By Mike Holt for EC&M Magazine

Here’s the follow up to yesterday’s newsletter. This includes all of the answers to the questions sent, so you can see how you did.

Q1. The Code prohibits “objectionable currents”; what is this, what are some examples, and how can it be prevented.

A1. Section 250.6 of the NEC addresses objectionable current. First of all, the Code states that to prevent a fire, electric shock, or improper operation of circuit overcurrent devices or electronic equipment, electrical systems and equipment must be installed in a manner that prevents objectionable neutral current from flowing on metal parts [250.6(A)]. The Code further states that temporary currents from abnormal conditions, such as ground faults, aren’t to be classified as objectionable current [250.6(C)].

Likewise, currents that introduce noise or data errors in electronic equipment are not considered objectionable currents for the purposes of this section. Circuits that supply electronic equipment must be connected to an equipment grounding conductor [250.6(D)].

OBJECTIONABLE CURRENT

Objectionable neutral current occurs because of improper neutral-to-case connections or wiring errors that violate 250.142(B).

Improper Neutral-to-Case Connection [250.142]
Panelboards. Objectionable neutral current will flow when the neutral conductor is connected to the metal case of a panelboard that’s not used as service equipment.

Separately Derived Systems. Objectionable neutral current will flow on conductive metal parts and conductors if the neutral conductor is connected to the circuit equipment grounding conductor on the load side of the system bonding jumper for a separately derived system.

Disconnects. Objectionable neutral current will flow when the neutral conductor is connected to the metal case of a disconnecting means that’s not part of the service equipment.

Wiring Errors. Objectionable neutral current will flow when the neutral conductor from one system is connected to a circuit of a different system.

Objectionable neutral current will flow on metal parts when the circuit equipment grounding conductor is used as a neutral conductor such as where:
•     A 230V time-clock motor is replaced with a 115V time-clock motor, and the circuit equipment grounding conductor is used for neutral return current.
•     A 115V water filter is wired to a 240V well-pump motor circuit, and the circuit equipment grounding conductor is used for neutral return current.
•     The circuit equipment grounding conductor is used for neutral return current.

DANGERS OF OBJECTIONABLE CURRENT

Objectionable neutral current on metal parts can cause electric shock, fires, and improper operation of electronic equipment and overcurrent devices such as GFPs, GFCIs, and AFCIs.

Shock Hazard. When objectionable neutral current flows on metal parts, electric shock and even death can occur from the elevated voltage on those metal parts.

Fire Hazard. When objectionable neutral current flows on metal parts, a fire can ignite adjacent combustible material. Heat is generated whenever current flows, particularly over high-resistance parts. In addition, arcing at loose connections is especially dangerous in areas containing easily ignitible and explosive gases, vapors, or dust.

Improper Operation of Electronic Equipment. Objectionable neutral current flowing on metal parts of electrical equipment and building parts can cause electromagnetic fields which negatively affect the performance of electronic devices, particularly medical equipment. For more information, visit www.MikeHolt.com, click on the “Technical Link,” and then on “Power Quality.”

When a system is properly grounded and bonded, the voltage of all metal parts to the earth and to each other will be zero.

When objectionable neutral current travels on metal parts because of the improper bonding of the neutral to metal parts in violation of the NEC, a difference of potential will exist between all metal parts. This situation can cause some electronic equipment to operate improperly.

Operation of Overcurrent Devices. When objectionable neutral current travels on metal parts, tripping of electronic overcurrent devices equipped with ground-fault protection can occur because some neutral current flows on the circuit equipment grounding conductor instead of the neutral conductor.

Q2. What are the requirements when terminating equipment grounding conductors, grounding electrode conductors, and bonding jumpers? 

