|NEC Questions and Answers
|By Mike Holt, for EC&M Magazine
Q1. I have a 1600A switchboard that is protected with ground-fault
protection in accordance with 230.95. The only neutral load on the switchboard
is from two 225A lighting panelboards. What size feeder, neutral, and bonds
wires are required for this installation if we use rigid nonmetallic conduit
Ungrounded Conductor - According to 240.4(C), the
ungrounded conductors must have an ampacity not less than 1600A. This can be
accomplished by any of the following parallel sets (conductors rated at least
Six sets of 300 kcmil = 285A x 6 = 1710A
Five sets of 400 kcmil =
335A x 5 = 1675A
Four sets of 600 kcmil = 420 x 4 = 1680A
sets of 500 kcmil is only rated 1520A (380A x 4), therefore it cannot be used.
Neutral Conductor - Section 220.22 states that the neutral demand load
shall be "the maximum unbalance computed load between the neutral and any one
ungrounded conductor." I will assume that all of the loads on two 225A
panelboards to be nonlinear line-to-neutral loads.
Based on this worst
case assumption, we cannot apply the "over 200A, 70% demand factor" contained in
220.22. Therefore the neutral must be sized at 100% of the line-to-neutral
loads, which in this case is 450A.
Assuming that we parallel the feeder
in four raceways, the neutral conductor in each raceway must have an ampacity of
no less than 450A/4 = 113A. According to Table 310.16, 2 AWG has a rating of
115A x 4 = 460A. But, the parallel rules contained in 310.4 require each
parallel neutral conductor to be sized no smaller than 1/0 AWG.
Fine Print Note No. 2 of 220.22 states "A 3-phase, 4-wire, wye-connected power
system used to supply power to nonlinear loads may necessitate that the power
system design allow for the possibility of high harmonic neutral currents." This
means that "good design" dictates that we should consider "up-sizing" the
neutral conductor to accommodate the odd triplen harmonic currents, which do not
The industry practice is to "double the neutral." If we followed
this practice, the neutral conductor would then be sized based on 450A x 2 =
900A. We can accommodate this by installing a 4/0 AWG neutral conductor in each
of the four parallel raceways.
Equipment Grounding (bonding) Conductor-
An equipment grounding (bonding) conductor must be installed in each of the
rigid nonmetallic conduits. Each shall be sized to the circuit's overcurrent
device protecting rating, in accordance with Table 250.122. For a 1600A
protected feeder, this would require a 4/0 AWG bond wire in each of the
Q2. A metal wire trough, used as a pull point, is installed
above an electrical panel. The inspector wants us to terminate all of the
equipment grounding (bonding) conductors to the metal trough instead of pulling
them continuous to the panel ground bar. This just does not make sense to me.
What do you think?
A2. The rule on terminating equipment grounding
(bonding) conductors was clarified in the 2002 NEC. 250.148 states that "where
conductors are spliced or terminated on equipment within a box, the equipment
grounding conductors associated with those circuit conductors shall be spliced
or joined within the box or to the box with devices suitable for the use."
Since the circuit conductors run through the trough without spice or
termination, the equipment grounding (bonding) conductors are not required to
terminate to the trough. Terminating the grounding conductor twice (the
panelboard and trough) is just asking for trouble.
Q3. What is the
working space requirement for 120V industrial machinery?
conditions of maintenance and supervision ensure that only qualified persons
will service the installation, the working space from live parts for 120V
circuits shall be a minimum of 2½ ft [670.5].
Exception: Where the
enclosure requires a tool to open, and where only diagnostic and troubleshooting
testing is involved, the clearances can be less than 2½ ft. How much less? This
is a judgment call by the authority having jurisdiction.
Note: The 2½ ft
working space rule only applies to 120V circuits. If the equipment is rated 208,
240, 277, or 480V, then the working space requirements shall comply with the
more stringent requirements contained in 110.26.
Q4. I'm under the
impression that all raceway and cable support fittings must be listed for the
purpose. I keep telling my boss that we can only use listed nonmetallic sheath
cable stables. Am I correct?
A4. Nope. The NEC does not require support
systems (including cable trays) to be listed. However, the NEC requires all
equipment to be approved by the authority having jurisdiction [110.2]. In
addition, 90.4 specifies that the authority having jurisdiction have the
responsibility for deciding on the approval of equipment and materials.
What's does "approved" mean? According to Article 100, "approved" means
acceptable to the authority having jurisdiction.
Note: I don't think UL
lists supports systems. Does anybody know for sure?
Q5. Section 310.4
defines conductors in parallel as "electrically joined at both ends to form a
single conductor." If we have two sets of parallel conductors in a single
raceway (total of six physical conductors), must we apply the ampacity
adjustment factors contained in 310.15(B)(2)(a)? Some engineers feel that if we
electrically join the conductors to form a single conductor, we only have three
conductors, instead of six!
A5. I hate these simple questions. They are
not that simple to explain because we have to consider conductor size to
accommodate continuous loading, overcurrent protections as well as conductor
ampacity. But here I go.
