©
2009 Magnum Energy Inc
Page 18
2.3.1 DC Wire Sizing
It is important to use the correct DC wire to achieve maximum ef
fi
ciency from the system and
reduce
fi
re hazards associated with overheating. Always keep your wire runs as short as practical
to help prevent low voltage shutdowns and keep the DC breaker from nuisance tripping (or open
fuses) because of increased current draw. See Table 2-3 to select the minimum DC wire size (and
corresponding overcurrent device) required based on your inverter model. The cable sizes listed in
Table 2-3 for your inverter model are required 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 greater than 5 feet, the DC wire will need
to be increased. Longer distances cause an increase in resistance, which affects the performance
of the inverter. Continue to use the overcurrent device previously determined from Table 2-3 and
then refer to Table 2-4 to determine the minimum DC wire size needed for various distances based
on your inverter model.
2.3.2 DC Overcurrent Protection
For safety and to comply with electrical code regulations, you must install a DC overcurrent pro-
tection device in the positive DC cable line to protect your DC cables. This DC overcurrent device
can be a fuse
or 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 C
lass-T type or equivalent is
recommended
. This fuse
type is rated for DC operation, can
handle the high short-circuit currents and
allow for momentary
current surges from the inverter without opening. However, because the fuse can be energized
from both directions, if it is accessible to unquali
fi
ed persons, the NEC requires that it be installed
in a manner that the power can be disconnected on both ends of the fuse before servicing.
See Table 2-3 to select the DC overcurrent device needed based on the minimum wire size ac-
cording to your inverter model.
Table 2-3, Recommended DC Wire/Overcurrent Device
Inverter Model
Maximum
Continuous
Current
1
NEC
Current
2
Recommended DC
Wire Size
3
(conduit rating)
Recommended DC
Overcurrent
Device
DC Grounding
Electrode Wire
Size
5
MS4024PAE
222 amps
278 amps
#4/0 AWG
(260 amps)
250 amps
4
#6 AWG
MS3748PAEJ
102 amps
128 amps
#2/0 AWG
(195 amps)
175 amps
#6 AWG
MS4448PAE
122 amps
153 amps
#2/0 AWG
(195 amps)
175 amps
#6 AWG
Installation
12345
Note 1 - Maximum Continuous Current is based on continuous power rating at the lowest input voltage.
Note 2 - NEC Current is based on the Maximum Continuous Current rating with a 125% NEC de-rating for
sizing the overcurrent device (when not continuous duty) to prevent it from being operated at more than
80% of rating.
Note 3 - 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 (if needed).
Note 4 - The next larger standard size overcurrent device may be used if the derated cable ampacity falls
between the standard overcurrent device sizes found in the NEC.
Note 5 - 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.
