What gauge wire needed?

[jvs]

First of all ,i know nothing about electrical things thats why i hope you can help me.

I have two 12volt 300 watt solar panels and i want them to run a waterpump,the pump is a 12 volt 3500 GPH bilge pump made by Seaflo.

I also have a PWM solar charge controller,voltage rating 12/24 volts,rated current 20 amps.

The distance between the panels and the charge controller is 35 meters.

So far so good.

From what i understand it would be best if i wired the panels in parallel to keep things 12 volts.

Parallel means higher current so bigger wires needed?

The salesmen told me i should run a separate wire for each panel,he recommended 2,5 as wire size.

That is where i decided i needed to get a second opinion,why the need of two wires?One heavier gauge would be better?

Also not sure if the charge controller could convert 24 volts back into 12 volts if i would connect in series.

What is the best way to wire this system and in each case(12 or 24 volts) what wire size should i use?

There is no option to shorten the distance between panels and controller.

We would like to water the garden with this system and it will be off grid.

I would like to have a very small voltage drop if possible.

Please help me.

[KMartinHandyman]

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VOLTAGE LOSS TABLES FOR AWG WIRE

WIRE LOSS TABLES FOR SOLAR ELECTRIC SYSTEMS

Includes 12, 24, and 120 volt charts and a metric to AWG size conversion table.

This is a five percent table which means at these amperage ratings at the listed distances, 5% of the power would be lost to resistance. Five percent is normally acceptable in low voltage systems, but if you want a 2% figure, divide the given distances by 2.5. For a 10% loss multiply the distance by 2. For distances at 48 volts, double the 24 volt distances for a 5 percent loss figure. For 240 volt 5% loss, double the 120 volt distances. These distances include the NEC requirement for current over sizing of 25%.

Example: For a pump drawing 9 amperes at 24 volts, located 88 feet from the battery bank: look at the center table for 24 volts. In the far left column find the next number higher than 9 (which is 10) and follow that line across the table until you find a distance figure greater than 88. At the top of the column find the gauge of wire (#6) that should be used. This method insures that wire losses are kept to an acceptable level without spending too much money on extra-heavy cable. Using a heavier wire than indicated, however, will result in even higher efficiencies and we do sometimes invest in the next larger gauge. Wire can get expensive, and it may not be worth the money to get that last 1% if you have to go to a much larger wire size.

SOME OF THE NEWER GRID TIE SYSTEMS INVERTERS, SUCH AS THE SUNNY BOY, USE UP TO 600 VOLTS DC. GENERALLY IN THESE SYSTEMS LOSS IN WIRE IS NOTHING TO WORRY ABOUT. HOWEVER - YOU WILL HAVE TO BE MORE CAREFUL ABOUT SELECTING AND INSTALLING THE WIRING - HIGH VOLTAGE DC IS NOT SOMETHING YOU WANT TO DO A 2ND RATE WIRING JOB ON. MAKE SURE THE INSULATION IS RATED FOR 600 VOLTS, AND THAT THERE IS NO DAMAGE TO THE WIRE OR INSULATION.

All distances are in FEET

DO NOT USE ANY WIRE SIZES THAT MIGHT FALL INTO THE RED ZONE - THIS WOULD EXCEED THE AMPERAGE RATING OF THE WIRE AND IT MAY OVERHEAT AND BURN.

