electronics
Why 12 V LED Strips Dim at the Far End: Voltage Drop Explained
Current in a 12 V LED run travels out and back, so the far end gets less voltage. The round-trip formula, worked numbers, and when thicker wire or 24 V fixes it.
You run an LED strip along a shelf, and the end nearest the power supply is bright white while the far end looks dimmer and a little warmer in colour. The strip isn’t faulty. Every metre of copper between the supply and the LEDs is a small resistor, the current passes through it twice (out on one wire and back on the other), and at 12 V there is very little voltage to spare. A half-volt loss hardly matters on a 120 V circuit. On a 12 V strip it is enough to make the dimming visible.
This post covers the formula, works through real numbers from our voltage drop calculator, explains why an LED dims much more than the voltage loss suggests, and lists the fixes that work: thicker wire, feeding power at both ends, or switching to 24 V.
The formula, and the easy-to-miss factor of 2
For DC and single-phase AC, the voltage lost in the wiring is:
Vdrop = 2 × L × I × R
L = one-way length (source to load), in km
I = load current, in A
R = conductor resistance, in Ω/kmThe 2 is there because the current makes a complete loop. The International Association of Electrical Inspectors puts it this way: the multiplier 2 “is added into the formula for the resistance of the conductor to the load and back.” The positive wire drops voltage on the way out, and the negative wire drops the same amount on the way back. The load sees what is left.
Because of that factor, our calculator asks for the one-way length and does the doubling itself. If you type the total length of both wires, the result will be twice the real drop. The same code handles three-phase circuits, where the multiplier is √3 instead of 2, and it applies a power-factor term for AC motors. For a 12 V strip you choose DC, and the formula reduces to the one above.
Why 12 V is so unforgiving
The drop in volts depends on the wire and the current. It does not depend on the system voltage. What you notice is the drop as a percentage of the system voltage:
| Drop | On a 12 V system | On a 120 V system |
|---|---|---|
| 0.5 V | 4.17 % | 0.42 % |
Low voltage also costs you a second time. For the same power, a 12 V load draws ten times the current of a 120 V load, so it drops ten times as many volts in the same wire. A 60 W load draws 5 A at 12 V and 0.5 A at 120 V. Over 10 m of 18 AWG copper, our calculator gives a 21.2 % drop at 12 V and 0.21 % at 120 V, a factor of 100. That is why RV, marine and solar wiring uses such thick cable for fairly modest loads.
Worked numbers: 18 AWG feeding a strip
18 AWG is a common gauge for the leads that come with LED strips and controllers. Here is what our calculator reports for copper 18 AWG. Unless noted, the conductor temperature is 75 °C, the calculator’s default (more on temperature below).
| Run (one-way) | Current | Supply | Drop | Drop % | Voltage at strip |
|---|---|---|---|---|---|
| 5 m | 3 A | 12 V | 0.76 V | 6.4 % | 11.24 V |
| 5 m | 3 A | 12 V (conductor at 20 °C) | 0.63 V | 5.2 % | 11.37 V |
| 5 m | 1.5 A | 24 V | 0.38 V | 1.6 % | 23.62 V |
| 10 m | 5 A | 12 V | 2.55 V | 21.2 % | 9.45 V |
| 10 m | 2.5 A | 24 V | 1.27 V | 5.3 % | 22.73 V |
The rows compare like for like: a 24 V strip of the same wattage draws half the current, so it drops half as many volts, and those volts are also a smaller share of a supply that is twice as high. Together that cuts the percentage to a quarter. QuinLED, a maker of DIY LED controllers, reaches the same conclusion and recommends that anyone starting from scratch “purchase 24v strip and power supplies.”
A 21 % drop is far beyond any guideline, but it’s an easy mistake to make when the power supply sits in a cupboard 10 m away. If you enter the 10 m, 5 A, 12 V run into our wire size calculator, it goes through the stocked gauges from thinnest to thickest and returns the first one that meets your limit:
| Limit | 12 V, 5 A, 10 m | 24 V, 2.5 A, 10 m |
|---|---|---|
| 3 % | 8 AWG (2.09 %) | 14 AWG (2.10 %) |
| 5 % | 10 AWG (3.32 %) | 16 AWG (3.34 %) |
| 10 % | 14 AWG (8.40 %) | 20 AWG (8.44 %) |
At every limit, the 24 V version gets away with a wire six gauge numbers thinner (8 → 14, 10 → 16, 14 → 20). A quarter of the drop allows a quarter of the copper, and every three AWG numbers halve the cross-section. (The calculator leaves out odd gauges such as 9 AWG, which are hard to buy, so it never recommends a size you can’t find.)
