Calculate Voltage Drop

When feeder circuits are routed over long distances (when you get close to a 100 feet run from power source to the load), then you have a voltage drop that you need to account for. All wires present a certain amount of resistance to the current going through them, and if you increase the amount of wire (longer run) then you're increasing the overall resistance through that circuit.

In order to prevent damage to the equipment or wire, you need to increase the size of the wire (diameter) to decrease or eliminate the voltage drop. The amount of voltage drop usually allowed per NEC is up to 3 % for branch circuits, and 5 % for the overall voltage drop (from the power source to the farthest outlet).

Ohm's Law, Electrical Math and Voltage Drop Calculations

Calculating Voltage Drop - 1 Phase

I (Amperage - also known as Current)

VD (Voltage Drop)

R (Resistivity of wire - taken from NEC Chapter 9 Table 8)

L (Length of run - we typically calculate the drop per 1000 ft lengths)

CM (Circular Mils of Wire - measure of the diameter (thickness) of the wire - again available from NEC tables)

K (Resistivity Constant - 12 for Copper, 18 for Aluminum)

Example (1) using VD = (2 * L * R * I) / 1000 ft

Find the voltage drop on a # 6 THWN copper, 3 wire, 120/240 Volt, single phase feeder circuit of 150 foot length having a 60 A load.

Use the formula above, and substitute the given values.

VD = (2 * 150' * .491 * 60) / 1000' = 8.838 Volts

The voltage drop is 8.838 Volts, we now have to check the % from the overall voltage.

% = (VD / V) * 100     

Substituting values gives us: % = 8.838 Volts / 240 Volts = 0.0368 %

This falls above the 3 % max. allowed, so you would check to see if you can use a # 4 THWN copper. (check NEC Chapter 9 Table 8 - Conductor Properties - column- ohm / kft  = 0.308)

Do the same as the above example, substituting the resistivity value for # 4 copper, and if the VD falls under the permitted 3 % range then use it!

Example (2) using VD = (2 * L * K * I) / CM

Find the voltage drop on a # 4 THWN copper, 3 wire, 120/240 Volt, single phase feeder circuit of 200 foot length having a 50 A load.

Use the formula above, and substitute the given values.

Use 12 for the resistivity constant for copper

VD = (2 * 200' * 12 * 50) / 41740 = 5.75 Volts

The voltage drop is 5.75 Volts, we now have to check the % from the overall voltage.

% = (VD / V) * 100     

Substituting values gives us: % = 5.75 Volts / 240 Volts = 0.0239 %

This falls under the 3 % max. allowed, so you can use it!

Calculating Voltage Drop - 3 Phase

When calculating the voltage drop in 3 phase circuits, do the same as in the single phase but the voltage drop must be multiplied by 0.866. Basically the voltage drop between any two phases conductors is multiplied by (1.732 / 2) = 0.866.

Example

Find the voltage drop and size THWN copper, 120/208 Volt, 3 phase feeder circuit of 200 foot length having a 150 Amp load.

Try 3/O copper with resistivity of 0.0766

Step (1)  

VD = (2 * L * R * I ) / 1000 ft = (2 * 200 * 0.0766 * 150) / 1000 ft = 4.596 VD

Step (2)  as per 215-2(b)

VD = 4.596 * .866 = 3.98

Step (3)

208 Volts * 3 % = 6.24 Volts (max. allowed)

So, the VD = 3.98 Volts which is below the 6.24 Volts allowed.

Even though this size wire would work, try the next lower size wire to see if that would also work.

The reason is that if you can do the job with a smaller size wire, it will cost you less in supplies and leave you with more profit. (Check the 2/0 copper!)

Voltage Drop Calculator

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