A new Code article covering lighting systems operating at 30V or less will modify some and help standardize other low-voltage lighting designs. Additional requirements, however, apply if wet contact is a factor.
Low-voltage lighting has been in use for about 30 years, with widely differing acceptance by local inspection authorities due to its ambiguous coverage in the NEC. Now it has its own article, the new Art. 411. Underwriters Laboratories (UL) is now writing a new standard (UL 2108) to cover these systems. When the standard is complete, probably within the year, we will see products listed accordingly.
There are, however, products available now that are listed and happen to comply with the new article, although not expressly listed to those terms. At present, they are being listed to the general incandescent lamp standard (UL 1571), although when the new standard is finalized, these listings will change over to the new standard. These products are being listed under Art. 725 of the NEC by incorporating power supplies that work under either Class 2 power limitations [by using a listed Class 2 power supply per Sec. 725-41(a)] or power-limited Class 1 circuits meeting the 1000VA, 30V limitations in Sec. 725-21 (a). The requirements in the new article are summarized in Fig. 1.
The new requirements
The most important thing to remember about these systems is that they are exactly that, systems. Although these may be assembled (and disassembled) in the field, each of the components will be listed as part of an overall system. Be careful of Sec. 411-6, which imposes a 20A branch-circuit restriction. The restriction applies to what the systems as a whole are connected to, and not to the ratings within a given lighting system.
These systems use an isolating power supply that must not exceed 30V (42.4Vpk) under any load condition, even while open circuited. The secondary circuits must have some form of current limitation such that the current cannot exceed 25A. Note that this creates an effective power limitation of 750VA (30V x 25A). This is less than the power4imited Class 1 threshold of 1000VA. The restriction is based on practical concerns involved with the desired physical flexibility for these systems; if the current went up above 25A, the wire size would increase to No. 10. Many manufacturers didn't think that was practical. For this reason, the 25A limitation applies even at lower voltages where the resulting VA is lower; at 12V for example, the VA limitation would be (12Vx25A) = 300VA.
Nevertheless, some manufacturers have been producing listed systems in the range between 750VA and the 1000VA maximum in Sec. 725-21(a), particularly for 24V indoor applications. They have been willing to pay the price in cumbersome conductors for being able to get longer runs, particularly for aisle pathway systems in large movie theaters, for example. Note that at 24V, the VA differential is even greater; 24V x 25A = 600VA.
It now appears that after the new UL 2108 standard takes effect, these listings will no longer be able to continue. This may have a significant impact on lighting layouts that extend long distances in only one direction. In some cases, it may be possible to center-feed these layouts with multiple circuits originating at a common power supply. Sec. 411-2 clearly allows for this, recognizing one or more circuits originating at the isolating power supply.
These systems, per Sec. 411-3, must be listed. The listing process will include additional restrictions. For example, if these systems are used outdoors or where wet contact could occur, the voltage limitation becomes 15V maximum, with most systems using 12V as a result. This restriction has been in Art. 725 for Class 2 systems for many years, and now is in the notes for Chapter 9, Table 11. For other applications, the 30V limitation is adequate to assure that there won't be a shock hazard. The other provisions of Art. 411 are primarily designed to prevent fires.
These systems must not be extended through a wall, unless using conventional wiring as covered in Chapter 3 of the Code. Presumably, this requirement Sec. 411-4(1) is also supposed to apply to a floor penetration, but the only term used is "building wall." This limits the possibility of heat being retained around the conductors. In general, these systems didn't need Chapter 3 wiring methods, having been covered under Art. 725 which, as part of Chapter 7, can supplement or modify the requirements in Chapter 3. Now that the rules are in Art. 411, correlating language probably needs to be included in Sec. 300-1(a).
The other restriction addresses the possibilities of sparks from short circuits. Although Sec. 411-5(c) allows the use of bare conductors, they must be high enough (7 ft) so that inadvertent bridging between the wires with conductive objects is very unlikely. If they are lower than 7 ft, then they must be specifically listed for this use. There are sophisticated power supplies that shut themselves off if the power demand varies from very close tolerances, and testing laboratories have evaluated them in conjunction with these applications. Otherwise, the conductors for these circuits will be insulated.
In addition, if these systems are used in hazardous (classified) locations, then they must comply with the usual rules for those locations. Just because these circuits have some degree of power limitation doesn't mean they are inherently safe for these locations. An energy release of just 0.25 mW-sec will ignite a suitable methane-air mixture; this is obviously far below the capability of one of these lighting systems.
System restrictions
The circuits supplied by these systems, in addition to having current and voltage limitations, also have important system grounding and insulation limitations to reduce the possibility of fire. The secondary must be fed from an isolating transformer. These transformers have been recognized in Sec. 680-5(a) for many years for use with underwater lighting fixtures in swimming pools, and in Sec. 680-51(a) Ex. for use with fountains. They have a grounded metal barrier between the primary and secondary windings.
