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Old 24-03-2016, 20:37   #1
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Installing a 50W solar panel - fuse and wire gauge question

We're going to install a small 50W Renogy mono panel this weekend and I want to make sure I've got everything down. We're on a mooring during the season and have all LEDs, and I've calculated that 50W should be plenty for us (for now).

The 50W panel has 12 AWG output cables, and we will have 20 ft of 12 AWG running from the panel to a Genasun 65W controller. We may not need all 20 feet - we'll find out when we get to the boat.

Am I correct that we should use 12 AWG wire from the controller to the battery? As for the in-line fuse between the battery and the controller, the Renogy has a 15A max series fuse rating, so does that mean I need a 15A fuse?

I know these are pretty basic questions, and I'm sure I'll have more, but we've all had to start somewhere.

Thanks.
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Old 24-03-2016, 20:42   #2
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Re: Installing a 50W solar panel - fuse and wire gauge question

Blue seas have an app for your phone that helps you figure out fuse and wire size, it's great and free!
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Old 24-03-2016, 21:46   #3
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Re: Installing a 50W solar panel - fuse and wire gauge question

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Originally Posted by alctel View Post
Blue seas have an app for your phone that helps you figure out fuse and wire size, it's great and free!
I'll check it out. Thanks!
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Old 24-03-2016, 22:35   #4
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Re: Installing a 50W solar panel - fuse and wire gauge question

Don't forget'the distance of wire you need and the size is for a round trip. If it is 20 feet from the panels to the controler you need 40 feet of wire. 20 For positive and 20 for negative. Place the fuse between the controller and the panel. The single most important thing to remember is wire load to source ie: from the battery to the controller then hook up the panels. Hooking the panels to the controller first will likely damage the controller. In all things power always go load to source. For final hookups.
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Old 24-03-2016, 22:46   #5
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Re: Installing a 50W solar panel - fuse and wire gauge question

My 20A controller wanted a 30A fuse to the batt. I would think your controller's optimum fuse rating would be slightly more than your expecting to see thru it with use, so a 10A fuse might be a better match than 15A on a 65W controller. Sounds like it's protected up to 15A though, so if those are easier to come by then why not?

Does the controller manual run through the wire size figures? Maybe not a bad idea to go larger if you plan to add panels to the same wire run down the road.
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Old 25-03-2016, 01:26   #6
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Re: Installing a 50W solar panel - fuse and wire gauge question

A fuse must be between the battery and the controller, towards the battery side. The battery is a much larger source than the solar panels, and the wires must be protected from the battery.

A fuse on the panel side of the controller does virtually nothing, because the max current the wire could possibly see is the max power of the panel.
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Old 25-03-2016, 08:52   #7
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Re: Installing a 50W solar panel - fuse and wire gauge question

You might check your controller to see what gauge wire it will accept. Mine would not accept larger than 10AWG yours may not accept more than 12.
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Old 25-03-2016, 09:37   #8
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Re: Installing a 50W solar panel - fuse and wire gauge question

Regarding the wire, #12 will be fine for your 40-ft round-trip panel/controller connection. #12 is OK for the controller/battery run if the distance is short. This all assumes that you have a 12V system.

Here's how you figure it:

The maximum current from your panel is around 3A (50W / 17V = 2.94A)
#12 wire has a resistance of 1.588 milliOhms / ft, so 40 ft = 0.0635 Ohms
0.0635 Ohms x 3 Amps = 0.19V voltage drop in the wires, which is 1.11% of 17V

We generally figure that a 2% voltage drop is good enough. Less is better, but 2% is probably near the point of diminishing returns. I used 17V in my calculations, rather than 12V, since 17V is approximately the Maximum Power Point (MPPT) of your panel, and the Genasun controller will be loading the panel to this voltage.

The wiring between the controller and the battery will be carrying slightly more than the raw panel current (the controller boosts the current and drops the voltage), but it's going to be under 5A. If you have (for example) 10 ft round-trip of #12 wire between the controller and battery, that gives you a 0.0795V drop at 5A, which is an additional 0.66% at 12V. This gives you a total percentage voltage drop of 1.77%, which is still pretty good. You will have connector losses, but these should be manageable at these power levels.

There isn't a compelling reason to put a fuse between the panel and the controller, since the short-circuit current will be limited to the maximum panel output which is not much more than 3A. I have a breaker in my panel wiring, but this is more of a convenient disconnect point should I need to work on the charging system. If your panel were to be feeding the battery directly (not using a controller) then a fuse would be essential.

Your charger will probably never feed more than 5A into the batteries. 50W at 10V (a very dead battery) is 5A. You generally fuse to protect the wiring, and for #12 wires a 15A fuse should be fine.

As has been mentioned, if you plan to increase your solar panel capacity you might as well run heavier wires now.
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Old 25-03-2016, 09:43   #9
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Re: Installing a 50W solar panel - fuse and wire gauge question

You normally fuse the wiring-not the load(equipment)
The load(equipment) should have it's own fuse provided by the mfgr.

The above rule applies to loads such as: radio,radar,etc.equipment.

In your case,your "equipment" (solar panel & regulator) likely doesn't have their own fuses.
You still fuse the 12Ga wire for it's rated current carrying capacity,regardless of it's length-round trip included. If the 12Ga wire will cause significant voltage drop over it's round trip length,you go with heavier wire.

These specs (max. amp rating & voltage drop at full capacity) are provided in tables easily accessed online & should always be checked before starting job.

In your case-12Ga wire is rated 20Amps max current & should be fused 20A.There is nothing to stop you using a lower amp fuse but there is no neccessity-12Ga will safely carry up to 20Amps.

