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

[Surfpath]

 
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."

[vtmaps]

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.

--vtMaps

[Surfpath]

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. 

[vtmaps]

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

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   

--vtMaps

[Surfpath]

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.
 

[Surfpath]

Hi All,
It has been a few years since the last post on this thread, but I am now making this panel addition (more home time thanks to Covid 19!). I have bought the midnite breakers as VT Maps recommended. The 5 of them should fit in the MNPV 4 combiner box fine. I just wanted to now check the numbers on the wire run between my combiner box to the charge controller.

Right now I have #6 AWG wire running ~50 feet (or 100 ft in a circuit) in 1.25” conduit with 3 conductors total. I get about 103 volts maximum on my controller (my panels are in series of 3).

Here’s my question: If I were to add this 5th series of panels (ie go from 12 panels to 15), would the 6AWG on my combiner to controller run be enough (to handle the increase in current)?

Cheers,
Surfpath

Panels
SONALI 250 Watts
VMP: 31.32
IMP: 7.98
VOC: 37.2
ISC: 8.45

 

[vtmaps]

Here’s my question: If I were to add this 5th series of panels (ie go from 12 panels to 15), would the 6AWG on my combiner to controller run be enough (to handle the increase in current)?

Your voltage drop (50' run at full power) will go from 1.37% to 1.71% when you add a fifth string.  At full power you will be burning 64.2 watts in the cable.  Your combined Isc will be 42.25 amps.  I think that is within the ampacity of #6 copper.

--vtMaps

[Surfpath]

Thanks VTMaps for the calculation. So we should be within the wire amperage maximum.

Out of curiosity, on the tables that are commonly used for the NEC maximums, there are several levels of copper wire amperage maximums, depending on the conductor temperature ratings. So, for example, 6 AWG copper wire is rated at 55/65/75 Amps for conductors that are rated at 60/75/90 degrees C. I have read elsewhere that these temperature ratings are called “terminal temperature limits” and are usually marked on the cable (if not one must assume the lowest rating).   

My questions is: The 6AWG Ancor primary cable that we are speaking about in this thread is marked as rated for 105 degrees C. Does this mean that it is actually rated for a maximum of 75amps?
Thanks,
SP
*I read in the NEC that Ambient temperature (we are consistently <=28 C), and number of conductors in conduit (we are 3),also impact this maximum, but our conditions don’t seem to. 
 

[Vic]

Surf..,

Am NOT vtMaps,   but,   in general you are correct.   The limit of permissible temperature rating of a cable is that of the terminal to which the cable connects  --  a circuit breaker,  often.

Most of the breakers that we use do have a terminal temp limit of 75 C.   Probably true for many busbars,   as well.

IMO,   good question.    Vic

[Surfpath]

Hi Vic,
Thanks! Nice clarifications. Per Kurt at Outback, the FM80’s terminals are rated at 75C so I will now assume 65 amps to be my current combiner-to-controller limit.

Again, I don’t plan to test this, my Midnite combiner limits me to 5 breakers and an ISC of roughly 42-45. I’m just curious.
SP