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©
2020 Sensata Technologies
Installation
2.4.1
DC Wire Sizing
It is important to use the correct sized DC wire to achieve maximum ef
fi
ciency from the system
and to reduce
fi
re hazards associated with overheating. Always keep your wire runs as short as
practical to prevent low voltage shutdowns and to keep the DC breaker from nuisance tripping (or
open fuses) because of increased current draw. See Table 2-1 to select the minimum DC wire size
(and corresponding overcurrent device) required based on your inverter model. The cable sizes
listed in Table 2-1 are required in order to reduce stress on the inverter, minimize voltage drops,
increase system ef
fi
ciency, and ensure the inverter’s ability to surge heavy loads.
If the distance from the inverter to the battery bank is >5 feet, the DC wire will need to be increased.
Longer distances cause an increase in resistance, which affects the performance of the inverter.
Use the overcurrent device previously determined from Table 2-1 and then refer to Table 2-2 to
determine the minimum DC wire size needed for various distances, based on your inverter model.
2.4.2
DC Overcurrent Protection
DC overcurrent protection is not included in the inverter—for safety reasons and to comply
with
NEC (National Electrical Code) regulations
—it must be provided as part of the installation. The DC
overcurrent protection device must be installed in the positive DC cable line, it can be a fuse or a
circuit breaker and must be DC rated. It must be correctly sized according to the size of DC cables
being used, which means it is required to open before the cable reaches its maximum current
carrying capability, thereby preventing a
fi
re.
Batteries can deliver thousands of amps in an instant during a short. Therefore, you are required
to install a DC-rated fuse (or circuit breaker) that has a interrupt current rating (known as Amps
Interrupting Current, or AIC) that can withstand the short-circuit current without explosion
or damage. If a fuse is used as an overcurrent device, a Class-T type or equivalent is highly
recommended when used with inverters. A Class-T fuse is rated for DC operation, can handle very
high short-circuit currents (up to 100,000 amps), and has a time delay that allows for momentary
current surges from the inverter without opening the fuse. In some installations, if the combined
short-circuit current of the battery bank is determined to be 2,700 amps or less, then an ANL type
of fuse may be used—if in doubt, use a Class-T fuse. See Table 2-1 for the DC overcurrent device
(coordinated with the DC wire size) recommended for your inverter.
*Electrical systems in mobile installations typically do not require using a DC disconnect, although
an overcurrent protection device is still required. These installations also do not normally use
conduit, so the fuse must be installed in the ungrounded conductor within 18 inches (45.7 cm) of
the battery—to protect the DC wiring system.
Table 2-1, Recommended DC Wire/Overcurrent Device for Rated Use
Inverter
Model
Maximum
Continuous
Current
1
Minimum DC Wire Size
(rating)
2
Maximum DC
Fuse Size
3
DC
Grounding
Wire Size
4
MSH3012RV
400 amps
#4/0 AWG
(107.2 mm
2
)
405 amps
400 amps
with time delay
#6 AWG
(13.3 mm
2
)
Note
1
–
Maximum continuous current is based on the inverter’s continuous power rating at the
lowest input voltage with an inverter ineffi ciency factored in.
Note
2
–
Copper wire rated with 194°F (90°C) insulation at an ambient temperature of 86°F (30°C),
with a multiple cable fi ll factor (0.8) de-rating in free air (if needed). May require larger conductor,
refer to your application wiring requirements, the DC grounding wire may be required to be the
same ampacity as the positive cable to the inverter.
Note
3
–
The next larger standard size overcurrent device may be used if the de-rated cable ampacity
falls between the standard overcurrent devices found in the NEC.
Note
4
–
Per the NEC, the DC grounding electrode conductor can be a #6 AWG (13.3 mm
2
) conductor
if that is the only connection to the grounding electrode and that grounding electrode is a rod, pipe,
or plate electrode.
