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Old 11-08-2012, 09:11   #1
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Help with Cable Sizing Calculators

Planning an install of 2 x 135 Watt solar panels. Details are:
Peak Power 135W
Rated Voltage 18.4V
Open Circuit Voltage 24.1V
Short Circuit Current 8.69A


Question is, when using a cable sizing calculator, what voltage and power values do I use, in this case, do I use 12V (nominal) or 18.4, and what amperage.

Also, for cable length, is it the length of the run from the panel to controller, or do I double that value (there and back).
Want to minimise any voltage drop, so dont want to undersize, but on the overhand, dont want to waste money on copper.

Any help as always is appreciated
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Old 11-08-2012, 09:21   #2
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Re: Help with cable sizing calculators

Electrical wire is gauged by the amount of current you expect to run through the wire, the distance of the run, if the wire is bundled or not and the maximum amount of heat you expect. The insulation is according to the maximum voltage and other environmental conditions you might expect.

The run is always measured from the source to the load and back to the source. In your case the source would be the battery or the correct sized breaker where you plan on connecting the load to. If you are unsure then measure from the battery to be safe.

For DC, dividing the power (watts) by the potential (volts) will give you current (ampres).

You then go to a table like this to determine what gauge wire you need....


See here for the color coding for the proper wire gauge...
http://www.westmarine.com/webapp/wcs...ge=Marine-Wire

Use the 3% voltage drop table and not the 10% voltage drop table.
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Old 11-08-2012, 10:19   #3
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Re: Help with cable sizing calculators

Quote:
Originally Posted by nigel1 View Post
...

Question is, when using a cable sizing calculator, what voltage and power values do I use, in this case, do I use 12V (nominal) or 18.4, and what amperage.

Also, for cable length, is it the length of the run from the panel to controller, or do I double that value (there and back).
Want to minimise any voltage drop, so dont want to undersize, but on the overhand, dont want to waste money on copper.

Any help as always is appreciated
Cheers
Nigel
Amps is Amps. Volts don't matter for this purpose as long as other components in the circuit are rated for the operating voltage or greater. See other post with diagram.

Length of circuit = total round trip length of cable. Bigger wire gauge = less voltage drop. I suggest sizing up a gauge or two not only to minimize drop, but also for future use (like adding another panel).
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Old 11-08-2012, 11:29   #4
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Re: Help with cable sizing calculators

Thanls, so let say I wire two x 135W in parallel, then thats about 16 amps.
I was using this calculator
DC Cable Sizing Tool - Wire Size Calculator - MM2 & AWG - solar-wind.co.uk
with a cable run of say 25 feet round trip.
Based on 2% loss, looking at 3AWG (25mm2). But to be on safe side, will use 2 AWG (35mm2) or, is that a bit overkill??
Not plannning on adding other panels in that area.
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Old 11-08-2012, 12:40   #5
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Re: Help with cable sizing calculators

Quote:
Originally Posted by nigel1 View Post
... But to be on safe side, will use 2 AWG (35mm2) or, is that a bit overkill??
Not plannning on adding other panels in that area.
I would go with larger cable (2 AWG). For that short of a run you are not looking at big cost difference and bigger is better.
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Old 11-08-2012, 12:49   #6
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Re: Help with cable sizing calculators

Quote:
Amps is Amps. Volts don't matter for this purpose as long as other components in the circuit are rated for the operating voltage or greater.
Volts really do matter for determining the proper conductor size.

The basic formula for determining conductor size is:

CM = (K x I x L)/∆E
Where:
CM = area of the conductor in circular mils
K = constant for resistivity of copper; 10.75
I = current in amps
L = circuit (source to load and return to source) length in feet
∆E = desired voltage drop in volts

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Old 11-08-2012, 12:57   #7
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Re: Help with cable sizing calculators

For a boat this size, 12 1/2 ft from panels to controller to batts seems short.
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Old 11-08-2012, 13:54   #8
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Re: Help with cable sizing calculators

Quote:
Originally Posted by CharlieJ View Post
Volts really do matter for determining the proper conductor size.

...

