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SECTION 8 | Installation
Conductors are protected with insulating material rated for specific temperature e.g. 75˚C/167˚F. As current
flow produces heat that affects insulation, there is a maximum permissible value of current (called
“Ampacity”) for each size of conductor based on temperature rating of its insulation. The insulating material
of the cables will also be affected by the elevated operating temperature of the terminals to which these are
connected. Ampacity of cables is based on the National Electrical Code (NEC)-2014. Please see details given
under “Notes for Table 8.2”.
The DC input circuit is required to handle very large DC currents and hence, the size of the cables and
connectors should be selected to ensure minimum voltage drop between the battery and the inverter.
Thinner cables and loose connections will result in poor inverter performance and will produce abnormal
heating leading to risk of insulation melt down and fire. Normally, the thickness of the cable should be such
that the voltage drop due to the current & the resistance of the length of the cable should be less than 2% to
5%. Use oil resistant, multi-stranded copper wire cables rated at 75ºC / 167°F minimum. Do not use aluminum
cable as it has higher resistance per unit length. Cables can be bought at a marine / welding supply store.
Effects of low voltage on common electrical loads are given below:
• Lighting circuits - incandescent and Quartz Halogen: 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.
• AC induction motors - These are commonly found in power tools, appliances, well pumps etc. They exhibit
very high surge demands when starting. Significant voltage drop in these circuits may cause failure to
start and possible motor damage.
• PV battery charging circuits - These are critical because voltage drop can cause a disproportionate loss
of charge current to charge a battery. A voltage drop greater than 5% can reduce charge current to the
battery by a much greater percentage.
8.4.4 Fuse Protection in the Battery Circuit
A battery is an unlimited source of current. Under short circuit conditions, a battery can supply thousands
of Amperes of current. If there is a short circuit along the length of the cables that connects the battery
to the inverter, thousands of Amperes of current can flow from the battery to the point of shorting and
that section of the cable will become red-hot, the insulation will melt and the cable will ultimately break.
This interruption of very high current will generate a hazardous, high temperature, high-energy arc
with accompanying high-pressure wave that may cause fire, damage nearby objects and cause injury. To
prevent occurrence of hazardous conditions under short circuit conditions, the fuse used in the battery
circuit should limit the current (should be "Current Limiting Type"), blow in a very short time (should be
Fast Blow Type) and at the same time, quench the arc in a safe manner. For this purpose, UL Class T Fuse
or equivalent should be used (As per UL Standard 248-15). This special purpose current limiting, very fast
acting fuse will blow in less than 8 ms under short circuit conditions. Appropriate capacity of the above
Class T fuse or equivalent should be installed within 7” of the battery Plus (+) Terminal (Please see Table
8.2 for fuse sizing). Marine Rated Battery Fuses, MRBF-xxx Series made by Cooper Bussmann may also be
used. These fuses comply with ISO 8820-6 for road vehicles.