Selection & sizing

How many lights per driver

Do not size to the nameplate. For constant voltage derive run length from 80 percent of driver power; for constant current derive series LED count from the voltage window, leaving margin for Vf spread and temperature drift. Worked examples for 60/100/150 W and 350/700/1050 mA, plus an acceptance check.

Last updated：2026-11-01

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The one-sentence rule: constant voltage by power, constant current by voltage window, both at 80 percent
Constant voltage: derive length from 80 percent power
Constant current: derive series count from the voltage window
The two topologies at a glance
Acceptance checklist

"A 100 W driver, 14.4 W/m strip, so nearly 7 metres" - sizing to the nameplate like that fails within three months. The correct method keeps 20 percent in reserve and derates to 80 percent load. The reserve is not caution; it absorbs LED Vf spread, temperature drift and inrush.

The one-sentence rule: constant voltage by power, constant current by voltage window, both at 80 percent

Constant-voltage driver: how many strip sections, look at total power, but cap at 80% of rated power
Constant-current driver: how many LEDs in series, look at whether the output voltage window covers the string Vf, with at least 2 V margin for Vf spread

Constant voltage: derive length from 80 percent power

L_max about 0.8 x P_driver / P_per_m

P_driver    driver rated power (W)
P_per_m     strip power per metre (W/m)
0.8         80 percent load (reserve for drift, rising Vf, inrush)

For a 24 V, 14.4 W/m strip:

CV driver	Rated power	80% usable	Max length
60 W	60 W	48 W	3.3 m
100 W	100 W	80 W	5.5 m
150 W	150 W	120 W	8.3 m

The most counter-intuitive row is that a 60 W driver cannot fully feed even one 5 m strip - 5 m is 72 W, already over the 48 W 80% cap. Many sites where "the driver runs hot and the strip dims over time" were sized to 100%. Constant-voltage units such as K01, H01 and S01 follow this formula.

Constant current: derive series count from the voltage window

A constant-current driver fixes the current; how many LEDs you can series depends on the output voltage window, not the current. Formula:

N_led no more than (V_out_max - V_margin) / Vf

V_out_max   driver output voltage maximum (V)
V_margin    reserve, use 2 V or more (Vf spread and drift)
Vf          single LED forward voltage (V)

With a 12-48 V window and Vf 3.0 V:

CC driver	Rated current	Output window	Max series LEDs
350 mA	350 mA	12-48 V	15 (46 V no more than 48-2)
700 mA	700 mA	12-48 V	15
1050 mA	1050 mA	12-48 V	15

Current level does not change the series count; it changes per-LED brightness and heat. Vf is not fixed: even one batch spreads about plus/minus 0.1 V, and 15 in series stacks to plus/minus 1.5 V. That is why the margin must be 2 V or more - otherwise the worst batch hits the output ceiling and the driver drops into constant-voltage current limiting, with unstable brightness. Constant-current units such as R01 and the high-current H01 follow the same window.

The two topologies at a glance

Left: CV parallel, sections limited by power. Right: CC series, count limited by window

Acceptance checklist

CV: take strip watts per metre, apply L_max about 0.8 x P_driver / P_per_m, stay under the 80% cap
CV: when paralleling sections, add their powers; the sum must still be no more than 80% of P_driver
CC: take single Vf from the spec (no data, use 3.0 V conservative), apply N_led no more than (V_out_max - 2) / Vf
CC: after Vf spread (plus/minus 0.1 V times count), the worst case must still sit inside the output window
In the first month feel the driver case; at 80% CV load it should not be too hot to hold (about 60 degrees C or above means recheck the margin)
When margin is short, add another driver to share the load rather than exceed 80%

Power and current checks are done in the LED driver sizing calculator; lead-wire drop for long runs is in the 12 V/24 V voltage drop and cable size table. Why the margin matters and how heat eats lifetime is in lifetime, tc temperature and thermal. Constant-voltage units K01, H01 and S01 and constant-current unit R01 load by the two formulas above; do not apply the CV power rule to a constant-current driver.

Not sure how many strip sections to parallel or how many LEDs to series? Send us the driver model, strip watts per metre or single Vf, and your intended total length and we will derate to 80 percent and show the margin. Contact →