Application Note
Determining the Maximum LED Load on
a Constant Voltage Power Supply
1.0 INTRODUCTION
This application note has been written as a guide to determine the maximum recommended
LED module load once a constant voltage LED power supply has been selected. Unlike the
lamp load for electronic ballasts, the maximum LED load for a constant voltage power supply
depends on several factors, including:
• Remote mounting distance of the power supply
• Output wire gauge size
• Ambient temperature range
• Accessories, such as the OPTOTRONIC® OT DIM or OT RGB control modules.
Because of the various factors mentioned above, your specific LED design application may
require a de-rating of the maximum wattage you can put on the power supply. This is
particularly important if you are using power supplies with a maximum output power that is at
the limit of the maximum allowed output current per UL Class 2 limits (50W for a 10V supply
and 100W for a 24V supply).
This process shows the importance of calculating the additional losses that may be
encountered during an LED system installation. Since LED power supplies limit the maximum
current and power output, the de-rating process ensures that the system will remain stable
under all operating conditions.
De-rating means that the maximum LED load on a power supply should be less than the
maximum load the supply is designed to handle. The difference between the maximum
output power and the maximum LED load is used up in resistive losses of the wiring,
increased power drawn by the LED module at lower temperatures, and secondary devices
(such as PWM dimming modules for dimming or colormixing applications). Using the derating process will help ensure that your LED application maintains a uniform light level along
the entire length of the LED load and does not flicker in cold temperatures or at maximum
LED load levels (dimming or colormixing). Should you have any questions regarding the
above process kindly contact your SYLVANIA representative.
2.0 DETERMINING THE MAXIMUM RECOMMENDED LOAD FOR YOUR
POWER SUPPLY
2.1 Determine the voltage system from the product literature for your LED
application (10V or 24V).
2.2 Select an OPTOTRONIC® power supply for your LED design application.
LED Voltage
10 Volt
1
24 Volt
2.3 Pick a wire gauge from the following table and note the resistance per foot.
Wire Gauge
14AWG
16AWG
18AWG
20AWG
22AWG
Wire gauge selected =
Resistance per foot
0.0025
0.0040
0.0064
0.0101
0.0161
Resistance per foot (R/ft) =
2.4 Calculate the maximum output current rating of your power supply.
Maximum power supply output current = Rated output power/Rated output
voltage
2.4.1
Divide the Maximum rated power output of the selected power supply by
its rated output voltage. This will give you the maximum output current
capability of the power supply.

For example, the OT75/120-277/24 has a rated maximum output wattage of
75 and the rated output voltage is 24.
Maximum output current for OT75/120-277/24 = 75/24
= 3.1 Amperes
Record this number for your selection below.
Maximum rated power output= ________
Rated voltage = ________
Calculated maximum output current (Imax) = ________
2.5 Calculate the resistive losses in the wiring.
Resistive power loss in watts = (Imax)^2 x (R/ft) x (Remote distance) x 2
2.5.1
Multiply the square of the maximum output current in step 2.4 by the
resistance per foot (selected in step 2.3) and the desired remote
mounting distance (in feet).
Note: Do not exceed the recommended maximum remote mounting distances as
seen in OPTOTRONIC® data sheets or installation guides.
2.5.2

Multiple this result by 2. The resulting number is the maximum power
loss you can expect because of resistive losses in the output wires.
Continuing with the example of the OT75/120-277/24 power supply, let’s
assume a wire gauge selection of 18AWG and a remote mounting
distance of 8ft.
The maximum current from step 2.4 was 3.1 Amperes. The resistance
per foot from the table in step 2.3 is 0.0064 ohms/ft.
Thus the resistive power loss = (3.1)^2 x (0.0064)x(8)x2
= 0.98 watts
2
Remote mounting distance (in ft) for your application = ________
Resistance per ft (R/ft) from step 2.3 = ________
Maximum output current (Imax) from step 2.4 = ________
Resistive loss for your application = ________
2.6 Calculate de-rating for applications in cold temperatures.
If your application will be outdoors and you expect extremely cold temperatures
(down to -5º F or - 20º C), a de-rating of 10% of the maximum rated output power is
recommended. This de-rating is required because LED modules draw high power in
colder temperatures. Not taking this de-rating into account may result in the LED
module flickering or turning off when the ambient temperature becomes extremely
cold.
Cold temperature de-rating = Rated output power x 0.10.
2.6.1

To calculate the de-rating, multiply the power rating of your
OPTOTRONIC® supply by 0.10.
For our example of the OT75/120-277/24 supply, this would result in a
de-rating of 7.5W (75 multiplied by 0.10).
Record the cold temperature de-rating for your application here = ________
2.7 Calculate the recommended LED load for your application.
LED load = Maximum power rating - Resistive power loss - Cold temp. derating
2.7.1

Subtract the resistive power loss in step 2.5 and the cold temperature
de-rating in step 2.6 from the maximum power rating of your
OPTOTRONIC® power supply. This is the recommended LED load for
your selected power supply.
For our example, LED load = 75- 0.98 – 7.5 = 66.52 watts
IF YOUR APPLICATION DOES NOT INVOLVE DIMMING OR COLORMIXING, YOU CAN STOP HERE.
3.0 ADDITIONAL DE-RATING FOR DIMMING OR COLORMIXING
APPLICATIONS
3.1 Calculate de-rating required for OT DIM or OT RGB controller interfaces.
Note: If your application involves the use of an OT DIM or an OT RGB control
interface from SYLVANIA, don’t forget to account for the additional power draw
required by these modules. The graphs below show the expected power
requirements.
For colormixing and/or dimming, maximum LED load = LED load in step 2.7
– Power requirement for OT DIM or OT RGB
3

Lets assume for our sample case that the LED load will be dimmed with
the OT DIM control interface.
Maximum LED load = 66.52 – 4 = 62.52 watts.
4.0 CHARTS
Power loss curve for OT DIM
CHART 1
Power loss curve for OT RGB Control Interfaces
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