Circuit breakers in a Combiner Box?

[Diemjoe]

I have 6 200w 12v panels and 6 100W Panels with the following specs:

Power watts	Pm	200​	100​
Max Pwr voltage	Vmp	20.4​	18.5​
Max current amps	Imp	9.8​	5.4​
Open Circuit Vltg	Voc	24.3​	23.2​
Short Crct amps	Isc	10.2​	5.7​
Max Voltage	DC	1000​	600​
Max Series Fuse	Amp	15​	10​
PMAX Coefficient		-0.46​
Voc Coefficient		-0.36​
Isc Coeffient		0.05​

Based on some on-line calculators, it looks like my best Combiner setup is

All identical panels wired in SERIES, then all series strings wired in PARALLEL:
Max power output:	1687.2W
Max power voltage (Vmp):	111V
Max power current (Imp):	15.2A
Loss:	6.21%​

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With that data in-hand, I bought a Midnight solar MNPV6 combiner box and 2 15amp 150VDC breakers. I was wondering if the calculated amperage of 15.2amps for each parallel circuit or on the combiner bus bar. Am I cutting it too close on the 15amp circuit breaker?

Also, I am trying to figure out the DC Miniature Circuit Breaker. Am I correct in my thinking is that they serve to protect the wire from the combined amperage coming out of the 2 circuit breakers. Is there anything special about the MCB's or are the just higher capacity fuses. I would also like to get a surge protector. Midnight Solar's solution has to be replaced if it trips. Is there anything similar that can reset?

Thanks

Don Miller

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[Horsefly]

When I first read that you were going to put the like kind in series and then combine the two series strings at the combiner I thought for sure that was wrong. But looking at the Vmp of both panels, it does seem like that would work, and would actually be the best way to wire them.

The circuit breakers should be above the Isc of each string, and I personally think it is wise to have the breaker a bit below the fuse size for the panels. In your case the max current out of the two strings will be different: 10.2A and 5.7A. So strictly speaking, you should probably be using a 12A breaker on the 200W string, and a 8A breaker on the 100A string. You probably can't find breakers that size. Seems to me like the 15A may be fine for the 200W string, but maybe a 10A would be better on the 100W string.

I don't actually know what you are talking about the the "DC Miniature Circuit Breaker" so I can't really answer that question.

The SPD (Surge Protection Device) is not something you should have to reset or replace short of something really going wrong (like a lightning strike). I've got 3 Midnight SPDs on my system (one at the breaker box). They are fine after 6 years, and I don't expect to ever have to replace them, I will admit I have one spare for the solar array, just in case.

 

[Hedges]

(Thermal) breakers or fuses should be sized 1.56x Isc (of string or array they have to carry.)
That's the usual 1.25x to avoid nuisance tripping x 1.25x for cloud edge effect (full sun plus additional light.)
That's written into NEC.

Also, no higher amperage rating than what PV panel label says, and no higher than ampacity of wire.

(Also run a ground wire from inverter/SCC to PV panel frames (or array that frames bond to, if proper bonding like WEEB.)

One or two strings in parallel normally doesn't require OCP. If you have two strings of similar voltage but different amperage connected in parallel, then at least the lower amperage string ought to have fuse or breaker.

I would also like to get a surge protector. Midnight Solar's solution has to be replaced if it trips. Is there anything similar that can reset?

They absorb brief spikes of 1000's of amps, absorb thousands of Joules (watt-seconds.) Resettable parts can't do that. They can take a number of smaller hits without failing. Nice thing about the Midnight is they have LED that show "protecting" or "failed", at least in AC applications. The MOV inside you could probably replace, available from DigiKey for a few dollars. I bought a bunch, going to put on my inverter PV inputs. Some day.