Conductor Ampacity - If you have six physical
conductors in a raceway, then you have six conductors in the raceway for
conductor ampacity adjustment purposes. It's really just that simple. The
definition of a parallel conductor contained in 310.4 only applies to 310.4 for
the purpose of applying the requirements contained in 310.4.
Can 3/0 AWG THHN, paralleled in a single raceway, and protected by a 400A
protection device be permitted to supply a 320A continuously load on a
Continuous Load - The maximum continuous load shall not
exceed 80 percent of the conductor ampacity before the application of any
adjustment factors [215.2(A)(1)]! The 3/0 AWG THHN parallel set has an ampacity
of 400A at 75C before ampacity adjustment; therefore it meets the continuous
load size requirements.
Conductor Ampacity - The ampacity of six 3/0 AWG
THHN conductor installed in the same raceway must be adjusted by the 80%
multiplier contained in Table 310.15(B)(2)(a).
3/0 AWG THHN Adjusted
Ampacity = 225A* (at 90C) x 0.80 = 180A x 2 = 360A
adjustment is based on the conductor ampacity at the temperature rating of the
conductor insulation (in this case 90C), not the 75C terminal rating of the
equipment. Read the last sentence in the first paragraph of 110.14(C).
Overcurrent Protection - Section 240.4(B) permits the 360A rated
conductors to be protected by a 400A protection device, "next size up rule."
Q6. Are hospital grade receptacles required in doctor, chiropractic, or
dentist examination rooms? What about isolated ground receptacles?
Oh good! An easy one. No and No.
Hospital Grade Receptacles - Hospital grade
receptacles are only required for patient bed locations, defined in 517.2 as an
inpatient sleeping bed; or the bed or procedure table used in a critical patient
Isolated Ground Receptacles - Isolated ground receptacles,
receptacles incorporating an isolated grounding connection intended for the
reduction of electrical noise [250.146(D)], are never required by the NEC.
In fact the FPN to 517.16 cautions against the indiscriminate use of
receptacles with insulated grounding terminals since such a practice forfeits
the benefit of parallel grounding paths that otherwise would occur.
Author's Comment: 517.13(B) requires all receptacles installed in
"patient care areas" (including not only patient bed locations but also doctor,
chiropractic and dentist exam rooms) to be grounded by an insulated copper
conductor connected to the grounding terminal of the receptacle.
Section 250.56 states that "A single electrode consisting of a rod that does not
have a resistance to ground of 25 ohms or less shall be augmented by one
additional electrode." Where did the 25 ohm value come from?
A7. I have
no clue, but my research indicates that it was added to the 1918 NEC. Since my
personal Code book collection only goes back to 1925, I'm not sure if this
statement is true. Maybe someone knows where I can get a 1918
Q8. What is the minimum size branch circuit conductor to a motor
and what is the largest breaker I can use to protect these conductors?
A8. Motor circuit conductors shall be sized no smaller than 125% of the
motor full-load current rating listed in Table 430.147 through 150 [430.6(A)(1)
and 430.22(A)]. The maximum size inverse time circuit breaker for short-circuit
ground-fault protection shall not exceed 250% of the motor full-load current
Example: 10 hp, 230V, three-phase motor having a
Service Factor 1.15 used for continuous duty application has a FLC rating of 28A
Note: Terminals rated 75C.
Conductors - 28A x 1.25 = 35A. Typically this would be 10 AWG THHN/THWN.
Branch Short-Circuit and Ground-Fault Protection - 28A x 2.5 = 70A
Yes, it's okay to have the 10 AWG conductors protected by the 70A
protection device [240.4(G)] because the motor is protected by an overload
protection device (heaters) sized not larger than 125% of the motor nameplate
rating [430.6(A)(1) and 430.32].
Q9. How do you size the protection
device and conductor for a 40 hp, 230V, three-phase fire pump motor?
Overload protection is not permitted for the fire pump motor circuit [240.4(A)
and 695.6(D)], but short circuit protection is required and it must be set to
carry indefinitely the locked-rotor current of the fire pump motor
Table 430.151(B) lists the locked-rotor current for the 40
hp, 230V, three-phase motor as 580A. Therefore the short-circuit protection
device must be sized at least 600A.
The branch circuit conductors for a
fire pump motor are sized just like any continuous duty motor, 125 percent of
the motor full-load current rating as listed in Table 430.150 [695.6(C)(2)].
For a 40 hp, 230V, three-phase motor (FLC or 104A), the branch circuit
conductors shall have an ampere rating of not less than 104A x 1.25 = 130A.
According to Table 310.16, this would work out to be a 1 AWG conductor
So the short-circuit protection device is set at
least 600A, and the conductors to the fire pump motor are sized no smaller than
Note: Voltage drop considerations might require the branch
circuit conductors to be sized larger than 1 AWG. See 695.7 for details.
Q10. Can I use one single-pole 15A, 125V AFCI breaker and one 15A, 125V
nonAFCI circuit breaker with 14/3 nonmetallic sheath cable to supply a AFCI
protected dwelling unit bedroom circuit and another circuit that is not AFCI
A10. No. A single-pole AFCI circuit breaker, just like a
single-pole GFCI circuit breaker, is not designed to operate on a multiwire
branch circuit. I suggest you use the new 14/4 NM cable that is manufactured for