Although there is a grounded barrier, that does not mean that the secondary circuit is grounded, and Sec. 411-5(a) prohibits grounding this circuit. This is a system grounding restriction; that is, you cannot intentionally connect one of the conductors "to earth or to some conducting body that serves in place of the earth," as defined in Art. 100.
There are important ramifications to this limitation that aren't obvious from reading the new article. Sec. 250-5(a)(1) requires system grounding if the supply transformer of a 50V or lower system is itself supplied from a source exceeding 150V to ground. Therefore, you cannot connect one of these new systems directly to a 277V branch circuit. You would need to first step the 277V circuit down to a 120V grounded system [and it would need grounding, per Sec. 250-5(b)(1)] and then supply the new Art. 411 lighting system.
The system grounding limitations also affect equipment grounding. Since these systems must fall within the parameters of Class 2 or Class 1 power-limited circuits, for the purposes of the Code, they would fall within the equipment grounding rules in Sec. 250-43(i). That section requires equipment grounding for limited-energy systems if system grounding is required in Part B of Art. 250. As we have seen, these systems cannot be grounded. Therefore, these lighting systems don't require any provisions for equipment grounding.
Swimming pool lighting
Sec. 411-4(2) defers to Art. 680 on the use of this lighting within 10 ft of a pool, spa, or fountain. This turns out to be quite complicated. Sec. 680-6(b)(1) generally allows fixtures to be as near as 5 ft to a pool (but no closer unless at least 12 ft above the water level). However, there are additional requirements that apply to fixtures closer than 10 ft. Both Sec. 680-6(b)(2) for lighting in the 5-10 ft zone, and the two exceptions to Sec. 680-6(b)(1) that allow closer distances for existing lighting and for indoor pools 7 1/2 ft or more above the water level require GFCI protection for such fixtures.
As noted in Fig. 2, GFCI devices as presently listed won't operate on a circuit with two ungrounded conductors. Therefore, these lighting systems are excluded between the 5-ft to the 10-ft radius around a pool unless the fixtures are rigidly attached to a structure at a point at least 5 ft above the maximum water level. They cannot be run at all, even on an existing structure, closer than 5 ft from the pool.
Nevertheless, we do see these lights used closer to swimming pools. These lights aren't installed under the provisions of Art. 411. This type of lighting operates on Class 2 power-limited circuits evaluated for wet-contact applications. Therefore, you install this type of lighting using listed Class 2 power supplies that typically don't exceed 75VA.
Landscape lighting
Another application of limited-energy lighting is in the path and accent lighting commonly used outdoors, as well as indoors in shopping malls, hotel atriums, and similar locations. These systems are listed to another standard entirely (UL 1838, Low Voltage Landscape Lighting Systems), and there are no plans to convert these systems over to the new UL 2108 standard. Because wet contact could be involved, these systems are limited to 15V, and UL already imposes the same 25A limitation as in the new Sec. 411-2. The output of one of these power supplies must never be connected in series or in parallel, and the maximum number and ratings of lighting units must never exceed the quantity specified in the listing and in the accompanying product directions.
The other issue with these systems is the location of the power supply. Installers need to be aware of the restrictions that come with various markings on these power supplies. For example, the "Outdoor Use Only" power unit cannot be used indoors. Although this seems obvious, we are used to assuming that if something can be used outdoors, it surely must be usable indoors. In this product category, only a power unit marked "Indoor Use Only" or "Indoor/Outdoor Use" (the other two possibilities) can be used indoors.
The power supplies will be arranged for a Chapter 3 wiring method connection, except outdoor units may be arranged for a cord- and plug-connection. In this case, the receptacle must be arranged to be weatherproof with the cord and plug connected, in accordance with Sec. 410-57(b).
These landscape lighting systems may also be used to supply submersible fixtures in fountains. In these cases, the transformer will have an isolated secondary to comply with Sec. 680-5(a), as required in Sec. 680-51(a) Ex. of the NEC. The lighting system will be marked "For Use With Submersible Fixtures Or Submersible Pumps." Be careful here! This product category is only for decorative fountains not intended for swimming or wading.
Note that underwater lighting fixtures intended for use in swimming pools are listed in a different category entirely. These fixtures, even if below 15V, must meet additional requirements. As in the case of Art. 411 systems, this wiring will originate at an isolating transformer. The secondary conductors from this transformer must never enter raceways or other enclosures with other conductors, however, in accordance with Sec. 680-5(c). GFCI protection is not required for these systems.
In general, however, these systems will be wired in the same way and using the same wiring methods as 120V lighting in the same environment. About the only other difference is that a potted, flush deck box can be used [Sec. 680-21(a)(4) Ex.], instead of the elevated swimming pool junction box required at line voltage. In any case, you must use a fixture listed as one of the forms of "Underwater Lighting Fixture For Swimming Pool." UL reserves the term "submersible" for underwater use in fountains, etc, not swimming pools and spas.