A2.  Equipment grounding conductors, grounding electrode conductors, and bonding jumpers must terminate in one of the following methods [250.8(A)]:
(1)  Listed pressure connectors
(2)  Terminal bars
(3)  Pressure connectors listed for direct burial or concrete encasement [250.70]
(4)  Exothermic welding
(5)  Machine screws that engage at least two threads or are secured with a nut.
(6)  Self-tapping machine screws that engage at least two threads
(7)  Connections that are part of a listed assembly
(8)  Other listed means
 Connection devices or fittings that depend solely on solder aren’t allowed [250.8(B)].
.
Q3. What are the requirements regarding ground detection systems and labeling for ‘ungrounded’ systems.

A3. Systems that aren’t required to be grounded in accordance with 250.20(B) [250.21(A)(4)] must have ground detectors installed [250.21(B)(1)]. This ground detection sensing equipment must be connected as close as practicable to where the system receives its supply [250.21(B)(2)].

Ungrounded systems must be legibly marked “Ungrounded System” at the source or first disconnecting means of the system, with sufficient durability to withstand the environment involved [250.21(C)].

Q4. What are the requirements related to the use of a ground rod for the grounding electrode system, including sizing of the grounding electrode conductor?

A4. Ground rod electrodes must not be less than 8 ft in length in contact with the earth [250.53(G)].
Rod-type electrodes must have a diameter of at least 5/8 in., unless listed [250.52(A)(5)(b)].

Author’s Comments:
•  The grounding electrode conductor, if it’s the sole connection to the ground rod, isn’t required to be larger than 6 AWG copper [250.66(A)].
•  The diameter of a ground rod has an insignificant effect on the contact resistance of a ground rod to the earth. However, larger diameter ground rods (¾ in. and 1 in.) are sometimes installed where mechanical strength is desired, or to compensate for the loss of the electrode’s metal due to corrosion.

Rod, pipe, or plate electrodes must meet the following requirements [250.53(A)]:

(1) If practicable, rod, pipe, and plate electrodes must be embedded below the permanent moisture level and be free from nonconductive coatings such as paint or enamel.

(2) A single rod, pipe or plate electrode must be supplemented by an additional electrode that’s bonded to one of the following:

Ex: If a single rod, pipe, or plate grounding electrode has an earth contact resistance of 25 ohms or less, the supplemental electrode isn’t required.

Q5. Please explain how I am to ground a service on a building that has a ground ring used for the lightning protection system grounding electrode.

A5.  If a lightning protection system is installed on a building/structure, it must be bonded to the building/structure grounding electrode system [250.106].

Author’s Comment: The grounding electrode for a lightning protection system must not be used as the required grounding electrode system for the buildings or structures [250.60].

Q6. What is the Code rule for sizing the equipment grounding conductor for feeder taps, and can you give me an example?   

A6. Equipment grounding conductors for feeder taps must be sized in accordance with Table 250.122, based on the ampere rating of the overcurrent device ahead of the feeder, but in no case is it required to be larger than the feeder tap conductors [250.122(G)].

Example: A 400A breaker protects a set of 500 kcmil feeder conductors. There are three taps fed from the 500 kcmil feeders that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size equipment grounding conductors for these taps?

The equipment grounding conductor, based on the 400A feeder protection, would be required to be 3 AWG by Table 250.122, however 250.122(G) states that the equipment grounding conductor run with feeder taps is not required to be larger than the feeder tap conductors in each raceway.

•   200A: 3/0 AWG is rated 200A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°. The equipment grounding conductor would also be 3 AWG.
•   150A: 1/0 AWG is rated 150A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°. The equipment grounding conductor would also be 3 AWG.
•   30A: 8 AWG is rated 50A at 75°, and is greater than 10 percent of the ampacity of 500 kcmil, which is rated 380A at 75°. Anything smaller than 8 AWG can’t be used, as it will have an ampacity of less than 10 percent of 380A (38A) in the 75° column of 310.15(B)(16). The equipment grounding conductor would be 8 AWG.

