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Old 19-01-2016, 15:31   #1
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Someone Check my Math

I'm planning to install 2 new solar panels on my boat and I think I'm doing the wiring math correctly but want to make sure. I will be doing a home run for each panel and running each one on its own controller.

The panels I'm looking at are between 250-285 watts and run 30V. So they put out about 9-10A short circuit.

The distance from the panels to the solar controllers is about 30ft including horizontal and vertical runs. I plan on using 10 gauge wire as I understand it can carry about 700A ish @ 30V. So for a 30ft run (really 60 for both pos and neg) I should be good for 11.8A @30V, which is over the 9 or 10 the panels will put out? Is this correct?

A second question I have is about fusing. I plan to fuse the wires from the controllers to the batteries, but wondering about the runs from the panels. Should I fuse from the panels as well?
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Old 19-01-2016, 15:56   #2
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Re: Someone Check my Math

10 gauge can not carry 700amps. Not for long anyway.

For your two 240-250 watt panels, that would be about 20 amps at 24 to 30 volts. If your using a MPPT controller for a 12V system then I would use 8 guage from the panels to the controller, or possibly 6 guage. This limits voltage drop, which improves conversion efficiency at the controller. From the controller to the batteries you will have roughly 30-34 amps. So that would be at least #8 wire. Least that's what I would do.

50 amp Fuses at the battery connections would be required. 30 amp fuses at the solar panels would be smart too.
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Old 19-01-2016, 16:22   #3
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Re: Someone Check my Math

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Originally Posted by sailorchic34 View Post
10 gauge can not carry 700amps. Not for long anyway.

For your two 240-250 watt panels, that would be about 20 amps at 24 to 30 volts. If your using a MPPT controller for a 12V system then I would use 8 guage from the panels to the controller, or possibly 6 guage. This limits voltage drop, which improves conversion efficiency at the controller. From the controller to the batteries you will have roughly 30-34 amps. So that would be at least #8 wire. Least that's what I would do.

50 amp Fuses at the battery connections would be required. 30 amp fuses at the solar panels would be smart too.
I was basing my math on these numbers.. Its 700A/ft.
Wire Size Chart.1 - Read /w "Ohm's Law & You" Cruisers & Sailing Photo Gallery

I think you may also have missed that I'm doing 2 home runs so EACH panel will have its own run. EACH panel will have its own MPPT controller. That means under 10A per panel!
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Old 19-01-2016, 21:31   #4
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Re: Someone Check my Math

a 30 ft run gives you 60 ft of wire.
#10 wire has 0.00102 Ohms/ft resistance.
60 ft of #10 has a resistance of 0.0612 Ohms.
10A through 0.0612 Ohms gives you a voltage drop of 0.612 volts.
The wire will safely carry the current, but is this voltage drop acceptable?

This is a 2% drop at 30V panel voltage, but is this the actual MPPT panel voltage? (Actually, 30V sounds about right for a "24V" panel.)

Anyway, 2% isn't too bad, so the #10 wires should be OK.
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Old 19-01-2016, 22:26   #5
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Re: Someone Check my Math

google voltage drop chart. Easier and less likely to make a math mistake.
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Old 19-01-2016, 22:35   #6
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Re: Someone Check my Math

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a 30 ft run gives you 60 ft of wire.
#10 wire has 0.00102 Ohms/ft resistance.
60 ft of #10 has a resistance of 0.0612 Ohms.
10A through 0.0612 Ohms gives you a voltage drop of 0.612 volts.
The wire will safely carry the current, but is this voltage drop acceptable?

This is a 2% drop at 30V panel voltage, but is this the actual MPPT panel voltage? (Actually, 30V sounds about right for a "24V" panel.)

Anyway, 2% isn't too bad, so the #10 wires should be OK.
Thanks Paul.. I appreciate the worked out math. It matches up with what I figured. Looks like I'm pulling 10/4 marine wire..
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Old 20-01-2016, 09:42   #7
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Re: Someone Check my Math

It's my understanding that if the circuit between the controller and battery is disconnected before the solar panel, the controller will be damaged. Therefore I question having fuse between controller and battery?
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Old 20-01-2016, 10:32   #8
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Re: Someone Check my Math

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It's my understanding that if the circuit between the controller and battery is disconnected before the solar panel, the controller will be damaged. Therefore I question having fuse between controller and battery?
I'm not sure that is true.

