Someone Check my Math

[travellerw]

Originally Posted by Paul Elliott

Can you clarify this? Most solar panels have a high short-circuit current. My 100W panels put out around 6A when shorted. The panel wiring can easiy handle this (it's not much more current than the MPPT current), so I don't have a fuse on the panel, but the short-circuit current is hardly zero.

Thank you.. I knew the answer just couldn't recall it. There is no point fusing between the panels and the controllers as the wire is big enough to handle the short circuit current. There is no way enough current could flow through the wires to actually pop a fuse unless the controller back fed from the batteries!

[twinboat]

Fuses on panel to controller runs are only needed If you have mutiple panels that can produce higher amps then the panel wires can handle.

If the output of 2 or more panels can backfeed and overload a shorted panels wire run, from the junction point, all panels will need overload protection.

Example, 4 ,15 amp panels. 1 shorts at the panel. The other 3 will send 45 amps to the short.

Installing a switch in the panel feed, near the controller, is handy when doing any kind of work.

[hellosailor]

Consider that an MPPT controller will take any excess voltage, and convert it into MORE AMPERAGE to charge your batteries.


So if you are losing a couple of percent of the voltage in your wires--that's a couple of percent more power, more amperage, that the MPPT controller will not have available to turn into AMPERAGE to charge your batteries.


10AWG may be sufficient...but you will be throwing out power that the MPPT's could be putting to good use.

[travellerw]

Originally Posted by hellosailor

Consider that an MPPT controller will take any excess voltage, and convert it into MORE AMPERAGE to charge your batteries.


So if you are losing a couple of percent of the voltage in your wires--that's a couple of percent more power, more amperage, that the MPPT controller will not have available to turn into AMPERAGE to charge your batteries.


10AWG may be sufficient...but you will be throwing out power that the MPPT's could be putting to good use.

Yup.. A good point and suggested to me by someone else.

However, my math only shows I will be throwing away about 6.2 watts (a loss of 2.65% with 10AWG). Considering that 8AWG will still throw some away, just not as much, the difference is really small.

Like I said, I will probably pull 8AWG if i can source it. It seems that dual boat wire 8AWG its out of stock in most places. I'm not keen on using primary wire and pulling 4 wires.

[bluegreen]

If you look at the size of the pins that your connecting too, there is no real need to guage a run thicker then that. The 700 amp line for a 10 amp short seems excessive to me. Go with the 14 guage and save a hundred bucks in copper. Just sayin.

Electronics background.

[sailorchic34]

One note on charge controllers. If your using a 10 amp controller with 24V batteries it should be fine. But if your house bank is 12V that 10amp controller may not be enough. This as amps listed are generally a rating for output amps to the batteries and not the input amps to the mppt controller.

So 9 amps at 24 volts, may be 18 amps at 12V. A 10 amp mppt controller might be fine or it might have a reduced life due to higher junction temperatures in the FET's. Depends on heat sink design and rated voltage of the FET's

Just Fyi.

[Paul Elliott]

Originally Posted by bluegreen

If you look at the size of the pins that your connecting too, there is no real need to guage a run thicker then that. The 700 amp line for a 10 amp short seems excessive to me. Go with the 14 guage and save a hundred bucks in copper. Just sayin.

Sorry, this is completely wrong. I can't say it any nicer than that.

The pin size is important, but hardly the determining factor. The resistance of the entire panel to controller circuit is the important thing, and the wire mostly determines that. If you used 60 ft of #14 wire at ten amps you would lose about 1.5 volts. In this case that's 5%, and way more loss than you want.

Fairly small pins (or connectors) can carry a lot of current. As long as they are clean and tight they are probably OK. You typically use larger connectors because they let you use larger wire. Some equipment only lets you use relatively small connectors or wires, and this is fine for short wire runs. For longer runs I sometimes use a short #12 or #14 jumper, connected by a terminal strip to the longer run of heavier wire. The short run of small wire can carry the current, and the voltage drop is negligible because the length is so short.

[bluegreen]

Im completley wrong because im out 5%... ok.
If his panels are not 90 degrees perpendicular to the sun and are most likely flat he will loose 30 percent. Inverters are 87 percent efficient. Battery size, quality, lifepo4 vs lead acid or agm make bigger differences, led or incadessent, the list goes on.
Im not at all "completley wrong". Im saying that being able to run a single LED for an extra 5 min is not worth the cost of 30 or more feet of 8 guage wire and heavy crimping tools and bla bla bla.
But im having fun being a part of a chat about boats. So im happy and being a happy liveaboard os what im doing and its fun. It rains almost every day here in Vancouver so im getting a windmill because my Midnite controller can do that. The system came with 2 , 280 watt pannels and wires that are quite thin. The dirt on the surface makes up for much more loss then electron battles in the skinny wire.

