Voltage Drop Percent

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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.


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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