Section 4: Calculating Voltage Drop

Introduction

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 by using the NewVolt spreadsheet calculator, which is available from the TxDOT Traffic Operations Division (TRF).

Maximum Allowable Voltage Drop

Typical service line voltage for illumination
are 240V or
 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 drops are derived as follows:
  • 480V 0.08 = 38.4V
  • 240V x 0.08 = 19.2V.
The drivers in LED luminaires operate on a range of voltages. The typical ranges are 120V-277V and 347V-480V. Although LED luminaires can operate on larger voltage drops than 8%, TxDOT recommends designing circuits with a maximum allowable voltage drop at 8% for LEDs also.

Formula

Voltage (V) is equal to current (I) times resistance (R), expressed as:
(click in image to see full-size image)
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.

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.
Design Amperes for Various TxDOT Standard Luminaires
Luminaire Wattage
and Type*
— Line Voltage —
120 V
240 V
480 V
150W HPS
1.67 A
0.83 A
0.42 A
250W HPS
2.50 A
1.25 A
0.63 A
400W HPS
3.75 A
1.88 A
0.94 A
150 W EQ LED
0.83 A
0.42 A
0.21 A
250W EQ LED
1.42 A
0.71 A
0.35 A
400W EQ LED
2.08 A
1.04 A
0.52 A
12-400 W HPS HM
45.0 A
22.5 A
11.3 A
6-400W LED HM
30.0 A
15.0 A
7.50 A
150 or 165 W IF
1.4 A
0.71 A
n/a
*HPS = High Pressure Sodium; LED = Light Emitting Diode; IF = Induction Fluorescent; HM = High Mast

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 Gauges (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.
Wire Resistance by Gauge
Wire Size
Loop Wire Resistance*
(AWG)
(ohms/foot)
(ohms/meter)
12
0.003360
0.011023
10
0.002036
0.006680
8
0.001308
0.004291
6
0.000820
0.002690
4
0.000518
0.001700
2
0.000324
0.001063
0
0.000204
0.000670
00
0.000162
0.000532
* Values shown are for uncoated copper conductors in conduit at 25°C
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 400W EQ LED fixtures. The conductor is 8 gauge wire.
Using data from the tables provided in this section, we obtain the following information:
  • current in the run = 2 × 0.52 amps
    or
     1.04 amps
  • loop resistance of the conductor = 0.001308
Using the formula for calculating voltage drop, we find
(click in image to see full-size image)
and therefore
(click in image to see full-size image)

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.

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.