Topics discussed in our "Code Forum" column involve complicated issues requiring extensive analysis. However, not every Code question warrants such in-depth treatment. In this issue, we're introducing "Code Quandaries," a new department with short answers to questions posed on our website.
Coverage includes: Sec. 240-4(b)(1), Sec. 400-5, Sec. 240-4(a), Sec. 725-21(b), Sec. 370-22, Sec. 555-4, Sec. 350-10(a), Sec. 410-77(c), Sec. 422-16(b), Sec. 411-2, Sec. 411-3, Sec. 384-13, and Sec. 386-16(b).
Q. Assume a factory-made and UL-listed fixture hanger kit comes with a No. 16-3 cord set having a pre-wired plug, as well as the necessary hook/loop and fittings. I believe you can connect this to a circuit of up to 30A per Sec. 240-4(b)(1).
If I make my own cord set using SO or SJO cord, a plug, and common generic components (not supplied specifically for use with that particular fixture), must I now comply with Sec. 400-5, and use No. 12-3 for connection to a 20A circuit? On a similar note: Say a typical tool or appliance, like a bench grinder, comes with a No. 16-3 cord. If I were to wire an identical motor using flexible cord, would I need to comply with Sec. 400-5 and base the cord size on the branch-circuit rating?
A. Sec. 240-4(b)(1) applies whether the cord is field wired or not; provided the load equipment is listed. However, the cord arrangement is subject to approval. This means the Authority Having Jurisdiction (AHJ) must find it acceptable. Your reading of the branch-circuit ampacities in the rule is correct. However, you may have a problem in the case of a stand-alone motor, based on the literal text of the exception.
This is because motors, as opposed to appliances, are seldom listed. As soon as the load equipment is unlisted, then go back to the main rule in Sec. 240-4(a), which is Table 400-5A and -5B. Note, however, you can use supplementary overcurrent protection, which could include a small fuse arrangement within the equipment. This would be another way of getting to the No. 16 cord.
Q. I recently had a client ask me why I always use 120V control power on a 480V motor controller. He asked me if it was a Code requirement or personal preference. And if so, what code? I thought it was the NEC, but I couldn't find it. I recall having clients (mostly oil companies) requesting 120V control because it was safer for an operator to have his or her hands in contact with 120V and not 480V, should the unlikely grounding of one of the conductors occur. In my 25 years as an electrical engineer, I have run into a number of things done for no Code reason; just general industry practice. Is this an actual rule?
A. I also prefer 120V control circuits, but it is a design preference. These circuits are Class 1 control circuits, and Sec. 725-21(b) allows up to 600V. I have done a lot of wiring in plants with 600V distributions. Particularly for simple start-stop control circuits, I have wired these across the line. That's how the job was engineered, and there was (and still is) no Code reason to object. One other point: This is an NEC answer. If NFPA 79 (NFPA's Electrical Standard for Industrial Machinery) applies to your work, Sec. 9-2.1 generally limits control circuit voltages to 120V.
Q. Can you run raceway out of an extension ring, and put line and load conductors in the same raceway for transformers or motors? If I have some power in a block wall and no access at the ceiling or the floor, can I add an extension ring to the box in the block wall and pipe out of it?
A. Yes, you can extend from an extension box, mounted and secured per Sec. 370-22. Line and load can be in the same raceway, as long as you meet the customary ampacity derating requirements for the numbers of conductors in a single raceway.
Q. What rules cover the safe working distance requirements for outdoor receptacles mounted on 480V switchgear enclosures? The electrical enclosures provide up to 4000A of shore-to-ship cables via single- or 3-pole receptacles.
A. There are no specific NEC work space requirements for these receptacles, since they would not normally be serviced while energized. In this regard, they are similar to other industrial pin-and-sleeve type high-amperage plugs and receptacles. There are additional requirements for shore power electrical distribution in NFPA 303 (Chapter 3), Fire Protection Standard for Marinas and Boatyards. The 1999 NEC now incorporates one of those rules as the new Sec. 555-4. Your shore power receptacles now need a switch or circuit breaker ahead of them, located within sight of the shore power connection.
Q. Where there are large numbers of recessed light fixtures installed in a grid ceiling, does the Code allow you to wire from fixture to fixture with flexible conduit? We have two local inspecting agencies that allow this type of installation. They both say as long as the flex is properly supported and the equipment grounding conductor is used when the flex is more 6 ft in length, it's okay. It seems to me the purpose of the flex conduit is to extend from the junction box to the fixture, and not to completely wire groups of fixtures with flex only.
