|By Mike Holt for EC&M Magazine
Q1. When can hospital grade MC cable of the interlocked
type be used in a doctor or dentist examining room.
A1. There is no such
product as "hospital grade Type MC cable," you must be referring to Type AC
cable that contains an insulated equipment grounding (bonding) conductor, marked
as HFC- Health Care Facility Cable. Type MC of the interlocked type (looks like
Type AC cable) is never permitted in the patient care area of a health care
Branch circuits serving patient care areas, like an examining
room, shall be installed in a metal raceway or listed cable having a metallic
armor or sheath that qualifies as an effective ground-fault current path in
accordance with 250.118 [517.13(A)].
The metal armored sheath of Type AC
cable is listed as a suitable ground-fault current path because it contains an
internal bonding strip in direct contact with the metal sheath of the cable
[250.118(8)]. However, the outer metal sheath of interlocked Type MC cable is
not listed as a ground-fault current path [250.118(10]); therefore, it shall not
be used to supply branch circuits in patient care areas of health care
facilities. Figure 517-4
Q2. My understanding of Article 645 is that for
a room containing data processing equipment to be classified as an Information
Technology Equipment Room, all of the provisions of 645.2 must be complied with.
This means that if a disconnecting means is not provided to disconnect power to
all electronic equipment and HVAC systems serving the computer room, then the
room is not an information technology room, and none of the requirements of
Article 645 must be complied with. So the way I read it, a disconnecting for the
electronic equipment and HAVAC system is an option for a computer room! Am I
A2. Yes, you are correct. If a disconnecting means is not
installed for the electronic and HVAC equipment in accordance with 645.10, then
the electrical installation is not required, as well as not permitted, to be
installed in accordance with Article 645. Let me see if I can explain this
better, Article 645 relaxes some of the installation requirements that one would
typically expect in Chapter 3, but only if the room is classified as an
Information Technology Room. For the room to be classified as an information
technology room ALL of the following shall be provided:
means complying with 645.10 is provided.
heating/ventilating/air-conditioning (HVAC) system is provided for information
technology equipment and it separated from other areas of the occupancy.
Only listed information technology equipment is installed in the room.
The room is only occupied by those persons needed for the maintenance and
operation of information technology equipment.
The room is separated from
other occupancies by fire-resistant-rated walls, floors and ceilings, with
Note: An information technology equipment room is an
enclosed area specifically designed to comply with the construction and fire
protection provisions of NFPA 75 - Standard for the Protection of Electronic
Q3. Can 460V branch-circuit
conductors be installed in the same raceway with 120V branch-circuit
A3. Yes, as long as the circuit conductors of the different
systems are identified by color-coding, marking tape, tagging, or other approved
means, and the identification system must be permanently posted at each branch
circuit panelboard [210.4(D)].
Also 200.6(D) requires one system
grounded (neutral) conductor be identified by a continuous white or gray outer
finish, and the other grounded (neutral) conductor shall be identified by a
continuous white finish with a readily distinguishable color stripe, other than
green, or white or gray outer finish. Traditionally, the 120V grounded (neutral)
conductor has an outer finish of white, and the color gray is used to identify
the 277V circuit grounded (neutral) conductor.
One last point, all
circuit conductors shall have an insulation voltage rating not less than the
480V circuit voltage. Since THHN/THWN building wiring is rated 600V, this is not
Q4. Our emergency generator serves a 3000A main lug
distribution board (eight sets of 500 kcmil). Are the conductors from the
generator to the distribution board considered service or feeder conductors? In
other words, are we to use Table 250.66 or Table 250.122 to size the grounding
conductor in each raceway?
A4. According to the definitions contained in
Article 100, "service conductors" originate from the electric utility that
delivers electric energy, to the premises served. Whereas "feeders" originate
from service equipment, the source of a separately derived system (transformer),
or other power-supply source (generator that is not a separately derived
So, since the conductors from a generator are always a feeder,
the equipment grounding (bonding) conductor from the generator to the
distribution board shall be sized in accordance with 250.122(F)(1), based on the
3000A protection device size. In your example, a 400 kcmil equipment grounding
(bonding) conductor must be installed in each of the eight parallel
Q5. Is it against the Code to GFCI-protect temporary lighting
circuits? I have seen exposed temporary lighting wire (wire nuts fell off or
were missing) touching metal studs thereby presenting a potentially deadly
A5. The NEC is silent on this issue, so this means,
it's okay to GFCI-protect temporary lighting circuit conductors. However, to
prevent placing a construction site in the dark by a ground fault, temporary
lighting shall not be installed on the branch circuit that supply receptacles
Note: Receptacles rated 15A or 20A, 125V used to supply
temporary power for construction, remodeling, maintenance, repair, or demolition
of buildings, structures, equipment, or similar activities shall be GFCI
Q6. I am wiring a patio on a home, where three of the
walls will be sliding glass doors, with nothing but a 4" post in between them.
