circuit protection of small solar system

LAST EDITED ON Jul-13-04 AT 06:06�PM (EST)

LAST EDITED ON Jul-13-04 AT 05:51�PM (EST)

Fuses and breakers mainly protect the wires in any type of electrical system. The size of the protection depends on the wire size. Although, when conducting low voltage dc electricity sometimes larger wire is used than is nesscesary for the amperage being conducted in order to keep voltage drop to a minimum. Such is the case when a PV array is a long distance from the battery it is charging. Size the fuse to the current being conducted in this case.
To start, look at the short circuit or Isc rating of the PV module. Chances are one module may not be much more then 10 amps Isc. at 12 vdc nominal.
#14 awg copper would carry this current safely with a 15 amp fuse but, if the PV module is more than 20 feet from the battery you might want to use #12 awg copper but keep the 15 amp fuse. Your PV module may be less than 10 amps. If you want to be NEC & UL compliant then you would add 156% to the Isc before sizing wire and fuse's. Thats 125% for NEC X 125% for UL X Isc = ________.
I would place an inline fuse between the PV module and the charge controller as well as the charge controller and the battery.
As for the inverters vdc input, I would take the maximum wattage and divide that by the the lowest voltage before it shuts itself of. This will give you the largest wire size it needs and I would size the fuse according to the size of the wire. For example, if the inverters continuos rated output is 600 watts and it shuts itself off at 10.5 vdc this would come to 57 amps. #6 awg copper is rated at 60 amps so a 60 amp fuse would be good. Again sometimes wire size is larger in order to keep voltage drop to a minimum if the inverter is a long distance from the battery. #4 awg copper wouldnt hurt but keep the 60 amp fuse.
As for the vac, having just one or two devices plugged directly into a small inverter I would not fuse its vac output. Chances are it has its own protection built in. If it where a large inverter powering many loads through out a house then I would protect the branch circuits.
As for where to place the protection. Remember that its protecting the wire but the fuses need to be protected as well from physical damage.
I would place the PV source fuse close to the module as long as its out of the weather (both sun and rain) and the charge controller and inverter fuses just outside of the battery compartment if conveinent. Be sure that the fuse's and fuse holders are UL listed with vdc ratings. Most automotive fuse's and fuse holder's are not rated for PV systems because the open circuit voltage of the PV module can be much higher than a alternator.
Fuse's and fuse holder's are not consider as a means of disconect. So if any switches or fused disconects are used they too need to be UL listed with dc voltage ratings equal to or higher than the open circuit voltage of the PV module and have amperage ratings no less than the fuse size.
Be sure these divices are for copper wire and not aluminum only. Personally I would not want any aluminum any where in a low vdc system.
The same goes for breakers and breaker panels. Square D - QO breakers and panels are UL listed with vdc ratings of upto 48 vdc. Thats not to say that one can use them in a 48 vdc nominal PV system because of the PV modules open circuit voltage. Some 24 vdc nominal system would not be compliant.
Every situation is different. So check with a qualified electrician. Have him look at the situatuation personally.