Designers of low voltage power circuits are often unaware of the implications of voltage drop and wire size.

In conventional home electrical systems (120/240VAC), wire is sized primarily for safe amperage carrying capacity (ampacity). The overriding concern is fire safety. In low voltage systems (12, 24, 48VDC) the overriding concern is power loss. Wire must not be sized merely for the ampacity, because there is less tolerance for voltage drop (except for very short runs). For example, a 1V drop from 12V causes 10 times the power loss of 1V drop from 120V.

Use the following chart as your primary tool in solving wire sizing problems. It replaces many pages of older sizing charts. You can apply it to any working voltage, at any percent voltage drop.

Universal Wire Sizing Chart

This chart works for any voltage or voltage drop, American (AWG) or metric (mm2) sizing. It applies to typical DC circuits and to some simple AC circuits (single-phase AC with resistive loads, not motor loads, power factor = 1.0, line reactance negligible).

Step 1 – Calculate the following:

Step 2 – Determine appropriate wire size from the chart below. Compare your calculated VDI with VDI in the chart to determine the closest wire size. Ampacity rating of wire size must be at least 125% of the continuous current passing through it.

NOTES: AWG = Amercan Wire Gauge. Ampacity is based on the National Electrical Code (USA) for 30°C (85°F) ambient air temperature, for no more than three insulated conductors in raceway in freee air of cable types AC, NM, NMC and SE; and conductor insulation types TA, TBS, SA, AVB, SIS, RHH, THHN and XHHW. For other conditions, refer to National Electric Code or an engineering handbook.

Determining Tolerable Voltage Drop for Various Electrical Loads

A general rule is to size the wire for approximately 2-3% drop at typical load. When that turns out to be very expensive, consider some of the following advice. Different electrical circuits have different tolerances for voltage drop.

LIGHTING CIRCUITS, INCANDESCENT AND QUARTZ HALOGEN (QH): Don't cheat on these! A 5% voltage drop causes an approximate 10% loss in light output. This is because the bulb not only receives less power, but the cooler filament drops from white-hot towards red-hot, emitting much less visible light.

LIGHTING CIRCUITS, FLUORESCENT: Voltage drop causes a nearly proportional drop in light output. Flourescents use 1/2 to 1/3 the current of incandescent or QH bulbs for the same light output, so they can use smaller wire. We advocate use of quality fluorescent lights. Buzz, flicker and poor color rendition are eliminated in most of today's compact fluorescents, electronic ballasts and warm or full spectrum tubes.

DC MOTORS may be used in renewable energy systems, especially for water pumps. They operate at 10-50% higher efficiencies than AC motors, and eliminate the costs and losses associated with inverters. DC motors do NOT have excessive power surge demands when starting, unlike AC induction motors. Voltage drop during the starting surge simply results in a "soft start".

AC INDUCTION MOTORS are commonly found in large power tools, appliances and well pumps. They exhibit very high surge demands when starting. Significant voltage drop in these circuits may cause failure to start and possible motor damage. Follow the National Electrical Code. In the case of a well pump, follow the manufacturer's instructions.