|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.
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,
Bathroom, 210.8(A)(1) and (B)(1)
Carnivals, Circuses and Fairs, 525.23
Elevator Pits, 620.85
Health Care Facilities,
Marinas and Boatyards,
Portable or Mobile Signs, 600.10(C)(2)
Swimming Pools, 680.22(A)
Temporary Installations, 527.6
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
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.
Note: Some local jurisdictions specify a maximum length, but most do
Q5. What size branch-circuit conductor and
short-circuit/ground-fault protection is required for a 7 1/2-hp 230V,
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
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
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
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
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)].
grounding electrode conductor is not required to be larger than 6 AWG because
the resistivity of the soil prevents the possibility of a current flowing that
is higher than what 6 AWG can handle. The purpose of this conductor is to carry
lightning to the earth, not help clear a fault. Like I say in my presentations,
lightning doesn't know the size of the service on a building, so a 6 AWG to a
ground rod for a 200A or 1200A service is fine.
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.