Q7. If I have a feeder to a remote building and I run the conductors in PVC, what are the grounding and bonding requirements for this installation?

A7.  Each building/structure’s disconnect must be connected to an electrode of a type identified in 250.52 [250.32(A)].

Author’s Comments:
•  The grounding of the building/structure disconnecting means to the earth is intended to help in limiting induced voltages on the metal parts from nearby lightning strikes [250.4(A)(1)].
•  The Code prohibits the use of the earth to serve as an effective ground-fault current path [250.4(A)(5) and 250.4(B)(4)].

A grounding electrode isn’t required where the building/structure is served with a 2-wire, 3-wire, or 4-wire multiwire branch circuit [250.32(A) Ex].

The grounding electrode conductor must terminate to the grounding terminal of the disconnecting means, and it must be sized in accordance with 250.66, based on the conductor area of the ungrounded feeder conductor [250.32(E)].

Question: What size grounding electrode conductor is required for a building disconnect supplied with a 3/0 AWG feeder?
(a) 4 AWG      (b) 3 AWG         (c) 2 AWG          (d) 1 AWG
Answer: (a) 4 AWG [Table 250.66]

Author’s Comment: If the grounding electrode conductor is connected to a ground rod, the portion of the conductor that’s the sole connection to the ground rod isn’t required to be larger than 6 AWG copper [250.66(A)]. If the grounding electrode conductor is connected to a concrete-encased electrode, the portion of the conductor that’s the sole connection to the concrete-encased electrode isn’t required to be larger than 4 AWG copper [250.66(B)].

In regard to the bonding portion of your question, to quickly clear a ground fault and remove dangerous voltage from metal parts, the building/structure disconnecting means must be connected to the circuit equipment grounding conductor, which must be one of the types described in 250.118. If the supply circuit equipment grounding conductor is of the wire type, it must be sized in accordance with 250.122, based on the rating of the overcurrent device [250.32(B)].

Caution: To prevent dangerous objectionable neutral current from flowing onto metal parts [250.6(A)], the supply circuit neutral conductor isn’t permitted to be connected to the remote building/structure disconnecting means [250.142(B)].

Q8. What is the requirement for attaching the equipment grounding conductor to metal boxes?

A8.  If circuit conductors are spliced or terminated on equipment within a metal box, the equipment grounding conductor associated with those circuits must be connected to the box in accordance with the following [250.148]:

Ex: The circuit equipment grounding conductor for an isolated ground receptacle installed in accordance with 250.146(D) isn’t required to terminate to a metal box.

Author’s Comment: Wire connectors of any color can be used with equipment grounding conductor splices, but green wire connectors can only be used with equipment grounding conductors.

Author’s Comment: Equipment grounding conductors aren’t permitted to terminate to a screw that secures a plaster ring.

Q 9. What are the minimum cover requirements for PVC and UF cable in dwelling occupancies?  

A 9. PVC and UF cable in dwelling occupancies that are rated 120V or less with GFCI protection and maximum overcurrent protection of 20A are allowed with 12 in of cover minimum.

Q 10. What are the minimum cover requirements for PVC and USE cable in commercial occupancies?  

A10. In commercial occupancies, the minimum “cover” requirement is 18 in. for PVC and 24 in. for USE cable in accordance with Table 300.5. Note 1 to Table 300.5 defines "cover" as the distance from the top of the underground cable or raceway to the top surface of finished grade.

Q11. Can I pull Type NM cable in a PVC sleeve located in a ground floor slab for the kitchen counter island?

A11. No.  Type NM cable must not be used in wet or damp locations [334.12(B)(4)].
The interior of enclosures or raceways installed in an underground installation are considered to be a wet location [300.5(B)]. Cables and insulated conductors installed in underground enclosures or raceways must be listed for use in wet locations according to 310.10(C).

