Formulas to Calculate Voltage Drop

The following formula is used to calculate voltage drop for linear lighting with even spacing and same wattage.

VD% = length of run x wattage on run x cable constant multiplier

For Example:

Using 13-volt tap, 100 ft. (12/2 cable) x 75-watts x 0.00100 = 7.5% voltage drop

• Voltage drop calculations are based on wattage & length information per run, not per transformer.
• The best way to minimize voltage drop is to centralize the transformer, and have multiple runs coming out of it.
• Each run should be the shortest distance as possible & carry a minimized load.

A) Straight Run

VD% = ft x W x Cable Constant

12V VD = 60’ x 300w x 0.0007 = 12.6%

15V VD = 60’ x 300w x 0.00055 = 9.9%

24V VD = 60’ x 300w x 0.000189 = 3.4%

B) Point Source

VD% = ft x W x Cable Constant x 2

Remote Wiring

If positioning the transformer in a remote location be sure to run a jumper wire that meets all national and local codes. The jumper wire is the cable between the transformer and the start of the lighting run, or between two illuminated sections.

For jumper wire (or non illuminated section) voltage drop calculations use the lighting load as a point source.

Example #1

Ambiance® Rx Lighting

12V VD% = 20’ x 150w x 0.0007 x 2 = 4.2%

The above example calculates the voltage drop on a 10 AWG jumper wire between the 12v transformer and the start of the Ambiance® Rx run with 150w load.

Example #2

Ambiance® Landscape Lighting
1 single 50w fixture

14V VD% = 50’ x 50w x 0.00060 x 2 = 1.5%

C) Combination of Non-Illuminated and Illuminated Sections

For linear lighting systems where the transformer is remotely mounted and the non-illuminated section is significant, a voltage drop calculation should be performed on both sections: Non-Illuminated and Illuminated.

Treat each section separately and perform a voltage drop calculation separately.

1. Non-Illuminated Section:

(26v) VD% = 40’ x 300w x 0.000175 x 2 = 4.2% = 1.1v

Note: Be sure to verify in the field that the voltage at the 1st light is not over 25v.

26v less 4.2% Voltage Drop (= 1.1 volts) = 24.9 volts at start of lighting run (or illuminated section).

2. Illuminated Section:

Note: When the starting voltage falls between two voltages, use the lower voltage to determine the cable constant.

(24v) VD% = 6’ x 300w x 0.000189 = 0.3%
The Voltage Drop between the first and last lamp is less than 1%, so there is no noticeable difference of light output between first and last lamp.

Multi-Tap Transformer Advantage

The advantage of using a Multi-Tap (12v, 13v, 14v, and 15v) or (24v, 26v, 28v, and 30v) transformers:

You can choose the appropriate output tap for a particular set of fixtures, (so the voltage at each lamp is between: 11.0-volts and 12.5-volts for 12 volt lamp; and between 22.0 volts and 25.0 for 24 volt lamps).