[Following is reprinted from Mike Holt's Newsletter]
By Mike Holt, to be printed in EC&M magazine
Q1. A main lug only distribution panel is fed through an automatic transfer
switch. The automatic transfer switch, the generator, and normal power are all
located remotely from the distribution panel. I was told that the distribution
panel does not require a main circuit breaker (disconnect). I contend that in
order to disconnect the power to the distribution panel, you have to open both
the normal and generator circuit breakers (cannot be disconnected by a single
breaker). Therefore a main breaker (disconnect) is required to be installed
"in" the disruption panel, but I cannot support this with a specific
NEC rule. Am I right?
A1. No. Section 384-16(a) does not require a disconnecting means for a
panelboard. But it does require panelboard with a neutral that has more than
10 percent of its overcurrent devices rated 30 amperes or less to be protected
against overcurrent by a device that has a rating not greater than that of the
panelboard. Exception No. 1 to this rule states that the overcurrent
protection is not required to be located in the panelboard, if the panelboard
feeder has overcurrent protection not greater than the rating of the
panelboard. In your case if the panelboard has proper overcurrent protection,
the installation meets the NEC requirements.
Q2. Can I make a splice within a panel?
As. Yes. Section 373-8 states that enclosures for overcurrent devices shall
not be used as a junction box or raceways, unless adequate space is provided.
Conductors inside a panelboard shall not fill the wiring space at any cross
section to more than 40 percent, and splices and taps shall not fill the
wiring space at any cross section to more than 75 percent, Figure 1.
Note: Graphics not part of this e-mail.
Q3. How do I ground a transformer in a building where there is no accessible
effectively grounded structural metal member of the structure and effectively
grounded metal water pipe is a over 200 feet from the transformer?
A3. Drive a ground rod. Section 250-30(a)(2) requires a grounding electrode
conductor to connect the grounded (neutral) conductor of the transformer to a
grounding electrode. Section 250-30(a)(3) requires the grounding electrode to
be as near as practicable and preferably in the same area as transformer.
Where effectively grounded structural metal member of the structure or
effectively grounded metal water pipe within 5 ft from the point of entrance
into the building is not available, then a ground rod installed in accordance
with the requirements of Sections 250-52 can be used.
Q4. I have encountered a design specification, which states: "Pull points
shall be provided in each conduit run to conform with the requirements of the
NEC ". I have always installed "pull boxes" 100 feet from each
other. I tried looking this up in the Code but was unable to find anything in
reference to the distance between pull boxes. Is there any rule in the NEC
that pertains to this?
A4. No. The NEC does NOT contain any requirement limiting the distance between
pull points. Conductors cannot be spliced within a raceway [300-13(a)], so the
maximum distance between pull boxes is dependent on the length of the
The Code specifies (this is contained in each of the raceway articles) that
there shall not be more than the equivalent of four-quarter bends (360 degrees
total) between pull points. In addition, the Code also alerts us to the fact
that “raceway fill” is based on common conditions where the length of the
pull and the number of bends are within reasonable limits. It states that for
certain conditions, a larger size conduit or a lesser conduit fill should be
considered [FPN to Table 1 of Chapter 9].
Q5. I recently had an electrical inspector require a 75-kVA transformer be
removed from a drop ceiling grid area. He claims that the Code does not allow
a 75-kVA transformer to be installed above a suspended ceiling. Our position
is that this is not a Code violation if the transformer complied with all the
clearance, access and ventilation requirements of the Code. Does the Code
allow a 75-kVA transformer to be installed above a drop ceiling grid area?
A5. No. The general requirement of Section 450-13 is that transformers must be
readily accessible to qualified personnel for inspection and maintenance.
However, dry-type transformers rated not over 600 volts and not over 50 kVA,
can be located above a suspended ceiling (not readily accessible), if not
permanently closed in by structure and provided they meet the ventilation
requirements of Section 450-9 and separation from combustible materials
requirements of Section 450-21(a) [450-3(b)]. Since your transformer is
greater than 50 kVA (75 kVA), it cannot be installed above the suspended
ceiling space because it is not readily accessible, Figure 2.
