At EC&M, we know NEC issues are very important to our readers. That's why we've dedicated a department to answering your latest Code questions and concerns. When you find yourself stumped by the Code, just e-mail your question to the author at [email protected] for future consideration in Code Quandaries.
Q. I'm installing 3-in. intermediate metal conduit (IMC) with an extended run length of about 300 ft to 350 ft. My coworkers told me I have to install a pull box about every 100 ft, but I can't find a reference to this in the NEC. Is there a rule in the Code that specifies the maximum distance between pull points?
A. No. The Code does not specify when a pull box is required, but Sec. 314.28 does specify how to properly size a pull box. You can run the raceway as long as you want, but the Code does not permit more than the equivalent of four quarter bends (360∞ total) between pull points [Secs. 362.26, 342.26, 344.26, 352.46, 358.26, 348.26, 350.26, and 356.26].
Q. I just set a service on a new house where I used ½-in. electrical metallic tubing (EMT) to encase the grounding electrode conductor (GEC) from the meter base to the ground rod. The inspector said I have to bond both ends of the metal raceway to the GEC. He also said I should use nonmetallic conduit in the future. Is he correct?
A. Yes. According to Sec. 250.64(E), you can use a metal enclosure to encase the GEC, but you must maintain electrical continuity between the point of attachment to cabinets or equipment and the grounding electrode. You must also securely fasten it to the ground clamp or fitting and make metal enclosures that are not physically continuous from cabinets or equipment to the grounding electrode electrically continuous by bonding each end to the GEC [Sec. 250.92(A)(3)]. This ensures they will be in parallel with each other. Failure to bond both ends could create a condition where the reactance of the raceway becomes so high that it creates an “inductive choke” that would limit the GEC's ability to carry current. If you used nonmetallic conduit to encase the GEC, you don't have to worry about maintaining the electrical continuity of the metal raceway.
Q. What is the maximum secondary conductor length permitted by the NEC from a transformer to a main breaker in an office building?
A. Twenty-five ft. The rules in Sec. 240.4 require you to protect all conductors against overcurrent in accordance with their ampacities as specified in Sec. 310.15. However, Sec. 240.4(E) allows you to run unprotected secondary conductors up to 25 ft if you meet the following conditions of Sec. 240.21(C)(6):
The secondary conductors shall have an ampacity that, when multiplied by the ratio of the secondary-to-primary voltage, is at least ⅓ of the rating of the overcurrent device protecting the primary of the transformer.
Secondary conductors terminate in a single circuit breaker or set of fuses that limit the load current to not more than the conductor ampacity permitted by Sec. 310.15.
The secondary conductors are suitably protected from physical damage.
The 1999 NEC is not very clear about secondary conductor lengths over 10 ft, but not more then 25 ft for nonindustrial facilities. Fortunately, Sec. 240.21(C)(6) has been added to the 2002 NEC and clearly explains how to properly size and install secondary conductor taps.
Q. I'm working on a project that includes hooking up a listed kiln in a ceramics classroom. The kiln is rated 100A at 208V. The flexible metal raceway from the kiln to the 100A disconnect includes two 1 AWG conductors, one 6 AWG grounded (neutral) conductor, and one 12 AWG equipment ground. I can't find a section of the NEC that allows the grounded (neutral) conductor to be reduced in size as it is. Am I missing something?
A. Yes. There are no Code rules that specifically tell us how to size the grounded (neutral) conductor for branch circuits, but there is one for feeders (220.22). You only need to size the grounded (neutral) conductor to carry the maximum unbalanced load between it and any ungrounded conductor. Because this is a listed product, you only need to wire the appliance according to the instructions.
However, the size of the equipment grounding conductor in the flexible metal raceway is cause for concern. This equipment grounding conductor may not be smaller than shown in Table 250.122, based on the rating of the circuit overcurrent protection device. For a 100A protection device, the equipment grounding conductor may not be smaller than 8 AWG.
Q. I've heard an insulated equipment grounding conductor for an isolated grounding circuit installation should float from the electrical system. I've also heard it must terminate at the neutral point of a transformer or at the neutral-to-case connection point at service equipment. Which way is correct?
A. Neither. The NEC doesn't require the isolated equipment grounding conductor for electronic equipment to terminate to the neutral point of a transformer or at the neutral-to-case connection point at service equipment — nor does it permit you to float the equipment grounding conductor from the electrical system [250.96(B) and 250.146(D)].
