PV panels' cable question

[Iceni John]

I'll soon be installing the main feed cables for my PV solar panels, to bring their power down from the roof to the charge controllers.   There will be two banks of panels, each bank feeding its own controller that will charge half the house batteries  -  this way I'll have essentially two separate systems for redundancy, so if anything goes wrong I should still have half my system working.   (I'll be 100% dependent on solar, and my little generator is for absolute emergency use only.)   Each bank of panels will be four 240W, nominal 12V, i.e. theoretically 80A total output going to each charge controller.   In practice there probably won't be 80A, because panels don't produce their rated output most of the time and because they produce more than 12V most of the time.   I'm guessing that real-world output to each controller will probably be closer to about 50 or 60A.   Each cable will be about 12 feet long.

Ordinarily I would look up ampacity charts to choose a cable that gives the least voltage drop for that current over that distance, but there are two complications:  A) the cables will be inside conduit, and B) they will get hot at times due to their location.   They will be inside the 1.5" gap between the outer and inner roof panels, then they will run next to some blanked-off windows that will probably get very hot on the outside.   I haven't found any information for temperature correction factors, or for running cables inside conduit.

I looked at some Ancor marine cables at West Marine  -  they have 4 gauge for $3 per foot, and 2 ga for $5.   Ouch.   Someone suggested using heavy jumper cables instead, but they're not intended for continuous loads or for use in enclosed spaces that could get hot.   Would 600V welding cable work instead?   Blue Seas chart for USCG-recommended ampacities suggests 2ga or 4 ga, but are those sizes applicable for what I'll be doing?

Help!   I'm confused!   I want to do this right, and if I have to spend $5 a foot so be it, but I want to be sure I'm doing it right the first time.   Any electricians out there who'd know?

Thanks, John   

[Len Silva]

THHN or THWN are good for 90°C or 194°F.  That should be fine.  2Ga will give you about a .2 volt drop.  I wouldn't go any smaller.  If you are going to have all four cables in the same conduit, I think I would go up to 1Ga or 1/0.
Welding cable in conduit is probably not a good idea.

[Iceni John]

Thanks, Len.   There will be only two feed cables, not four, and I plan on running each one inside its own conduit.   Is 2 gauge still good for that?   Just out of curiosity, is the Ancor marine tinned stranded cable equivalent to THHN or THWN?   Does it make any difference to the ampacity if the conduit is metal or plastic?   Plastic will be easier to install inside the curved roof, but I'm worried about the heat doing bad things to it (and I can't inspect it when it's in place).

Thanks, John   

[luvrbus]

Why not welding cable one of the largest seller of solar supplies in the US Northern AZ Sun and Wind welding cable is their first choice ?

[harleyman_1000]

 What is your estimated cost of your solar?

[Ralph7]

         I have 840 watts and a Morningstar MPPT controller, connected all in series, using welding cable from the roof. 2 separate combiner boxes on the roof, 1 for + and 1 for --,,,Also I fused input and output from controller,
        Got all my stuff from Northern Wind and Sun in Flagstaff
        One major item IF you do not wire heavy and loose any Volts it will affect total system!!

[Len Silva]

Why not welding cable one of the largest seller of solar supplies in the US Northern AZ Sun and Wind welding cable is their first choice ?

The only question is welding cable in conduit.  If it is in open air or a cable tray there would be no problem.
Because most welding cables do not have a UL type number, and the ones that do are SJ or similar, they are not approved for conduit.  On the other hand, THE NEC does not really apply to low voltage so it's probably not an issue.

I would be concerned about heat dissipation in a conduit. It's really kind of nit picking and for 12 feet likely wouldn't make any difference.

[Iceni John]

What is your estimated cost of your solar?

Large panels are now less than $1 per watt, cheaper than ever before.   My gut feeling is that PV will soon start climbing in price, as either anti-dumping tariffs are enforced or as demand picks up generally.   I'll buy the panels this year, even if I don't install them for another year or three.   Two MPPT charge controllers and all the other associated wiring and so forth may end up costing as much as the panels themselves.   Mind you, once I have it installed I'll have free electricity until I'm a doddering grizzled old fart.   Oh wait, I already am.

Another incidental benefit to covering most of my roof with panels is shade  - there'll be much less sunlight reaching the roof.   I'll also make some solar water heater panels to at least preheat the water before the water heater, and in the summer I may not need to use the WH at all.   The main reason for me to have PV is simply to not have to run a generator every time I need power  -  I really HATE the sound of generators!