Mike’s Comment: Section 450-13 should be reworded so that it’s intent
could be easily understood.
Q6. What is the maximum size hole an electrician can drill in a ten-inch floor
joist? I have been in commercial construction most of my life and always left
the structural supports intact. When having a house built I was surprised to
see a four-inch hole cut into the two outside joists for an exhaust fan. I do
not believe there was any harm done to my structure, but does the NEC cover
A6. No. The National Electrical Code does not specify how much of the framing
member can be removed, because this is a building code requirement.
Q7. For purposes of computing the lighting load for feeders and services, many
engineering firms in our geographical area disregard Table 220-3(a), and
instead use 125% of the actual installed lighting load. When the bulk of the
light fixtures employ T8 fluorescent lamps with electronic ballasts, such a
computation results in a lower load than Table 220-3(a) would require. Does
the National Electrical Code permit this?
A7. No. Section 220-3(a) is very clear in that it states that the “lighting
load of not less than that specified in Table 220-3(a) shall constitute the
minimum lighting load for each square foot of floor area”. Example D3 in
Appendix D contains a note that clarifies that “where the actual connected
lighting load (at 125%) is less than 125% of the load from Table 220-3(a),
“the minimum lighting load from Table 220-3(a) shall be used in the
calculation. Had the actual lighting load been greater than the value computed
from Table 220-3(a), 125% of the actual connected lighting load would have
Q8. I am confused as to how to size the neutral conductor for commercial
service and feeder conductors. Am I required to size the neutral conductor at
125% for continuous loads?
A8. Yes. NEC Section 215-2 for feeders and Section 230-42 for services require
the ampacity of the conductors (before the application of any adjustment or
correction factors) to have an ampacity of not less than 125 percent of
continuous loads, plus the sum of noncontinuous load. Since the neutral
(grounded) conductor is part of the feeder or service, it must also comply
with the same requirements, therefore the neutral must be sized not less than
125 percent of continuous loads, plus the sum of noncontinuous load in
accordance with Section 220-22.
Example: If the load on the feeder/service consisted of 150 amperes of
three-phase 208 volt loads and 150 amperes of single-phase 120 volt loads, the
feeder/service conductors would be, 3 500 kcmil and 1 No. 3/0.
Feeder Conductor = 500 kcmil copper, rated 380 amperes at 75 degrees C
150 amperes + 150 amperes = 300 amperes X 1.25 = 375 amperes
Neutral Conductor = No. 3/0 copper, rated 200 ampere at 75 degrees C
150 amperes X 1.25 = 188 amperes
Q9a. We recently moved a large circuit breaker for some heavy industrial
equipment. Now the load conductors will not reach the bottom of the circuit
breakers. The customer wants us to back-feed the circuit breaker (line
conductors terminate on the bottom of the breaker and the load conductors
terminate to the top of the breaker) instead of running new (and expensive)
wire. The manufacturer of the heavy industrial equipment insists that we can
connect the line or load to either side of the circuit breaker. However, many
of electrical people here at the facility feel that the NEC requires the
supply to terminate to the top terminals and the load to terminate to the
bottom terminals. I haven't been able to find this regulation in the NEC.
Q9b. I have a question regarding the proper connection to a circuit breaker
and I can't find anyone who will make a statement with confidence or who can
steer me in the right direction. I understand that it is common practice to
connect the line side to the top of the breaker and the load side to the
bottom of the breaker. I was also told that an inspector always assumes that
the breakers are wired this way. However, if you have an underground service
entering a CT cabinet and where the main breaker is located directly above the
CT cabinet, why must you still wire the line side to the top of the breaker?
In this situation, it is obvious where the line side originates. Doesn't the
breaker function the same way whether it is fed from the top or the bottom?
Does the NEC cover this?
