# Correct Voltage Drop Calculation

Discussion created by rstadtman on Jul 19, 2016

For years I've questioned the way we calculate voltage drop.  The old 10% voltage drop rule made no sense and no one really new why we did it.  Now we are told that per UL we need to start voltage drop calculations at 20.4 volts dc.  Why? Because UL says that the control panels and power supplies etc. must work between 85 and 110% of their listed voltage.  So 85% of 24 is 20.4 vdc right?

Yes and no.  If I read it correctly UL is requiring the panel to work at 100% efficiency when the voltage is between 85% and 110% of listed voltage or 20.4 - 26.4 vdc (when on batteries).  This statement has nothing to do with voltage drop calculations. In reality the panel would need to put out a minimum of its listed voltage of 24vdc to each NAC circuit even when the input voltage is 20.4 vdc.  And to my knowledge to be UL listed it does.  There is no requirement (or need) for derating the output.

Based on all of the above the voltage drop calculations started at the regulated nominal 24 volts dc and the ending voltage shall not be less than 16 volts dc. 24vdc-16vdc=8vdc (usable voltage). To allow for variations in the actual field cable installation ending voltage should not be lower than 18 volts dc. This leaves 25% spare voltage to cover field requirements.

Looking for opinions, Ron