NEC Questions and Answers Based on the 2008 NEC
November 2009 – Part 2

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 local inspector says that receptacles in walls of a multifamily dwelling must be spaced 24" apart from each other when they are installed back to back. Can you help me understand where he is coming from?
A1. Because the walls are fire-resistance rated, 300.21 applies. Openings in fire-rated walls, floors, and ceilings for electrical equipment must be fire-stopped using methods approved by the authority having jurisdiction to maintain the fire-resistance rating of the fire-rated assembly.
Outlet boxes must have a horizontal separation not less than 24 in. when installed in a fire-rated assembly, unless an outlet box is listed for closer spacing or protected by fire-resistant “putty pads” in accordance with manufacturer’s instructions.

Q2. Can pneumatic tubing be in the same raceway with 24V remote control circuits?
A2. 300.8 would prohibit this practice if it were a lighting or power circuit, but because it is control wiring a different set of rules apply. Only those sections contained in Article 300 specifically referenced In Article 725 apply to Class 1, 2, and 3 circuits [725.3]. Because 300.8 is not referenced in 725.3, it does not apply to Class 2 or Class 3 circuits.

Q3. Can a service rated automatic transfer switch be installed between the utility meter and first disconnect?
A3. Yes, if it is listed as “suitable for use as service equipment” [230.66]. If it were not listed as such, it could not be used because electrical equipment must not be connected to the supply side of the service disconnect enclosure, except for the equipment listed in 230.82. Transfer switches are not one of the items listed in that section.

Q4. What are the requirements in locating transformers above a suspended ceiling?
A4. Dry-type transformers, rated not more than 50 kVA, are permitted above suspended ceilings or in hollow spaces of buildings, if not permanently closed in by the structure [450.13(B)]. Dry-type transformers not exceeding 50 kVA with a metal enclosure can be installed above a suspended ceiling space used for environmental air-handling purposes [300.22(C)(2)].

Q5. What is the maximum height for a disconnect switch?
A5. Switches and circuit breakers used as switches must be capable of being operated from a readily accessible location [404.8(A)]. They must also be installed so the center of the grip of the operating handle of the switch or circuit breaker, when in its highest position, isn’t more than 6 ft 7 in. above the floor or working platform [240.24(A)]. It’s also worth pointing out that the disconnecting means for a mobile home must be installed so the bottom of the enclosure isn’t less than 2 ft above the finished grade or working platform [550.32(F)].

Q6. If a manufacturer states that a multi-motor chiller has a minimum circuit ampacity of 357A, a full load of 343A, a minimum circuit breaker size of 400A and a maximum circuit breaker size of 400A, can I feed this with 500 kcmil?
A6. Yes, 500 kcmil, having an ampacity of 380A [Table 310.16] would be fine. For a her­metic refrigerant motor-compressor, the rated-load current marked on the nameplate of the equipment is to be used in determining the rating of the disconnecting means, the branch-circuit conductors, the controller, and the branch-circuit short-circuit and ground-fault protection [440.6(A)].

Q7. Can service conductors (between the meter and the service disconnect) be tapped to feed a fire pump?
A7. Yes, a separate service or a connection located ahead of (but not within) the service disconnecting means is allowed as the power source for a fire pump [695.4(A)].

Q8. I recently saw a service disconnect that was being used as a conduit for two feeder circuits. Is this legal?
A8. 230.7 requires that service conductors not be installed in the same raceway or cable with feeder or branch-circuit conductors. This rule doesn’t prohibit the mixing of service, feeder, and branch-circuit conductors in the same service equipment enclosure, however, so what you describe is permitted.

Q9. We have the circuit for an A/C and also a GFCI circuit in the same raceway, but the GFCI needs to be mounted a few feet away. Can the A/C disconnect be used as a raceway for the circuit?
A9. Cabinets, cutout boxes, and meter socket enclosures can be used as a raceway for conductors that feed through if the conductors don’t fill the wiring space at any cross section to more than 40 percent [312.8].

Q10. When calculating box fill, does the mechanical ground and the isolated ground count as one or two wires?
A10. All equipment grounding conductors in a box count as a single conductor volume in accordance with Table 314.16(B), based on the largest equipment grounding conductor that enters the box [314.16(B)(5)]. Insulated equipment grounding conductors for receptacles having insulated grounding terminals (isolated ground receptacles) [250.146(D)] count as an additional single conductor volume in accordance with Table 314.16(B).

Q11. How many feet of track lighting can be on a 20A circuit?
A11. As many feet as you would like. The feeder or service load calculations of 220.43(B) do not limit the number of feet of track on a circuit, nor do they limit the number of luminaires mounted on an individual track [410.151 FPN].

Q12. Where is the high leg conductor really supposed to terminate? I keep seeing it on phase “C” on the utility side of the meter, and on phase “B” in the service disconnect (and everywhere downstream).
A12. Panelboards supplied by a 4-wire, delta-connected, three-phase (high-leg) system must have the high-leg conductor (which operates at 208V to ground) terminate to the “B” phase of the panelboard [408.3(E)]
Exception: The high-leg conductor can terminate to the “C” phase when the meter is located in the same section of a switchboard or panelboard.

Warning: The ANSI standard for meter equipment requires the high-leg conductor (208V to neutral) to terminate on the “C” (right) phase of the meter socket enclosure. This is because the demand meter needs 120V, and it gets it from the “B” phase.

WARNING: When replacing equipment in existing facilities that contain a high-leg conductor, use care to ensure that the high-leg conductor is replaced in the original location. Prior to 1975, the high-leg conductor was required to terminate on the “C” phase of panelboards and switchboards. Failure to re-terminate the high leg in accordance with the existing installation can result in 120V circuits inadvertently connected to the 208V high leg, with disastrous results.

Q13. Are the adapters that are available to convert a lampholder to a receptacle legal?
A13. No, they are not legal. Lampholders of the screw-shell type must be installed for use as lampholders only [410.90]

Q14. Do I need a grounding conductor for a pole light, or is the ground rod good enough?
A14. You absolutely need an equipment grounding conductor. Metal parts of electrical raceways, cables, enclosures, or equipment must be bonded together and to the supply system in a manner that creates a low-impedance path for ground-fault current that facilitates the operation of the circuit overcurrent device [250.4(A)(5)].
Because the earth is not suitable to serve as the required effective ground-fault current path, an equipment grounding conductor is required to be run with all circuits.

Question: What is the maximum fault current that can flow through the earth to the power supply from a 120V ground fault to metal parts of a light pole that is grounded (connected to the earth) via a ground rod having a contact resistance to the earth of 25 ohms?
(a) 4.80A      (b) 20A         (c) 40A       (d) 100A
Answer: (a) 4.80A
I = E/R
I = 120V/25 ohms
I = 4.80A

Danger: Because the contact resistance of an electrode to the earth is so high, very little fault current returns to the power supply if the earth is the only fault current return path. Result—the circuit overcurrent device will not open and all metal parts associated with the electrical installation, metal piping, and structural building steel will become and remain energized.