Tool / Run and driver
A voltage drop calculator that sizes the driver at 80 % and flags any feed over 3 %.
This is the LED strip voltage drop calculator we use on every load sheet. Pick the strip, enter the run in metres, the wire gauge and how far the driver sits from the first cut. You get total watts, the driver size, the current on the feed and the drop in volts and percent. The formula is printed underneath so you can check our arithmetic.
LED strip driver sizing, worked live.
Numbers update as you move a slider. The starting values are the 7.2 m kitchen run from our worked sheet: COB strip at 10 W/m and a 4 m feed of 18 AWG. That comes out at 72 W, a 100 W driver and 2.1 % drop.
Feed drop is over 3 %. Go up one wire gauge, move the driver closer, or split the run onto a second feed. The far end will look warmer and dimmer than the near end if you leave it.
Feed drop under 3 %. No change needed on this feed.
Send this sheet copies the figures into the spec field of the parts list form. Nothing is ordered and nothing is priced until we reply in writing.
Total load W = run length (m) × strip watts per metre
Driver minimum rating = W ÷ 0.8, rounded up to a stocked 100 W or 150 W unit, or a multiple of them
Current I = W ÷ V, where V is 24 or 48
Feed drop Vd = 2 × L × I × R, where L is feed distance in metres and R is ohms per metre of one conductor (18 AWG 0.02095, 16 AWG 0.01317, 14 AWG 0.00828). The 2 is there because current goes out on red and back on black.
Drop % = Vd ÷ V × 100, kept under 3
Feeds = number of separate runs × ceil(length per run ÷ max single-feed length), where max single-feed length is 10 m on 24 V strip and 20 m on 48 V strip
Before the numbers
Measure the run where the strip will sit, not the counter.
The counter is always longer than the light. Cabinets have end panels, a fridge housing, a corner blind. Run the tape along the underside of the cabinets, set back about 50 mm from the front edge where the channel goes, and tick each end. That is your run length.
Then measure the feed. People measure the wall from driver to cabinet and forget the cable goes up a stud bay, across a soffit and down again. A 1.5 m straight line is often 4 m of cable. On 18 AWG at 3 A that difference alone moves the drop from 0.8 % to 2.1 %.
Round the run length down to the nearest cut point. COB strip cuts every 50 mm, so 2,437 mm becomes 2,400 mm. The calculator does not care, but the reel does.
What each wire gauge carries inside 3 %.
Longest one-way feed that stays under 3 % drop at the load shown, on 24 V. Double every distance for 48 V strip, because the same watts pull half the current.
| Load | Current | 18 AWG | 16 AWG | 14 AWG |
|---|---|---|---|---|
| 40 W | 1.67 A | 10.3 m | 16.4 m | 26.1 m |
| 60 W | 2.50 A | 6.9 m | 10.9 m | 17.4 m |
| 80 W | 3.33 A | 5.2 m | 8.2 m | 13.0 m |
| 120 W | 5.00 A | 3.4 m | 5.5 m | 8.7 m |
L = 0.72 ÷ (2 × I × R). The 80 W row is the full working load of a ZV-D100 driver; the 120 W row is the working load of a ZV-D150P.
Common mistakes
Four ways the sums go wrong on site.
- 01Sizing the LED strip power supply 24 V at 100 %. 72 W of strip on a 75 W driver works on day one. By month ten it is flickering in a cabinet with no air. We round 72 W up to 90 W and then to the 100 W unit.
- 02Forgetting the return conductor. Voltage drop happens on both red and black, which is why the formula has a 2 in it. Leave it out and every answer is half what it should be.
- 03Using one feed for a U-shaped kitchen. Three runs joined by jumper wires behind cabinets look like one run on paper and behave like three long feeds in series. Feed each bank separately.
- 04Adding 12 V strip from another supplier to the same driver. It will not work, and we do not stock it. For long runs use the 48 V long-run strip, which goes 20 m on one feed.
Got a number you do not trust?
Send the run lengths and where the power comes in. We check the sheet by hand and reply with a parts list and a written USD quote. Your figure and ours should match; when they do not, the sheet says why.