A9. Yes. NEC Section 110-3(b) states “Listed or labeled equipment shall be
installed and used in accordance with any instructions included in the listing
or labeling.” According to Underwriters Laboratories Inc. (UL) “General
Information Directory,” breakers that are marked “Line and Load” are
intended to be wired so that the supply (line) terminates to the “Line”
terminal and the feed (load) terminates to the “Load” terminal. Circuit
breakers that do not have “Line and Load markings can be wired in either
direction, Figure 3.
Q10. I installed a panel on the wall of a bathroom of a commercial building,
where the distance from the front of the panel to the toilet is 5 feet. My
question is, is it legal to install a panel in a commercial bathroom?
A10. Yes. Section 240-24(e) only prohibits overcurrent devices in bathrooms*
of dwelling units and guest rooms of hotels and motels. Since this is not a
residential bathroom, the installation of the panel in the bathroom is
permitted. Careful, Section 230-70 prohibits the installation of the service
disconnecting in any bathroom.
*A bathroom is an area including a basin with one or more of the following: a
toilet, a tub, or a shower [Article 100 definition].
Q11. My question is about the use of 3” pancake boxes (½” deep). It seems
to me as that there is not enough volume in the box to handle a light fixture
connection consisting of a hot, neutral, and a ground. Does the canopy of the
light fixture have any effect when calculating volume requirements?
A11. Yes. Section 370-16(b)(1) Exception states that equipment grounding
conductor and not over four fixture wires smaller than No. 14 are not required
to be counted when they enter a box from a domed fixture, Figure 4.
Q12. NEC Section 250-56 states, “A single electrode consisting of a rod,
pipe, or plate that does not have a resistance to ground of 25 ohms or less
shall be augmented by one additional electrode.” How do you determine if the
electrode meets this requirement, and why don't inspectors require testing of
A12. The resistance of a ground rod can be determined by using the “fall of
potential method”. For more information visit http://www.mikeholt.com/Newsletters/GroundResistance.htm
There is no need for an inspector to measure the resistance of the ground rod,
if there are two ground rods installed in accordance with Sections 250-52 and
Q13. Can you put a panel with circuit beakers in a walk-in closet if the panel
is installed behind the door?
A13. Maybe. This is a judgment call by the inspector. Section 240-24(d)
prohibits overcurrent devices (circuit breakers) from be located in the
vicinity of easily ignitible material, “such as in clothes closets”. To me
the key is “easily ignitible materials”, but my experience is that most
inspectors focus on “such as in clothes closets” and will probably
prohibit this installation.
Q14. If a wraparound style fluorescent fixture is secured to a T-bar ceiling,
can you run a cord out the top of the fixture to a receptacle located in the
ceiling area, if the ceiling space is not used as a plenum area in an office
A14. Maybe. Section 410-14(a) permits electric-discharge lighting fixtures
supported independent of the outlet box to be connected to the branch circuit
by flexible cord if:
1. The fixture require adjusting or aiming [410-31(a)] (Not in this case)
2. The fixture is located directly below the outlet box and the flexible cord
is visible for its entire length outside the fixture, and the cord is not
subject to strain or physical damage [410-41(c)].
Q15. When installing an outdoor antenna, can I 'ground' the antenna and
lead-in cable to a new 8 foot grounding rod?
A15. No. Telecommunications systems [800-40(b)], antennas and lead-in cables
[810-21(f)], CATV [820-40(b)], and network-powered broadband communications
systems [830-40(b)] must all be bonded to one of the following locations
1. Building or structure grounding electrode system as described in Section
2. Interior metal water pipe meeting the requirements of Section 250-104(a).
The limitation of 5 feet in Section 250-50 does not apply.
3. Metal service raceway.
4. Service equipment enclosure.
5. Building or structure grounding electrode conductor.
6. Metal enclosure enclosing the building or structure grounding electrode
7. Accessible bonding means such as six inches of No. 6 copper conductor
connected to the service equipment or raceway [250-92(b)].
