12 V Floodlight Voltage Drop on a Long Workshop Run
A vocational trainee is installing a 12 V, 60 W LED floodlight at the far end of a workshop. The 12 V, 10 A SMPS is mounted 18 m away along the cable route. The floodlight is wired with 2.5 mm² twin-core copper cable. With only the floodlight running, a clamp meter reads 4.9 A and a DMM at the lamp terminals reads 11.1 V; the SMPS terminals read 12.3 V. A 4 A bench load is then plugged into a socket fed from the same cable at the far end. The floodlight dims visibly and its terminal voltage falls to 10.2 V. The trainee proposes replacing the 10 A SMPS with a 15 A unit. The supervisor asks for a measurement-based judgment before any replacement and reminds the trainee that the cable is buried in conduit and cannot easily be enlarged.
1. Calculate the cable loop resistance from the first measurement and predict the lamp voltage when both loads run. Does the evidence support a cable-drop explanation?
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Use the first test: the drop is 12.3 V - 11.1 V = 1.2 V at 4.9 A. Loop resistance R = V/I = 1.2/4.9 = 0.245 Ω. The per-conductor resistance is half this, 0.1225 Ω over 18 m, i.e. about 0.0068 Ω/m, which is close to 2.5 mm² copper. With both loads, the cable carries 4.9 + 4.0 = 8.9 A. Predicted drop = 8.9 × 0.245 = 2.18 V, so lamp voltage ≈ 12.3 - 2.18 = 10.1 V. That matches the measured 10.2 V. Cable loss = I²R = 8.9² × 0.245 ≈ 19.4 W. The SMPS terminal voltage may sag slightly, but the dominant effect is the long cable. So yes, the evidence supports cable drop, not an SMPS current-capacity problem.
2. Evaluate the proposal to fit a 15 A SMPS. Recommend a practical remedy and justify it with one calculation.
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The existing 10 A SMPS already has more current capability than the 8.9 A load. A 15 A unit would not reduce the 2.18 V cable drop; it could even regulate a little differently, but the cable is the bottleneck. To keep drop to 0.5 V at 8.9 A, the loop resistance would have to be R ≤ 0.5/8.9 = 0.056 Ω. The present 0.245 Ω is about 4.4 times too high. Even 10 mm² copper over 18 m would give roughly 0.066 Ω loop, still about 0.59 V drop, so a cable change alone is marginal. The practical fix is to move the 12 V SMPS close to the floodlight and run the 230 V primary to it, or use a local 12 V supply; keep a short 2.5 mm² tail. Alternatively, use a 230 V LED floodlight. The 15 A SMPS should not be purchased as the fix.
3. What checks and safety points should be completed before altering the installation?
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Measure supply voltage and lamp voltage at the same time, and clamp the current in each conductor. Check terminal tightness, connector rating, fuse/breaker rating, cable insulation and whether the cable is rated for the conduit temperature. Confirm the SMPS can deliver 8.9 A continuously and that its terminals accept 2.5 mm². The 19.4 W cable loss is heating the conduit, so do not simply increase the fuse. Isolate and prove dead before re-terminating; 12 V is not a shock hazard but the 230 V primary is. Verify polarity, glanding and strain relief. After changes, repeat the load test and check that lamp voltage stays within the LED driver's range.