Voltage Drop Percent
Voltage Drop Percent and the NEC
The NEC does not require voltage drop to be any specific number in percent.
This is a huge area of confusion for most electricians. Because there are two
fine print notes which mention 3% for branch circuits and 3% for feeders, and 5%
overall, electricians think this is a requirement. But keep in mind that fine
print notes are explanatory and not code.
Now there is a left handed, indirect requirement in the NEC which does put a
limit on voltage drop. It is 110.13B. So how does that article apply? Well when
conductors are sized to power a load, if that load requires voltage to be at
least some specific amount for efficiency of operation, then the circuit
conductors must be sized to provide the minimum design voltage.
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Voltage drop percent is always a comparison to the source voltage. So the
simplest calculation is based on measurement of voltage at the service point or
main panel. Then go on to measure the load voltage when the load is energized
and working.
Subtract the load voltage from the source voltage. Then divide this
difference by the source voltage. What results is the voltage drop as a decimal
value. To convert to a percentage, move the decimal to the right and add the %
sign.
To repeat for clarification, all percentages of voltage drop are a
comparison to the source voltage.
Find practice problems HERE.
There are many worksheets on this topic.
Find audio support HERE.
Voltage Drop Percentage Formulas
There are many different formulas which can be used to find the percentage
of voltage drop for the various aspects of a circuit. Here is a collection which
may come in handy. Copy them over into your notebook of formulas. They are also
reprinted in Reference Formulas Appendix. Details HERE about RFA.
Find practice problems HERE.
Find audio support HERE.
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