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When voltage at the beginning of a length of cable (at the transformer) is higher than the voltage at the end, the difference is known as "voltage drop". The visual indicator of voltage drop is dim light fixtures at the end of a run of cable. The closer the light fixtures are to the transformer, the higher the voltage and brightness. Voltage drop can be minimized through proper selection of the cable size based on the total wattage load and the length of the run. (See formula below).
Formula using 12 gauge cable for Accent or Path lighting with even spacing and same wattage on each fixture: 0.0011 x Total Watts on Cable x Length of Cable = Voltage Drop in % Example: 8 fixtures x 16.2 Watts (T 5 WB) = 130 Watts ..............0.0011 x 130 (Total Watts) x 50 Feet (Length of Cable) = 7.2% Voltage Drop
Formula using 12 gauge cable for single fixture on a length of cable: 0.0022 x Fixture Wattage x Length of Cable = Voltage Drop in %
To achieve optimum lighting effects, do not allow more than 8% voltage drop (75% light output) on each fixture. Halogen lamps' light output and life are severely affected by more than 10% voltage drop. Non-halogen lamps may be used with any voltage drop. However, higher voltage drop results in lowered light output and the color of the light becoming yellow. In some applications, up to 18% voltage drop (50% light output) may be acceptable, where at the level of illumination is not objectionable. The lower voltage will extend the non-halogen lamp life. For example, a 10% drop in voltage will extend a non-halogen lamp's life approximately four times, while reducing illumination levels about 30%. To achieve even illumination on an accent system at points where two different lighting segments meet, do not allow for more than 5% difference in voltage drop at ends.
To minimize voltage drop:
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