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NEC Questions 014

By Mike Holt for EC&M Magazine

Q1. A meter and main breaker panel is connected by a PVC and the neutral is bonded to the meter enclosure and the panel enclosure. The service conductors enter the meter enclosure through a hub with rigid metal conduit. Is there any more bonding required for the service?

A1. Additional bonding is not required for this service. The metal riser is bonded to the meter enclosure by the threaded hub, provided the connection is made up wrench tight as required in 250.92(B)(2). The meter enclosure is bonded to the grounded (neutral) conductor because the neutral terminal is bonded to the meter enclosure in this case [250.92(B)(3)]. The service disconnect is bonded by the main bonding jumper that is located in the panel enclosure [250.28 and 250.92(B)(3)].

While the Code required bonding has been provided [250.92], the service must be grounded to a suitable electrode in accordance with 250.24(A) and 250.50.

Q2. On a 1,600A 277/480V service, is the neutral conductor from the utility transformer to the service disconnect considered a current carrying conductor if the only line-to-neutral loads is a 200A lighting panel?

A2. The neutral (grounded) service conductor for this installation is not considered a current carrying conductor (for conductor ampacity adjustment purposes) [310.15(B)(2)(a)]. The grounded (neutral) conductor of a three-phase, 4-wire wye system is only required to be counted as a current carrying conductor when a major portion of the load is nonlinear [310.15(B)(4)(c)]. In this case, the maximum neutral load is only 12.5% of the service rating.

Q3. Can Type AC health care cable be use for emergency system branch circuits in a hospital?

A3. No, 517.30(C)(3) requires emergency system branch circuit conductors be protected by a nonflexible metal raceway or by MI cable.

Q4. Is it permissible to install line and load wiring in the same raceway?

A4. The NEC does not restrict the installation of line and load conductors within a common raceway, but it does prohibit the installation of service conductors within raceway with non-service conductors [230.7].

Q5. Is GFCI protection required for a hard-wired spa?

A5. GFCI protection is required for both cord-and-plug and hard-wired spas containing single-phase equipment.

Q6. I have a 4-wire, three-phase, 480V, 600A feeder (parallel 350 AWG THHN in 2 conduits) serving a 600A panel that supplies nonlinear loads. If the ambient temperature in the plant is 100°F, is this installation correct?

A6. The installation is fine as long as the calculated load does not exceed 510A. Lets go through the steps:
Step 1 Determine the ampacity of the conductor in accordance with the conductor insulation temperature rating.
350 AWG THHN is rated 350A at 90°C.

Note: When performing conductor ampacity, use the conductor ampacity based on the insulation rating of the conductor NOT the ampacity based on the terminal rating of the equipment!

Step 2 Determine the adjusted ampacity based on four current-carrying conductors [310.15(B)(2)(a)] located in an ambient temperature of 100°F [Table 310.16].
Conductor Bundle Adjustment Factor: 0.80
Ambient Temperature Correction Factor: 0.91

Conductor Adjusted Ampacity = 350A x 0.80 x 0.91
Conductor Adjusted Ampacity = 255A x 2 = 510A

Q7. Do the Tables in Annex C take into consideration conductor ampacity adjustment for conductor bundling?

A7. No.

Q8. Does the NEC permit the installation of a transfer switch between the meter and the service disconnecting means? However, the transfer switch is not marked "suitable for use as service equipment" as required by 230.66. Is this legal?

A8. No. A transfer switch is not permitted ahead of the service disconnecting means [230.82], however a transfer switch that is listed as "suitable for use as service equipment" (means it have a main with overcurrent protection, and a main bonding jumper) is permitted.

Q9. Is a chain link fence surrounding an outdoor 480V standby generator required to be grounded?

A9. No, the NEC does not require this fence be grounded, and frankly, it serves no purpose to ground the fence to anything.

Q10. Is an antishort bushing required for Type MC cable of the interlocked type?

A10. No, the NEC does not require the use of antishort bushings for the terminations of MC cable, but 320.40 requires the termination fittings to be listed and identified for the terminations of this type of cable. However, many manufacturers of MC cable provide antishort bushings with the cable, and they do provide protection of the conductors.

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