John

[harleyman_1000]

Im buying and picking up my first bus in Phoenix, and would like to have solar installed before I start fulltiming in it. Can anyone tell me of someone that can install it for me? I have rewired my harleys, but this solar stuff is all new to me, and truthfully I don't understand how it works?

[Iceni John]

Im buying and picking up my first bus in Phoenix, and would like to have solar installed before I start fulltiming in it. Can anyone tell me of someone that can install it for me? I have rewired my harleys, but this solar stuff is all new to me, and truthfully I don't understand how it works?

As Clifford also suggests, Northern AZ Sun & Wind in Flagstaff is well respected for honest business practices and fair pricing, not something one can take for granted in the solar industry!   See what they have to offer  -  I think they have complete packages which would take a lot of the guesswork off your shoulders.   Their price for 25 feet of 2 gauge welding cable is way less than I can buy 4 ga marine cable for locally (even with S&H), so they'll be getting some of my business.   Their prices on panels are also pretty good, not quite as low as some companies, but what recourse does one have with some fly-by-night outfit if a panel is damaged or defective?   Their website has lots of useful info.

John

[Ralph7]

             I agree contact Northern Wind n Sun for install advice, several in Az. do the work but from what I have read most do not wire properly! And the systems do not work to the max.

[sommersed]

Wire the panels in series up to the limit of a good MPPT controller or two, say 24 or even 48 volts (or whatever).

The controller will then do it's thing and input the voltage the battery's are configured for. Personally, I use a 24 Volt battery bank because there are converters, inverters, and such readily available in 24 Volt Batt banks.

The smaller cabling is a lot easier in the higher voltages!


Ed

[harleyman_1000]

I'll soon be installing the main feed cables for my PV solar panels, to bring their power down from the roof to the charge controllers.   There will be two banks of panels, each bank feeding its own controller that will charge half the house batteries  -  this way I'll have essentially two separate systems for redundancy, so if anything goes wrong I should still have half my system working.   (I'll be 100% dependent on solar, and my little generator is for absolute emergency use only.)   Each bank of panels will be four 240W, nominal 12V, i.e. theoretically 80A total output going to each charge controller.   In practice there probably won't be 80A, because panels don't produce their rated output most of the time and because they produce more than 12V most of the time.   I'm guessing that real-world output to each controller will probably be closer to about 50 or 60A.   Each cable will be about 12 feet long.

Ordinarily I would look up ampacity charts to choose a cable that gives the least voltage drop for that current over that distance, but there are two complications:  A) the cables will be inside conduit, and B) they will get hot at times due to their location.   They will be inside the 1.5" gap between the outer and inner roof panels, then they will run next to some blanked-off windows that will probably get very hot on the outside.   I haven't found any information for temperature correction factors, or for running cables inside conduit.

I looked at some Ancor marine cables at West Marine  -  they have 4 gauge for $3 per foot, and 2 ga for $5.   Ouch.   Someone suggested using heavy jumper cables instead, but they're not intended for continuous loads or for use in enclosed spaces that could get hot.   Would 600V welding cable work instead?   Blue Seas chart for USCG-recommended ampacities suggests 2ga or 4 ga, but are those sizes applicable for what I'll be doing?

Help!   I'm confused!   I want to do this right, and if I have to spend $5 a foot so be it, but I want to be sure I'm doing it right the first time.   Any electricians out there who'd know?

Thanks, John   

So you will have 2 banks of 960 each, or a total of 1920 watts? Is that 160 amps going to the batteries?

[Iceni John]

So you will have 2 banks of 960 each, or a total of 1920 watts? Is that 160 amps going to the batteries?

There will be two separate banks of almost 1,000 W each, each bank feeding its own charge controller which will charge four of the eight golfcart batteries.   Each bank will be independently tiltable to improve performance, especially in winter when the sun is lower.   The more I think about this, the more I realize that it's going to be a lot less than the nominal 80-ish amps going down each feed cable, but everyone stresses the critical necessity of minimizing voltage loss.   While I can probably get away with 4 ga cables without a problem, I think I'll use 2 ga  -  in the big scheme of things an extra few dollars for heavier cables is peanuts compared to everything else I'm doing!

John

[Lee Bradley]

I don't know how your banks are configured but I would configure them so I had each 12 volt series separately cabled down to the controller and parallel them at that
point on a large bus bar.  Smaller wiring and easier to drop a leg if you a cell.  Just a thought but keep us updated I can see this in my future. About how big is a 1,000 watt panel?