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) 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 greater than 1.5 m (5 ft), 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 as previously identi
fi
ed 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 the 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, standard
safety practices require 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 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, standard safety practices require that
the power 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 industry standard
requirements).
Table 2-1, Recommended DC Wire/Overcurrent Device for Rated Use
Inverter
Model
Maximum
Continuous
Current
1
Using Conduit
In Free Air
DC
Grounding
Electrode
Wire Size
4
Minimum DC
Wire Size
(rating)
2
Recomm.
DC Breaker
Size
3
Minimum DC
Wire Size
(rating)
2
Maximum
DC Fuse
Size
3
MS4124PE
273 amps
107.2 mm
2
(#4/0 AWG)
260 amps
250 amps
5
67.4 mm
2
(#2/0 AWG)
300 amps
300 amps
with time
delay
13.3 mm
2
(#6 AWG)
MS4348PE
143 amps
67.4 mm
2
(#2/0 AWG)
195 amps
175 amps
33.6 mm
2
(#2 AWG)
190 amps
175 amps
with time
delay
13.3 mm
2
(#6 AWG)
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 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
3
– The next larger standard size overcurrent device may be used if the de-rated cable ampacity falls
between the standard recommended overcurrent devices.
Note
4
– As recommended, the DC grounding electrode conductor can be a 13.3 mm
2
(#6 AWG) conductor
if that is the only connection to the grounding electrode and that grounding electrode is a rod, pipe, or plate
electrode.
Note
5
– May not allow continuous operation at full rated power.
