Fuse and Cable Selection for a 12 V LED Sign
Your team is installing a 12 V DC LED sign for a shop. The sign has three parallel strips, each rated 0.8 A at 12 V. The 12 V, 5 A power supply will sit 6 m from the sign. The only wire in the store is 0.5 mm² (current rating about 3 A, resistance about 36 mΩ per metre per conductor) and 1.5 mm² (rating about 10 A, about 12 mΩ per metre per conductor). A 3 A glass fuse is available. A trainee proposes using the 0.5 mm² wire because 3 A is greater than the 2.4 A total load, and fitting the 3 A fuse. Before approving, you must check voltage drop at the sign and whether the fuse will hold. The LED strips are sensitive to supply voltage; below about 11.0 V they dim noticeably. The power supply has a short-circuit shutdown but no remote sensing.
1. Calculate the total load current and the voltage drop at the sign for both wire sizes. Which wire meets the 11.0 V limit?
Read the worked answer
Total load = 3 × 0.8 A = 2.4 A. The current travels 6 m out and 6 m back, so the loop is 12 m.
For 0.5 mm²: R = 0.036 Ω/m × 12 m = 0.432 Ω; Vdrop = 2.4 A × 0.432 Ω = 1.04 V. The sign sees 12.0 − 1.04 = 10.96 V, which is below 11.0 V.
For 1.5 mm²: R = 0.012 Ω/m × 12 m = 0.144 Ω; Vdrop = 2.4 × 0.144 = 0.346 V. The sign sees 11.65 V, which meets the limit with margin.
Only the 1.5 mm² wire is acceptable. The 0.5 mm² result is marginal and will get worse if the supply is slightly low or the connections add resistance.
2. Evaluate the proposed 3 A fuse. Recommend a wire and fuse combination, with reasoning.
Read the worked answer
At 2.4 A, a 3 A fuse is loaded to 80% of rating. Glass fuses are normally derated for continuous load (about 75%), and LED drivers can draw inrush current, so nuisance blowing is likely.
Use 1.5 mm² wire, which is rated about 10 A. A 4 A fuse protects that wire and gives headroom for inrush. A 3 A slow-blow fuse could be used only if the measured inrush is below 3 A. Do not use the 0.5 mm² wire.
After installation, measure the actual current and the voltage at the sign under load. A voltage check at the power supply alone is not enough because it does not include the cable drop.
3. The trainee also suggests a 5 A fuse because the supply is rated 5 A. Why is that unsafe?
Read the worked answer
A fuse protects the wire, not the power supply. A 5 A fuse on 0.5 mm² wire rated 3 A would allow 5 A to flow during a fault, overheating the wire before the fuse opens. The supply can deliver 5 A, so it will not limit the current.
A 5 A fuse is also poorly matched to a 2.4 A load: it is loaded to only 48%, so a partial fault may not blow it quickly. The fuse rating must be above the normal load but not above the wire ampacity. That is why 1.5 mm² wire plus a 4 A fuse is the safer choice.