# Sizing breakers for RV system, math check

Solar Expert Posts: 121 ✭✭✭✭✭
Putting together my NAWS order and want to make sure I get the correct breakers.

I guess I will use a MS baby box even though one of the example systems on the NAWS site seems to use the Blue Sea marine surface mount DC breakers which would be a lot more space saving. Those are thermal though and the Midnite stuff is magnetic so maybe better?

My system consists of 4 270 watt panels wired 2s2p feeding into a Midnite Classic 150 which charges a 24V battery bank consisting of two Lifeline 125AH 12V AGM batteries.

The panel specs are VMPP 31.5V, IMPP 8.58A, VOC 38.5V, ISC 9.17A

Paralleled then series the solar output at MPP would be 63V at 17.16A

The peak charging amps to the battery at about 24V (if they were seriously drained) would be 45 amps.

Of course you get those rare times where the panels are cold and the sun is just right and you may get higher power output.

I am thinking I need a 20 amp breaker for the panel output and a 60 amp breaker for the CC output. They don't make a 25 amp breaker and 30 amps seems too large. Actually I don't really understand the point of a breaker on the solar side of things at all...why not just use a switch? The solar cells can't produce a near infinite amount of current like a AGM or lithium battery...short circuit current is very close to maximum power point current. Maybe the solar side breaker is just meant to be a switch to allow you to shut down the solar power to service the CC.

• Solar Expert Posts: 3,045 ✭✭✭✭
Re: Sizing breakers for RV system, math check

Hi Skyko,

Your thinking on the breaker sizing seems good. Technically the MidNite breakers need to be sized at 125% of Isc for the PV side, and 125% of max current (because these breakers are rated to carry rated current without tripping) on the battery side. So, you are correct 25 A would have been nice for the PV input, but 30 A is should be -- the MN breakers are certainly better than any thermal breaker.

You simply must size the conductors to be able to carry the current of the circuits.

The Baby Box is very small. With only two breakers in one, you can often get by, BUT large conductors are difficult to route in such a small box. The Big Baby is larger and not much more expensive.

The MidNite breakers are inexpensive, and are rated to break rated current about 10,000 times, without failure. It is very difficult to find a DC-rated switch for anything close to the cost of the MN Breakers.

Any, yes, when you update the FirmWare on a Classic, you will want to switch off, the PV input, and then the battery breaker on the output of the Classic. Then turn on the battery breaker, and next the PV breaker (this is to be gentle on the CC). So, it is primarily a switch. But, in the case of a catastrophic fault, the breaker is there to protect the cable for overcurrent and possible fire.

My opinions. Vic
Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
• Solar Expert Posts: 121 ✭✭✭✭✭
Re: Sizing breakers for RV system, math check
Vic wrote: »

Any, yes, when you update the FirmWare on a Classic, you will want to switch off, the PV input, and then the battery breaker on the output of the Classic. Then turn on the battery breaker, and next the PV breaker (this is to be gentle on the CC). So, it is primarily a switch. But, in the case of a catastrophic fault, the breaker is there to protect the cable for overcurrent and possible fire.

My opinions. Vic

Thanks Vic, I will get those breakers.

I still don't understand how the breaker protects anything on the solar side. If you use 10 gauge wire, which can easily handle 30 amps without melting and you have solar cell panels with a short circuit current rating of less than 20 amps total, then the only way there could be a fire (non arc fault which a normal breaker would not protect you anyway) is if you drive under a giant magnifying glass with your RV and the panels produce 200 to 300% of rated power.
• Solar Expert Posts: 3,123 ✭✭✭✭
Re: Sizing breakers for RV system, math check
Skyko wrote: »
Thanks Vic, I will get those breakers.

I still don't understand how the breaker protects anything on the solar side. If you use 10 gauge wire, which can easily handle 30 amps without melting and you have solar cell panels with a short circuit current rating of less than 20 amps total, then the only way there could be a fire (non arc fault which a normal breaker would not protect you anyway) is if you drive under a giant magnifying glass with your RV and the panels produce 200 to 300% of rated power.
The breaker is not there to protect against short circuit current from the panels damaging any of the wiring. It is there so that the full set of other strings or back current from the battery in the case of a failed CC cannot damage an already defective string by sending current backwards through its panels and its wiring.
SMA SB 3000, old BP panels.
• Solar Expert Posts: 3,738 ✭✭✭✭
Re: Sizing breakers for RV system, math check
Skyko wrote: »
I still don't understand how the breaker protects anything on the solar side.

It doesn't if it is designed correctly. The array shouldn't be able to produce enough current to overheat the wire or trip the breaker. The breaker functions as a switch. If it is a polarized breaker, the plus (or line) side receives the combiner cable and the negative (or load) side of the breaker goes to the controller.

In order for that breaker to interrupt battery current, there would have to be 2 faults, one fault would be a short circuit between the PV plus and PV minus cables and the other would be a fault in the controller. If this were to happen the breaker in the PV line, if polarized, would be of the wrong polarity to break the current. If battery current were to get through a faulty controller to a shorted panel, it would still have to pass through the breaker between the controller and the battery, and that breaker, if polarized, would be polarized properly to interrupt battery current.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 175 ✭✭✭
Re: Sizing breakers for RV system, math check
Skyko wrote: »
.............................snip..................................

I still don't understand how the breaker protects anything on the solar side. If you use 10 gauge wire, which can easily handle 30 amps without melting and you have solar cell panels with a short circuit current rating of less than 20 amps total, then the only way there could be a fire (non arc fault which a normal breaker would not protect you anyway) is if you drive under a giant magnifying glass with your RV and the panels produce 200 to 300% of rated power.
It is more a case of code requirements, more for grid tie systems then RV. But if a camper maker wants to include solar and keep their RIVA cert. the breakers or fuses are also required.
I used them on my system because of "Murphy"