Distance(length of wire round trip) is available from a table. Look up 12Ga.
I'm quite sure your 40ft round trip at max 15Amps is OK for 12Ga. but check it out.

Watts = Amps x Volts Amps = Watts / Volts Volts = Watts/Amps



Your 50watt panel will supply a max of approx. 50Watts / 14Volts(working voltage) = 3.57 Amps or 4 Amps nominal. No problem for 12 Ga wire.
You can fuse the wire from panel to controller with any size from from 6-20 Amps-doesn't matter.

The battery can output 1000's of Amps if the 12Ga wire between the bat. & controller gets shorted,so it's very important to fuse this wire at or close to the bat. connection. The normal current coming into bat. from will never exceed 4 Amps,so you can fuse from 6-20 Amps-doesn't matter.

If this wiring was a branch supply from your Dist. Panel to equipment,lights,etc.,I would recommend Fusing 12Ga at the full 20Amp at panel,regardless of actual equipment load draw. Again-you are fusing the wire-Not the equipment.

Cheers/ Len
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Old 25-03-2016, 09:45   #10
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Re: Installing a 50W solar panel - fuse and wire gauge question

We have a much larger array but the concept is the same. Study of this and 2 lightening strikes in 6 years have taught me to connect everything with double pole switches. This includes solar panels. For that, I use 2-pole circuit breakers between each panel and the controller and between the controller & battery. This replaces the fuses. In an active electric storm we open all breakers to orphan all devices with an air gap. You can use any breaker. Amps is amps - 12 or 120 volts so the selection is easy. Breakers and fuses protect wires first and devices second. This means you select the breaker to be no larger than the wire gauge requires. This will not be a problem for you because 12 gauge is rated for more than 15 amps. You will perhaps upsize the wire in order to reduce the losses from a long run of wire. The short of it is, use the breaker size according to the controller requirement. Likewise, the wire from your controller to the battery might also be larger. Bigger wires is the easiest, least expensive way to maximize the output of the system. PHOTO: 3x10 amp panel breakers; 1x40 amp controller output breaker. These are DIN-rail mount breakers. Find them on-line Amazon or most contractor electrical supply outlets. Also, McMaster-Carr. I find them by the hundreds at a local industrial used & surplus. The biggest rookie mistake is wires too long and/or too small. Use tinned marine grade wire.
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Old 25-03-2016, 11:04   #11
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Re: Installing a 50W solar panel - fuse and wire gauge question

What Paul said. Except, if the output from the controller is never expected to exceed 5 amps, I'd put in a 10A fuse not a 15. Either one should blow in a problem, and some folks would even go down to a 7.5A fuse, as being the next full step above the maximum amperage that your system should be providing.


The idea is that a lower amperage fuse will blow sooner and faster if there is a problem. And sometimes, there are problems where amperage on a line is "excessive" to varying degrees, so the lower amperage fuse shuts that down while a problem is still smaller.
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Old 25-03-2016, 12:24   #12
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Re: Installing a 50W solar panel - fuse and wire gauge question

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Originally Posted by Nicholson58 View Post
We have a much larger array but the concept is the same. Study of this and 2 lightening strikes in 6 years have taught me to connect everything with double pole switches. This includes solar panels. For that, I use 2-pole circuit breakers between each panel and the controller and between the controller & battery. This replaces the fuses. In an active electric storm we open all breakers to orphan all devices with an air gap. You can use any breaker. Amps is amps - 12 or 120 volts so the selection is easy. Breakers and fuses protect wires first and devices second. This means you select the breaker to be no larger than the wire gauge requires. This will not be a problem for you because 12 gauge is rated for more than 15 amps. You will perhaps upsize the wire in order to reduce the losses from a long run of wire. The short of it is, use the breaker size according to the controller requirement. Likewise, the wire from your controller to the battery might also be larger. Bigger wires is the easiest, least expensive way to maximize the output of the system. PHOTO: 3x10 amp panel breakers; 1x40 amp controller output breaker. These are DIN-rail mount breakers. Find them on-line Amazon or most contractor electrical supply outlets. Also, McMaster-Carr. I find them by the hundreds at a local industrial used & surplus. The biggest rookie mistake is wires too long and/or too small. Use tinned marine grade wire.
Check the DC rating of the breakers. (use DC rated breakers)
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Old 25-03-2016, 12:41   #13
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Re: Installing a 50W solar panel - fuse and wire gauge question

"For that, I use 2-pole circuit breakers between each panel and "
I wonder. Breakers are known to arc over and weld shut when their amperage rating is exceeded. Would a lightning strike carry sufficient power to jump any gap and simply fuse (weld) the breaker shut?
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Old 25-03-2016, 14:51   #14
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Re: Installing a 50W solar panel - fuse and wire gauge question

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Place the fuse between the controller and the panel.
Wrong

Place the fuse between the controller and the batteries - as close to the batteries as convenient.

The panels are self limiting but the batteries aren't.

Size fuse at 25 or 30 amps - less that 12 ga can handle.
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Old 25-03-2016, 15:06   #15
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Re: Installing a 50W solar panel - fuse and wire gauge question

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In your case-12Ga wire is rated 20Amps max current & should be fused 20A.There is nothing to stop you using a lower amp fuse but there is no neccessity-12Ga will safely carry up to 20Amps.
12 ga 105c marine wire has an ampacity of 38 in an engine space and 45 outside engine spaces.

A larger fuse has less voltage drop than a smaller fuse.

Even a 100 amp fuse would blow in the blink of an eye if the battery shorted.

I would use a 30 amp fuse near the battery and keep the controller to battery run as short as you can.
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