All questions and answers are based on the 2005 NEC.
Q. 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 such as a ground rod?
A. Structural metal is only an electrode when it meets one of the following [250.52(A)(2)]:
(a) 10 feet 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) As shown in the Figure, 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 (AHJ).
It is unlikely that your structural metal meets (a), so it is only an electrode if it's bonded to another electrode, such as a concrete-encased electrode.
Q. Can I run underground telephone line in the same trench with underground UF cable without separation?
Q. Does 200.6 intend that the color gray be used to identify the neutral conductor of one system while white is used to identify the neutral of another system when conductors of different systems are installed in the same raceway?
A. Basically, the answer is yes. The NEC states that where neutral conductors of different wiring systems are installed in the same raceway, cable, or enclosure, each neutral conductor must be identified to distinguish the systems by one of the following [200.6(D)]:
A continuous white or gray outer finish along its entire length.
The neutral conductor of the other system must have a different outer covering of continuous white or gray outer finish along its entire length. A white conductor with a colored stripe (other than green) running along the entire length of the conductor is also permitted.
Other means of identification as permitted by 200.6(A) or 200.6(B) that will distinguish each system neutral conductor.
The means of identification must be permanently posted at each branch-circuit panelboard.
Note: Where a premise has branch circuits supplied from more than one voltage system, each ungrounded conductor must be identified by system [210.5(C)].
Q. Is there a limit to the number of NM cables that can be run through the same drilled hole in wooden floor joists?
A. No. However, if you bundle them continuously for more than 24 inches, they must have their ampacity adjusted in accordance with 310.15(B)(2)(a).
Example: Let's say we're working with a 20A circuit. If we bundle three 12/2 cables and one 12/3 cable (nine current-carrying 12 THHN conductors), the ampacity for each conductor (30A at 90°C, Table 310.16) is adjusted by a 70% adjustment factor [Table 310.15(B)(2)(a)].
Adjusted conductor ampacity = 30A × 0.70 = 21A
Q. How many NM cables can be secured to a panel by each NM box connector?
A. 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 referencing the manufacturer's Web site.
Q. Does the NEC limit motor control circuits to 120V?
A. No. Motor control circuits are covered by Part VI of Art. 430, and motor controllers are covered by Part VII of Art. 430 — neither of which provides a voltage limit on motor control circuits.
Q. How deep does rigid metal conduit need to be run under a road on industrial plant property?
A. The minimum cover is 24 inches for raceways under roadways [Table 300.5].
Q. Can I install a receptacle below 18 inches in a dwelling unit garage?
A. Yes. The NEC does not specify any height for receptacle outlets in dwelling unit garages.
Q. Can I use NM for wiring inside wooden stud walls of a dwelling unit garage, which will be exposed?
A. If it is judged by the AHJ to be subject to physical damage, the cable must be protected from physical damage by rigid metal conduit, intermediate metal conduit, Schedule 80 rigid nonmetallic conduit [352.10(F)], electrical metallic tubing, guard strips, or other approved means [334.15(B)].
Q. The voltage to ground momentary drops from 110V to 102V when the 240V air conditioner kicks in and the lights dim. Is this an NEC violation?
A. No. The purpose of the National Electrical Code is the practical safeguarding of persons and property from hazards caused by the use of electricity [90.1(A). The NEC doesn't consider voltage drop to be a safety issue in this application.
Q. Can manufactured wiring systems, which can be unplugged, be used for emergency lighting and exit lights?
Q. How do I size the conductors and the breaker for a motor?
A. There are two components to this question: Sizing the conductors and sizing the ground fault and short-circuit protection. Let's do this step-by-step:
Step 1: Conductors to a single motor must be sized not smaller than 125% of the motor FLC rating as listed in [430.22(A)]:
Table 430.247 (direct current motors)
Table 430.248 (single-phase motors)
Table 430.250 (3-phase motors)
Step 2: Each motor branch circuit must be protected against short-circuit and ground faults by a protection device sized no greater than the percentages listed in Table 430.52 [430.52(C)(1)].
Example: What size branch-circuit conductor, ground-fault, and short-circuit protection device is required for a 7½-hp, 230V, 3-phase motor with a nameplate of 19A?
Step 1. Motor Branch-Circuit Conductor Size
Motor FLC = 22A [Table 430.250]
Conductor's size = 22A × 1.25
Conductor's size = 27.5A, 10 AWG, rated 30A at 75°C [Table 310.16]
Step 2. Branch-circuit short-circuit and ground-fault protection device using an inverse time breaker.
22A × 2.5 = 55A, next size up = 60A [240.6(A) and 430.52(C)(1), Exception No. 1].
Q. How many conductors can I install in a wireway?
A. The maximum number of conductors permitted in a wireway is limited to 20% of the cross-sectional area of the wireway. When more than 30 current-carrying conductors are installed in any cross-sectional area of the wireway, the conductor ampacity, as listed in Table 310.16, must be reduced according to the adjustment factors listed in Table 310.15(B)(2)(a) [376.22].
Example: 20A circuit. If the wireway has more 32 current-carrying THHN conductors, the ampacity of each conductor is adjusted by a 40% adjustment factor [Table 310.15(B)(2)(a)]
Adjusted conductor ampacity = 30A × 0.40
Adjusted conductor ampacity = 12A