Sizing guide
How to size a DCDB box for a 3kW, 5kW or 10kW solar system
Two numbers decide most of the sizing decision: how many strings you are running, and what voltage each string sits at. Everything else follows from those.
We get this question often enough that it deserves its own guide, rather than a one line answer over the phone. If you are speccing a DCDB box for a residential or small commercial job, here is how to think through it.
Step one: count your strings, not your kilowatts
It is tempting to size a DCDB purely off system capacity, a 3kW system gets a small box, a 10kW system gets a bigger one. That is roughly true but it skips the number that actually matters: string count. A 3kW system wired as a single string needs a simple 1 in 1 out DCDB. A 3kW system split across two smaller strings, which happens more often than people expect on roofs with shading or multiple orientations, needs a 2 in 1 out configuration instead, even though the total capacity is identical.
So the real first step is working out the array layout, not the kilowatt rating. Once you know the string count, the DCDB size mostly falls into place.
Rough starting points by system size
These are starting points, not fixed rules, since every roof and inverter combination is a little different.
- 3kW, single string: a 1 in 1 out DCDB is usually enough, with one DC MCB or fuse and one surge protection device.
- 5kW, commonly two strings: a 2 in 1 out configuration, combining both strings into one protected feed before the inverter.
- 10kW and above: often three or more strings depending on the inverter's MPPT inputs, which means a multi-string DCDB built to that exact count.
Notice that MPPT count matters as much as raw kilowatts here. An inverter with two MPPT trackers effectively wants two separate string inputs even at a smaller system size, because each tracker performs best with its own dedicated string.
Step two: check the string voltage
Once you know the string count, the second number is the voltage each string will run at, which depends on how many panels you have wired in series. This decides the voltage rating the DCDB's MCBs and surge protection device need to handle safely. Getting this wrong is a quieter failure than getting the string count wrong. It will not stop the system from working on day one, but it leaves the protection undersized for the voltage actually present, which shows up as a problem only when something goes wrong.
Step three: think about future expansion
If a client mentions they might add panels later, even informally, it is worth sizing the DCDB with one or two spare ways rather than exactly matching today's string count. Swapping a whole distribution box a year later, because the client added a second array, costs more in labour than the small difference in price would have cost upfront.
What we need from you to quote a DCDB
When you send us a sizing request, three details get you an accurate quote on the first pass: the string count, the string voltage, and whether the installation is rooftop or ground mount, since mounting affects the enclosure rating we recommend. Pair it with a matching ACDB box sized to the inverter's AC output, and an earthing kit for the array structure, and the protection side of the job is fully specified in one enquiry.
Send your string count and voltage through the contact page or over WhatsApp, and we will confirm the DCDB configuration before you place the order.