Wire Sizes And Maximum Length Determination Chart

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Wire Sizes and Maximum Length Determination
(7/5/2007)
Wire sizes become important at low voltages. At 12 volts DC a loss of more than 10% in
voltage across the length of the wire can mean the difference between the inverter
running or not running. The currents can get high and any voltage drop becomes
significant. In general at 12 Volts DC one should run the inverter close to the battery and
then pipe the 120 Volts AC to the point of use on smaller wire.
The general rule is at low voltages pay attention to voltage drop and at high voltages pay
attention to maximum current caring capacity for the size of wire.
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, a 1V drop from 12V causes 10 times the power
loss of 1V drop from 120V.
Use the following charts as your primary tool in solving wire sizing problems.
Determining tolerable voltage drop for various electrical loads
A general rule is to size the wire for approximately 2 or 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.
DC TO AC INVERTERS: Plan for 3 to 5% voltage drop. In a push to shove situation
one can use up to a 10% voltage drop as a maximum.
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. A 10% drop in voltage is usually the max. Fluorescents use 1/2 to
1/3 the current of incandescent or QH bulbs for the same light output, so they can use
smaller wire.
DC MOTORS 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
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