A combiner box is a simple electrical
component for combining and housing the wiring coming from your solar
panels. Depending upon your particular solar system design you
may or may not even need one of these components. For many
systems the wires coming from the strings of solar panels are combined
inside the inverter so there is no need to use a combiner box.
However, if the distance between your panels and your inverter is
significant, or if you have more than 2 or 3 strings in your solar
system, then a combiner box may be useful.
Combiner boxes are usually small rectangle boxes made of sheet
metal or fiberglass. If you are going to use one then it will
usually be mounted near the solar panels, either on the roof or just
below where the wiring exits from the panels. Solar panels are
usually wired together in a string of somewhere between 6 to 10
panels. So if you have a significant system which has 18 or more
solar panels then you usually have 2 or 3 strings of wiring. The
exact number of panels to the string is usually dependent upon the
characteristics of your inverter, and whether or not you are putting
in a grid-tied or off-the-grid solar system. The function of the
box is straightforward. Multiple string inputs are combined in the box
and then a single set of wires (positive, negative and ground) go out
of the box to the inverter.
A combiner box will usually have the following components:
- Outer Housing - The combiner box has either a metal or
fiberglass housing that holds the wiring. The housing protects
the wires from the weather and has to be completely water tight.
Combiner boxes should be certified as compliant with the National
Electrical Code (Article 690.4D) and they are one of the components
that most inspectors will look at when inspecting a solar system.
The housing also helps to prevent people from getting close to the
wiring who should not have access. Most boxes will either
include a key or have a hole for padlocking the box so there is no
unauthorized access. One feature that you should look for in
the combiner box is what is called a dead front. This
is simply a metal or polycarbonate plate that covers up the wiring
and has a cut out for the fuses to stick through. This type of
feature allows a user to get at the fuses or breakers as needed
without being exposed directly to any of the wiring. This
feature has probably protected more than a few people from a nasty
shock when trying to flip a breaker.
- Overcurrent Protection - All combiner boxes will have a
set of either fuses or circuit breakers (also referred to as a an OverCurrent Protection Devices (OCPD)
which prevent a short from sending too much current to the
inverter. Fuses are more common than circuit breakers because
voltages in grid-tied systems can get up near 600 volts and circuit
breakers are usually only rated for about 150 volts DC. Usually there is one fuse for each string coming
from your system. The number of fuses depends upon the model.
For most systems 4 - 8 fuses should be enough but you could buy
models with many more.
- Wire Terminals - Usually a plastic bar that has holes it
it for you to insert the wires which come from your solar panel. Most terminals
are designed to hold wires of different sizes. The number of
holes depends on the model you choose but most models will have
enough wiring inputs for at least 8 strings. Bigger combiner
boxes can handle up to 20-30 strings.
Combiner boxes can provide a number of benefits. If the run
from the panels to the inverter is fairly significant then combining
them first will reduce the amount of wiring you need overall thus
reducing costs. Combiner boxes can also simplify the physical
wiring, particularly if you have 4 or more strings. In
off-the-grid systems it is not unusual to divide the panels into more
strings in order to keep the voltages closer to that of the batteries.
In this scenario the combiner box simplifies the wiring. Using a
combiner box also makes it easy to provide overcurrent protection
(fuses) for each individual string which can help prevent overloading
should one of the strings fail and backload into another string.
While such an event is unlikely it is good to have this added
protection. Another advantage found in some combiner boxes is the
ability to disconnect a single string easily which makes them easier
to service. You can just shut off the string you want to service
while leaving the other connected.
Combiner boxes are not particularly expensive components.
They can range in price from $40 up to about $150 depending upon how
many breakers or fuses you need. All major solar equipment
providers should carry them. Combiner boxes will be rated as to
the Maximum DC Voltage (Mas VDC) they are designed to handle.
Most grid tied systems are designed to stay under 600 volts by code so
a box for a grid-tied system should be rated to at least this level.
If the inverter is designed for an off-grid system where the voltages
are usually lower the combiner box may be rated as low as 150 volts.
Combiner boxes are expandable in most cases so if you think you may be
adding more strings of solar panels to your system at a later date
then you should probably buy a combiner box with more inputs and more
voltage than you have to have. That way you can easily expand
your system later.