Overcurrent protection for flexible cord A reader was familiar with factory-made and listed fixture hanger kits supplied with 16-3 cord sets with pre-wired plugs, as well as the necessary hook/loop and fittings. First, he noted the kits seemed to be capable of connection to a circuit of up to 30A per Sec. 240-4 Ex 1. Then, he asked if he made his own cord set using SO or SJO cord, a plug, and common generic components (not supplied specifically for use with that particular fixture), would he be required to comply with Sec. 400-5 (i.e., use 12-3 for connection to a 20A circuit)? He also wondered about a bench grinder or similar appliance; could he replace a similar 16-3 cord set instead of sizing the replacement to the branch-circuit?
The Code Forum panel's response. We think Sec. 240-4 Ex. 1 applies whether the cord is field wired or not, provided the load equipment is listed. The cord arrangement is, however, subject to approval, which means being acceptable to the AHJ. We agree with the reader's assessment of the branch-circuit ampacities in that exception. We think there may be a problem in the case of a stand-alone motor. This is because motors, as opposed to appliances, are seldom listed. As soon as the load equipment is unlisted, you can't apply the exception. Then, you default to the main rule, which is Table 400-5A and -5B. Note, however, you can use supplementary overcurrent protection, which could include a small fuse arrangement within the cord cap/equipment. This would be another way of getting to the No. 16 cord.
Grounding different services in large buildings The Code recognizes the special problems involved when you install electrical service to buildings of large area. For example, Sec. 230-2 Ex. 5 allows (by special permission) multiple services to these buildings. We received a question asking for assistance with sizing a grounding conductor to connect two main services in the same building that were approximately 700 ft apart.
The Code Forum panel's response. Refer to Sec. 250-54 and either use the same electrode (this would be feasible if you're using effectively grounded building steel as the electrode, for example), or bond the two different electrodes together. The minimum size bonding conductor would be that size meeting the worst case for any of your single services. For example, suppose one service is 2000A and the other is 700A, and suppose there is a good water pipe electrode near the 2000A service. Go from the 2000A service to the water pipe with 3/0 and to the same pipe from the 700A service with 2/0, and yes you'll have to go all 700 feet.
Now, suppose the water pipe is in the middle. Go from service #1 with 3/0, and from service #2 with 2/0. If the pipe is near the smaller service, go the 700 ft with 3/0. If you have qualifying building steel, the problem is much easier. Make that the electrode, and bond from that to the water pipe, which you can do at the pipe location. That is, go from Service #1 to the steel with 3/0; go from service #2 with 2/0, and go from the steel to the pipe in the middle with 3/0, the worst case GEC. Note that nothing in the NEC sets a maximum length on a grounding electrode conductor.
Extensions from branch circuits in walls We often need to extend branch circuits from existing wiring. We received a question about doing this work while properly using extension rings, and whether there were any other Code issues involved, such as whether line and load conductors could run together in this kind of an extension.
The Code Forum panel's response. You can certainly extend from the existing wiring. An extension ring is one way to do it. The applicable requirements are in Sec. 370-22:
370-22. Exposed Surface Extensions. Surface extensions from a box of a concealed wiring system shall be made by mounting and mechanically securing a box or extension ring over the concealed box. Where required, equipment grounding shall be in accordance with Article 250.
Exception: A surface extension shall be permitted to be made from the cover of a concealed box where the cover is designed so it is unlikely to fall off, or be removed if its securing means becomes loose. The wiring method shall be flexible and arranged so that any required grounding continuity is independent of the connection between the box and cover.
The extension ring is preferred, although there has been a lot of interest recently over extensions from covers. The exception covers this point, and the last sentence assures the grounding continuity will be there even if the cover loosens. In addition to the grounding precautions, the panel wanted to be sure you didn't end up with exposed conductors if the cover fell off, even though an equipment bonding jumper was in place. The first sentence covers this entirely separate concept, although they are often confused. You need to use covers with round mounting holes or keyed slots that have a raised detent behind the final screw position. These designs require the screw to work its way out of the box, or to get very loose and to then have the cover lift and turn before it can come off.