Mike's Comment: When a ground rod is installed for a communications system
(your question), it must be bonded with a No. 6 copper or larger bare or
insulated conductor to the grounding electrode system at the building or
Q16. Does the Code allow Class 2 low voltage cable (CL2) to be run exposed in
occupied public space? Can the cable be run exposed in a non-public workspace,
i.e. mechanical room, etc.?
A16. Maybe. Section 725-7, which covers mechanical execution of work,
specifies that Class 1, Class 2, and Class 3 circuits shall be supported by
the building structure in such a manner that the cable “will not be damaged
by normal building use”. Cables installed in public places where they could
be damaged by normal building use would have to be protected or installed in a
Cables installed in mechanical rooms where they could be damaged by normal
building use would also have to be protected or installed in a suitable
raceway. The bottom line is that this is a judgment call by the “authority
having jurisdiction”, which is usually the electrical inspector.
Q17. Is it legal to run Uf feeder cable in the same ditch with water pipe?
A17. Yes. There is no NEC rule on the separation of underground raceways or
cables from other piping systems. However, underground conductors for
satellite dishes and antenna [810-18(a)], direct-buried coaxial cable
[820-11(b)], and network-powered broadband communications cables shall be
separated at least 12 in. from conductors of any light or power circuits.
Q18. My local inspector believes the bending radius on 500 kcmil conductors
entering the back of a 6 inch deep enclosure was violated. The inspector
passed the installation but said he would not the next time. Can a 6 inch deep
enclosure, such as a panel cabinet be used for 500 kcmil conductors entering
the back of the enclosure?
A18. Yes. Conductors shall not be deflected (bent) within a cabinet or cutout
box unless sufficient width (in this case dept) is provided in accordance with
Table 373-6(a). Table 373-6(a) required a minimum of 6 inches of width (depth)
in the cabinet so that 500 kcmil conductors can be properly deflected (bent),
Q19. A 230 volts central air conditioning specifies a maximum 45-ampere fuse
or HACR breaker, with a minimum circuit ampacity of 27.8 amps. Can I use No.
10 THHN with a 40-ampere HACR Breaker?
A19. Yes. Figure 7.
Conductors Sizing Section 440-33. Conductors supplying a motor-compressor with
additional load(s) shall have an ampacity not less 125 percent of the highest
motor or motor-compressor rating in the group, plus the other loads.
Example: A/C with the following loads: Compressor 18.7 amperes, OD Motor 1.8
amperes, ID Motor 2.6 amperes.
Conductor Nameplate Size = 18.7 amperes X 1.25 + 1.8 amperes + 2. 6 amperes
Conductor Nameplate Size = 27.8 amperes
Protection Sizing Section 440-22(b). The protective device having a rating or
setting not exceeding 175% of the largest motor-compressor rated-load current,
plus the other loads, shall be permitted. Where the protection at 175% is not
sufficient for the starting current of the motor, the protection rating can be
increased to 225% of the largest motor-compressor rated-load current, plus the
Example: A/C with the following loads: Compressor 18.7 amperes, OD Motor 1.8
amperes, ID Motor 2.6 amperes.
Protection Nameplate Size = 18.7 amperes X 2.25 + 1.8 amperes + 2. 6 amperes
Protection Nameplate Size = 46.48 amperes, next size down = 45 amperes
Q20. I am installing a 1200 ampere service (three sets of 600 kcmil per
phase). What is the largest grounding electrode conductor required for this
A20. As small as No. 6. Section 250-66 identifies that the sizing of the
grounding electrode conductor is based on the type of electrode used:
Ground Rod/Plate Electrode [250-66(a)]. Where the grounding electrode
conductor is connected to a ground rod or plate electrode [250-52(c)(d)], that
portion of the conductor that is the sole connection to the grounding
electrode shall not be required to be larger than No. 6 copper wire.
Concrete-Encased Electrodes [250-66(b)]. Where the grounding electrode
conductor is connected to a concrete-encased electrode [250-50(c)], that
portion of the conductor that is the sole connection to the grounding
electrode shall not be required to be larger than No. 4 copper wire.
