circuit protection of small solar system

1 Posts
Jun 22, 2004 12:35 pm
circuit protection of small solar system

I have a solar panel, MorningStar regulator, battery, and inverter in order to run a cash register and scale. Where do I need to put a fuse or breaker to protect the system? Does a fuse  within the DC circuit next to the battery also protect the circuit to the solar panel? Should the AC circuit have a fuse?

8 Posts
Jun 23, 2004 06:44 am
Re: circuit protection of small solar system

berry, it is always a good idea to install breakers or fuses within a system to disengage power to the components in order to work safely on them. I believe the Morningstar already has internal circuit controls to keep it and your batteries safe but this does not keep you safe from the rest of the equipment. It is best to set up your system just as you would any household electrical system. The solar panel is the power in, such as from the street to a meter, so install a breaker/switch between it and the controller, based on current output of panel. The batteries are the source of power after the meter to the load, so put a breaker in between the batteries and either the controller (if you are using the controller to attach your load) or a load breaker, sized on current draw of appliances,. So three breakers in all PV, batteries, load......attach output of load to inverter if you need 110V  
Jul 11, 2004 07:45 pm
Re: circuit protection of small solar system

LAST EDITED ON Jul-13-04 AT 07:05 PM (EST)

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.

Jul 18, 2004 10:39 am
Re: circuit protection of small solar system

Something I happened to think of later is Ground Fault Circuit Interuption or GFCI on the vac side of your inverter. I gathered from your post that you are operating a roadside fruit and or vegtable stand and that the remote PV/battery/inverter system your using is not grounded to Earth. If this is true then there is the possibility that someone could become that path to Earth and get a jolt of electricity if there is not GFCI protection on the vac.
Typically the neutral is bonded to the equipment grounding conductor or EGC (which should go to Earth via ground rod) in one location only. But GFCI protection on certain branch circuits is still Code if not just plain smart.
In the 120 vac (or any polyphase ac) system that your using the neutral has the potential of having 120 vac on it. This current will seek the path of least resistance to Earth.
For example, if you where standing on wet Earth barefoot and touched a metal part of the scales or register, and a neutral wire with a bare spot on it was touching metal (anywhere in the whole system if there is a third equipment grounding wire) you would become that path to Earth and get shocked by 120 vac. GFCI protection limits the duration of the shock to milliseconds by disconecting the circuits power supply. This could happen even in a system that has a ground rod because the current will take the path of least resistance.
So to make it short, use an inverter that has a GFCI receptical built into it. Or a plug in type, but not all types of GFCI devices are compatible with all inverters.
And again seek the advice of a licenced electrician with knowledge of PV and the balance of these systems.  

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