Fuse selection for solar panel DC control boxes

[Tradie]

What size fuses would you use in the DC control box?

If you have strings of 460 watt panels, wired with 4 mm solar wire and what considerations should be taken into effect?

You are protecting the wire, right so a 30 amp fuse would be used ?

Did you take the voltage worse case scenario in account ?

Do solar panels have built in overload protection ?

Does the inverter have built in protection, if so why even fit a DC control box ?

Something to think about on this cloudy Tuesday morning.

[GCE]

What size fuses would you use in the DC control box?

You are protecting the wire, right so a 30 amp fuse would be used ?

.

You making a simple decision complicated - Panel spec has a max fuse rating in the spec if you still want a warranty

Does the inverter have built in protection, if so why even fit a DC control box ?

Warranty conditions - Most inverter manufacturers insist on a DC combiner box with fuse protection and surge arrestors


SANS 10142-1
5 Fundamental requirements
5.1 General
All commodities in an electrical installation shall be installed in accordance
with the requirements in this part of SANS 10142 and with the manufacturer's
instructions, where applicable.
NOTE 1 This clause contains the general safety principles applicable to electrical
installations.
NOTE 2 The manufacturer's instructions may contain more stringent requirements.

[Tradie]

I agree, such a s simple decision, that is why I ask myself, if it is so simple why would someone use a 30 amp fuse, or even a 20 amp fuse as per the manufacturer's spec sheet.

If the short circuit current (Isc) 9.2 amps, and the max current is less than 9 amps, why would you install a 20 or 30 amp fuse, maybe, because electricians are trained to select overload protection as per wire size ( taking into account all the factors as per the SANS regs) ?

I ask these questions, because I am interested to know what is going on in a persons head when they are designing and installing installations. If someone doesnt ask the simple questions on an open forum, and use the responses to guide people we are never going to get it right.

You making a simple decision complicated - Panel spec has a max fuse rating in the spec if you still want a warranty

[Tradie]

For the record I am not in a pssing contest with anyone, and have no interest in proving anything to anyone, I just feel this industry needs help and the more information we can share, hopefully the information will help improve the industry.

To clear up something else, I know I tell people I am a DIY solar installer, but in reality I do have a few qualifications and some experience in the electrical industry.

[Justloadit]

Since PV panels are a current source, at no time will you overload the panel, even under short circuit conditions.
In my opinion, the purpose of the fuse is in the case the equipment being supplied by the solar panels develops a fault that it may feed back a voltage to the PV panels, having the fuse in line reduces the risk of the PV panels failing and causing a fire. So the fuse is not to protect the cable but rather to protect in case of a catastrophic failure. In my opinion the fuse should be approximately 50% higher than the PV panel current rating.

Now there is another situation, what happens when you start placing PV panels in series, and then placing the strings in parallel?
Well each string should have a suitably rates Schotky "Series Diode" to isolate each string from each other, and each string should have a fuse rated at no ore than 20% higher than the string short circuit current. Again the fuse is to protect the string in case there is a failure in the string or in the series diode. Just remember that the series diode is going to run hot, that is why I recommend a schotky diode, is volt drop across the junction can vary between 0.5V and 1.5V depending on the diode specification voltage. Just a quick calculation. If the panel is pushing 10A and the diode used has a volt drop of 1.5V then we are talking about 15W of heat generated by the diode, so a suitable heatsink is required. That is why the low cost combiners do not have any diodes, and rely on the fuse to do the protection of the PV string.