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Old 12-08-2014, 15:50   #16
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Re: Cable Size

P.S. It's not the charts or tables that serve as the answer (though ok for "rule of thumb" approach, but rather the voltage drop equation Vd=IxR. R=2xdxkft/1000ft x (ohm/kft). Vd=Ix2xd/1000ft/kft x ohm/kft and %Vd=Vd/Vnom x 100%=2xdxI/1000ft/kft x resistance of conductor (per resistance charts in NEC) divided by Vnom x 100%. But rule of thumb will probably get you there too.
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Old 12-08-2014, 17:23   #17
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Re: Cable Size

It's not clear to me if everyone is using panel voltage in their calculations or buss voltage. The difference between 13v and even 19v, never mind 50v would make a big difference in wire size. Weight savings would not be the issue but cost of the wire sure would. Never mind having such large wire above deck and going through the deck. I think they ask you to put the controller near the buss, if so that would really cut down for the need of large wire.
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Old 12-08-2014, 18:46   #18
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Re: Cable Size

When calculating the Vd, for the purposes of the system, nominal system voltage is used per the formula cited from the NEC, article 690 (per solar section,moderating calls, etc.). The higher voltages would not be used as they are generally open circuit voltages and relative to the panel and cell type, mono-crystalline, polycrystalline, cigs or amorphous. As the charging occurs from a potential of higher voltage to the lower nominal system voltage, it is a static picture of a dynamic situation that constantly adjusts itself, hence the value of MPPT I. Tracking on both sides of the system.. If one was able to bring the voltage in at a higher level because of the capability of the charge controller to take a higher series voltage from the panels to the controller and then reduce it to an input voltage considerably lower for nominal system voltage, that would be preferable but, again, the prospect of shading would lead to a potential loss of the entire array in some cases, depending on what cells might vibe shaded. Thus, it would seem to be optimal to maintain charging closer to the nominal system voltage level, run the panels to a centrally located buss bar, from there to a controller located the minimal distance necessary to the battery bank and call it good. Strategic minimization of distance, consequently wire sizing and expense are reflected in the choices and location of the balance of system components. Energy efficiency = cost efficiency.
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Old 13-08-2014, 15:26   #19
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Re: Cable Size

Quote:
Originally Posted by goboatingnow View Post
the sizing tool is correct as it adds a factor of 2. ( if you work it out from first principles)

dave
I think you have missed the point of my post.

This calculator does use the length of the cable from source to load and applies a factor of X 2 - but it doesn't tell you whether to use the single distance for the cable length or the return distance. All cable size charts seem to use the full return distance, and they tell you this, and all the calculators seem to use the single distance, but don't always make this clear. This can lead to confusion for people who do understand why you need to consider the return distance. If they put in double the length they get double the cable size - safer than having half the cable size needed - but many of these calculators are from cable retailers who are therefore making more money from your mistake.
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Old 13-08-2014, 18:20   #20
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Re: Cable Size

Here is a good calculator I use - it is not from a cable retailer.

AWG by wire length/amps calculator
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Old 14-08-2014, 04:23   #21
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Re: Cable Size

Quote:
Originally Posted by mitiempo View Post
Here is a good calculator I use - it is not from a cable retailer.

AWG by wire length/amps calculator
Many thanks for that - BUT - it has one very major failing - it says Maximum Current (Amps), what else could it be, but Distance: has no indication of feet or meters.

This is an international Forum so does anyone have an online calculator, not a chart, which is not misleading or confusing?????

Ohm's Law is so very simple but it's implication is often so very confusing, which is why Forums like this can be so useful.
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Old 14-08-2014, 20:28   #22
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Re: Cable Size

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Originally Posted by sailinglegend View Post
Many thanks for that - BUT - it has one very major failing - it says Maximum Current (Amps), what else could it be, but Distance: has no indication of feet or meters.

This is an international Forum so does anyone have an online calculator, not a chart, which is not misleading or confusing?????
The link is to a North American calculator so the distance is in feet - east to tell really as the wire is notated in AWG not mm2.
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