Wire question

[JHinman]

My 5er has an existing Zamp solar setup, using a 45watt panel, a PWM controller, and 8 AWG wires from roof to batteries.

I want to replace the tiny Zamp panel with three strings of two 100-watt panels, and put an MPPT controller near the batteries.

I think the existing 8 AWG wire is adequate, but would like input from this community before I start this project.

The way I figure it, the 100-watt panel strings will produce a maximum of 5.5 amps at 36 volts, so putting three strings in parallel will give me about 16.5 amps at 36 volts. I am not sure what the total one-way length from the combiner box to the batteries is, but it could be as much as 20 feet.

I think the existing wire can handle the amerge, with a voltage drop on the order of 2%. This should be suitable.

Am I calculating this correctly?

Thanks for any thoughts.

[hclarkx]

Your are golden.

The #8 is good for well over twice the current you will put on it.

Voltage drop between the panels and the solar controller is not a problem in that the solar controller will compensate for it.

Voltage drop between the panels and the solar controller is only an issue re power loss. I.e., some of your generated wattage will be lost in heat in the wire. That may be a few watts, but if you have #8 wire, you've done about all one would want to do to reduce those losses.

It's good to see a manufacturer provide decent conductor up to the roof.

An idle question ..... are you using 100W panels because they fit the available roof space? 200W panels are currently the sweet spot for dollars per Watt and might be a good alternative if they will fit the roof. I'm in the process of changing out my 100W panels (up to 7 years old) and doing a bit of re-arranging to get 200W panels up there. This was made possible in part because the latest crop of 200W panels are a bit more efficient and in the case of the ones I got, are 6" shorter than ones available a year or two ago (length was my constraint).

[bucks2]

A 50 amp service cord typically has 8 gauge wire..... I'll bet you can't put enough solar panels on the roof to overload 8 gauge wire.

[JHinman]

Originally Posted by hclarkx

An idle question ..... are you using 100W panels because they fit the available roof space?

Thanks for the feedback on the wire. I thought I was good, but lack confidence in my understanding of electricity.

The rationale for the 100-watt panels is getting higher voltage by putting panels in series, yet having pairs in parallel spread out to increase the chances that at least one pair will be in the sun and active.

I had three 170-watt panels spread out on a 27-ft TT, connected in parallel, and that seemed to work. (I did not get much experience with the set-up, as I used it once before finding a rig I liked better.)

My understanding is that MPPT controllers are more efficient when used with higher voltage arrays.

Renogy has some 100-watt panels for $100 right now. That looks pretty attractive.

[hclarkx]

Originally Posted by JHinman

Thanks for the feedback on the wire. I thought I was good, but lack confidence in my understanding of electricity.

The rationale for the 100-watt panels is getting higher voltage by putting panels in series, yet having pairs in parallel spread out to increase the chances that at least one pair will be in the sun and active.

I had three 170-watt panels spread out on a 27-ft TT, connected in parallel, and that seemed to work. (I did not get much experience with the set-up, as I used it once before finding a rig I liked better.)

My understanding is that MPPT controllers are more efficient when used with higher voltage arrays.

Renogy has some 100-watt panels for $100 right now. That looks pretty attractive.

Got it. Good plan. You are correct on the benefits of higher voltage AND parallel paths. And $100 for 100W Renogy panels is a good deal. My son and I got latest 200W name brand panels for $150 each though we bought 20 panels and did cash and carry. I was thinking four 200W panels, two parallel branches of two panels, would also give you two parallel branches and a bit more Wattage if they will fit and the price is right. That would be about 20 amps instead of 15, but you have the wire for it. The fact that I'm upgrading from 100W Renogy panels to 200W panels spurred my thoughts on going 200 out of the box.

[glennwest]

Originally Posted by bucks2

A 50 amp service cord typically has 8 gauge wire..... I'll bet you can't put enough solar panels on the roof to overload 8 gauge wire.

Depends on how it is wired.Higher voltage requires less wire gauge.

[Winemaker2]

Originally Posted by glennwest

Depends on how it is wired.Higher voltage requires less wire gauge.

Not true.
Ampacity charts dont list or worry about V.
8 ga good for 40 to 55 A depending on insulation... doesn't matter at what V

If you are talking wattage thats another story and A a reverse relationship w V. Double V and A is halved for same wattage.

Sent from my SM-T387V using iRV2 - RV Forum mobile app

[paul65k]

Originally Posted by Winemaker2

Not true.
Ampacity charts dont list or worry about V.
8 ga good for 40 to 55 A depending on insulation... doesn't matter at what V

If you are talking wattage thats another story and A a reverse relationship w V. Double V and A is halved for same wattage.

Sent from my SM-T387V using iRV2 - RV Forum mobile app

All true of course.....I think the issue is line loss and not wire capacity. The lower the voltage the higher the resistance and typically the reason for higher gauge wiring for Solar panels running at 12V (17ish actually). This comes up very often in these discussions and the reason to use more than just an ampacity chart when making wire choices for low voltage wiring

[Marine359]

You really don’t have much voltage drop over long solar cables. I use 30ft 10 gauge wire from my portable panel to a mppt controller mounted close to battery. I’ve measured to confirm spec 22v Voc at the panel, and measure 21.5v at the end of the 30ft cable. If you’re unsure, just use a multimeter to test if your panel is putting out factory spec, and test again at the end of your cable.