Perhaps I was a bit cavalier with "Amps is Amps" statement, but for a low voltage short run like this is it really going to make a practical difference in wire gauge selection?
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Old 11-08-2012, 14:44   #9
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Re: Help with cable sizing calculators

Quote:
Originally Posted by CharlieJ View Post
Volts really do matter for determining the proper conductor size.

The basic formula for determining conductor size is:

CM = (K x I x L)/∆E
Where:
CM = area of the conductor in circular mils
K = constant for resistivity of copper; 10.75
I = current in amps
L = circuit (source to load and return to source) length in feet
∆E = desired voltage drop in volts

Charlie
The voltage in the above formula is the voltage drop due to the resistance of the cable (R = K x L / CM), not the supply voltage. For all practical purposes, the OP does not need to concern himself with the supply voltage, only the "short circuit current", which is the maximum current his panel is capable of providing.
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Old 11-08-2012, 15:04   #10
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Re: Help with cable sizing calculators

Quote:
Originally Posted by nigel1 View Post
Thanls, so let say I wire two x 135W in parallel, then thats about 16 amps.
I was using this calculator
DC Cable Sizing Tool - Wire Size Calculator - MM2 & AWG - solar-wind.co.uk
with a cable run of say 25 feet round trip.
Based on 2% loss, looking at 3AWG (25mm2). But to be on safe side, will use 2 AWG (35mm2) or, is that a bit overkill??
Not plannning on adding other panels in that area.
According to the table I posted, 25 feet and 16 amps works out to 6 AWG. 2 AWG is overkill. I think that on the calculator that you used you entered 25. The units for distance for that calculator are meters. The correct run to enter would be 8 meters.

I think it is a somewhat of a waste of money to gauge for less than a 3% drop. Marine electronics are tolerant of greater voltage fluctuations. Also, the difference in efficiency between 1%, 2% and 3% is something that you will never notice.
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Old 12-08-2012, 05:35   #11
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Re: Help with Cable Sizing Calculators

Thanks for all the advice.

David, I used 8m for length, but opted for the 1% loss, on the understanding that with Solar, the least loss the better.
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Old 12-08-2012, 05:57   #12
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Re: Help with Cable Sizing Calculators

Quote:
Originally Posted by nigel1 View Post
Planning an install of 2 x 135 Watt solar panels. Details are:
Peak Power 135W
Rated Voltage 18.4V
Open Circuit Voltage 24.1V
Short Circuit Current 8.69A


Question is, when using a cable sizing calculator, what voltage and power values do I use, in this case, do I use 12V (nominal) or 18.4, and what amperage.
l
The panels won't produce the short circuit current. This is only seen at 0v.
The current to use or the calculations from the panels to the controler is the maximum power point current. This will be listed in the specs, probably about 7.8A for 135w. If using a MPPT controler this is correct current to use for calculations ( although in exceptional conditions you may briefly see slightly more than this). If using a non MPPT regulator the current from the panels and controler will be slightly more, but only very slightly.
From the controler to the batteries the current will be higher if you use a MPPT regulator.
In this case the best approximation of the maximum current is the wattage of the panel divided by the minimum likely charging voltage. Say 135/12.5 =10.8A.

So in summary with a MPPT regulator the maximimum current to use for voltage calculations would be 7.8A for the panel to controler and 10.8A from the controler to batteries. (for each 135w panel in parallel, so double these figures)

Hope this helps. Often the calculations are only done roughly (and remember these are maximums), but you asked, and if you want to calculate precisely these are the correct numbers to use.
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Old 12-08-2012, 06:39   #13
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Re: Help with Cable Sizing Calculators

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Originally Posted by nigel1 View Post
David, I used 8m for length, but opted for the 1% loss, on the understanding that with Solar, the least loss the better.
Many things conspire to reduce the average output from solar cells - the angle, a passing fluffy cloud, strategically-deposited gull poop... so the solar cells will rarely if ever work at max rated output. So 3% cable loss is trivial compared to these.
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Old 12-08-2012, 10:23   #14
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Re: Help with Cable Sizing Calculators

Thanks for the further explanations, and to remind me about the other variables, now I know what I need, and saved a few pennies.
Cheers
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