Lower temps cause higher voltage at the lamps. As copper wire cools, its resistance decreases by about 8% for every 40ºF drop (e.g. room temperature to freezing). This is insignificant for short runs but for longer runs connected to an 18 volt tap, the voltage increase due to the lower temperature would be about 0.5 volts – enough to shorten lamp life if voltage at the fixture exceeds 12 volts. This is one reason why installers in Northern climates should keep fixture voltages below 11.5 volts.

The formula to calculate new voltage at fixture due to temperature change is:

           NVF = TAPV – (ΔVI – (ΔVI x .002 x ΔT))
Where:

• NVF = Voltage at the fixture at new temperature
• TAPV = Voltage at transformer tap
• ΔVI = Voltage drop between transformer tap and fixture at time of installation
• ΔT = Drop in temperature compared to temperature at time of installation (ºF). Use negative value for increase in temperature.

Lower temps increase thermal shock at the lamps. Even at room temperature, the initial start-up voltage applied to a lamp filament results in a surge where amperage can be 10 to 14 times the nominal amperage during normal operation. This is why lamps nearly always burn out during the first few milliseconds after start-up. Sub-freezing ambient temperature exacerbates this effect even further. As a result, lamps may burn out before their expected lifetime during very cold winters.