The kitchen in this house came with a run of track lighting over the island, and two of the four heads had died by the time we'd been here a year. Replacing them looked like the easiest job on the list. I measured nothing, photographed nothing, and ordered three heads online that looked exactly like the ones on the ceiling. Two of them would not seat at all — they turned about a third of the way and stopped. The third clicked in beautifully and never lit. I had bought hardware for a different track system, which I did not know was a category of mistake you could make, and the fix started with reading a label I had never looked at in three years of standing under it.
Three profiles that look identical from the floor
Track is not a generic product with a generic slot. The residential market settled long ago on three incompatible profiles, each named after the company that developed it: H, after Halo; J, after Juno; and L, after Lightolier. From below, all three are an aluminum channel with a groove down the middle. Inside the groove, they differ in the thing that matters, which is where the electrical contacts sit.
H-type track carries three contacts, two live and one ground. J-type uses two contacts spaced differently from either of the others. L-type also uses two, in a layout unique to it. A head built for one profile lands its contacts in the wrong place on another, which produces exactly the three outcomes I collected: it refuses to twist in, it twists in and makes no connection, or — the one worth avoiding — it engages partially and makes a connection that was never designed or tested to be made.
The label that actually settles it
Track systems are evaluated and certified as systems rather than as loose parts, and the way that gets communicated is a pair of matching warnings. UL's Luminaires Marking and Application Guide describes them as correlation markings, and they work in both directions. Each track section is marked to say that only luminaire assemblies marked for use with a named track should be installed on it. Each head carries the mirror image, saying it is to be used only with that same named track. The blank in both markings is filled with the manufacturer's name and the series number of the track — not with a letter.
That distinction is the practical heart of this. Retail listings sell heads as H-compatible or J-compatible, and that shorthand is usually good enough to get you a head that mechanically fits. The marking on your ceiling is narrower than the shorthand, because it names a specific series from a specific maker. When the two disagree, the label on the track is the one that was tested.
The same guide notes that each carton of track parts ships with instructions identifying which system the part belongs to, and each track section's instructions carry the series number or model name, the electrical ratings, the mounting method, and the spacing between supports. All of which is to say the information exists in writing; it just tends to leave the house with the packaging.
Places the Code will not let track go
Track is a permitted wiring method with a short list of prohibitions, and several of them come up in ordinary houses. Section 410.151(C) rules out lighting track where it is likely to be subject to physical damage, in wet or damp locations, where exposed to corrosive vapors, in hazardous locations and storage battery rooms, where concealed, where extended through walls or partitions, and lower than five feet above the finished floor unless it is protected from damage or operating below 30 volts.
Translated into rooms: a covered porch or a bathroom ceiling over a shower is a damp or wet location and not track territory. Running one length of track through a wall so it continues in the next room is specifically not allowed. Neither is tucking track above a soffit or inside a valance where it ends up concealed. And in a garage or basement workshop, track mounted at working height rather than on the ceiling runs into the five-foot rule.
There's one more, and it's the one people trip over in bathrooms. Track is prohibited inside the zone that Section 410.10(D) draws around a tub or shower — three feet horizontally and eight feet vertically from the top of the tub rim or the shower threshold. That zone includes the space directly overhead, which rules out the pleasant-looking idea of a short run of adjustable heads above the tub.
Support is a weight question, not a screw question
Track has to be fastened securely, and Section 410.154 puts it in terms of load rather than hardware: each support must be suitable for the weight of the luminaires that can be installed on it. Note the phrasing — not the heads that are on it today, but what the track can hold. Heavier heads, or more of them, change the answer, and the spacing between supports comes from the track manufacturer's own instructions rather than from a rule of thumb about studs and joists. A track fastened only with drywall anchors is a common finding and a bad one, because the failure mode is the whole run coming down at once.
Where a track job stops being a homeowner job
Swapping a head on an existing, correctly matched track is a genuinely small task with the breaker off. Almost everything else on a track system has a marking that says otherwise. Only track systems designed for it may be cut to length in the field, and those ship with instructions describing how; if yours didn't, the run isn't meant to be shortened. Intercept connectors that turn a straight run into an X or a T, the kind provided with breakaway ground tabs or intended for field rewiring, are marked in plain language as intended for installation by a qualified electrician only. Track systems that need high-temperature supply wire are marked with the minimum rating that connection requires, which is a question about the cable in your ceiling and not about the fixture. And a track not meant to hang on stems or cables is marked not to be pendant mounted, which is worth reading before a design idea becomes a purchase.
How to buy heads without guessing
Do this before ordering anything. Turn the circuit off, take one existing head down, and read the marking on the head and on the track itself — the text is usually on the inside of the channel or near an end cap. Photograph both, and write the manufacturer, the series number, and the track's electrical rating into your house records, because that page is what makes the next replacement a five-minute errand. Count the heads you plan to run and check their combined wattage against the track's rating rather than against the breaker's. Then buy to the marking, not to the letter, and if a head resists going in, take it back out — a head that has to be forced is telling you it belongs to a different system, and the answer is never a firmer twist.