The equipment grounding conductor from electronic equipment may not be “floated, lifted, or isolated” from the electrical source. If the metal parts of an electrical system were truly isolated or floated from the electrical source, a line-to-case fault could not be cleared — there would be no path for electrons to return to the power supply — and the metal parts of the equipment would remain energized with dangerous touch voltage.
To ensure a safe electrical system, you must bond all metal parts of the electrical system together and to the utility grounded (neutral) conductor at service equipment or the neutral point of a separately derived system [Sec. 250.4(A)(4)].
According to several industry standards, the insulated equipment grounding conductor should originate at the neutral point at the transformer or neutral-to-case connection point of service equipment, and it should remain insulated from the metallic raceway and all other metal parts through its length. To maintain isolation from the metal parts of the electrical system, the isolated equipment grounding conductor may pass through metal enclosures [Secs. 250.146(D) and 408.20 Exception].
Real world experience demonstrates that the difference in ground potential between electronic equipment (which causes the problems with digital equipment) can be reduced to a satisfactory level by simply terminating all insulated equipment grounding conductors to a “single point” at the panelboard where all the circuits originate.
Q. Does the NEC require you to measure (test) the resistance of the grounding electrode to validate the ground resistance does not exceed 25 ohms?
A. No. The rules in Sec. 250-50 specify that a metal underground water pipe, the metal frame of a building or structure, a concrete-encased electrode, or a ground ring can be used as the required grounding electrode for services (Sec. 250-24), separately derived systems (Sec. 250-30), and remote buildings and structures (Sec. 250-32). The NEC does not require any of these electrodes have a resistance of 25 ohms or less (contrary to what many think), nor does it require you to measure their ground resistance.
However, if none of the electrodes specified in Sec. 250-50 are available, then for all practical purposes you need to install a ground rod (Sec. 250-52). Sec. 250-56 specifies that where a single electrode consisting of a ground rod has a resistance of more than 25 ohms, you must install an additional electrode. You must bond the additional electrode to the first electrode with a conductor that is not required to be larger than No. 6 cu. [Sec. 250-66(a)], and the additional electrode must be located at least 6 ft from the other ground rod [Sec. 250-56]. Once you have installed two ground rods, there is no need to measure their ground resistance, because the NEC does not require them to have a resistance of 25 ohms or less.
Q. What size copper equipment grounding conductor is necessary for a 5 hp Design Letter B, 208V, 3-phase motor protected by an inverse time circuit breaker, assuming no more than three current-carrying conductors in a raceway, ambient temperature of 30°C (86°F), and a circuit conductor length of 50 ft?
A. Number 12. According to Sec. 250-122, the equipment grounding conductors shall not be smaller than shown in Table 250-122, based on the rating of the circuit overcurrent protection device. But it does not have to be larger than the circuit conductors supplying the equipment. Follow these steps to determine the size:
Step 1. Size the circuit protection device (circuit breaker) per Sec. 430-52(c)(1).
Motor FLC = 16.7A [Table 430-150]
Protection device size = 16.7A x 250%
The next size up permitted is 45A [Secs. 240-6(a) and 430-52(c)(1) Exception No. 1].
Step 2. Size the motor-circuit conductors per Sec. 430-22(a).
Conductor size = 16.7A x 125%
The 75°C column in Table 310-16, calls for No. 12 [Sec. 110-14(c)(1)(d)].
Step 3. Size the equipment grounding conductor per Sec. 250-122(a) and (d).
The copper equipment grounding conductor required by Table 250-122 is No. 10, but the equipment grounding conductor does not have to be larger than the No. 12 circuit conductors.
Q. I'm a contractor in California with a limited specialty license relating to swimming pools and spas. I continually run into electricians who have a different interpretation of the GFCI protection requirements of Sec. 680-42 for spas and hot tubs. When is GFCI protection required?
A. According to Sec. 680-42, GFCI protection is required for the outlet that supplies:
A self-contained spa or hot tub.
A packaged spa or hot tub equipment assembly.
A field-assembled spa or hot tub with a heater load of 50A or less.
This section also states that GFCI protection is not required for:
A listed self-contained unit or listed packaged equipment assembly with integral GFCI protection provided for all electrical parts within the unit or assembly.
A field-assembled spa or hot tub rated greater than 250V or rated 3-phase.
A combination pool/hot tub or spa assembly.
Q. Does the NEC have any requirements about where a 120V smoke detector must be installed?
A. No. The NEC does not cover the location of smoke detectors. This is covered in other NFPA documents such as NFPA 72 Fire Alarm Code or NFPA 101 — Life Safety Code.