Does the NEC required receptacle outlets in front of the sliding
A6. To ensure that a general-purpose receptacle is conveniently
located to reduce the likelihood that an extension cord will travel across
openings, such as doorways or fireplaces, a receptacle outlet shall be installed
so no point along the dwelling unit wall space will be more than 6 ft, measured
horizontally from a receptacle outlet [210.52(A)].
210.52(A)(2), a wall space is:
Any space 2 ft or more in width, unbroken
along the floor line by doorways, fireplaces and similar openings.
occupied by fixed panels in exterior walls.
The space occupied by fixed room
dividers, such as freestanding bar-type counters or railings.
So no, a
receptacle is not required in front of the sliding door.
Q7. If I were to
replace a two-wire receptacle in a bathroom outlet that does not have a
grounding means, am I required to replace it with a GFCI receptacle?
No, the replacement receptacle is not required to be of the GFCI type, however,
it is required to be GFCI protected [406.3(D)(2)]. This can be accomplished by
the use of a GFCI receptacle, or a two-wire nongrounding type or three-wire
grounding type receptacle that is GFCI protected upstream by a GFCI circuit
breaker or GFCI receptacle in accordance with 406.3(D)(3).
Q8. Is a
disconnecting means within sight of an outdoor spa or hot tub for a
A8. A maintenance disconnecting means shall be
installed within sight from spa and hot tub equipment [680.12 and 680.42]. It
shall be located not less than 5 ft horizontally from the inside walls of the
spa or hot tub, unless separated by a solid fence, wall, or other permanent
In other than a "single-family dwelling," a clearly
labeled emergency spa or hot tub water recirculation and jet system shutoff
shall be supplied. The emergency shutoff shall be readily accessible to the
users and located not less than 5 ft away, but adjacent to and within sight of
the spa or hot tub [680.41]
The maintenance disconnecting means required
by 680.12 or a pushbutton controlling a relay located in accordance with this
680.41 could be used to meet the emergency shutoff requirement.
purpose of the emergency shutoff protects users, deaths and injuries have
occurred in less than 3 ft of water because individuals became stuck to the
water intake opening. This requirement applies to spas and hot tubs installed
indoors as well as outdoors.
Q9. I don't see where 300.4 limits the
number of NM cables through a given opening in wood or metal framing members. I
don't want to turn the framing members into Swiss cheese and reduce its
structural integrity by boring a hole for every NM cable. So how many NM cables
can I install in each bored hole?
A9. You are correct, the NEC does not
limit the number of cables within an opening, and we must not reduce the
structural strength of framing members below the requirements of the building
However, where multiconductor NM cables are stacked or bundled
longer than 24 in., the allowable conductor ampacity, based on 90ºC insulation
rating of the conductors as listed in Table 310.16 [334.80], shall be adjusted
in accordance with the multiplying factors contained in Table
So what should you do? Well, separate the cables after
they pass through the framing members so that they aren't' bundled more than 24
inches. But, you don't have to worry about this if no more than nine current
carrying 14 AWG, 12 AWG, or 10 AWG conductors are bundled together for more than
New Ampacity of 14 AWG = 25A x 0.70 = 17.5A, okay with 15A
New Ampacity of 12 AWG = 30A x 0.70 = 21A, okay with 20A
New Ampacity of 10 AWG = 40A x 0.70 = 28A, okay with 30A
Q10. A motor control center is located in a
building where the supply conductors originate from another building. Is a
disconnecting means required for the motor control center? If yes, where must it
A10. A disconnecting means is not required for the motor
control center, however a disconnecting means is required at the remote building
for all conductors that enter a building or structure [225.31]. The building
feeder disconnecting means, which disconnects the MCC shall be installed at a
readily accessible location, either outside the building or structure or inside
the building or structure, nearest the point of entrance of the
Note: Conductors are considered outside of a building or
other structure where they are encased or installed under not less than 2 in. of
concrete or brick [225.32 and 230.6].
But, where documented safe
switching procedures are established and maintained, the building/structure
disconnecting means can be located elsewhere on the premises if the disconnect
is monitored by qualified persons [225.32 Ex 1].
Note: A qualified person
is one who has the skills and knowledge related to the construction and
operation of the electrical equipment and installation, and has received safety
training on the hazards involved with electrical systems [Article
Q11. When our condo building was built, power-limited fire alarm
wiring was installed in raceways and outlet boxes. New power-limited audible
alarms cables are now being installed and they are not run in a raceway. At the
point where the new cable exits the existing junction box (knock-out), no
bushing, clamp or other protection is provided. Does the Code require
low-voltage or limited-energy cables that exit an outlet box to have some form
of protection at the knockout?
A11. I can't find any specific rule in the
NEC that specifically states that fire alarm or any other low-voltage or
limited-energy cable exiting an outlet box must be provide with a fitting that
protects the cable. Maybe because outlet boxes are not required for low-voltage
and limited-energy systems. There are rules in 300.16 and 300.17 for building
cables and knob-and-tub wiring, but these sections do not apply to low-voltage
or limited-energy cables.
