24 V LED Tape: Voltage Drop, Plenum Routing, and Class 2 Separation
A tenant fit-out in a St. Paul retail bay specifies 24 VDC LED tape under twelve shelf units. The listed Class 2 driver (100 W, 24 V, 4.2 A output) sits in a utility closet. The longest run is 62 ft of 14 AWG two-conductor cable to the first tape section, plus 11 ft of 18 AWG factory lead on the tape itself. The installer reports the far end of the tape is visibly dimmer than the near end and flickers when dimmed below 30 percent. Total connected tape load is 96 W. The ceiling over the sales floor is an open plenum used as a return-air path. The installer proposes three fixes: turning up the driver's output trimpot to compensate, running the remaining Class 2 cable through the plenum with the same CL2 riser-rated cable already on the truck, and relocating the driver into a plenum-rated junction box above the ceiling to shorten the 24 V run. The owner's standard requires 20 percent spare capacity on power supplies. The plan also shows the 24 V cable sharing a 1 in. EMT with the 120 V feed to the driver for 25 ft.
1. Quantify the voltage drop and evaluate the trimpot fix. What do you recommend and why?
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Compute the loop. 14 AWG copper is about 3.07 ohms per 1000 ft, so 124 ft of loop (62 ft out and back) is 0.38 ohm. The 18 AWG lead is about 6.5 ohms per 1000 ft; 22 ft of loop adds 0.14 ohm. Total 0.52 ohm. At 96 W / 24 V = 4.0 A, the drop is 4.0 x 0.52 = 2.1 V, roughly 8.7 percent. The far tape sees about 21.9 V while the near tape sees about 23.8 V, which is exactly the reported symptom.
The trimpot is the wrong fix. It raises both ends, drives the near tape above its 24 V rating, may exceed the driver's listed output adjustment range, and leaves the spread unchanged because the ratio is fixed by resistance. It also moves the installation outside the basis of the listing.
Options that work, cheapest first: split the load into two 48 W runs with shorter home runs; relocate the driver toward the middle of the load; or upsize to 12 AWG (0.24 ohm, about 1.0 V drop) or 10 AWG (0.15 ohm, about 0.6 V). Note the 100 W driver at 96 W leaves 4 percent spare against the owner's 20 percent standard, so a driver change is likely anyway. A single 150 W supply is no longer a Class 2 source (Class 2 output is capped at 100 VA), which changes the wiring method and separation rules; two 60 W Class 2 drivers is usually the cleanest compliant answer.
2. Evaluate the plenum routing and the proposal to relocate the driver above the ceiling.
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The ceiling is 'other space used for environmental air,' so 300.22(C) governs. The CL2 riser-rated cable is not listed for that space and cannot be installed exposed there; it must be a plenum-listed cable (CL2P/CL3P) or must remain inside a wiring method the section permits, such as EMT. That gives a useful nuance: if the cable stays completely inside continuous EMT, the raceway is what 300.22(C)(2) recognizes, so a riser-rated cable inside the raceway can be acceptable. The exposed whips to each shelf are the failing part of the proposal.
Relocating the driver above the ceiling needs more than a plenum-rated box. The driver must be identified for the ambient and the air space, and it must remain accessible for service under 314.29. Most LED drivers are not listed for plenum installation, and a plenum-rated box does not cure an unlisted driver. Keeping the driver in the closet, or in a listed enclosure outside the return-air path, is the defensible design. Ask the AHJ before the ceiling closes, because the return-air path is often the deciding fact in a fit-out inspection.
3. Is the 25 ft of shared EMT with the 120 V feed acceptable? Explain the basis.
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No, treat it as a violation rather than an exception. The driver is a listed Class 2 source, so everything on its output side is a Class 2 circuit, and 725.55 limits how Class 2 conductors may share a raceway or enclosure with electric light and power conductors. The recognized exceptions are narrow: factory-assembled equipment, and limited allowances connected with the power conductors that supply the Class 2 equipment, both of which are read strictly by reviewers and generally carry a voltage-insulation condition.
A field-installed 25 ft EMT containing the 120 V feed and the 24 V output is not one of those clean cases. The practical fixes are a separate raceway for the Class 2 output, a listed hybrid cable, or moving the driver so the outputs never enter the line-voltage raceway. Inside the driver enclosure, follow the manufacturer's own separation as listed. If you want to argue the exception, get it in writing from the AHJ before rough-in rather than defending it at final inspection.