We also think you can put line and load in the same raceway. Just be sure you meet the ampacity derating requirements for the numbers of conductors in a single raceway. Also, be sure to meet the box fill restrictions in Sec. 370-16 after considering the additional conductors. However, you'll be able to add the existing and the extension ring (assuming it to be marked with its capacity) volumes together in determining the allowable conductor fill.
Low-voltage track lighting One of our readers described a 12V track lighting system he had to install. The fixtures themselves came from Italy; the transformers were from local sources. The reader wanted to know if Art. 411 applied to the system since he obtained the transformers separately? He also wanted to know what would be the proper overcurrent protection for the transformer primary and secondary conductors?
The Code Forum panel's response. The system described is almost certainly in violation of the parameters in Sec. 411-2 and therefore unlistable as required in Sec. 411-3:
411-2. Lighting Systems Operating at 30 Volts or Less. A lighting system consisting of an isolating power supply operating at 30 volts (42.4 Vpk) or less, under any load condition, with one or more secondary circuits, each limited to 25 amperes maximum, supplying lighting fixtures and associated equipment identified for the use.
411-3. Listing Required. Lighting systems operating at 30 volts or less shall be listed for the purpose.
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. The reason is these systems have sophisticated fault sensing mechanisms that interrelate with the fixtures and conductors to assure coordinated protection. You cannot supply this in the field, and therefore the question about field-supplied overcurrent protection doesn't apply.
Wiring dishwashers/waste disposers We received several questions dealing with wiring dishwashers and waste disposers in residential occupancies. In general, people want to know when an additional disconnecting means needs to be installed in the area, particularly when the surrounding construction makes an additional disconnect problematic. In addition, we received questions addressing when flexible cord is required or permitted for connection.
The Code Forum panel's response. Taking the second part of the question first, any Chapter 3 wiring method is allowable provided it is otherwise suitable for the location and the relevant support and other installation requirements are met, as per the applicable article. We know there is a substantial divergence of opinion in the inspection community as to just what should be allowed, based on the perceived likelihood of physical damage. Some authorities allow Type NM cable with few restrictions; some allow it only if secured tightly to another surface such as a pipe drain or cabinet support. Some even insist on some sort of armored cable. Since this is a judgment call, consult local authorities first.
You can always use flexible cord under the constraints in Sec. 422-8(d), but watch out for the nameplate markings. Some waste disposers make outlandish horsepower claims to help sell the product. They can do this because test labs police the current ratings but not the horsepower claims.
Sec. 430-6(a) directs Code users to consider these horsepower ratings instead of current markings when evaluating motor circuits, which leads to oversized circuit elements. For appliances such as these, look for (and discuss with the local authority) a new exception in the1999 NEC that will allow you to base your calculations on the actual appliance current marking.
Remember Sec. 422-8(d) also restricts cord lengths to 3 ft in the case of waste disposers, and 3 ft to 4 ft for dishwashers. In the case of a dishwasher, that can be hard to live with if the wiring compartment is on the opposite side of the dishwasher from the sink base where you were planning to set the receptacle. The 1999 NEC will have some relief here as well, with an allowance to measure the cord length from where it comes out the rear of the dishwasher.
The reason we took the second question first is that a cord connection frequently provides the best way to address the first. A dishwasher needs to be disconnectable (per Sec. 422-20) in accordance with the provisions of Part D of Art. 422. If there is a qualified unit switch, then that in conjunction with an additional disconnecting means located in accordance with Sec. 422-25 is enough. Otherwise, you'll need the additional switch, or you can cord- and plug-connect the unit. Since you often won't know the exact dishwasher model at the time of rough-in (and many don't have these switches), a receptacle may be the best way to hedge your bets.