By Mike Holt for EC&M Magazine
Q1. My question concerns the termination of home automation control conductors that operate at less than 30V. Currently we run twisted pair Class 2 cables from the control panel to the 125V home automation switches. Some inspectors prohibit the Class 2 cable from entering the switch box, other say it is ok but they insist we maintain 1/4 in. clearance between the power and control wiring, and some want us to install a barrier between the Class 2 wiring and the line voltage wiring. What rules apply to this installation?
A1. The rule you are looking for is contained in 725.55(D), which permits Class 2 conductors to be mixed with power conductors (without a barrier), if the power conductors are introduced into the enclosure solely to connect to the Class 2 controlled equipment (switch in this case). But this is only permitted if a minimum of 1/4 in. separation is maintained between the Class 2 conductors and the power conductors.
Q2. Is GFCI protection required for a 125V, 15 or 20A receptacle supplying power to a cord and plug water cooler?
A2. GFCI protection is not required for the receptacle supplying a water cooler, unless the receptacle is located in an area where GFCI protection is required. Agricultural Buildings, 547.5(G)
Q3. What is the minimum mounting height for a panelboards, disconnect, or meter located in or on a building?
A3. The NEC does not mandate a minimum mounting height for panelboards, disconnects, or meter enclosures. But switches, and circuit breakers used as switches, shall be installed so the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, is not more than 6 ft 7 in. above the floor or working platform [404.8(A)]. Exception No. 2 permits switches and circuit breakers used as switches to be mounted higher than 6 ft 7 in. if they are located adjacent (next) to the equipment they supply and they are accessible by portable means.
Note: 550.32(F) requires the outdoor mobile home disconnect to be installed so the bottom of the enclosure is not less than 2 ft above finished grade or working platform.
Q4. How far can unfused service entrance conductors extend into a building?
A4. Because service entrance conductors do not have short-circuit or ground-fault protection, their lengths must be severely restricted inside a building. Section 230.70(A)(1) requires the service disconnect to be located at a readily accessible location either outside, or inside nearest the point of entry of the service conductors.
Q5. What size branch-circuit conductor and short-circuit/ground-fault protection is required for a 7 1/2-hp 230V, three-phase motor?
A5. Branch-circuit conductors are sized as 125% of the motor FLC [430.22 and Table 430.150] 22A x 1.25 = 27.5A, 10 AWG, rated 30A at 60°C [110.14(C) and Table 310.16].
The branch-circuit protection is sized in accordance with 240.6(A), 430.52(C)(1) Ex. 1, and Table 430.150. For an inverse-time breaker: 22 A x 2.5 = 55A, next size up = 60A
Note: The 10 AWG conductors are protected against overcurrent by the overload protection devices, which are sized between 115-125 percent of motor nameplate current rating [430.32].
Q6. When is an expansion fitting required for rigid
A6. An expansion fitting is required to compensate for thermal expansion and contraction where the length of the raceway change is expected to be 1/4 in. or greater in a straight run between securely mounted items such as boxes, cabinets, elbows, or other conduit terminations [352.44].
Listing instructions provided with expansion fittings indicate
that we should add 30ºF to the ambient temperature when the raceway is in direct sunlight.
Assuming a high ambient temperature of 90ºF (plus 30ºF due to solar heating) and a
low temperature of 0ºF (no solar exposure), the temperature change will be 120ºF.
Table 352.44 indicates that the total expansion and contraction length change for
120ºF would be 4.9 in. for a 100 ft run.
CAUTION: If the ambient temperature during installation is high, you should realize that the conduit is at its expanded range and will contract when the temperature drops. Of course, the opposite applies if the ambient temperature is low.
Q7. Why is interlocked MC Cable not permitted to be used for branch circuit wiring for the patient care area of doctors, chiropractic, and dentist examining rooms? This does not make any sense to me, because AC cable is permitted.
A7. Section 517.13 requires all branch circuits serving patient care areas to be installed in a metal raceway or cable that is identified as an acceptable grounding return path in accordance with 250.118 [517.13(A)]. In addition, the grounding terminals of all receptacles and conductive surfaces of fixed electric equipment in patient care areas shall be grounded by an insulated copper equipment grounding (bonding) conductor [517.13(B)] that is installed in the raceway or cable.
The NEC requires redundant grounding (two separate equipment bonding paths); one bond path shall be a mechanical bond (metal raceway or armored of a metal cable that is suitable as an effective ground-fault path), and the second bond path shall be an insulated copper equipment grounding conductor.
The metal sheath of AC cable is suitable as a ground-fault path because it contains an internal bonding strip of aluminum in direct contact with the outer metal sheath [250.118(9)]. However, the outer metal sheath of interlocked MC Cable is not listed as a suitable ground fault path [250.118(11)], therefore, MC Cable shall not be used to supply branch circuits in patient care areas of health care facilities.
Q8. I have an issue where a contractor used 15A, 125V receptacles on a 20A circuit with 12 AWG conductors. In addition, the contractor used 14 AWG tap conductors to the 15A receptacles. I feel that a 20A rated receptacle is required and the conductors to the receptacle shall be a minimum 12 AWG. Am I right?
A8. Almost. A 15A rated receptacle can be connected to a 20A branch circuit that supplies two or more receptacles (this would include a single duplex receptacle, because it contains two receptacles) [210.21(B)(3)]. However, the tap rule contained in 210.19(A)(4) Ex. 1(c) clearly states that branch circuit taps are not permitted for receptacles. So the 15A receptacles are fine, as long as there are at least two of them on a circuit, but the 14 AWG pigtails are a Code violation.
Q9. In our standard grounding detail for a 1200A service, we show a separate grounding electrode conductor to the building steel, water pipe, and ground rod. Some in our office insist that we only need a 6 AWG to the ground rod. I say it needs to be full sized in accordance with Table 250.66. Who's right?
A9. Not you. Where the grounding electrode conductor is
connected to a ground rod, that portion of the grounding electrode conductor that
is the sole connection to the ground rod is not required to be larger than 6 AWG copper
[250.53(E) and 250.66(A)].
Q10. What is the proper way to replace two prong receptacles with a grounding type receptacle in an older house where no ground wire is located in the outlet box?
A10. Where no grounding means exists in the outlet box, nongrounding-type receptacles can be replaced with a GFCI receptacle if marked "No Equipment Ground," or a grounding type receptacle, if GFCI protected and marked "GFCI Protected" and "No Equipment Ground." See 406.4(D)(3) for details.
Copyright © 2002 Mike Holt Enterprises,Inc.