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.

Question: What size dual-element fuse is permitted to protect a 5 hp, 230V, single-phase motor with a service factor of 1.15 and a nameplate current rating of 28A?
(a) 20A         (b) 25A         (c) 30A       (d) 35A

Answer: (d) 35A
Overload Protection [430.32(A)(1)]
28A x 1.25 = 35A

Q2. What is the Code rule for Motor control circuit overcurrent protection?

A2. Motor control conductors that aren’t tapped from the branch-circuit protective device are classified as a Class 1 remote-control circuit, and they must have overcurrent protection in accordance with 725.43 [430.72(A)]

Author’s Comment: Section 725.43 states that overcurrent protection for conductors 14 AWG and larger must comply with the conductor ampacity from Table 310.15(B)(16). Overcurrent protection for 18 AWG must not exceed 7A, and a 10A device must protect 16 AWG conductors.

Author’s Comment: The above limitations don’t apply to the internal wiring of industrial control panels listed in UL 508 Standard for Practical Application Guidelines.

Transformers for motor control circuit conductors must have overcurrent protection on the primary side in accordance with 430.72(C)(1) through (6) [430.72(C)]

Author’s Comment: Many control transformers have small iron cores, which result in very high inrush (excitation) current when the coil is energized. This high inrush current can cause standard fuses to blow, so you should only use the fuses recommended by the control transformer manufacturer.

If physical damage would result in a hazard, the conductors of a remote motor control circuit installed outside the control device must be protected by installing the conductors in a raceway or be protected from physical damage [430.73].

Motor control circuit conductors must have a disconnecting means that simultaneously opens all sources of supply when the disconnecting means is in the open position [430.75(A)]. If the control circuit conductors are tapped from the controller disconnect, the controller disconnecting means can serve as the disconnecting means for the control circuit conductors [430.102(A)].

If the control circuit conductors aren’t tapped from the controller disconnect, a separate disconnecting means is required for the control circuit conductors, and it must be located adjacent to the controller disconnect.

Q3. What is the NEC requirement for disconnects for a motor?

A3. A disconnecting means is required for each motor controller, and it must be located within sight from the controller [430.102(A)].

Author’s Comment: According to Article 100, within sight means that it’s visible and not more than 50 ft from one to the other. 

A motor disconnect must be provided in accordance either one of the following [430.102(B)]:

• A disconnecting means is required for each motor, and it must be located in sight from the motor location and the driven machinery location [430.102(B)(1)].
• The controller disconnecting means [430.102(A)] can serve as the disconnecting means for the motor, if the disconnect is located in sight from the motor location [430.102(B)(2)].

Ex to (1) and (2): A motor disconnecting means isn’t required under either condition (a) or (b), if the controller disconnecting means [430.102(A)] is capable of being locked in the open position. The provision for locking or adding a lock to the disconnecting means must be installed on or at the switch or circuit breaker, and it must remain in place with or without the lock installed.
(a)      If locating the disconnecting means is impracticable or introduces additional or increased hazards to persons or property.
(b) In industrial installations, with written safety procedures, where conditions of maintenance and supervision ensure only qualified persons will service the equipment.

Informational Note 2: For information on lockout/tagout procedures, see NFPA 70E, Standard for Electrical Safety in the Workplace.

Q4. How many 10 AWG conductors can I install in a trade size 1 PVC raceway?

A4. Raceways must be large enough to permit the installation and removal of conductors without damaging the conductors’ insulation, and the number of conductors must not exceed that permitted by the percentage fill specified in Table 1, Chapter 9 [352.22].

When all conductors in a raceway are the same size and insulation, the number of conductors permitted can be found in Annex C for the raceway type.

Question: How many 10 AWG THHN conductors can be installed in trade size 1 Schedule 40 PVC?
(a) 5              (b) 10           (c) 15         (d) 18

Answer: (c) 15 conductors [Annex C, Table C10]

Author’s Comment: Schedule 80 PVC conduit has the same outside diameter as Schedule 40 PVC conduit, but the wall thickness of Schedule 80 PVC conduit is greater, which results in a reduced interior area for conductor fill.

