Other means approved by the authority having jurisdiction.
It is unlikely that your structural metal meets (a), so it is only an electrode if it is bonded to another electrode, such as a concrete encased electrode.
Q9. How many NM cables can be secured to a panel by each NM box connector?
A9. Cable clamps or cable connectors must be used with only one cable, unless that clamp or fitting is listed for more than one cable [110.3(B)]. The best way to determine this is by using the manufacturer’s website. Here is an example from Arlington’s site: http://www.aifittings.com/c_19.htm
Q10. Does the National Electrical Code limit motor control circuits to 120V?
A10. No. Motor Control Circuits are covered by Part VI of Article 430 and Motor Controllers are covered by Part VII of Article 430, neither of which provides a voltage limit on motor control circuits.
Q11. How deep does rigid metal conduit need to be run under a road on industrial plant property?
A11. The minimum cover is 24 in. for raceways under roadways [Table 300.5].
Q12. Can I install a receptacle below 18 in. in a dwelling unit garage?
A12. Yes. The NEC does not specify any height for receptacle outlets in dwelling unit garages.
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