combiner boxes

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.