Why the LEDs dim more than the voltage does
A 6 % voltage loss doesn’t sound like much, so why is the dimming so easy to see? The answer is in how a 12 V strip is built. Waveform Lighting’s schematic notes describe the usual layout: “3 LEDs in series requires 9 volts for the LEDs and 3 volts for the resistor, bringing us to 12 volts”. Many of these three-LED groups are wired in parallel along the strip. The 24 V version uses six LEDs and two resistors per group.
Each group follows the same formula our LED resistor calculator uses:
I = (Vsupply − n × Vf) / RThe resistor only sees the headroom above the LED forward voltages, which is about 3 V out of 12. A voltage drop comes entirely out of that small headroom. With three 3.0 V LEDs and a 150 Ω resistor (the value our calculator picks for 20 mA at 12 V):
| Voltage reaching the group | Headroom | Current |
|---|---|---|
| 12.0 V | 3.0 V | 20.0 mA |
| 11.5 V | 2.5 V | 16.7 mA |
| 11.24 V (the 5 m, 3 A row above) | 2.24 V | 14.9 mA |
| 11.0 V | 2.0 V | 13.3 mA |
So a 6.4 % voltage drop becomes roughly a 25 % drop in current, and light output tracks current. In practice the effect is a bit smaller than this table shows, because an LED’s forward voltage also falls slightly as the current falls. Waveform’s page gives the example of an LED drawing less current when its voltage is reduced to 2.9 V. Still, the direction and the rough size are right: a resistor-limited strip amplifies voltage drop. HitLights lists the visible symptoms: warm-white strips turn “orangey at the end” and RGB strips show wrong colours.
The same calculator also shows how resistors are chosen. For three 3.3 V white LEDs at 20 mA on 12 V, the exact value is (12 − 9.9) / 0.02 = 105 Ω. Our calculator never rounds down, so it picks the nearest standard value at or above that: 110 Ω from the E24 series (19.1 mA) or 120 Ω from E12 (17.5 mA). Rounding down would push the LEDs past the current you asked for. It then recommends a power rating of at least twice the resistor’s actual dissipation. Here that is 40 mW, so a 1/8 W part is enough.
Fix 1: feed power at both ends
Many LED strip makers give the same advice: feed the strip from both ends. HitLights calls it the “best simple upgrade,” and QuinLED’s guide calls for a dual feed on 5 m strips. The arithmetic shows why it works so well. Our calculator treats the load as sitting at the end of the wire, so it can model the change: feeding from both ends turns one 10 m run carrying 5 A into two 5 m runs carrying 2.5 A each.
| Feed | Model | Drop at worst point |
|---|---|---|
| One end | 10 m, 5 A, 18 AWG, 12 V | 2.55 V (21.2 %) |
| Both ends | 5 m, 2.5 A, 18 AWG, 12 V | 0.64 V (5.3 %) |
The length halves and the current halves, so the drop falls to a quarter. The lowest voltage is now in the middle of the strip, not at the far end. The same quarter ratio holds for a strip’s evenly spread load. For a uniform load, the current in the conductor falls linearly to zero at the far end, so the drop at the end is exactly half of what a lumped load would give: 2 × r × I × L / 2. Feeding from both ends halves both L and I again, which gives a quarter of that.
One limit to be honest about: our calculator models the wire you choose, not the copper traces inside the strip. Those traces have resistance too. QuinLED describes cheap 12 V strips that fall from 12 V to “9v or even 8v” over 5 m of strip. Nothing you do to the feed wire removes that, so treat the strip maker’s maximum run length as a separate limit (HitLights says manufacturers often recommend about 5 m of continuous run at 12 V) on top of the wire calculation.
Fix 2: thicker wire, and why copper beats aluminium here
Resistance per metre is resistivity divided by cross-sectional area, so each step up in gauge buys a fixed improvement. Our calculator computes AWG resistance from the ASTM B258 diameter formula and annealed-copper resistivity (1/58 Ω·mm²/m). We checked those values against the resistances in NEC Chapter 9, Table 8 (2017 edition, as published here), which are given at 75 °C for uncoated solid copper:
| Size | Our value at 75 °C | NEC Table 8, solid Cu, 75 °C |
|---|---|---|
| 18 AWG | 25.48 Ω/km | 25.5 Ω/km |
| 14 AWG | 10.08 Ω/km | 10.1 Ω/km |
| 12 AWG | 6.34 Ω/km | 6.34 Ω/km |
| 10 AWG | 3.99 Ω/km | 3.984 Ω/km |
They agree to rounding. Stranded wire in the same table is about 2 % higher (7.95 vs 7.77 Ω/kFT for 18 AWG), which the calculator page mentions but does not add in.