A. The inspectors are correct; flexible metal conduit is a Chapter 3 wiring method. The question didn't specify the size of the flex, but you need to observe the restrictions in Sec. 350-10(a) on the use of 3/8-in. flex. Even for these restrictions, however, Sec. 410-77(c) makes modifications for paired fixture sections, allowing up to 25 ft of 3/8-in. flex instead of the normal 6-ft restriction. In addition, you need to observe any wire fill restrictions on the junction boxesthat are part of the lighting fixtures.
Q. I am confused regarding allowable wiring methods to residential dishwashers and waste disposers. Art. 422 says "shall be permitted," when referring to a cord and male cord cap configuration. Does this wording imply an option? What other wiring methods would then be permitted? Is hard wiring permitted direct via Type NM cable, or Type MC, etc.?
A. Any Chapter 3 wiring method is allowable, provided it is otherwise suitable for the location and you meet the relevant support and other installation requirements, as per the applicable article. Then, you can also use flexible cord under the constraints in Sec. 422-16(b).
Q. Our company recently had us install a 12V track lighting system. The fixtures came from Italy. We installed the transformers as furnished, which originated locally. Some were dry type, 250VA, 120/240-12/24V; others were open control transformers we installed in screw cover enclosures. Does Art. 411 still apply to the system, since the transformers were obtained separately? And, what would be the proper overcurrent protection for the transformer primary and secondary conductors?
A. The system you describe is almost certainly in violation of the parameters in Sec. 411-2 and therefore is unlistable as required in Sec. 411-3. Since all lighting systems operating below or equal to 30V must be listed for that purpose, and since this one couldn't be, it cannot be installed and still meet NEC requirements. Remember: The requirement is for a system listing, not a component listing. Why? These systems have sophisticated fault-sensing mechanisms that interrelate with the fixtures and conductors to assure coordinated protection. You simply can't assure this type of installation.
Q. I have a 200A single-phase service panel. There is one 100A subpanel fed from it already. One leg of the main service panel draws 100A, and the other draws 75A when everything is running full tilt. Can I add an additional subpanel to the main panel? How can I determine the largest allowable ampacity of this second subpanel?
A. Theoretically, you can add any size subpanel you want, even 400A, if you protect it according to Code (for example, with individual protection per Sec. 384-16 in the case of a lighting and appliance board, as is likely here). The only issue is how much load you connect. That's determined by Art. 220. The existing panel presumably has 200A protection, so it is protected. Your next step is a load calculation, to assure you aren't connecting excessive load to the 200A panel, as covered in Sec. 384-13. Some inspectors may be uneasy about a large additional panel, believing it invites excessive future load connections without adequate opportunity for inspection, but you do satisfy the Code with individual protection. Per Sec. 386-16(b), even if the parent 200A panel is a power panel, you need to provide protection for multiple small-branch circuits originating from it.
Q. What is the NEC ruling on using a DPST switch to control a 277V light and a 120V fan?
A. There isn't any objection in the NEC itself to doing this, because Sec. 380-8 (b) only applies to adjacent devices. However, Sec. 110-3(b) effectively incorporates by reference all restrictions in the UL guide card data. In this case, you'll find the following information (in guide card category WJQR on "snap switches") "Multipole, General Use Snap Switches have not been investigated for more than single circuit operation unless marked '2-circuit' or '3-circuit.'"
Some of us have been trying for many years to interest manufacturers in securing such listings on some of their double- and triple-pole switches with absolutely no success. To the best of my knowledge, there are no snap switches with the applicable listing. In fact, there don't appear to even be listings for two 120V circuits, such as might be applied to two oil burners. Even though the two-pole switch you'd like to use for that application is rated 240V, this eliminates any issue about potential between adjacent energized parts running above the nominal voltage rating.
Editor's Note:These answers are given by our panel of experts. This author chairs the panel. Other panel members include: Bill Summers, James Stallcup, Joe Ross, and Dan Leaf. The opinion expressed is that of the panel. If a panelist disagrees with the majority opinion, his explanation is printed following the answer. Although authoritative, the answers printed here are not, and cannot be relied on as formal interpretations of the NEC.