As some of you already know, I'm piecing together a solar system. Controller is a Morningstar 45A PWM. Panels will be two 12V 150W wired in parallel. I'm going to be using a combiner box on the roof, more than likely the AM Solar one that "connects" to the fridge vent. Basically, the panel wires will connect to the C-Box and then I want two wires from there to the controller. I estimate 15 feet for that run but might be longer. I intend to buy 25 ft of wire. I think that should do it. Considering getting this: Genuinedealz Flat Duplex Marine. Not sure what size wiring to get. My confusion is in how much voltage drop is ok. Needs to accommodate up to 450W in parallel total. Thanks all.

GD calculator says you lose .8 volts with #10 and .5 volts with #8 with 25 amps flowing. #10 is minimum for ampacity IMO.Say they are 18v Pmax panels you will still have 17+ volts hitting the controller so I would call it good. #6 would be killing it but could work after the controller if using the same coil of wire.

Hi Mena, I generally recommend designing a system around 2% (or less) voltage drop…Taking your 450W figure (I used 35A), and assuming 40 feet of wire (20 feet each of + and - conductors) you will need to 6ga wire to stay at 2%. 4ga would be slightly better, but IMHO would not worth the extra cost. With the 6ga wire you can safely select a fuse in the neighborhood of 75-80A to keep losses low and still have a good margin of safety.In terms of ampacity alone, you would be fine going as small as 10 or even 12ga… but personally I'd spend the extra $$$ to keep the voltage drop in the 2% range.Cheers-Mark

smkettner wrote:GD calculator says you lose .8 volts with #10 and .5 volts with #8 with 25 amps flowing. #10 is minimum for ampacity IMO.Say they are 18v Pmax panels you will still have 17+ volts hitting the controller so I would call it good. #6 would be killing it but could work after the controller if using the same coil of wire.This is good feedback.A better analysis would involve knowing the panel specs, each panel to combiner wire length and then the actual length combiner to controller and controller to batteries.

450/130 x8.2= 28a but less when flat.15 ft pos and neg, #8 about 1/2 a volt drophttp://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&phase=dc&noofconductor=1&distance=15&distanceunit=feet&amperes=28&x=74&y=6With 12v PWM the voltage drop panel to controller is less critical than from controller to battery. My old ASC manual says 1/2 volt array to controller is ok and 1/4 volt for controller to battery.IMO #8 is the right amount. In your over-all set-up you have already decided not to try to squeeze the last amp out of it, just be reasonable and leave it at that. So it would be strange to worry about a milli amp or two by using fatter wire than # 8 for this part of the set-up. I would agree with fat wire from controller to battery, epcecially if you can share with the inverter's wires for part of that path for instance.

You did - probably - right by going for 45A charger instead of 20A. This is 600W array that you may potentially have. If your roof allows more panels, don't do it wrong now by sizing your wiring to 300W only because you are buying 300W today.

Panel to Solar Controller Wire Size | Good Sam Community

57 Replies

CJW8
Explorer
Feb 20, 2014
This is what I used to parallel my panels coupler with the 2 #10 in one side and the #4 out the other side and covered in heat shrink. You could use it just before yout controller if the wire is too big. I really like those reducers mentioned above but most times there isn't room for them in the panel... But trimming some strands will work just fine too. Actually my controller lugs were not rated for #4 but I was able to squeeze it in them anyway.
mena661
Explorer
Feb 20, 2014
NinerBikes wrote:
I don't think Mena's wiring job is going to be under engineered, at this point in time. Now if he can just find a quality installer! Maybe RJ can do it, and then we can critique it? :W

You going to Quartzsite soon Mena661?
LOL! RJ, could probably do a better job than me on the install honestly. If I could figure out how to make a contraption like his, I could add another 300W.
NinerBikes
Explorer
Feb 20, 2014
I don't think Mena's wiring job is going to be under engineered, at this point in time. Now if he can just find a quality installer! Maybe RJ can do it, and then we can critique it? :W

You going to Quartzsite soon Mena661?
mena661
Explorer
Feb 20, 2014
BFL13 wrote:
Mena it says it takes #6 wire.
I'll just trim some strands from the wire. It'll fit! I know it's expensive but it's something I don't have to experiment with putting things together to see if they fit plus it has plenty of room to attach wires to (saves multiple trips to the store that's 9 miles away too). I'm not totally against plug n play. :)

EDIT:
BFL13 wrote:
I like those reducers. I use them to go in set screw terminals instead of welding wire with all those silky strands, where the set screw can't get a good hold on the strands

http://www.sonicelectronix.com/item_32009_Stinger-SPT522.html
Oh, those have set screws. Maybe I will pick up some of those.
BFL13
Explorer II
Feb 20, 2014
I like those reducers. I use them to go in set screw terminals instead of welding wire with all those silky strands, where the set screw can't get a good hold on the strands

http://www.sonicelectronix.com/item_32009_Stinger-SPT522.html
EsoxLucius
Explorer
Feb 20, 2014
BFL13 wrote:
Mena it says it takes #6 wire.
Stinger SPT522 4 Gauge to 8 Gauge Power Wire Reducer
westend
Explorer
Feb 20, 2014
mena661 wrote:
westend wrote:
The 4 AWG will work well for you. All sorts of lugs are available for #4 so connectivity is easy.

You're now getting into what Handy Bob advocates, also. :B

If you're going to use a combiner box, IMO the off-the-shelf boxes are expensive. I made my own but there are also cheaper ways to add two terminal strips into a plastic box.

My DIY box:
I'm probably going to get this one: AMSolar C-Box.

I dunno', yes it fits nicely under the lid of the fridge vent but only accepts 6 ga. wire and is expensive for what it is.

Nearly the same thing can be fashioned from a single or double gang box with common lug strips and good box entry clamps. The simple plastic boxes can be screwed to the vent housing or glued to the roof membrane with some sealant or VHB tape.

I used this box entry clamp for two MC4 cables. I sealed the cables into the rubber with sealant, also.
BFL13
Explorer II
Feb 20, 2014
Mena it says it takes #6 wire.
mena661
Explorer
Feb 20, 2014
westend wrote:
The 4 AWG will work well for you. All sorts of lugs are available for #4 so connectivity is easy.

You're now getting into what Handy Bob advocates, also. :B

If you're going to use a combiner box, IMO the off-the-shelf boxes are expensive. I made my own but there are also cheaper ways to add two terminal strips into a plastic box.

My DIY box:
I'm probably going to get this one: AMSolar C-Box.
westend
Explorer
Feb 20, 2014
The 4 AWG will work well for you. All sorts of lugs are available for #4 so connectivity is easy.

You're now getting into what Handy Bob advocates, also. :B

If you're going to use a combiner box, IMO the off-the-shelf boxes are expensive. I made my own but there are also cheaper ways to add two terminal strips into a plastic box.

My DIY box:

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Panel to Solar Controller Wire Size

57 Replies

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