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Old 16-07-2018, 06:12   #1
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Solar instalation: fuse & cable sizing

Installing solar panels and not sure about fuse & cable sizing.

Setup in parallel:
Port> 38W + 80W + Victron SmartSolar MPPT 75/15
Starboard> 38W + 80W + Victron SmartSolar MPPT 75/15

Panels tech data:
38W Voc 23.6V Isc 2.2A Vmp 17.6V Imp 2.15A
80W Voc 23.6V Isc 4.67A Vmp17.6V Imp 4.54A

MPPT 75/15 tech data:
Rated charge current 15A
Nominal PV power 220W
Maximum PV open circuit voltage 75V

Min. ANL fuse size for Mastervolt DC Distribution is 20A. Would it be fine? Or should I use 40A or 50A?

Cable length from panel to controller would be about 3m, from controller to DC Distribution additional 2m. Panel cable is 2m long and 2.1 sqmm, maybe it would be possible to connect directly to controller.
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Old 16-07-2018, 14:56   #2
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Re: Solar instalation: fuse & cable sizing

Size your fuse to the total output that you expect from the panel array. Then google wire size chart and download. It will tell you everything you need to know about wire size for any application (volts / amps / length). Really handy to have around!! I'll bet there is even an app these days to calculate wire size.



Sorry - teaching you to fish here instead of giving you a fish. Hope you appreciate it.
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Old 19-07-2018, 00:45   #3
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Re: Solar instalation: fuse & cable sizing

don't combine 2 different panels to one controller. that is my first advice


hmm maybe ok with exact same Vmp. find it odd 2 different panels would have exact same.

the max current of the controller is 15a. there is no way you would be putting a 40 or 50a fuse on that.

the max amps you would even see is under 10a per controller. I would fuse each at 15a. 20 is ok if that is the smallest fuse you can use. I'd wire with 10awg

I don't know how that distro works. but make sure you are connecting on the battery side of your main swtich. and not the load side. if you have some special fused bus, it's likely on the load side.
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Old 20-07-2018, 04:32   #4
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Re: Solar instalation: fuse & cable sizing

Fuses are sized to protect the wire. Use the largest fuse to limit voltage drop. You do not need fuses from the panel to controller. Fuse the controller to battery with a fuse at the battery positive. The wire size to the fuse panel determines the size of fuse, again at the battery positive. There are many charts that show the safe ampere rating for different class wire insulation.
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Old 20-07-2018, 05:49   #5
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Re: Solar instalation: fuse & cable sizing

Quote:
make sure you are connecting on the battery side of your main switch.
and not the load side. if you have some special fused bus, it's likely on the load side
Why?

Can't the PV be connected to controllers that are connected to a small Fused Battery Panel that is connected to the positive busbar? For an example diagram see
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Old 20-07-2018, 11:28   #6
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Re: Solar instalation: fuse & cable sizing

Any charge source should be connected to the battery directly.

Exceptions would be power output to a robust busbar close by, as long as a dedicated voltage sensor wire direct to the bank.

Also temp sensor wires.
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Old 20-07-2018, 13:20   #7
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Re: Solar instalation: fuse & cable sizing

There are many solar controllers with a load terminal. I can't comment on those. I keep all my charge sources wired direct to the battery bank. Solar is hard enough to get enough, I don't want another switching device in the way.
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Old 20-07-2018, 13:38   #8
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Re: Solar instalation: fuse & cable sizing

Quote:
Originally Posted by john61ct View Post
Any charge source should be connected to the battery directly.
Directly? There is full Mastervolt system onboard, so Battery>>MasterShunt>>Latching relay>>2xDC Distribution with all charge/load sources connected via fuses.
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Old 20-07-2018, 13:59   #9
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Re: Solar instalation: fuse & cable sizing

The wire size for a short run of solar to the regulator of say less than 6 feet would be a #6 ga wire or fatter.
The purpose of the fuse is to prevent the battery from melting the wire and causing a fire if something should short circuit in the wire and controller. The danger comes from the battery, not the solar system. Any fuse should be close to the battery but not necessarily in the battery compartment.
Fuse size should be sufficient to allow for the full SC current of the solar array, a 100W solar panel probably generates no more than 5A on SC. Circuit breakers have to be sized slightly higher as they get warm and can trip because of the heat.

The battery acts as a voltage limiter on the solar output, and prevents a high voltage being applied to any other device. Some electronics are not guaranteed above 10% of the working limit, say 14Volts, and if the solar had no battery load on it, other devices would experience a voltage surge So to prevent damage caused by switching off the main battery switch, the solar is often hard wired to the battery post. Make sure the fuse is in that wire just outside the battery box. If you need to work on the solar array, then disconnect it at the solar panel or nearby to prevent high voltages getting to the electronics.
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Old 21-07-2018, 05:08   #10
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Re: Solar instalation: fuse & cable sizing

Zil I am not understanding you, what is a solar controler with a load terminal? Also what is another switching device in the way?
I am showing the solar controlers wired to a fused battery panel that is wired directly to the pos busbar which is wirhin 2' of the bat +
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Old 21-07-2018, 05:11   #11
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Re: Solar instalation: fuse & cable sizing

