LED strip power supply and voltage drop

How big a power supply do I need for an LED strip?

Multiply the watts per foot by the length, then add some headroom. A 16 ft strip rated 4.4 W per foot uses 70.4 W, which is 5.9 A at 12 V. With 20% headroom the supply should deliver at least 84.5 W, or 7.0 A. Drivers last longer when they do not run at their full rating all the time; 20% is common vendor advice, not a standard. Use the power printed on the strip's label or data sheet, because the watts per foot depend on the LED chips and the density of the LEDs.

Why does the far end of my LED strip look dim or yellow?

The two copper rails of the strip have resistance, and the current for all the LEDs flows through the first stretch. The voltage falls along the strip, so the last LEDs get less than the first. With the example numbers the far end is at 10.4 V instead of 12 V when fed from one end, a drop of 13.6%. HitLights lists dimmer LEDs at the far end, color shifts (red dominates on RGB strips) and flicker as the signs. The usual fixes are to feed the strip from both ends, to use a heavier wire for the feed, or to use a 24 V strip.

What does feeding from both ends do?

It cuts the strip's own voltage drop to a quarter. In our model the current is spread evenly along the strip. Fed from one end, the first section carries the whole current and the drop at the far end is I × r × L ÷ 2, where I is the total current, r the resistance of both rails per foot and L the length. Fed from both ends, each end supplies half the current for half of the length and the lowest voltage is in the middle, I × r × L ÷ 8. In the example the drop in the strip falls from 1.41 V to 0.35 V. The feed wires must be separate and run all the way to each end; a link along the strip does not help.

Is 24 V better than 12 V for long runs?

For the same watts, 24 V draws half the current, and the drop in volts is half of that of 12 V while the supply voltage is twice as big, so the percentage falls to a quarter. In the example, the same strip fitted for 24 V (with the same rail resistance, which real 24 V strips do not always have) loses 3.4% instead of 13.6%. HitLights gives about 16 ft (5 m) as the longest single run at 12 V and about 32 ft (10 m) at 24 V. These are vendor figures, so check the data sheet of your own strip.

What wire gauge should I use to feed the strip?

Pick the wire so that the drop in it is small compared with the supply voltage. The wire has a loop of two conductors, so the drop is the current times twice the length times the resistance of one conductor. Copper 18 AWG is 6.38 ohms per 1,000 ft and 12 AWG is 1.59 ohms per 1,000 ft. For 5.9 A over 3 ft, the calculator asks for 20 AWG or thicker to stay under 3%; HitLights suggests 14 or 12 AWG instead of 18 AWG on long runs. For a feed of more than a few feet, use the wire size calculator or the voltage drop calculator.

Can I measure my strip's resistance myself?

Yes, and this is the best way if the data sheet does not give it. With the strip unplugged, join the plus and minus pads at the far end with a short wire. Then measure the resistance between the plus and minus pads at the near end with a multimeter. That is the resistance of the two rails in series over the full length; divide by the length to get ohms per foot, and enter it in the "both rails" field. Shorting the far end also shorts out the LEDs there, so the reading is the copper alone. Make sure the strip is unplugged and no controller is connected. This is our own method, not a manufacturer procedure.

How the numbers are worked out

Power is the watts per foot times the length: 4.4 × 16 = 70.4 W. Current is power divided by the supply voltage: 70.4 ÷ 12 = 5.9 A. The supply size is the power times one plus the headroom. For the voltage drop the calculator assumes that the current is spread evenly along the strip. That gives a drop at the far end of I × r × L ÷ 2 for one-end feeding, from integrating the current that is left after each LED along the length (our derivation). The feed wire adds I × 2 × length × R, where R is the resistance of one conductor of the wire: with 3 ft of 18 AWG that is 0.22 V. In total the example loses 1.63 V of 12 V (13.6%) fed from one end and 0.46 V (3.9%) fed from both ends.

Worked example: 16 ft of 12 V strip

Take 16 ft of a 12 V strip at 4.4 W per foot, with rails measuring 0.03 ohm per foot (an example value, not a measured one), and a 3 ft run of 18 AWG to the supply. The strip uses 70.4 W and 5.9 A and wants a 85 W supply. Fed from one end, its far end gets 10.37 V, a drop of 13.6%. Fed from both ends with two feed wires, the lowest voltage is 11.54 V, a drop of 3.9%. For a 5% drop in the strip alone, the longest run is 10.4 ft from one end and 20.9 ft from both ends. If your strip's rails have a different resistance, the answers move in proportion to it.

Limits of this method

The model is an upper bound. A strip with a resistor for each LED group draws less current as its voltage falls, so the drop in the real strip is somewhat lower than computed, and the LEDs at the end are dimmer by an amount that depends on the strip's design (constant-current chips behave differently from resistor strips). The rail resistance varies from one strip to another, and cheap strips use thinner copper; HitLights advises looking for 2 oz or thicker copper layers. The copper figures here are for 20 °C with annealed copper at 1.7241 × 10⁻⁸ ohm-metres, which is 2.6% higher than the 1.68 × 10⁻⁸ used by the wire resistance calculator, and warm wire has about 0.4% more resistance for each degree Celsius. Finally, the 20% headroom and the 5% drop limit are working figures that you can change, not rules; for mains-voltage wiring, the NEC and your local code decide, not this page.

Sources: HitLights, “Voltage Drop in Long LED Strip Runs” (a vendor article, read October 28, 2026: the 16 ft and 32 ft run limits, feeding both ends, 14 or 12 AWG feed wire and 2 oz copper; its figures are the vendor's); the 20% supply headroom is common vendor advice (a vendor technical reference seen in search results, not opened); ASTM B258 for AWG sizes and annealed copper at 100% IACS for the resistance of wire; the voltage drop and both-ends formulas are our derivation from Ohm's law. For the resistor of a single LED use the LED resistor calculator; for a long feed see the voltage drop calculator, the wire size calculator and Ohm's law calculator. How we check our calculators.