Underwriting Laboratories (UL), the independent safety testing lab for all things electrical, allows Low Voltage Transformer manufacturers like FX to produce transformers with up to a 15-volt output to help compensate for voltage drop through the system. Voltage Drop is the loss of electrical pressure from the electrical source (Transformer) to the Fixtures on any given circuit. For very long circuit runs, it isn’t uncommon for the circuit to lose 3 to 4 volts.
The primary objective is to minimize voltage drop by installing the proper size feeder cable (home run) to each zone and to make sure that each fixture on each cable run is receiving between 10.5 and 11.5 volts AC, or 10–15 volts AC for LED fixtures. To maintain 10.5 -11.5 volts at the last fixture (for incandescent fixtures) on the circuit (last fixture in Daisy Chain illustration below), the installer may have chosen to use the 14-volt tap on the transformer. Because of Voltage Drop, the circuit may lose 3 volts (or more) from the transformer to the last fixture. The only way the last fixture will receive 11 volts would be to use the 14 volt tap on the Transformer. If the system was installed in sequence (Daisy Chain illustration below) and the first fixture on the circuit is close to the transformer, it may very well be receiving 14+ volts, thus it is burning out with high frequency!
Daisy Chain Method (Recommended for LED systems):
Unfortunately, if the circuit was installed as diagramed above for incandescent fixtures, the first fixture will always receive more voltage than the last fixture on the circuit. Consequently, the first fixture will typically burn out much quicker than it should…especially if the circuit was wired up on the 14 volt tap. The solution to this scenario is to rewire the circuit as illustrated below. The “Tee” method of cabling a circuit will balance the voltage load far better than the Daisy Chain method. The Daisy Chain method works well for LED fixtures to maintain 10-15 volts AC at the fixtures.
Tee Method (Recommended for Incandescent systems):
Couldn’t I just move the cable down to the 11 volt tap?
Yes, this is an option. However, now the first fixture will be receiving the proper voltage…but the last fixture will be receiving 8 volts or less resulting in a fixture producing very dim light output.
