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.

Section 4: Calculating Voltage Drop