Solar - questions on cabling and fuses

[mvweebles]

See attached (some may recognize - I posted a question on MPPT controllers a couple days ago). Panels are on a hard top of trawler, panel is a 20-foot one way run to MPPT/solar control panel with breakers. I'm hoping for knowledgeable guidance on following questions.

1. 6awg cable appears to be fine for the 14A/27V run from the panel pairs (series) to the MPPT controller 20 feet away. I don't have the equipment on front of me - are the lugs on a modern MPPT large enough for 6awg cable? Of not, how are these connections made up? Obviously, stripping some strands would physically work, but wondering if there's a better practice. Also, the pigtails off the panel appear to be 10awg. Best practice in connecting two dissimilar sized wires that are partially exposed to weather?

2. Fusing of panels. In reading another active thread, there was conflicting opinion about fusing the panels. One post by a knowledgeable person seemed to suggest it was not needed if just a single panel, but if there were three, it became important, and should be done at the panel. Questions: should I install fuses between the MPPT and the panel? If so, a single fuse per string, or a single fuse per panel? Maybe an inline fuse in the 10awg pigtail for each panel (making sure each are reasonably accessible)?

Thanks in advance

Peter

[mvweebles]

Dang it. Forgot to attach diagram

[Pete7]

Peter, which model Victron MPPT do you have? The smaller models up to 100/20 have 6mm2 cable size limits. Larger ones bigger.

Originally Posted by mvweebles

In reading another active thread, there was conflicting opinion about fusing the panels. One post by a knowledgeable person seemed to suggest it was not needed if just a single panel, but if there were three, it became important, Peter

I think the important word was "String". Now I think I have 3 strings in my panel, but need to go and see it to be sure. However, I did fit a circuit breaker in between the panel and the MPPT. Not exactly cheap but it is a useful way of switching off the panel from the MPPT and therefore the charge in the battery leads if you want to do some work on the batteries. Otherwise those loose leads are still live even if disconnected from the battery.

Pete

[LLCoolDave]

I have a victron 100/30 and it fits 6awg cable and it seems to be the max.

[cofc]

Southwire has an app called Voltage Drop that is helpful when estimating wire size needed. For your application they recommend 10AWG. So you might be adding unnecessary headaches and costs.

[pjShap]

10AWG is safe for the wire, but gives 10% voltage drop meaning 10% of the generated power is dissipated in the wire. 6AWG is 5% voltage drop and the reduced losses might be worthwhile.

[GordMay]

FWIW:
The Blue Sea Systems "T-1" Circuit Breaker (surface-mount part nos. 7120 to 7133; panel-mount part nos. 7020 to 7033), has been recalled effective May 2003.
If you still have a T-1 circuit breaker, please contact Blue Sea Systems for a replacement. It will be replaced with a Bussmann 187 Circuit Breaker (Panel Mount or Surface Mount), which is a direct replacement.
The company received no reports of any incidents, but apparently found, through its own research, that “there is a possible fire risk if the operator holds the circuit breaker handle in the ‘on’ position, after the unit has tripped, causing the contacts to weld together.”
➥ https://www.bluesea.com/resources/541

[Gary Mc]

Those numbers look a little high. For 15 amps at 7 meters using #10 I get a 2.5% drop and with #6 I get a 1% drop.
I would go with #8 my self.

[mvweebles]

Originally Posted by Pete7

Peter, which model Victron MPPT do you have? The smaller models up to 100/20 have 6mm2 cable size limits. Larger ones bigger.

I think the important word was "String". Now I think I have 3 strings in my panel, but need to go and see it to be sure. However, I did fit a circuit breaker in between the panel and the MPPT. Not exactly cheap but it is a useful way of switching off the panel from the MPPT and therefore the charge in the battery leads if you want to do some work on the batteries. Otherwise those loose leads are still live even if disconnected from the battery.

Pete

Thanks Pete - I am going with 100/50 Smart MPPT controllers. With 27A on the panels in series, was just a little close for my tastes. That and the upgrade to 6AWG cable added over $250 to the install, but it's a one time cost to my eyes.

Is a fuse/CB needed between the panel and MPPT if I have a CB between the MPPT and the battery? The surface mount CBs you linked are great, though I'd have to figure out best way to mount without burying them.

Peter

[Pete7]

Originally Posted by GordMay

has been recalled effective May 2003.

Hopefully they are all out of existence now. Mine aren't Blue Seas but badged some other make.

Pete

[Pete7]

Originally Posted by mvweebles

The surface mount CBs you linked are great, though I'd have to figure out best way to mount without burying them.

Peter

I had the same problem and despite the claims that they were waterproof, I chose not to put them outside near the panels. Instead they sit next to the MPPT were I can access them and see what is going on. For me its a bit of belt and braces, but you have more panels that warrants thinking about.

Also, if you haven't thought about it, the cable from the MPPTs to the batteries may need to be a different size than the one from the panel to the MPPT, since the Ah will be higher with a lower voltage, so big beefy cables needed. There I put the fuse next to the batteries.

[mvweebles]

Originally Posted by Gary Mc

Those numbers look a little high. For 15 amps at 7 meters using #10 I get a 2.5% drop and with #6 I get a 1% drop.
I would go with #8 my self.

I'm only okay with electrics. My understanding is you need to double the run due to return path. So a 20-foot run between two components is actually 40-feet overall. When I look at the BSS chart, 40-feet at 15A is pretty solidly in the 6 AWG category. Am I reading this incorrectly? No harm done of course except perhaps being more expensive than needed, but would like to know for future use.

Peter

Wiring Size Diagram.jpg Views: 135 Size: 247.4 KB ID: 231660" style="margin: 2px" />

[rslifkin]

Solar panels are a current source and one of limited capacity. In my mind, if the wiring can handle the full short circuit current of the panels feeding it without melting down and the controller can handle it as well, you don't need a fuse (as it's impossible to produce feed enough power to cause a problem that a fuse would solve).

[mvweebles]

Originally Posted by Pete7

I had the same problem and despite the claims that they were waterproof, I chose not to put them outside near the panels. Instead they sit next to the MPPT were I can access them and see what is going on. For me its a bit of belt and braces, but you have more panels that warrants thinking about.

Also, if you haven't thought about it, the cable from the MPPTs to the batteries may need to be a different size than the one from the panel to the MPPT, since the Ah will be higher with a lower voltage, so big beefy cables needed. There I put the fuse next to the batteries.

Putting them next to the MPPTs would be simple enough, but they would be a 20-feet of wire away from the panels. My understanding is best practice is to put the fuse as close to the power source as possible, but I'm not sure about this. I guess it's better than nothing.

Solar goes to a bus bar that then goes to batteries. Cables are 4-O, so pretty big.

Peter

[Gary Mc]

Originally Posted by mvweebles

I'm only okay with electrics. My understanding is you need to double the run due to return path. So a 20-foot run between two components is actually 40-feet overall. When I look at the BSS chart, 40-feet at 15A is pretty solidly in the 6 AWG category. Am I reading this incorrectly? No harm done of course except perhaps being more expensive than needed, but would like to know for future use.

Peter

Attachment 231660

You are correct in that if you do the calculation manually you use the total conductor length which it there and back. Essentially you take the total length, and if using feet, then you need the resistance per foot of the conductor which will give you the total resistance of the conductor. Then use the max amps times the resistance and you get the voltage drop of the conductor.


However most tables and on line calculators use the distance to the load so they are already accounting for the return leg in their calculations.
Hope this helps.