Aluminium has 61 % of copper’s conductivity. The same 10 m, 5 A, 18 AWG run drops 35.0 % in aluminium against 21.2 % in copper, a ratio of about 1.65. Two AWG steps increase the cross-section by about 1.59×, so aluminium needs roughly two sizes larger to match copper.
Temperature matters too. Copper’s resistance rises by 0.00393 per °C above 20 °C, the constant our calculator uses. Table 8’s footnote gives the equivalent coefficient measured from 75 °C, 0.00323, and 0.00393 ÷ 1.216 = 0.00323, so the two agree. Between 20 °C and 75 °C the resistance of 18 AWG goes from 20.95 to 25.48 Ω/km, 21.6 % higher. Our calculator defaults to 75 °C, which its FAQ describes as the termination rating most sizing calculations assume. That makes the default the conservative case. If your wire is running cool, set it to 20 °C and compare, as in the 5.2 % vs 6.4 % rows above.
What “acceptable” means: 3 % and 5 % are advice, not law
The 3 % and 5 % figures come from the US National Electrical Code. The feeder version, 215.2(A) Informational Note No. 2 in the 2017 text, says conductors “sized to prevent a voltage drop exceeding 3 percent at the farthest outlet … and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, will provide reasonable efficiency of operation.” The same note points to 210.19(A) Informational Note No. 4 for branch circuits.
These are informational notes, which the NEC treats as explanatory material. As PDH Academy’s NEC 2023 overview puts it when summarising 90.5(C), such notes “are not enforceable portions of the Code.” Our calculator’s verdict reflects that. Its labels are “Within the 3 % branch-circuit guideline”, “Over 3 % — acceptable only if feeder + branch together stay within 5 %”, and “Over the 5 % total guideline”. Treat them as a design target. For LED lighting, the real test is whether you can see the difference.
Voltage drop is only half of wire sizing
A wire can be thick enough for voltage drop and still too thin to carry the current safely. Our wire size calculator sizes for voltage drop only and says so in a box under every result. Next to its pick it shows a reference ampacity for copper AWG, taken from the NEC Table 310.16 75 °C column (14 AWG 20 A, 12 AWG 25 A, 10 AWG 35 A, 8 AWG 50 A; values checked against a Thomas & Betts reprint of the table). It then caps those figures with the NEC 240.4(D) small-conductor limits, which in the 2017 text are “14 AWG Copper. 15 amperes”, “12 AWG Copper. 20 amperes”, and “10 AWG Copper. 30 amperes”.
Short, high-current runs are where this matters. Take 25 A at 12 V over 1 m with a 5 % limit. By voltage drop alone, 14 AWG passes (4.20 %). The calculator then warns that 25 A is more than 14 AWG’s 15 A reference limit and points you to 10 AWG. As a rule, long runs are limited by voltage drop and short runs by ampacity. Use whichever size is larger. The ampacity column is the building-code table for copper only. Our calculator doesn’t show ampacity for aluminium or metric sizes, and it doesn’t cover vehicle or marine wiring standards, bundling, or insulation ratings. The page recommends checking with your code or an electrician.
Sizing the rest of a 12 V system
- Start with the voltage drop calculator for the wire you already have, then use the wire size calculator to find the smallest stocked gauge that meets your limit.
- If you need the current from a strip’s watts-per-metre rating, the Ohm’s law calculator solves V, I, R and P from any two of them.
- If you are wiring bare LEDs instead of a finished strip, the LED resistor calculator gives the E12/E24 value, the actual current and the power rating, and the resistor color code calculator shows which bands that value carries so you can pick it out of a parts bin.
- For a battery-powered installation, the battery runtime calculator estimates hours from capacity, depth of discharge and converter efficiency, with an optional Peukert correction for lead-acid.
- If the lights have to stay on through power cuts, the UPS runtime calculator estimates backup time from the UPS battery and the load in watts.
All of these run in your browser. If a 12 V run is long or carries a lot of current, check it with the voltage drop calculator before you buy wire, and consider 24 V if you are choosing a new strip.