Q12. A large number of satellite professionals
ground the satellite system to the nearest available water hose bib. Is this
permitted by the NEC?
A12. No. The satellite mast [810.15] and discharge
unit (ground block) [810.20(C)] must be grounded (actually bonded) in accordance
with 810.20. The electrode that is suitable for this purpose includes the
nearest accessible [810.40(F)(1)]:
Building or structure grounding
electrode system [250.50].
Interior metal water-piping system, within 5 ft
from its point of entrance [250.52(A)(1)]. Figure 810-10
Metallic service raceway.
Service equipment enclosure.
Grounding electrode conductor or the grounding electrode conductor metal
Note: Grounding the lead-in antenna cables and the mast helps in
preventing voltage surges from static discharge from reaching the inner
conductor of the lead-in cable. Because the satellite sits outdoors, wind
creates a static charge on the dish as well as the wire attached to it. This
charge can build up on the dish and cable until it jumps across an air space,
often through the electronics inside the LNBF or receiver. Grounding the coaxial
cable and dish to the building grounding electrode system helps in dissipating
this static charge.
Nothing can prevent damage from a direct lightning
strike, but grounding these systems in accordance with the NEC with proper surge
protection (not NEC required) can help reduce damage to satellite equipment as
well as other related equipment from nearby lightning strikes.
third year apprentice came in the office the other day and wanted to know why we
mount garage receptacles outlets above 18 inches. I informed him the NEC
required this. But his question got me thinking, is there a minimum height for
receptacles in a dwelling unit garage?
A13. The NEC does not prohibit
receptacles within 18 inches of a dwelling unit garage floor. So if you wanted,
you could install them in the floor! However in commercial garages [Article
511.1], it would not be cost effective to install a receptacle within 18 inches
of the floor. Since this area is classified as a Class 1, Division 2 location,
the receptacle would have to be identified as suitable for use in a hazardous
Class 1 location. Or in other words, the receptacle and wiring method would have
to be explosionproof in accordance with Article 501.
Q14. My buddy told
me that a 208V motor could be connected to the 208V high-leg conductor derived
from a delta-connected transformer bank? I can't believe the load would work if
wired this way, but if it does, is this permitted by the Code?
motor will operate perfectly if wired in this configuration. However, a
single-pole circuit is rated 120/240V and it is only permitted to serve a
circuit where the nominal voltage of any one conductor to ground (actually the
metal case) does not exceed the lower of the two values [240.85]. So you can see
this would be a violation because the 208V high-leg circuit conductor exceeds
the single-pole (120V) voltage rating of the circuit breaker.
Q15. I am
installing a 3-phase feeder to 480V panel where all of the loads operate 480V
phase-to-phase. Is a neutral conductor required in the feeder raceway to the
A15. No. However, if the installation was a service raceway, then
we have a different ball game. Because electric utilities are not required to
provide an equipment grounding (bonding) conductor to service equipment, a
grounded (neutral) conductor is required to be from the electric utility
transformer to each service disconnecting means. The grounded (neutral)
conductor shall be bonded to the enclosure of each disconnecting means as
required by 250.24(B) [250.130(A)].
Caution: It is critical that the
grounded (neutral) conductor be run to service equipment from the electric
utility, even when there are no line-to-neutral loads being supplied at the
premises (i.e., a four-wire service drop for a three-phase service). In
addition, the metal parts of the service equipment itself must be bonded to the
grounded (neutral) conductor to ensure that dangerous voltage from a
line-to-case fault will be quickly removed by the operation of the circuit
protection device [250.4(A)(3) and 250.4(A)(5)].
Danger: Because of the
earth's high resistance and resulting low fault current, the circuit overcurrent
protection device will not open and clear the fault. As a result, all metal
parts associated with the electrical installation, as well as metal piping and
structural steel, will remain energized at a lethal level.
grounded (neutral) service conductor is required to serve as the effective
ground-fault current path, it shall be sized so that it can safely carry the
maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This
is accomplished by sizing the grounded (neutral) conductor in accordance with
Table 250.66, based on the total area of the largest ungrounded conductor
[250.24(B)(1)]. In addition, the grounded (neutral) conductors shall have the
capacity to carry the maximum unbalanced neutral current in accordance with
Example: What is the minimum size grounded (neutral) service
conductor required for a 400A, three-phase, 480V service where the ungrounded
service conductors are 500 kcmil and the maximum unbalanced load is 100A? Figure
(a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG
Answer: (d) 1/0 AWG
The unbalanced load requires the grounded (neutral) conductor
to be 3 AWG, which is rated 100A at 75°C in accordance with Table 310.16.
However, Table 250.66 requires the grounded (neutral) service conductor to be
sized no smaller than 1/0 AWG to accommodate the maximum possible fault current
likely to be imposed on it.
Copyright © 2004 Mike Holt Enterprises,Inc.