Page 14
©
2013 Magnum Energy, Inc
.
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
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. In a residential or commercial electrical installation, the NEC
requires both overcurrent protection and a disconnect switch. If a circuit breaker is used as the
overcurrent protection device, it can also be used as the required DC disconnect.
If a fuse is used as an overcurrent device, a Class-T type or equivalent is highly recommended. This
fuse type is rated for DC operation, can handle high short-circuit currents, and has a time delay
that allows for momentary current surges from the inverter without opening the fuse. However,
because the fuse can be energized from both directions, the NEC requires that it be installed in a
manner that the power must be disconnected on both ends of the fuse before servicing.
Use Table 2-1 to select the DC overcurrent device needed based on the recommended minimum
wire size for your particular inverter model (may not meet all local code or NEC requirements).
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
MSH3012M
400 amps
#4/0 AWG
(107.2 mm
2
) 405 amps
400 amps
with time delay
#6 AWG
(13.3 mm
2
)
MSH4024M
267 amps
#2/0 AWG
(67.4 mm
2
) 300 amps
300 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 inef
fi
ciency factored in.
Note
2
–
Copper wire rated with 90°C (194°F) insulation at an ambient temperature of 30°C (86°F), 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 (such as in boats), 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 conductor if that is the only
connection to the grounding electrode and that grounding electrode is a rod, pipe, or plate electrode.