120 Volts AC or DC Chart

Amps in Wire Watts at 120V #14 #12 #10 #8 #6 #4 #2 1/0 2/0 3/0

2 240 422 656

4 480 187 328 516

6 720 141 225 328 562

8 960 103 159 272 422 666

10 1200 84 131 216 337 534

15 1800 56 84 131 225 356 562

20 2400 65 103 168 272 422 675

25 3000 84 131 216 337 543

30 3600 65.63 112 178 281 450 722

40 4800 84 131 216 337 543 675

50 6000 67 103 171 272 431 543 684

24 Volts DC Chart

Amps

in Wire Watts

at 24V #14 #12 #10 #8 #6 #4 #2 1/0 2/0 3/0

1 24 169 262 412 675

2 48 84 131 207 337 532

4 96 37 66 103 169 267

6 144 28 45 66 112 178 282

8 192 21 32 54 84 133 216

10 240 17 26 43 67 107 169 270

15 360 11 17 26 45 71 112 180 289

20 480 13 21 37 54 84 135 217 270 343

25 600 17 26 43 67 108 172 217 274

30 720 13 22 36 56 90 144 180 228

40 960 17 26 43 67 108 135 171

50 1200 13 21 34 54 86 108 137

12 Volts DC Chart

Amps in Wire Watts at 12V #14 #12 #10 #8 #6 #4 #2 1/0 2/0 3/0

1 12 84 131 206 337 532

2 24 42 66 103 168 266 432 675

4 48 18 33 52 84 133 216 337 543 675

6 72 14 22 33 56 89 141 225 360 450 570

8 96 10 16 27 42 66 108 168 272 338 427

10 120 8.5 13 22 33 53 84 135 218 270 342

15 180 6 8.5 13 22 35 56 90 144 180 228

20 240 6.6 10 16 27 42 67 108 135 171

25 300 8 13 22 33 54 86 108 137

30 360 6.6 11 18 28 45 72 90 114

40 480 8 13 21 33 54 67 85

These are one-way distances, measured from point A to point B. The out and back nature of electrical circuits has already been included. For PV arrays, figure the entire run, from the panels to the charge controller to the batteries

Cross Reference of AWG (American Wire Gauge) Sizes to Metric (mm)

AWG mm2 AWG mm2 AWG mm2 AWG mm2

30 0.05 18 0.75 6 16 4/0 120

28 0.08 17 1.0 4 25 300MCM 150

26 0.14 16 1.5 2 35 350MCM 185

24 0.25 14 2.5 1 50 500MCM 240

22 0.34 12 4.0 1/0 55 600MCM 300

21 0.38 10 6.0 2/0 70 750MCM 400

20 0.50 8 10 3/0 95 1000MCM 500

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[jvs]

This is not the kind of info i need,the internet is full of this stuff but it makes no sense to me at all.

Thanks for posting anyway.

[Jdietz]

Only interesting metric is Amps. This decides the cable losses and heating. (That's why the voltage on distribution cables is so high, cheaper cable. More volts: less amps, same total wattage.)

Bigger is always better, especially on the 12 or 24V runs, as the most amps run at that segment. 24V would halve your amps, so can go with smaller cable.

Tables everywhere, but in the end you pick the one that:

a) Doesn't burn up

Minimizes losses over distance for the budget you have.

[Jdietz]

Here is a decent calculator. It advises 5mm^2 cable for 24V/30 meters, 12V would need 2 runs or a huge cable.

http://www.solar-wind.co.uk/cable-sizing-DC-cables.html

[Crossy]

300W @ 12V is 25A so simply on current rating 2.5mm² cable would do the job, just (but your charge controller is too small anyway).

But over 30m that same 2.5mm² cable will drop all of your panel voltage before it got to the controller

16mm2 cable would drop 2V, 25mm² would be better, getting expensive (don't forget that's two pairs of 25mm² you need).

What is the current rating of your pump? It may be better to put the charge controller and batteries at the panel end of the cable run.

Is it possible to get the panels nearer the pump, shorter runs will need smaller cables?

Or consider a 220V pump and an inverter.

[jvs]

Crossy ,Thank you!

Yes i can put the controller next to the panels,no problem.Please advice wire size?

The pump is 25 Amps max. current draw and is dual voltage 12/24.

Better to go 24 volts?Series or parallel?

I hope you can explain it so i can understand.

[Crossy]

If you have a dual voltage pump great, run everything at 24V (panels in series). I assume the 25A is the 12V current (about 300Watts), so the 24V current would be about 12.5A.

At 12.5A, 24V and 30m you should be able to use one run of 10mm² cable from the controller to the pump [if the pump is 25A @ 24V then use one run of 16mm² cable).

If you are not using the pre-wired solar connectors I would run 4mm² from the panels to the controller, or if you've bought a roll of 10mm² just use that

[jvs]

Thanks,this is the set up as it is now?I mean with the controller being 30 meters away from the panels,i can place the controller next to the panels,different scenario?

The pump draws maximum 25 Amps,how do i know this is at 12 or 24 volts?