Even if it is, I would rather have a dead controller than a fire!
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Old 20-01-2016, 10:35   #9
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Re: Someone Check my Math

I have been doing a lot of research on solar panels and this is a little long but well worth the read. if you read the FAQ's first then the RV Battery charging puzzle he answers many questions.

https://handybobsolar.wordpress.com/
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Old 20-01-2016, 11:07   #10
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Re: Someone Check my Math

Have you tried some of the solar string sizing tools?
http://string-calculator.morningstarcorp.com
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Old 20-01-2016, 11:24   #11
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Re: Someone Check my Math

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Have you tried some of the solar string sizing tools?
String Calculator » Morningstar Corporation
Ya... Frankly I found it easier to use the ohms law and table I posted. Most of the sizing tools seemed to be tied to one product or another.

I was just looking for a sanity check as I have a million projects going on. I have pretty much decided to pull 8AWG if I can get it, if not, 10AWG will be fine, just not %100 optimized.
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Old 20-01-2016, 12:02   #12
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Re: Someone Check my Math

Quote:
Originally Posted by travellerw View Post
I'm planning to install 2 new solar panels on my boat and I think I'm doing the wiring math correctly but want to make sure. I will be doing a home run for each panel and running each one on its own controller.

The panels I'm looking at are between 250-285 watts and run 30V. So they put out about 9-10A short circuit.

The distance from the panels to the solar controllers is about 30ft including horizontal and vertical runs. I plan on using 10 gauge wire as I understand it can carry about 700A ish @ 30V. So for a 30ft run (really 60 for both pos and neg) I should be good for 11.8A @30V, which is over the 9 or 10 the panels will put out? Is this correct?

A second question I have is about fusing. I plan to fuse the wires from the controllers to the batteries, but wondering about the runs from the panels. Should I fuse from the panels as well?
There is a standard durable solar panels cable - just use this one - up to 600W no issues at all... and do expect about 10amps per every 200W charging.. not much more...

Solar Panel Cable for MC4 Solar Panel connectors and MC3 4mm 1M Waterproof | eBay
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Old 20-01-2016, 12:42   #13
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Re: Someone Check my Math

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It's my understanding that if the circuit between the controller and battery is disconnected before the solar panel, the controller will be damaged. Therefore I question having fuse between controller and battery?
I think your understanding is incorrect.

Per ABYC, a fuse must be connected within 7" of the battery connection for any charger.

If the controller can't handle it, then it can't be ABYC complaint and shouldn't be on the boat.

That said, I have seen some 48 Vac 3~ wind generator controllers that cannot tolerate an open circuit between generator and controller.

The output of any voltage source that can cause a fire on wiring fault, should have some form of current limiting.

Most solar panels have short circuit current of 0, so fusing panel to charge controller is not required. The battery connection to the charge controller must be fused, to protect the charge controller or battery cables in the event of a short circuit. Since the charger wiring likely has the lower current carrying capacity at max output, the fuse should be sized to suit that.

Full Disclosure: I sell and install charging systems professionally. The post is offered as free advice and not intended to solicit business.

Ramblin Rod
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Old 20-01-2016, 13:50   #14
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Re: Someone Check my Math

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Originally Posted by vtomanov View Post
There is a standard durable solar panels cable - just use this one - up to 600W no issues at all... and do expect about 10amps per every 200W charging.. not much more...

Solar Panel Cable for MC4 Solar Panel connectors and MC3 4mm 1M Waterproof | eBay
This stuff has a cross-section of 4 square millimeters -- smaller than #10 AWG (which is 5.25 mm square). It's really too small for a 10A, 30 ft run (60 ft of wire).
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Old 20-01-2016, 14:23   #15
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Re: Someone Check my Math

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Originally Posted by ramblinrod View Post
Most solar panels have short circuit current of 0, so fusing panel to charge controller is not required.
Can you clarify this? Most solar panels have a high short-circuit current. My 100W panels put out around 6A when shorted. The panel wiring can easiy handle this (it's not much more current than the MPPT current), so I don't have a fuse on the panel, but the short-circuit current is hardly zero.
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