Low (& high) Voltage wire question

hillside · January 2016

I have a small system for backup power, 515 watt., 3000 w. inverter, and the question is, this system was set up 4 years ago using Romex type 110 v. house wire for the batteries to the charge controller.
Also, upon further review, the panels themselves were wired with Romex type...The system has worked well but now that I have it down for maintenance, should I replace it with low voltage wire. Is it that important? thanks

BB. · January 2016

First, you need to determine the working voltage (12/24/etc.) and maximum continuous current. That will let us size the copper wire.

Next, for exterior wire under/arround the solar panels, they usually use a cable rated for use in direct sun (UV rated). For cables in conduit on the roof, they use a higher temperature insulation for a wet location.

But, in general, there is no reason to use a "DC" rated cable for you installation. You can use standard house wiring with the appropriate gauge and insulation.

However, for fuse/breakers/switches--There is definitely a difference--DC rated switches are much heavier duty/larger than their AC counterparts (DC arcs are much harder to "stop" vs AC arcs--such as when switches are thrown).

-Bill

oil pan 4 · January 2016

I use THHN wire for most of my panel wiring. The romex looking wiring rated for out side I believe is wv-b, it's good for direct burial, can take sun light and is grey.

hillside · January 2016

It's a 24 volt system with 4 6v. batts., the wire is AWG12, rated to 600v., gray, sunlight resistant. Panels are 15 ft. away and the wire is in pvc conduit.
The system is not in continuous use. It's used for running a washing machine and propane clothes dryer a few times a week.
In an extended power outage it would be used a lot more...and probably will anyway..I live on the island that will default again soon!

BB. · January 2016

What is the Vmp/Imp of the solar panels, and how do you have them wired (series, parallel, series-parallel)? What kind of solar charge controller do you have?

Many "24 volt panels" have Vmp~30 volts. A single panel is not high enough voltage to efficiently/quickly charge a 24 volt lead acid battery bank (on hot days, the Vmp~30 volts will fall to 27 or even 24 volts. A flooded cell lead acid battery really needs around 29 volts to charge (at room temperature).

The "optimal" minimum Vmp-array for a 24 volt battery bank is Vmp-array~35 to 40 volts (standard test conditions). And this is the optimal array voltage for a PWM charge controller.

A true MPPT charge controller can use the above range, or even higher (depending on charge controller) to Vmp-array~100 volts for a typical higher end MPPT charge controller... You can put two Vmp~30 volt panels in series and everything will work just fine (plus you can use smaller AWG cable for wiring).

-Bill

hillside · January 2016

ok, I'll admit to ignorance of knowing what V-mp/lmp means.
I have 4- 70w. panels and 1- 235 w. panel wired together for this 24v.system.
Using a Stecca 30 amp. charge controller.
With the old inverter (now replaced with Samlex) the gauge would read 30 volts on occasion . The new inverter doesn't have a gauge, but I read 27 volts recently at the battery bank terminals.
This setup is operating in tropical heat.
Could we get back to my question about wire size and stranded copper vs. solid copper?

vtmaps · January 2016

hillside said:
Could we get back to my question about wire size and stranded copper vs. solid copper?

I didn't realize that was your question. In order to answer that question I need to know whether the Steca controller is PWM or MPPT. I also need to know how the panels are configured (series, parallel, series-parallel) as well as their Imp and Vmp.

To calculate the wire size, I need to know the current of the combined panels and the distance (which you mentioned is 15 ft). The reason I need those numbers is so I can calculate the voltage drop. Assuming that your panels are configured in a way that the combined Vmp is about 30 volts, 12 gauge wire will give you a 3% voltage drop. 10 gauge would be better.

btw, if your combined Vmp is really 30 volts, then as Bill mentioned you do not have the ability to get a full charge on your batteries.

--vtMaps

BB. · January 2016

Voltage Maximum Power (Vmp) and Current Maximum Power (Imp).