[travellerw]

Originally Posted by sailorchic34

One note on charge controllers. If your using a 10 amp controller with 24V batteries it should be fine. But if your house bank is 12V that 10amp controller may not be enough. This as amps listed are generally a rating for output amps to the batteries and not the input amps to the mppt controller.

So 9 amps at 24 volts, may be 18 amps at 12V. A 10 amp mppt controller might be fine or it might have a reduced life due to higher junction temperatures in the FET's. Depends on heat sink design and rated voltage of the FET's

Just Fyi.

They are 25A controllers with FETs and diode bridges upgraded to 60A (couldn't help tinkering). The heat sinking in these controllers is insane. The FETs are pressed to the case with silpad. The case is like 1.5 pounds of aluminum. I think I'm good on the controller side.

If you look at the size of the pins that your connecting too, there is no real need to guage a run thicker then that. The 700 amp line for a 10 amp short seems excessive to me. Go with the 14 guage and save a hundred bucks in copper. Just sayin.

Electronics background

Yup super wrong.. Not to be a dick, but setting up and arduino is not an "Electronics background". That is all I have to say about that!

[hellosailor]

The change from 10 AWG at 60' round trip (about a 2% loss) to 8 AWG (about a 1.27% loss) isn't great. But if you were to change panels at some future date, going to a conventional "12" volt panel (17-22vdc max output) or a higher amperage...then you might want to reconsider and drop in heavier wiring now, as opposed to doing it over later.


But if the panels are going to be "for sure" right, then your math seems to be OK.
www.genuinedealz.com is probably the most beloved supplier of marine cable (tinned, jacketed, duplex) with fast shipping, good prices, and professional crimping at a minimal added cost.

[ramblinrod]

Originally Posted by Paul Elliott

Can you clarify this? Most solar panels have a high short-circuit current. My 100W panels put out around 6A when shorted. The panel wiring can easiy handle this (it's not much more current than the MPPT current), so I don't have a fuse on the panel, but the short-circuit current is hardly zero.

Sorry, brain fart, you are correct, when the panel is shorted the voltage drops to zero so output power drops to zero, not current.

[Paul Elliott]

Originally Posted by bluegreen

Im completley wrong because im out 5%... ok.

Sorry if I was too blunt. You're wrong not because of 5% vs 2%, but because of the concept of sizing wires to match some random connector vs sizing wires for overall resistance.

In high-voltage runs (such as 240VAC applications) a 1V drop due to wire resistance isn't particularly critical -- it's a small percentage of the system voltage. In these cases wires are sized for temperature rise, especially when multiple circuits are run in a common conduit.

In low-voltage applications, such as with solar panels, the currents are high and the voltages are low. A 1V drop will have a big impact in a 12V system. Short runs can use relatively smaller wires, but long runs require thicker wire. The connectors are close to irrelevant, as long as they are clean and tight, and not *too* small.

[bluegreen]

Not some random connector. The wires from the pannel itself are not very big. My 30 volt 9 amp panels are in series. So i have 66.7 volts of pannel with rather small wire from them to the controller. At that point it goes to 24 volt system where i run the heavier guage to the batteries. In all the research ive done, the wire size seemed to be the very least of all the powerloss problems ive ran into. And if i want to add more pannels in series it dosent affect the current on the pannel side. Same amps just volts increase. The controller does the rest. So traveler having roughly the same pannels with a mppt controller swapping 60v 9a to 12v 50a, his controlers efficiency is far more critical then the pannel wire size. Thats all im saying. The 1.5 volts lost will only account for 10 watts on either side. Having a small shadow pass over the pannels will effect it more then that. Its just how i see it. Electronics is a lot more rough n dirty then i would have thought prior going to electronics engineeringin college back in 1998. Its getting foggy for sure so i like reading this thread and voice an opinion when i can.
Maybe i will try heavier wire and see if it makes any difference. I however believe the batteries internal resistance make the bigger difference. But let me know how this works out. Im interested for sure. And good luck to all.

[keninmed]

I have been looking at this from a Motorhome (RV) point of view.
The advice seems to be to fuse both input AND output from controller.
The input fused near controller is to disconnect when playing with 12v
side. Apparently the controllers do not like running open circuit!

[travellerw]

Originally Posted by keninmed

I have been looking at this from a Motorhome (RV) point of view.
The advice seems to be to fuse both input AND output from controller.
The input fused near controller is to disconnect when playing with 12v
side. Apparently the controllers do not like running open circuit!

I think I would opt for the blanket over the panels! Simple, cheap and less terminations to have problems with.

Motorhomes don't have to deal with living on the water in the tropics. On a boat, terminations are ALWAYS the failure point.