# What size fuse or circuit breaker to use

Posts: 4Registered Users
I am getting ready to install solar panels on my RV. I would like to fuse the positive line from the solar panels to the charge controller but don't know what size fuse should be used. There is only about 15% difference between short circuit current and max power current for the panels I have. Suppose my nominal noon day current is 15 amps, if I fuse at 20 amps, a short in the positive line would not draw enough current to blow the fuse. If I fuse at 15 amps I might blow fuses every day. Am I missing something here? What should I use?

Jim

• Posts: 17,615Banned ✭✭
Re: What size fuse or circuit breaker to use

Welcome to the forum.

What we really need to know here is the panel specs, how many there are, and how they are connected (all parallel, all series, or a combination thereof).
• Posts: 4Registered Users
Re: What size fuse or circuit breaker to use

There are two 12V, 140 watt panels connected in parallel. Short circuit current is 8.95 amps. Max power current is 7.73 amps.

Jim
• Posts: 3,121Solar Expert ✭✭✭✭
Re: What size fuse or circuit breaker to use
AvoJim wrote: »
There are two 12V, 140 watt panels connected in parallel. Short circuit current is 8.95 amps. Max power current is 7.73 amps.

Jim

Since you have only two panels, there is not much point in putting in a circuit breaker at all, except to act as a disconnect switch.

The CB will not protect the panels, since they can deliver short circuit current continuously with no damage. The CB will not protect the wiring, since it can carry the full Isc anyway.

The only thing that the CB would do is protect against a fault in the inverter allowing full battery voltage to be applied back toward a damaged panel. Not likely, and better handled by using a single breaker after the point where the two panels are combined.

If you have 3 or more panels, then you would use a fuse of CB for each, going into the combiner, to protect that panel in case a fault in that panel tries to draw current in reverse from all of the other panels. It could be sized well above the Isc of the panel.
SMA SB 3000, old BP panels.
• Posts: 4Registered Users
Re: What size fuse or circuit breaker to use

inetdog

I was more concerned about the positive side from the panels shorting to something that could not carry the current load without excessive heating and might cause a fire. But, since the max current and the nominal current are so close I don't think it makes sense. I assume fuses/CB or not that precise.

I didn't understand your reference to inverter, did you mean charge controller. I do plan to put a breaker between the battery and the charge controller.

Jim
• Posts: 3,121Solar Expert ✭✭✭✭
Re: What size fuse or circuit breaker to use
AvoJim wrote: »
inetdog
I didn't understand your reference to inverter, did you mean charge controller.

Yes. The biggest, most hazardous source of power is the batteries. There is a limit to how low you can fuse between the battery and an inverter because of normal load current, but between the CC and the batteries you can fuse at or just over the maximum output current from the charge controller. And that, in turn, will protect the panels in case the CC fails in a way that feeds battery voltage back through its input.
SMA SB 3000, old BP panels.
• Posts: 3,121Solar Expert ✭✭✭✭
Re: What size fuse or circuit breaker to use
AvoJim wrote: »
inetdog
I was more concerned about the positive side from the panels shorting to something that could not carry the current load without excessive heating and might cause a fire. But, since the max current and the nominal current are so close I don't think it makes sense. I assume fuses/CB or not that precise.
Jim

1. Yes, they are not that precise, and the point at which they will trip depends strongly on how long that current continues.
2. The difference in damage to external equipment that Imp from the panels could do is not significantly different from the damage caused by Isc.
SMA SB 3000, old BP panels.
• Posts: 4Registered Users
Re: What size fuse or circuit breaker to use
inetdog wrote: »
2. The difference in damage to external equipment that Imp from the panels could do is not significantly different from the damage caused by Isc.

That was my reason for asking the question. It just seemed "unprofessional" to not put some kind of protection into the PV circuit. I guess I will just pay extra attention to the wire runs to protect them from some kind of invasive action. Thanks for the responses.

Jim
• Posts: 3,121Solar Expert ✭✭✭✭
Re: What size fuse or circuit breaker to use
AvoJim wrote: »
That was my reason for asking the question. It just seemed "unprofessional" to not put some kind of protection into the PV circuit. I guess I will just pay extra attention to the wire runs to protect them from some kind of invasive action. Thanks for the responses.

Jim

Glad you asked. There are quite a few electrical professionals who get into unfamiliar water when trying to deal with solar. Engineers included..... :-)
SMA SB 3000, old BP panels.