What is an MC4 connector and an MC4 extension cable? If you're asking this question, you've probably noticed that most modern high power solar modules are manufactured with wire leads that have MC4 connectors on the ends. Years ago, solar modules were built with a junction box on the back that required the installer to manually attach wires to the positive and negative terminal posts. This method is still used, but it's slowly becoming a thing of the past. Modern solar modules tend to use the MC4 connectors because they make wiring your solar array much simpler and faster. The connectors come in both male and female types which are designed to snap together. They meet the requirements of the National Electric Code, they're UL certified and the preferred connection method of electrical inspectors. Due to the locking mechanism of the MC4 connectors, they will not come unplugged and are well suited for outdoor environments. The connectors can be separated but it requires a special MC4 unlocking tool.

 

Wiring MC4 Equipped Modules in Series:

If you have two or more solar modules to wire in series, the MC4 connectors make it very simple. Take a look at the first module and you'll notice that it has two wires extending from the junction box. One wire is the DC positive (+) and the other is the DC negative (-). Generally, the male MC4 connector is associated with the positive lead and the female connector is associated with the negative lead. This may not always be the case, so it's always a good idea to look at the markings on the junction box or test the polarity with a digital volt meter. A series connection is when you wire the modules together by connecting the positive lead on one module to the negative lead on another module. The male connector will snap directly into the female connector. Here's a simple diagram illustrating this.

 

 

 

 

 

 

 

 

 

 

 

As you can see, the two modules are now connected together by the two leads. This increases the voltage of the circuit. For example, if your modules are rated for 18 volts at maximum power (Vmp), then two of them connected in series will measure 36 Vmp. If you connected three modules in series, the total Vmp would be 54 volts. The current at max power (Imp) will be constant when wiring a series circuit.

 

Wiring MC4 Equipped Modules in Parallel:

 

Parallel wiring requires the positive leads to be connected together and the negative leads to be connected together. This method will increase the current at max power (Imp) while keeping the voltage constant. For example, let's say your modules are rated for 8 amps Imp, and 18 volts Vmp. If you connect two of them in parallel, your total amps would be 16 amps Imp and the voltage would remain at 18 volts Vmp. When wiring two or more modules in parallel, you will need some additional equipment. If you're only using two modules, the easiest method is to use MC4 multibranch connectors. You obviously can't connect two male connectors or two female connectors together, so we use the multibranch connectors to accomplish that. There are two different multibranch connectors. One type accepts two male MC4 connectors on the input side and has a male MC4 connector for its output. The other type accepts two female MC4 connectors and has a female MC4 connector for its output. Essentially, you've stepped down the number of wires from two positive and two negative to one positive and one negative. Here's a diagram so that you can see what it's doing.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

If you are paralleling more than two modules or you're paralleling strings of modules, that requires a device called a PV combiner box. You no longer need the multibranch connectors because the combiner box will be performing the same function. The multibranch connectors are only suitable for paralleling two modules together. If you need to connect more than two modules or strings in parallel, a combiner box is used. The total number of modules that can be combined will depend upon the electrical rating and physical size of the combiner box. Whether you've combined your modules with the multibranch connectors or with a combiner box, you'll need to understand how to select and use MC4 extension cables.

 

ref:http://www.solar-electric.com/how-to-use-mc4-connectors-cables.html