Ground Rings [250-66(c)]. Where the grounding electrode conductor is connected
to a ground ring [250-50(d)], that portion of the conductor that is the sole
connection to the grounding electrode shall not be required to be larger than
the conductor used for the ground ring.
Mike’s Comment: A ground ring is required to be bare copper not smaller than
No. 2, but often it is sized much larger.
Other Electrodes. Where the grounding electrode conductor is connected to any
other electrode [250-50], the conductor shall not be less than given in Table
So the answer depends on the electrode. It would not have to be larger than
No. 6 copper to a ground rod, No. 4 copper to concrete-encased steel (Ufer),
No. 2 copper to a ground ring, or No. 3/0 copper for all other electrodes.
Q21. I'm installing a feeder in parallel. Can I use to different types of
insulation on the different phases? Example: A phase - THHN, B phase - XHHW,
and C phase - THHN.
A21. Yes. Section 310-4. When conductors are run in parallel (joined together
at both ends electrically, thereby forming a single electrical conductor), the
currents need to be evenly divided between the individual parallel conductors
so that each conductor is evenly heated. This is accomplished by ensuring that
each of the conductors within a parallel set has the same impedance, material,
cross-sectional area, and insulation type, and that all conductors terminate
in the same manner.
The FPN to this Section in the 1996 NEC indicated that conductors of one phase
or grounded (neutral) conductor were not required to have the same physical
characteristics as those of another phase or grounded (neutral) conductor to
achieve even current distribution. In the 1999 NEC, this text was relocated
from the FPN into a positive Code requirement for Section 310-4.
Example: A 400 ampere feeder that has a neutral load of 240 ampere can be
wired with the following: Figure 8:
Phase A, 2 250 THHN Aluminum (rated 410 ampere), 100 feet long.
Phase B, 2 No. 3/0 THHN Copper (rated 400 ampere), 104 feet long.
Phase C, 2 No. 3/0 THHN Copper (rated 400 ampere), 102 feet long.
Neutral, 2 No. 1/0 THHN Aluminum (rated 240 ampere), 103 feet long.
Ground, 2 No. 3 THHN Copper, 101 feet long.
Q22. Is a refrigerator required to be on a dedicated circuit in a house? What
about a refrigerator in an office kitchen?
A22. No, No. The NEC does not require a separate circuit for a refrigerator.
Section 210-23(a) allows a refrigerator (or any appliance) to be on a 15 or 20
ampere circuit with other loads if the appliance does not exceed 50 percent of
the branch-circuit rating. Section 210-52(b)(1) identifies that the 20 ampere
small appliance circuit for a dwelling unit can supply refrigeration
equipment, but only if the refrigerator does not exceed 50% of the small
appliance circuit rating in accordance with Section 210-23(a).
Q23. Does the NEC required the primary and/or secondary supply for a
transformer to be installed in a flexible wiring method?
A23. No. Flexible wiring methods are used for installation convenience,
reduction of audible noise from vibration, and where flexibility is required.
Generally a flexible wiring method is used for the primary and secondary
supply for a transformer, because is much easier to install flex then EMT or
PVC. However, electrical metallic tubing, rigid nonmetallic conduit, or any
other fixed raceway could be use for the transformer primary or secondary
supply, Figure 9.
Q24. I'm terminating No. 14 THHN stranded wire to a circuit breaker. One of
the electricians on the job said that terminating stranded wire to a breaker
was a Code violation and a fork connector was required whenever we terminate
stranded No. 14 or No. 12 wire. Is this true?
A24. No. Circuit breakers, receptacles, switches and other devices are all
listed for the termination of stranded wire on terminal screws.
Q25. Do switches for bathroom/shower lights have to be GFCI protected? Is
there any distance that the switch must be from the bathtub or shower space?
A25. No. The NEC only requires that a switch not be installed within the wet
location of a tub or shower spaces, unless the switch is part of a listed tub
or shower assembly, Figure 10.
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