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How to calculate wire size for your power inverter or battery system

How to Size Wiring for Your power inverter System


Properly sized wire can make the difference between inadequate and full charging of a battery system, between dim and bright lights, and between feeble and full performance of tools and appliances. Designers of low voltage power circuits are often unaware of the implications of voltage drop and wire size.

In conventional home electrical systems (120/240 volts ac), 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, at a constant wattage load, a 1V drop from 12V causes 10 times the power loss of a 1V drop from 120V.

Universal Wire Sizing Chart
A 2-Step Process

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:


VDI = Voltage Drop Index (a reference number based on resistance of wire)
FEET = ONE-WAY wiring distance (1 meter = 3.28 feet)
%VOLT DROP = Your choice of acceptable voltage drop (example: use 3 for 3%)

STEP 2: Determine Appropriate Wire Size from Chart

Compare your calculated VDI with the VDI in the chart to determine the closest wire size. Amps must not exceed the AMPACITY indicated for the wire size.

Wire Size
Area mm2
Not Recommended


Metric Size
by cross-sectional area
(VDI x 1.1 = mm2)
(VDI x 1.7 = mm2)
Available Sizes: 1 1.5 2.5 4 6 10 16 25 35 50 70 95 120 mm2


20 Amp load at 24V over a distance of 100 feet with 3% max. voltage drop
VDI = (20x100)/(3x24) = 27.78
For copper wire, the nearest VDI=31.
This indicates #2 AWG wire or 35mm2

NOTES: AWG=Amercan Wire Gauge. Ampacity is based on the National Electrical Code (USA) for 30 degrees C (85 degrees 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.


Use the subsequent chart as your principal tool in solving cable sizing problems. It replaces a range of webpages of more mature sizing charts. you could possibly perhaps make use of it to any working voltage, at any percent voltage drop.

Determining tolerable voltage decrease for a range of electric loads

A simple guideline could well be to dimension the cable for roughly several or 3% decrease at regular load. When that turns out being extremely expensive, look at some while using next advice. a range of electric circuits have a range of tolerances for voltage drop.

LIGHTING CIRCUITS, INCANDESCENT AND QUARTZ HALOGEN (QH): might not cheat on these! A 5% voltage decrease prospects to an approximate 10% using up in illumination output. this critically is mainly as the bulb not just receives much less power, however the cooler filament drops from white-hot towards red-hot, emitting extremely much much less noticeable light.

LIGHTING CIRCUITS, FLUORESCENT: Voltage decrease prospects to a almost proportional decrease in illumination output. Flourescents use 1/2 to 1/3 the present-day of incandescent or QH bulbs for that precise same illumination output, so they could use more small wire. We advocate utilization of most effective quality fluorescent lights. Buzz, flicker and bad color rendition are eliminated in the significant largest % of today's small fluorescents, digital ballasts and cozy or full spectrum tubes.

DC MOTORS may perhaps be utilized in eco-friendly energy systems, specifically for h2o pumps. They operate at 10-50% higher efficiencies than AC motors, and eradicate the expenditures and losses linked with inverters. DC motors might not have too much energy surge demands when starting, in contrast to AC induction motors. Voltage decrease for that duration of the starting surge merely outcomes in the "soft start".

AC INDUCTION MOTORS in many cases are present in large energy tools, home appliances and nicely pumps. They exhibit extremely higher surge demands when starting. significant voltage decrease in these circuits may perhaps lead to incapacity to begin and achievable engine damage. abide by the nationwide electric Code. within situation of the nicely pump, abide by the manufacturer's instructions.

PV-DIRECT photo voltaic h2o PUMP circuits should be sized not for that manageable voltage (ie. 24V) but for that real working voltage (in that situation roughly 34V). with out a power battery to sustain the voltage down, the working voltage are planning being covering the optimum energy period voltage while using PV array.

PV power battery CHARGING CIRCUITS are vital mainly because voltage decrease can lead to some disproportionate using up of ask for current. To ask for a battery, a producing device should make use of a higher voltage than previously exists within of the battery. that's why most PV modules are created for 16-18V optimum energy point. A voltage decrease higher than 5% will minimize this vital voltage difference, and can minimize ask for present-day towards the power battery with a extremely much higher percentage. Our simple suggestion right here could well be to dimension for just about any 2-3% voltage drop. if you actually really feel that this PV assortment may perhaps be expanded within future, dimension the cable for long-term expansion. Your client will enjoy that when it arrives time to put towards the array.

WIND GENERATOR CIRCUITS: At most locations, a wind generator delivers its full rated present-day only for that duration of occasional windstorms or gusts. If cable sized for diminished using up is large and extremely expensive, you could possibly perhaps look at sizing for just about any voltage decrease as higher as 10% in the rated current. That using up will only happen occasionally, when energy is most abundant. seek advice from the wind system's training manual.

More techniques for cost reduction

ALUMINUM cable may perhaps be much more cost-effective than copper for some principal lines. energy firms apply it mainly since it could perhaps be more affordable than copper and lighter in weight, even however a bigger dimension should be used. it could perhaps be secure when installed to value with AL-rated terminals. you could possibly perhaps desire to look at it for long, pricey works of #2 or larger. The cost distinction fluctuates while using metals market. it could perhaps be rigid and difficult to bend, and never rated for submersible pumps.

HIGH VOLTAGE PV MODULES: look at producing utilization of higher voltage modules including a MPPT photo voltaic ask for controller to straight down change towards the computer software voltage (e.g. 12, twenty-four and 48V) to compensate for too much voltage drop. In some situations of lengthy distance, the improved module cost may perhaps be reduce compared to cost of bigger wire.

SOLAR TRACKING: power inverter system Use a photo voltaic tracker (e.g. Zomeworks or Unirac) to make certain that your more small assortment may perhaps be used, particularly in higher summer-use situations (tracking gains between the most energy in summer time once the sunlight requires the longest arc by way of the sky). the higher small PV assortment will require more small wire.

WATER nicely PUMPS: look in a slow-pumping, diminished energy computer software getting a storage space tank to gather water. This lessens the two cable and pipe dimensions precisely where lengthy lifts or works are involved. A PV array-direct pumping computer software may perhaps eradicate a lengthy cable operate by producing utilization of the independent PV assortment situated near towards the pump. a range of of our photo voltaic h2o pumps are extremely effective DC pumps that can be found as much as 48V. We also make AC variations and converters to permit utilization of AC transmitted much more than excellent distances. These pumps draw much less running current, and much much less starting present-day than traditional AC pumps, therefore significantly reducing cable dimension requirements.

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