How to Calculate Voltage Drop

Voltage drop is caused by the resistance or impedance that a current occurs between the point of power (the circuit breaker) and the electrical device receiving the power. The most common cause of voltage drop is a long distance between the two. In order to remedy this, the circuit conductor must be increased in order to maintain the current between the points.

According to NEC 215.2(A)(4) fine print note No. 2, the voltage drop for feeders cannot exceed 3% and the voltage drop for branch circuits cannot exceed 5%.

So, in a situation where there is a long distance between the power source and the electrical device, a voltage drop calculation must be performed in order to determine if conductor up-sizing is required. The calculations for a single phase circuit and a three phase circuit differ slightly. Listed below are the voltage drop calculations for the two:

Single Phase Feeder/Circuit

Three Phase Feeder/Circuit

VD  =  The Voltage Drop (conductor temp of 75°C) in volts
VD%  =  The percentage of voltage drop (VD ÷ source voltage x 100). It is this value that is commonly called "voltage drop" and is cited in the NEC 215.2(A)(4) and throughout the NEC.
L  =  One way length of the circuit's feeder (in feet)
R  =  Resistance Factor per NEC Chapter 9, Table 8, in Ohm/Ft
I  =  Load Current (in Amperes)
Source Voltage  =  The voltage of the branch circuit at the source of power. Typically the source voltage is either 120, 208, 240, 277, or 480.

Let's take a look at a couple examples:

(A.) At an apartment complex, a 120-volt, 20-amp convenience receptacle is required at the bbq area. The distance of the circuit from the panelboard to the receptacle is 125ft. and we will assume the load is 13amps. The feeder is copper.
13amps = #12awg per NEC 310.15(B)(16)
R = 1.98 per NEC Chapter 9, Table 8


* Since the branch circuit's voltage exceeds 5% the wire size must be upsized from #12awg to #10awg.

(B.) A compressor pump is to be designed 300' away from an industrial warehouse. The source of power is within the warehouse and the pump operates at 480v 3-phase, 60amps. The feeder is copper.
60amps = #6awg per NEC 310.15(B)(16)
R = 0.491 per NEC Chapter 9, Table 8


* Since the branch circuit's voltage does not exceed 5%, no upsizing is required.

The examples above were bot applied to branch circuits but the calculations are the same for panelboard / distribution feeders. It's also important to note that both examples involved copper as the conductor type because copper is used in most circuit feeders. But the examples would apply to aluminum feeders as well, just with different resistance factors.

Finally, it is the design professional's responsibility to properly size the circuit feeder for voltage drop, even if it is on the cusp of what the code requires. This means that if the voltage drop for a branch circuit is 4.99%, it is a good idea to upsize the wire. When in doubt, upsize.

And if you don't want to use the calculation every time voltage drop is in question, try our Voltage Drop Calculator.

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