Sizing PV Battery Cables
Choosing the proper size battery cables for a Photovoltaic (PV) System can be a bit ambiguous. You
can calculate the maximum continuous current on the cables by taking the rated power of the Inverter,
and then dividing that by the nominal voltage of your battery bank and the efficiency of the Inverter.
Mathematically, that is:
Inverter’s Rated Power (watts )
Max. Cable Current (amps) = Battery Bank Voltage (volts) x Inverter efficiency (a fraction < 1)
Since DC Power (watts) equals Voltage (volts) times Current (amps), you can calculate the power draw
of the inverter under different circumstances by multiplying it’s Output Current by it’s Output Voltage
(typically 120VAC or 240 VAC). This will usually give you a close approximation of the actual AC
Power, but it ignores the AC Power Factor. This value is called the VA (or KVA) rating, and it is,
actually, more pertinent to our battery cable calculations. Again, mathematically we have:
DC Power (watts) = Voltage (volts DC) x Current (amps DC)
and,
AC Power (watts) = Voltage (volts AC) x Current (amps AC) x Power Factor (a fraction <= 1)
Knowing this is useful because sometimes the inverter specs will list a maximum continuous current at
a specific voltage, instead of the power. Also, usually an inverter can surge a higher power than it can
put out continuously. Sometimes the specs will list a one minute to a thirty minute surge capacity in
amps or watts, as well.
So knowing how much current your battery cables will need to carry is relatively straightforward.
Where things get a bit vague is knowing how much they can carry. A simple way is to look up the
current carrying capacity, called Ampacity, of different sizes of wire in the National Electrical Code
(NEC). Normally you would look up the values in the column for 75o C. This will tell you what size
cables will “safely” carry the current you need for your inverter. The drawbacks to this approach result
from the fact that the NEC Ampacities are for relatively long runs of wires carrying AC current through
building walls and crawl spaces, and they are, appropriately, very conservative. You will end up
spending a tremendous amount of money to purchase wires that are probably much larger than you
need. These large wires will be cumbersome to install, and may not even fit into all the equipment you
need to connect to your system, probably because the manufacturers know that battery wires sized per
the NEC are a complete overkill.
Many people use welding cables to connect their batteries, so it would be natural to look at those
amperage ratings. However, welding ratings have the opposite problem than building wires. They may
be rated for currents that would be too much for battery cables in a PV system. Welding equipment
typically has a Duty Cycle Percentage Rating, which is the number of minutes out of ten that it can be
used continuously before it has to cool down. Typically these vary from 20% to 80%, and even though
a lot of modern welding equipment is rated at a 100% Duty Cycle, welding processes do not tend to run
continuously due to practical limitations.
Another way of evaluating wires is by their Marine Ampacity rating. This specification only applies to
cables with their associated fittings that are designated for Marine applications, so unless you purchase
Marine cables and fittings, these ampacities may not, technically, apply to your system. However, they
do provide values that usually lie between the NEC Ampacity ratings and Welding Wire ratings, so I
use them as a rough guide.
I will use my system calculations as a practical example. I have four Magnum MS4448-PAE inverters
connected in parallel. They are rated at 4,400 watts each, so the combined output should be about
17,600 watts. At 240VAC each inverter can put out 18.3 continuous amps (73.2 amps total).
Additionally, each inverter can provide 4800 surge watts for 30 minutes, and 5400 surge watts for 5
minutes. That would be total outputs of 80 amps and 90 amps, respectively. The Battery Bank is 48
volts DC, and the Inverter’s efficiency at maximum output is 88%. My main battery cables are each
about 8½ feet in length.
Using the continuous amperage figure for my calculations yields:
Current on Main Battery Cables =
73.2 amps x 240 volts
48 volts x 0.88
17,568 watts
= 42.24 volts
= 415.9 amps
According to NEC Table 310-16, I would need battery cables that are 600 kcmil (or MCM). Each
cable would need to be over an inch in diameter at a cost of over $16 per foot (in 2023).
According to Lincoln Welding, an AWG 3/0 welding cable can carry 400 amps for a one-way distance
up to 25 feet, and maintain a 75 o C temperature. That amperage capacity ought to work in my system.
Each cable would measure about 5/8 inch in diameter, and would cost about $5.50 at current prices.
For single cables, not bundled or in conduit, the allowable Marine amperage for AWG 4/0 cable at 75oC
is 360 amps, but that goes down to 252 amps if they are bundled two or three cables together. I was
unable to find consistent data for cables larger than 4/0 in marine applications, which just underscores
the difficulty associated with identifying proper size battery cables. For comparison, the Welding
amperage of 4/0 cable is more than 600 amps @ 60% Duty Cycle but somewhat less than 600 amps @
100% Duty Cycle, according to Lincoln Welding. The current price is about $7.50 a foot.
Eventually, I decided to go one size larger than AWG 4/0 cable, and purchased 250 kcmil (MCM)
welding cable, which is over ¾ inch in diameter, and costs about $11.00 per foot. According to the
NEC table, it is rated for 255 amps in the 75o C column and 290 amps in the 90o C column, both of
which are significantly below the maximum continuous amperage of my combined inverters. I
couldn’t find any reliable specs for Marine 250 kcmil (MCM) cable, but the Supplier’s specs listed for
the actual welding cable I purchased claim it can carry 577 amps @ 90o C, much more than I need.
However the same product listing says that for this general type and size of cable the ampacity rating is
445 amps (just to add to the general confusion on the subject).
To be on the safe side, I could put a smaller AC main circuit breaker on my system. This would limit
the continuous current on the battery cables to:
60 amps x 240 volts
Current on Main Battery Cables =
48 volts x 0.88
=
340 amps
(Which would allow higher surges but probably not the 4 x 5400 watts for 5 minutes, or the 4 x 4800
watts for 30 minutes that the equipment is capable of producing.)
Other possibilities are:
65 amps x 240 volts
Current on Main Battery Cables =
48 volts x 0.88
=
369 amps
70 amps x 240 volts
48 volts x 0.88
=
398 amps
Current on Main Battery Cables =
Conclusion
So, according to the Supplier’s (and, presumably, the Manufacturer’s) specs, my main battery cables
should be more than large enough for the maximum load, but by other measures, specifically the NEC
cable ratings, they are insufficient. This is where the situation is ambiguous. The 250 kcmil (MCM)
cables I bought were about the largest I could readily purchase as an average consumer, and they will
be difficult to install because of their size. Another option would be to run two smaller cables in
parallel for each main battery connection, but that would also be cumbersome to install. It would be
nice if there was an NEC or Industry standard ampacity rating specifically for PV Battery Cables, that
accounted for the unique conditions of this particular type of application.
Article written by Gregory Robert Hill 1/2024