"Any charge source should be connected directly to the battery"
Mainesail shows fused charger, and alternator attached to pos busbar. What's wrong with that?
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Old 21-07-2018, 05:25   #12
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Re: Solar instalation: fuse & cable sizing

"..and if the solar had no battery load on it, other devices would experience a voltage surge So to prevent damage caused by switching off the main battery switch, the solar is often hard wired to the battery post."
This is useful explanation for thinking and reasons for wiring direct to batt pos post before the disconnect switch, but doesn't the solar controler limit the voltage? Yes I understand that the PV will continue to output even with the batt disconnect switch off. So if the PV is wired through a Fused Battery Panel, I would have to also remove the solar fuses before working on the system. Right now I just unplug the pv from the deck fitting.
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Old 21-07-2018, 06:35   #13
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Re: Solar instalation: fuse & cable sizing

Some solar controllers have two outputs.

One is for charging, goes to the bank.

Also a pair of terminals for loads, limited current, has an LVD that cuts output off when the bank gets too depleted.
Quote:
Originally Posted by rgleason View Post
"Any charge source should be connected directly to the battery"
Mainesail shows fused charger, and alternator attached to pos busbar. What's wrong with that?
As I wrote, the dedicated sensor wires on the battery is what allows for output to be indirect.
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Old 28-07-2018, 22:36   #14
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Re: Solar instalation: fuse & cable sizing

Quote:
Originally Posted by rgleason View Post
"..and if the solar had no battery load on it, other devices would experience a voltage surge So to prevent damage caused by switching off the main battery switch, the solar is often hard wired to the battery post."
This is useful explanation for thinking and reasons for wiring direct to batt pos post before the disconnect switch, but doesn't the solar controler limit the voltage? Yes I understand that the PV will continue to output even with the batt disconnect switch off. So if the PV is wired through a Fused Battery Panel, I would have to also remove the solar fuses before working on the system. Right now I just unplug the pv from the deck fitting.
Answer:'
Once you disconnect the battery from the solar controller, the solar is in a no-load condition and its terminal voltage will rise to 21 volts say, the controllers job is to control current flow from the solar to the battery or other load, not to control the voltage, It is not a voltage controller unless it has been designed to switch the solar to another load. That's a bit of a waste, so better to just disconnect the solar array at the array, or at least prior to the controller.
Yes it is important that the solar array cannot directly connect to the instrument panel as it could damage all the electronics which are rated for only 15 volts max usually. So before disconnecting the battery (switched?) disconnect the solar array at the solar panels. To avoid any mistake always connect the panels to the battery terminals beyond the switch, so they cannot be separated from the battery. The solar panel output doesn't need a fuse in its wire to the controller.
The only possible hazard is if the wires from the battery to the controller were to short circuit and the wires melted causing a fire. Make sure that in the wiring there is no connection from the controller output to any internal bus such as the instrument panel, the controller output must go directly to the battery post. A fuse rated for 20% higher than the maximum solar array output current, should be in the positive lead close to the battery box but outside it. Then if anything in the solar controller wiring shorts the battery current that flows will blow the fuse and disconnect the battery from the wiring and the solar output from the instruments which are connected to it via the battery positive feed to the instrument panel. The instruments will be protected from the surge that happens when the solar array has no load on it (the battery.)

Before working on a solar system disconnect the solar panel output that goes to the controller.

Cheers

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Old 28-07-2018, 22:57   #15
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Re: Solar instalation: fuse & cable sizing

Quote:
Originally Posted by waeshael View Post
Answer:'
Once you disconnect the battery from the solar controller, the solar is in a no-load condition and its terminal voltage will rise to 21 volts say, the controllers job is to control current flow from the solar to the battery or other load, not to control the voltage, It is not a voltage controller unless it has been designed to switch the solar to another load. ....
That might have been true of a cheap PWM controller 20 years ago, but certainly not of the modern MPPT controller that the OP is proposing to use. I, for instance, have a similar unit and it simply disconnects the solar from the batteries (it reports it is in 'silent' mode as opposed to bulk/absorb/float) when the battery voltage exceeds the maximum setpoint (14.4 in my case). At that time the terminal voltage on the controller input runs up to 47V (panel's Voc) but the batteries never see more than 14.4V because the controller limits voltage output as well as current.

Since any MPPT controller includes a DC-DC power supply that does voltage conversion one of the key setpoints is the output voltage and it is quite easy to keep the voltage on the output side from rising above the desired level.

And where you connect makes no real difference in terms of electronics safety, the voltage at your control panel should be essentially the same as that at your battery post otherwise the system is poorly designed. Barring some very minor losses from cabling the voltage is the same everywhere in the system. The only way to avoid this would be to disconnect the load side of the system while charging, and that's not really practical except maybe on a dock queen.

I do prefer to connect to a charging bus at the battery because I think it makes it easier to handle maintenance and changes to the system, but certainly not a requirement if it means pulling lots of cable through difficult areas.
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