Question: How many 10 AWG THHN conductors can be installed in trade size 1 Schedule 80 PVC conduit?
(a) 5   (b) 10      (c) 13      (d) 18
Answer: (c) 13 conductors [Annex C, Table C9]

Cables can be installed in PVC conduit, as long as the number of cables doesn’t exceed the allowable percentage fill specified in Table 1, Chapter 9.

Q5. What is the Code rule for the securing and supporting of PVC raceways?

A5.  PVC conduit must be securely fastened and supported in accordance with the following [352.30]:

• PVC conduit must be secured within 3 ft of every box, cabinet, or termination fitting, such as a conduit body [352.30(A)].
• PVC conduit must be supported at intervals not exceeding the values in Table 352.30, and the raceway must be fastened in a manner that permits movement from thermal expansion or contraction.

PVC conduit installed horizontally in bored or punched holes in wood or metal framing members, or notches in wooden members, is considered supported, but the raceway must be secured within 3 ft of termination [352.30(B)].

Question: What is the minimum interval required between supports of trade size 1 PVC?
(a) 3 ft           (b) 5 ft          (c) 6 ft        (d) 8 ft
Answer: (a) 3 ft [Table 352.30]

Question: What is the minimum interval required between supports of trade size 3 PVC?
(a) 3 ft           (b) 5 ft          (c) 6 ft        (d) 8 ft
Answer: (c) 6 ft [Table 352.30]

Q6. What are the rules on the use of expansion fittings for a PVC raceway?

A6. If PVC conduit is installed in a straight run between securely mounted items, such as boxes, cabinets, elbows, or other conduit terminations, expansion fittings must be provided to compensate for thermal expansion and contraction of the raceway in accordance with Table 352.44, if the length change is determined to be ¼ in. or greater [352.44].

Author’s Comment: Table 352.44 was created based on the following formula:

Expansion/Contraction Inches =
Raceway Length/100 x [(Temp Change/100) x 4.00]

Example: How much will a 25 ft run of PVC conduit contract located on the roof where the ambient temperature range is between 10F and 150F?
(a) 0.5 in. (b) 0.6 in. (c) 0.7 in. (d) 1.4 in.

Answer: (c) 1.4 in.
Expansion/Contraction Inches = Raceway Length/100 x ((Temp °F Change/100) x 4.00)
Expansion/Contraction Inches = (25/100) x ((140*/100) x 4.00)
Expansion/Contraction Inches = 1.40 in.

Q7. Is an equipment grounding conductor always required in a PVC raceway between the utility meter and service disconnect?

A7. Usually an equipment grounding is required in PVC, and when that is the case, a separate equipment grounding conductor of the wire type must be installed within the conduit [300.2(B) and 352.60].

However, there is an exception that deals with service raceways:

Ex 2: An equipment grounding conductor isn’t required in PVC conduit if the neutral conductor is used to ground service equipment, as permitted in 250.142(A) [250.24(C)].

Q8. How do I size the grounding electrode conductor for a service that is fed by parallel service conductors?

A8. Except as permitted in (A) through (C), a grounding electrode conductor must be sized in accordance with Table 250.66 [250.66].

If the grounding electrode conductor is connected to a ground rod as permitted in 250.52(A)(5), that portion of the grounding electrode 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 grounding electrode conductor that’s the sole connection to the concrete-encased electrode isn’t required to be larger than 4 AWG copper [250.66(B)].

 If the grounding electrode conductor is connected to a ground ring, the portion of the conductor that’s the sole connection to the ground ring isn’t required to be larger than the conductor used for the ground ring [250.66(C)].

Author’s Comments:
•  A ground ring encircling the building/structure in direct contact with the earth must consist of at least 20 ft of bare copper conductor not smaller than 2 AWG [250.52(A)(4)]. See 250.53(F) for the installation requirements for a ground ring.
•  Table 250.66 is used to size the grounding electrode conductor when the conditions of 250.66(A), (B), or (C) don’t apply.

Question: What size grounding electrode conductor is required for a service consisting of three 600 kcmil conductors per phase in parallel?
(a) 2/0 AWG                      (b) 3/0 AWG     (c) 4/0 AWG    (d) 300 kcmil
Answer: (b) 3/0 AWG

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