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Pre-Wired Combiner box (MNPV4)- adding a 5th string?

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Hi Midniters,
I have a standard TYCO-MNPV4 prewired combiner box (it came with 4 fuses), but I currently only use 3 of those fuses for my 3 strings.  I am now planning to add 1 or 2 more strings of similar 250 watt grid tie panels (so expanding my current total of 2000w of PV to either 2750 or 3500 watts on a 48v system).

Adding the 4th string should be easy because I have the spare bay. Go midnite prewired combiner box, whoo hoo!

But what about a 5th string?

I havent looked in the box recently, but is there room for a 5th fuse (or breaker, ideally).  I remember from a previous discussion about the MNPV3 enclosure that Robin said a 4th string could be added if a larger din rail was installed.  See quote below.  Is there enough room on the prewired MNPV4 for a 5th string (ie breaker)?

Robin: "Many people have asked about this in the past, so I may as well come clean. When I designed the original MNPV3 years ago, part of the design was the deadfront. The deadfront is actually large enough for 5 breakers, but the din rail was made for three. Things would get very cramped for space with the output wire too since we always hang it off one side or the other. I did want to accommodate a possible future change, so if you look on the back of the deadfront, you will see that there are outlines for two more breakers. After a couple years of shipping the deadfront with this feature, I started to feel guilty that the din rail would only hold 3 breakers. I finally changed it to hold five breakers a few years ago. The busbar will only accommodate three breakers, but as you have discovered, you can get the 6 position busbar and cut it down. You will need to get a saw to cut out the extra hole (s) for tha additional breaker, but it will be a pretty inexpensive way to upgrade your present PV3 to a PV4 or PV5."

What is your string voltage?  If you can switch over to 150 volt breakers, I think that your combiner will hold 6 breakers in the same space as 4 fuses.


String voltage high is 108volts on the FM80 readout.

I kinda wished the MNPV prewired combiner came with breakers (you can open and close them under load, right?).  I think I could have ordered it that way, but the tyco boxes were being 'discontinued'....and I had a rushed order...

Anyway I get the drift, switch the fuses to breakers and 5 or 6 breakers could fit on the rail. 


--- Quote from: Surfpath on January 27, 2015, 12:26:04 PM ---I kinda wished the MNPV prewired combiner came with breakers (you can open and close them under load, right?).  I think I could have ordered it that way

--- End quote ---
IIRC, I advised you of this over at the NAWS forum.  It's not too late to make the switch... The only advantage to fuses is their higher voltage rating.   Grid tie strings tend to be hundreds of DC volts, too much for a small-sized breaker.  In a low voltage system like yours, breakers could make you very happy   :)


OK, I will switch over to breakers, I know it makes sense. Lemme just finish a few other solar projects for my family and buddies.


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