wire sizing

htwheelz67 · August 2010

I'm installing 2 100 watt panels (23.0 vmp) on the roof of my toyhauler in parallel, the controller (Morningstar mppt 15) will be approx 25'-30' from the panels and the controller will be max 5' from my batts 2 -6 volt crown 230ah.
I may add a 3rd panel but not now.
Should 8-10 gauge wire suffice?

niel · August 2010

Re: wire sizing

for a 30ft run that is 60ft of wire and is over 3% for the voltage drop even with the #8. #6 will be better at just over 2%. all this was at imp of 4.3a x 2 = 8.6a.
for the run between the controller it is possible to hit the current max of 15a through downconversion and mppt gains at times so i used 15a in my calculations and i don't recommend another pv being added to this controller with a 12v output as 200w is its max rating. the 5ft run equates to 10ft of wire. with #8 it would present an additional voltage drop percentage of about 1%, and this is additive to the other v drop loss percent for the system btw. 1% or lower is recommended to have for between the critical run between the cc and the batteries, but even though the #8 meets that criteria, why change now as you should keep this all at #6. the #6 will give about a .63% drop or about .076v. this small drop still allows the battery to get a full charge even though what the controller sees is more than what it actually is at the battery because of that v drop.
you can try dabbling with the voltage drop calculator yourself to see how current, voltage, wire size, and distance at 90 degrees c (recommended) interrelate in keeping voltage drop losses to a minimum. it's in the faqs, links, and info area above.

Kamala · August 2010

Re: wire sizing

If you wire the panels in series you can use smaller gauge conductors to the controller. The voltage drop over 60 ft of #10 would be less than 1% according to the calculator at Blue Sea Systems.

htwheelz67 · August 2010

Re: wire sizing

I just went and measured and it is about 30ft, I have a place to put the controller 1-2 feet from the battery but under my sink in the kitchen would be the best place which is 10' away from the batts.

I am going to try the 200 watts to start with, basically it is just to keep my batts charged and when we dont have shore power or cant or dont want to run the genny we can use the system.

I have decided to use my 1000 watt xantrex inverter vs my 2000 watt chicago electric cheapo and at most I will run a 24"tv 75watts, 22"tv 60 watts and a directv box 25watts and maybe a light or two not constantly, I will be installing a few LED bulbs in certain fixtures and maybe my laptop.

so if I use say 1250 watts in a night then in 10 hours at say 150 watts I should be able to recharge the batts?

So maybe 200-250 watts for 4-5 hours tops, I may have found a place to put 2 more 6 volts as well to bump the battery capacity to 460 AH.

If I add 2 more panels I will just add another controller or get a bigger one.

I can run in series as shading shouldn't be a problem I have a spot at the pass side rear which should never be shaded for the panels, so with an mppt controller is there a benefit between series or parallel in optimal conditions besides the smaller gauge wire/less loss?

niel · August 2010

Re: wire sizing

as kamala pointed out putting the pvs in series is a good option and i concur #10 will work, but at 1.37% as it is rating it at a higher wire temperature. what i said on the controller to the batteries still applies as the output is 12v and high current. there definitely is an advantage to running higher voltages in terms of keeping losses down to a minimum and/or allowing for the use of smaller gauge wires being able to pass the same amount of power.

under your sink is not a good place to put electronics of any kind. if you must place long runs of wire between the battery and cc then you will need thicker wires to offset the voltage drop.

be careful of how you state things as you are confusing watts and watt hours and even me to a point. the battery bank is 230ah at 12v so to preserve good battery life you don't want to exceed 1/2 of that or 115ah. at 12v this becomes 12v x 115ah = 1380wh. don't forget to add in the losses that running the inverter will bring as for instance your 60w tv will draw considerably more than that from the battery. a good battery monitor showing the current flow will reveal a more accurate accounting and as it will also show the volts of the batteries it is simple to multiply the volts times the current to get the wattage.

i would not add more batteries to the battery bank unless you obtain more pvs and another controller to properly charge it as it would be in the neighborhood of about 3% for the charge rate and we recommend it be at least 5% and if you can swing even more pvs then we safely recommend up to a 13% rate of charge. if you must get another one then get one that will fit all of what you have in mind on running, but i would understand too if you bought another of the same to parallel so as not to waste your first purchase if 2 will cover your total needs now and in the future. 2 of them with a 12v output would accommodate 400w of pv.

mike95490 · August 2010

Re: wire sizing

htwheelz67 wrote: »

......so if I use say 1250 watts in a night then in 10 hours at say 150 watts I should be able to recharge the batts? ........

10 hours of solar charging light ? They dont even get that in Alaska. Count on 5 good hours of light summer, 4 hours winter.

htwheelz67 · August 2010

Re: wire sizing

I'll be in Parker AZ, lots and lots of sun.

Kamala · August 2010

Re: wire sizing

mike90045 is right. No matter where you are (with regard to latitude) it is unreasonable to expect more than five or, at an uncomfortable stretch of wishful thinking, six hours of usable sunlight. The angle of incidence is the key. Just because the sun is up and visible doesn't necessarily mean that you can efficiently harvest its energy.

htwheelz67 · August 2010

Re: wire sizing

I got it..............so I decided on #8 wire, I will run in parallel to start but I did install a junction box between the panels so if I want to I can wire them in series by popping 4 screws if shading is not a problem.

Another question, I couldnt find a #8 fuse holder anywhere only a #10, will about 3" of #10 right at the batteries hurt anything? could I use a circuit breaker instead?

mike95490 · August 2010

Re: wire sizing

This is when I plug another vendor other than our host, who does not carry the item. Look for the Blue Seas Systems "Terminal Fuse Block ".

No wires, bolts to the battery terminal, and you bolt your cable to it. Awesome.

wire sizing

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