Vmp and Imp are the (roughly) optimum working voltage and current for the solar panels. Note that, again roughly, the useful output voltage of a solar panel (under weak sun to full sun) is from zero voltage (dead short) to Vmp. The current itself (Imp under full sun) is pretty much proportional to amount of sun...

So, the panel can be at 10-15 volts (based on battery voltage), but the current will be pretty much fixed (based on amount of sun at that instant of time). Technically, solar panels are "current sources", "fixed" output current and variable voltage; whereas batteries are "voltage sources" with fixed output voltage and variable output current.

Ideally, you want the solar panels to replace somewhere around 100% to 125% of the Amp*Hours used overnight by the next day's sun. And you don't want to take the deep cycle lead acid battery much below ~50% state of charge overnight, for longer battery life.

Note that regular car batteries are designed to only be cycled by ~15% of their AH capacity (to 85% state of charge) for long life. Marine-RV are not much better.

-Bill

hillside · January 2016

Ok, not sure about the Stecca CC. It is this one :
http://www.ebay.com/itm/like/321637018622?rmvSB=true
I had a pro install the panels, so I'm not sure of the series/parallel question. But it is 4- 70w.'s + 1 235w (5 total) wired to produce 24v.
While the system is down, (waiting for new batteries) , should I just rewire the panels with stranded copper wire, say, 8 ga. replacing the Romex 12 awg solid wire?

Xizang · January 2016

@hillside - You sound like you're in a great place! I'm in the States but looking to move to a coastal location in Belize or Panama soon.
As for the wire, I understand that electric current actually flows on the SURFACE of individual conductors. So if you consider the circumference of each of those tiny little strands and add up all the strands in the wire, you have a whole lot more surface area to conduct current than does a solid round wire. Plus, the stranded wires are more flexible and put much less strain on the terminals of solar panels, breakers, charge controllers, inverters, etc. And Marine-grade stranded wire is made up of much finer strands, making it even much more flexible than commercial and household wiring. And the Marine-grade wire strands are almost always tin-plated, making them corrosion resistant. Marine grade wiring comes in various voltage ratings.

BB. · January 2016

To clarify, current cowering in the surface of the conductor. ... That is called the "skin effect". The current only moves to the skin of the conductor of the current is AC. DC does not move current to the skin of the wire.

Also, skin depth is frequency dependent (higher frequency, the more current is pushed to the surface).

For our discussion, skin depth conduction issues only is a concern at 120 Hz or so on 0000 (4/0) cable.

And, ideally, each strand would need to be insulated so that each stand would carry its share of current.

Another issue with marine wire, it is SAE gauge which is slightly smaller diameter than AWG used for house/commercial wiring (may not be a big issue for most applications). Also house wiring has standards for insulation types / quality (temperature, oil/sun rating, etc).

Mixing "standards" can get you into trouble sometimes.

-Bill

highvoltpower · April 2016

BB. said:

To clarify, current cowering in the surface of the conductor. ... That is called the "skin effect". The current only moves to the skin of the conductor of the current is AC. DC does not move current to the skin of the wire.

Also, skin depth is frequency dependent (higher frequency, the more current is pushed to the surface).

For our discussion, skin depth conduction issues only is a concern at 120 Hz or so on 0000 (4/0) cable.

And, ideally, each strand would need to be insulated so that each stand would carry its share of current.

Another issue with marine wire, it is SAE gauge which is slightly smaller diameter than AWG used for house/commercial wiring (may not be a big issue for most applications). Also house wiring has standards for insulation types / quality (temperature, oil/sun rating, etc).

Mixing "standards" can get you into trouble sometimes.

-Bill

thanks for sharing knowledge..

mike95490 · April 2016

PV panels produce DC and have no senstivity to "skin effect"
4/OOOO wire does not exhibit "skin effect" until 125Hz
Standard 120 or 240 AC is 60Hz well below any "skin effect" region. Don't worry about it
http://www.powerstream.com/Wire_Size.htm

Low (& high) Voltage wire question

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