Author’s Comment: The definition of a “Wet Location” as contained in Article 100, includes installations underground, in concrete slabs in direct contact with the earth, locations subject to saturation with water, and unprotected locations exposed to weather. If raceways are installed in wet locations above grade, the interior of these raceways is also considered to be a wet location [300.9].

Q12. What is the smallest conductor allowed by the NEC for branch circuits?

A12. The smallest conductor permitted for branch circuits for residential, commercial, and industrial locations is 14 AWG copper, except as permitted elsewhere in the Code [Table 310.106(A)].

Author’s Comment: There’s a misconception that 12 AWG copper is the smallest conductor permitted for commercial or industrial facilities. Although this isn’t true based on NEC rules, it may be a local code requirement.

Q13. What is the smallest conductor allowed by the NEC for Class 1 control and signal circuits?

A13. Conductors of sizes 18 AWG and 16 AWG installed in a raceway, enclosure, or listed cable are permitted for Class 1 circuit conductors  if they don’t supply a load that exceeds the ampacities given in 402.5. Conductors of 14 AWG and larger must not supply loads greater than the ampacities given in 310.15 [725.49(A)].
 Class 1 circuit conductors must have a 600V insulation rating and must comply with Table 310.104. Conductor Insulation for sizes 18 AWG and 16 AWG must comply with Table 402.3 [725.49(B)].

Q14. Can I make splices, taps, or install feed-through conductors in a panel?

A14. Cabinets, cutout boxes, and meter socket enclosures can be used for conductors as feeding through, spliced, or tapping off to other enclosures, switches, or overcurrent devices where all of the following conditions are met [312.8]:

Q15. When can I make a splice in a conduit body?

A15. Splices are permitted in conduit bodies that are legibly marked by the manufacturer with their volume and the maximum number of conductors permitted in a conduit body is limited in accordance with 314.16(B) [314.16(C)(2)].

Question: How many 12 AWG conductors can be spliced in a 15 cu in. conduit body?
(a) 4                    (b) 6                   (c) 8                 (d) 10

Answer: (b) 6 conductors (15 cu in./2.25 cu in.)
12 AWG = 2.25 cu in. [Table 314.16(B)]
15 cu in./2.25 cu in. = 6

Capped elbows, handy ells, and service-entrance elbows aren’t permitted to contain any splices.

Q16. What is the Code rule regarding plugging unused openings in boxes and other enclosures?

A16. Openings through which cables or raceways enter must be adequately closed [314.17(A)].

Author’s Comment: Unused cable or raceway openings in electrical equipment must be effectively closed by fittings that provide protection substantially equivalent to the wall of the equipment [110.12(A)].

Q17. What is the Code requirement for handhole enclosures?   

A17. Handhole enclosures must be identified for underground use, and be designed and installed to withstand all loads likely to be imposed on them [314.30(D)].

Handhole enclosures must be sized in accordance with 314.28(A). For handhole enclosures without bottoms, the measurement to the removable cover is taken from the end of the raceway or cable assembly. When the measurement is taken from the end of the raceway or cable assembly, the values in Table 312.6(A) for one wire to terminal can be used [314.28(A)(2) Ex].

Underground raceways and cables entering a handhole enclosure aren’t required to be mechanically connected to the handhole enclosure.

Splices or terminations within a handhole must be listed as suitable for wet locations [110.14(B)].

Handhole enclosure covers must have an identifying mark or logo that prominently identifies the function of the enclosure, such as “electric.” Handhole enclosure covers must require the use of tools to open, or they must weigh over 100 lb.

Metal covers and other exposed conductive surfaces of handhole enclosures must be connected to an equipment grounding conductor sized to the overcurrent device in accordance with 250.122 [250.102(D)]. Metal covers of handhole enclosures containing service conductors must be connected to an equipment bonding jumper sized in accordance with Table 250.66 [250.92 and 250.102(C)].

For more NEC Practice, purchase Mike Holt's NEC Exam Practice Questions book based on the 2011 NEC.