LED Cable Size for 12V and 24V Signs: Calculate Current and Voltage Drop

1. Start with the load on each branch

Draw the driver, each feed cable and the modules or strip connected to that feed. Write down the actual DC voltage and the watts on each branch. Use the selected product’s current specification where available; watts divided by volts gives a useful nominal estimate. Do not divide a whole-sign load equally unless the branches really carry equal loads.

For example, an illustrative 48W branch draws 4A at 12V. An equivalent 48W load designed for 24V draws 2A. This comparison is between compatible products at their specified voltages. It is never permission to connect 12V modules to a 24V supply. Include controllers, dimmers and connectors in the current-capacity check.

Power-supply headroom and cable selection solve different problems. A larger driver can provide more current, but it does not remove resistance in a long thin cable. Confirm the supply’s capacity and operating conditions using its own datasheet, then assess each outgoing circuit separately.

2. Calculate the outgoing and return cable loss

For a simple two-conductor copper feed with its load at the far end, an illustrative estimate is voltage drop = 2 × one-way length × current × copper resistivity ÷ conductor area. Use metres, amps and square millimetres. A commonly used approximate copper resistivity at 20°C is 0.0175 ohm·mm²/m. Actual resistance depends on material, temperature and the cable specification; use the manufacturer’s resistance figures for design.

The factor of two includes both the positive and negative conductors. A 5m route therefore contains about 10m of conductor in the circuit. This simplified estimate excludes terminals, connectors and internal strip resistance. It also assumes a concentrated end load: distributed modules and multiple feed points need a calculation appropriate to their actual arrangement.

The examples below are arithmetic illustrations, not approved cable ratings or a universal acceptable-drop table. Check the voltage required at the LED input against its manufacturer’s limits, and allow for real connection losses and operating temperature.

3. Check current capacity independently

Passing a voltage-drop calculation does not establish that the cable can carry the current safely. Obtain the exact cable’s conductor construction, insulation temperature limit, permitted installation conditions and current-rating information. Check how enclosure temperature, grouping and the proposed route affect the rating. A cross-sectional area printed on a reel is only part of that assessment.

Check every part of the path: power-supply terminals, distribution connectors, controller channels, joints and the product’s own lead wires. A thicker feed does not increase the rating of a small downstream connector. Branch protection needs to suit the complete circuit and fault conditions, as determined by the competent installer.

Glowfy lists a red-and-black 2-core 0.5mm² cable for LED projects. Treat that as a particular cable option, not proof it suits every load or length. Confirm the current product information and request any missing cable data before specification. Colour helps identify polarity but does not establish conductor material, insulation suitability or current capacity.

4. Improve the layout before oversizing everything

If the calculated loss is excessive, compare a shorter feed, a larger conductor or a split into separately supplied branches. Moving the driver closer can reduce cable loss but must preserve appropriate access, cooling and environmental protection. Keep the driver within its installation instructions rather than hiding it in an unsuitable small space.

For a nominal 48W sign divided into two genuinely equal 24W branches at 12V, each branch draws about 2A. Each cable’s length and rating still need checking, and the common upstream path must carry their combined current. Splitting a drawing into two lines only helps if the physical circuit and protection are actually designed that way.

Additional strip feed points must follow the strip and controller instructions. Do not parallel independent power-supply outputs unless the manufacturer explicitly supports that operation. For detailed feed arrangements, use Glowfy’s voltage-drop and power-injection guide alongside the chosen product’s installation documentation.

5. Verify under the real operating load

After installation, a competent person should check voltage at the driver output and the load input under representative operating conditions. An unloaded cable measurement can look normal because very little current is flowing. Record the lighting state, connected load and measurement locations so a later comparison means something.

If the measured loss is higher than expected, investigate joints, conductor construction, routing and the real current. Localise the loss rather than turning up the driver beyond its permitted setting or the LEDs’ voltage limits. Check for loose, corroded or damaged connections with the equipment safely isolated as appropriate.

The HSE explains that electrical equipment must suit its use and environment and be maintained in good condition. Low DC voltage does not make high-current wiring immune to overheating. Mains connections, protective measures and electrical verification belong with a person competent for the work.

• Record branch watts, nominal voltage and design current.
• Record the one-way route length and complete circuit arrangement.
• Keep cable resistance, conductor area and current-rating evidence.
• Check all connector ratings and the required branch protection.
• Measure driver and load voltage under load and retain the readings.
• Label the branches and keep an as-installed wiring sketch.