You may be surprised at how many conductors can pass through many fixture wiring compartments without requiring identification for through-wiring suitability.
We commonly wire from one recessed fixture to the next, rather than going back to an outlet box for each fixture. You'll often want to do that in order to save on materials and labor. This process, often called "daisy chaining," involves using a Chapter 3 wiring method to go from fixture to fixture. We often see this process misinterpreted as requiring the recessed fixtures to have junction boxes identified for through-branch circuit wiring.
You'll find the general concept in the second paragraph of Sec. 410-11, which allows through-wiring as long as the fixture itself is "identified" for this purpose. Art. 100 defines this as being generally recognizable and suitable for a particular application, and doesn't necessarily mean a marking. In this case, however, the terms of applicable product listings require actual markings on the fixtures. In both a raceway or junction box, you must know how to apply marking provisions for through-wiring when you see them.
Testing laboratories take a narrow view of what through-wiring actually is. They routinely evaluate fixture wiring compartments assuming you'll be wiring from one fixture to the next. You only have to look for a through-wiring listing if you're planning on running additional conductors through the fixture compartment not related to the fixture.
There is a corollary to this principle. Suppose you have evaluated your fixture for a specified number of through-wiring conductors. In figuring that number, you can safely ignore the circuit conductors supplying the lampholder or ballast. In fact, you cannot only ignore those supply conductors that come into the fixture, you can ignore their counterparts that leave the fixture and go on to the next.
For example, suppose you want to carry a 3-phase 4-wire multiwire circuit through a wiring compartment, in addition to the actual lighting circuit supplying this fixture and then going on to the next. Assume you're using a raceway wiring method, and the conductors carried straight through are unspliced. We see a number of plausible alternatives as to how to count those wires against a conductor limitation displayed on the fixture:
Take the wires as numbering four. Usually the reasoning here is Sec. 370-16(a)(1) counts such unspliced conductors only once. But this is beside the point. The applicable product standards count such conductors as entering the box and again as leaving the box. Put another way, the conductor fill marking limitations don't discriminate between raceway and cable wiring methods, where all the conductors necessarily terminate. Note: Fixtures that have these allowances usually undergo mechanical stress tests to be sure they'll take the forces involved in pulling wire. However, a manufacturer need not submit the fixture for these tests, if it will be marked as suitable for cabled wiring only.
Take the wires as numbering12, to include the actual lighting circuit, in and out. Usually the reasoning here also goes back to Sec. 370-16(a)(1), since similar conductors are counted. True again, but this is also beside the point.
Take the wires as numbering eight. That's the correct answer, but be sure you got it the right way. First, ignore the supply and load side conductors of the branch circuit actually supplying the fixture. Also ignore the fixture wire entering the compartment. Then, count the through-circuit conductors once upon entering, and again on exit, for the result of eight.
Fig. 1, on page 48D of the original article, shows the sort of label you will need to look for on this fixture. Testing labs usually require this type of label to show a fixture has been tested to recognize the heat contributions under load of other branch circuit conductors passing through the wiring compartment.
Multiwire branch circuits add another dimension to this (see box below). Sec. 210-4(a) allows, but does not require, you to consider multiwire branch circuits as multiple circuits. Current testing laboratory interpretations effectively consider a multiwire branch circuit as one circuit, so the entire multiwire branch circuit doesn't have to be counted as through-wiring.
For example, take a typical configuration involving a 3-circuit 4-wire multiwire branch circuit (208Y/120V, 20A circuits) used for commercial lighting. If you want to "daisy chain" the fixtures, look at eight No. 12 AWG conductors, four in and four out, at least near the beginning of the run. All of these conductors are part of the same multiwire branch circuit.
Because of this, you should classify none of them as "through-wiring." In other words, a recessed fixture junction box that is listed for eight through-branch-circuit conductors is actually rated to carry eight (four in and four out) through-branch circuit wires in addition to the conductors actually supplying the connected fixtures. Don't make the mistake of counting the conductors of the lighting circuit, including conductors from other legs of the same multiwire branch circuit, against the conductors against the through-conductor listing limitation. If you do, you'll be needlessly shortchanging yourself in terms of the capacity of the fixture wiring compartments just installed.
If you're using a raceway wiring method, such as conduit or EMT, this means you might be looking at a second run of pipe in order to supply loads downline from the fixture installation. Actually, the run of conduit connecting the fixtures together could have served the dual purpose of connecting the fixtures as well as enclosing additional circuit conductors-perhaps even loads unrelated to the lighting.
Obviously, if an inspection authority rejects your installation because of the misinterpretation, you'll have to appeal or rewire. And you'd just as soon do neither. So why is there such confusion in the field about through-branch circuit wiring? Where does it stem from? One problem is in the language of Sec. 410-31 of the NEC. Both contractors and inspectors often misinterpret the intent of its language, negating the usefulness of the through-branch circuit rating. Sec. 410-31 (Fixtures as Raceways) states "fixtures shall not be used as a raceway for circuit conductors." This is a flat negative statement; however, the section's intent is really in its three exceptions that follow. Ex. 2 gets to the heart of the matter:
...fixtures designed for end-to-end assembly [as in troffers] to form a == continuous raceway or fixtures connected together by recognized wiring methods...shall be permitted to carry through conductors of a 2-wire or multiwire branch circuit supplying the fixtures.
This recognition of some additional circuits by right, without requiring identification as a raceway, is why the circuits (2-wire or multiwire) directly associated with the fixture have been immune to through-wiring fill restrictions. Sec. 410-31 does not cover many recessed fixtures because you won't use them as a raceway, even though you may plan on using them for through-wiring. Nevertheless, you'll find manufacturers and testing labs using this approach for stand-alone fixtures as well. The concept of through-wiring doesn't necessarily imply use as a raceway, although it certainly can.
You may find old-work fixtures equally confusing. Through-wiring applications don't involve a remodeling-type (old-work) fixture connected to a length of cable. For that reason, these fixtures may have no conductor fill marking. However, don't jump to the conclusion that you can't "daisy-chain" these fixtures. As we've seen, you can because those conductors don't count as through-wiring.
The drawings show actual labels from recessed fixtures. Eight conductors, unrelated to the fixture circuit conductors (multiwire or otherwise), could be added to wiring compartments.
Because of product standard inconsistencies over time, the word "through" may or may not be present. Under the applicable product standards, both labels mean the same thing. However unintuitive, testing laboratories judge the label that says "maximum of 8 No. 12 AWG branch circuit conductors" to allow an additional multiwire or 2-wire circuit in and out of the fixture.
Electrical contractors and inspectors must be aware that through-wire markings indicate the number of conductors that can be passed through an outlet box and continued on, and should not count as part of the wires supplying the fixtures, even in the case of multiwire branch circuits. If you keep this firmly in mind, you'll use the full electrical system capacity meant to be achieved, while eliminating job site confusion and delays.