Section 4: Calculating Voltage Drop

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This section explains voltage drop and how to calculate it for roadway illumination branch circuits.

Voltage drop can be calculated manually, using the methods described in this section or automatically using NEWVOLT, a program set up on Microsoft Excel.

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Maximum Allowable Voltage Drop

Typical service line voltage for illumination is 480 VAC. However, since copper wire has some amount of resistance, a voltage drop (or loss) will occur in the wire itself. This energy is lost in the form of heating in the wire.

Magnetic regulator ballasts for HPS of the type specified for roadway lighting (and shown on Roadway Illumination Details) will operate properly at 10 percent under rated line voltage. (This is not true for all electrical equipment. For equipment other than roadway lighting, see the equipment manufacturer’s documentation.) Good design practice allows the utility company 2 percent variation from rated line voltage, leaving 8 percent available for voltage drop in branch circuits. Therefore, the maximum allowable voltage drop for a 480 volt circuit would be 38.4 volts, derived as follows:

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Voltage (V) is equal to current (I) times resistance (R), expressed as

Therefore, voltage drop (Vd) in any given run may be calculated as

 (click in image to see full-size image)

Discussions of each of the factors in this formula follow.

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Current in the Run

When calculating voltage drop manually, the designer must determine the current in each run (that is, from the last light pole to the next-to-last, etc., all the way back to the service pole). The current depends on the number and type of fixtures. The following table shows the current required for the various types of fixtures.

Anchor: #i1003936Design Amperes for Various Luminaires

Lamp Wattage and Type*

— Line Voltage —

120 V

240 V

480 V

100 W MV

1.2 A

0.6 A

0.3 A

175 W MV

1.8 A

0.9 A

0.5 A

100 W HPS

1.2 A

0.6 A

0.3 A

150 W HPS

1.8 A

0.9 A

0.5 A

200 W HPS

2.4 A

1.2 A

0.6 A

250 W HPS

3.0 A

1.5 A

0.75 A

400 W HPS

4.2 A

2.2 A

1.2 A

40 W F

0.37 A



150 or 165 W IF

1.4 A

0.71 A


* MV = Mercury Vapor; HPS = High Pressure Sodium; F = Fluorescent; IF = Induction Fluorescent

NOTE: The amperage shown for MV and HPS fixtures is for magnetic regulator type ballasts only. If other types of ballasts are used, refer to the manufacturer’s specifications.


Conductor Resistance

To calculate voltage drop, you need to know the resistance of the conductor (wire) used in the branch circuit. Resistance is a function of wire size and length. Resistance for both wires going to the luminaire must be considered.

The following table shows wire resistance for various American Wire Gages (AWG). Since both wires are the same size in typical circuits, the table shows “loop resistance”; thus the designer need only calculate the distance between luminaire poles.

Anchor: #i1004012Wire Resistance by Gage

Wire Size

Loop Wire Resistance*




























* Values shown are for uncoated copper conductors in conduit at 25°C.

NOTE: Loop resistance accounts for the wire run in both directions, requiring the designer to measure only the one-way distance between luminaire poles.

Larger wire sizes have lower resistances. Using larger wire is one way to reduce the voltage drop in the circuit.

Length of Run

When using the preceding table to obtain conductor resistance per meter or foot, the “length of the run” used in the voltage drop formula will simply be the one-way distance between the poles.

Because of the way luminaires are wired, the height of the pole is of no consequence in voltage drop calculations. Only at the last pole would the height be a factor, and then only if the pole were very tall (high mast, for instance).

Calculation Example

On a 480 volt branch circuit, the run from the last light pole to the next light pole is 200 feet. The twin-arm light pole supports two 400 watt HPS fixtures. The conductor is 8 gage wire.

Using data from the tables provided in this section, we obtain the following information:

  • current in the run = 2 × 1.2 amps or 2.4 amps
  • loop resistance of the conductor = 0.001308

Using the formula for calculating voltage drop, we find

and therefore

Total Voltage Drop

Each run of the branch circuit will have a voltage drop. Therefore, as you work toward the electrical service, the total voltage dropped in the wiring increases as the drop for each successive run is added. This total must not exceed 8 percent at the pole farthest from the electrical service. The 8 percent is based on a mag-reg ballast that operates at plus-or-minus 10 percent line voltage. The allowable voltage drop should be adjusted to accommodate the specific ballast used.

Split Branch Circuit

Sometimes a branch circuit splits and runs in two directions. When this happens, the designer must remember that each run split off the circuit has a separate voltage drop.

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