Power Inverters DC cables
The link between the battery and the inverter carries a very high current; it should be maintained as short as possible; ideally no more than 12 feet although it is not rare to find lengths of 30 feet.
A good practice would be to set the inverter in a different compartment than the battery or bank of batteries, so that there is a trade-off between the link length and different compartments.
There would be an appreciable voltage drop in the cables if the cables cross section area is not enough. For more battery-inverter distance, more cable cross section area is needed. It is recommended to follow the instructions for cables specified by the inverter manufacturer; otherwise what follows could be applied.
An acceptable drop in both cables combined (positive and negative) is 0.5V for a 12V system and 1.0V for a 24V system but it is customary to consider 0.5V drop in both systems.
The next expressions calculate the wire cross section area Wa[mm2] in square millimeters for 12 volt and 24 volt systems as a function of the inverter output power P[watt] in watt and the distance battery-inverter L[ft] in foot. It is considered a cable temperature of 75°C (167°F) and an inverter efficiency of 85%.
12 volt system: Wa[mm2] = 0.0025 x P[watt] x L[ft]
24 volt system: Wa[mm2] = 0.0012 x P[watt] x L[ft]
For distances less than 5' use L[ft] = 5'.
The results are in [mm2] in order to have linear expressions.
The following table transforms [mm2] to AWG#
(American Wire Gauge number).
Note
The size of these cables is independent of the
inverter AC voltage and frequency output
(120 VAC/60 Hz; 220 VAC/50 Hz).
Example
If we want to determine the cross
section area of the DC cables as AWG# for a battery or
bank of batteries of 12V and an inverter of P[Watt] = 2000 Watt
with a distance between the battery and the inverter
of L[ft] = 12 feet, the application of the formula is:
Wa[mm2] = 0.0025 x 2000 x 12 = 60 mm2
From the table:
60 mm2 => AWG# = 2/0
Important
With those determined AWG#'s the
voltage drop in the pair of cables combined is around 0.5V or
less at nominal power and for the selected length.
Cases with power surge, even for inverters with "soft-start", could require an AWG# that corresponds to a higher cross section area in order to avoid an inverter shut down at the moment of the surge (even more when the battery is not fully charged).
Besides there are other resistances in the DC circuit that provoke voltage drops, like the internal battery resistance and the terminal contact resistances. So, it is helpful to minimize the voltage drops where we have access to do it (DC cable pair drop) in order to eliminate unexpected inverter shut downs.
For those cases the selection should be made using the power surge in place of the nominal power.