A little confused about inverter cable sizing

BeeSamps

New Member
Joined
Apr 30, 2021
Messages
7
Hey guys, forgive my newbness. I ordered a renogy 3000 W inverter/charger about a month ago. I'm now realizing that because my system is 12v I'm going to need some fairly beefy cables to it. I've been trying to calculate the cable size needed and thought I had it figured out. I picked up some good 1/0 welding cable locally yesterday. However, now I'm thinking I miscalculated and need an even bigger gauge. Can somebody help me figure this out? Thanks in advance.
 

wkyongae

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May 30, 2021
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22
I use 4/0 welding cables for everything. Costly yes, voltage drop no. Missouri wind and power original owner said it like this. Ac travels on the inside of the wire. Hence thats the reason solid copper wire can be used with it and you can run long distances. However, dc travels on the outside of the wire. Therefore, the more strains of wire, less voltage drop over longer runs. I use 4/0 welding cable from my combiner box to my charge controller of over 50 feet with no voltage loss. Amazon is the cheapest place I found.
 

Zil

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May 27, 2020
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Until we get to extremely high voltages or high wave lengths, the stories about electrons traveling on surface of wire or deep in the heart of copper are wives' tales. The conductivity of wire depends on the square area of the copper. Solid wire is used in common construction because it doesn't move nor need the flex of stranded wire, plus the big one, it is cheaper. One should at the least learn to use a voltage drop table to select dc wiring. And to use an ampacity table to select the fuses. Using cable that is way oversize makes jobs more difficult to string and steals money from the beer fund. Otherwise do not much harm. Sometimes bigger can be better.
 

chrisski

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Aug 14, 2020
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One should at the least learn to use a voltage drop table to select dc wiring. And to use an ampacity table to select the fuses.
I have found that for inverter to battery cables and also battery cables, voltage loss is not an issue as much as the ampacity of wire. Calculating for voltage loss only on these short runs often gets you wire that is not rated for that ampacity.

I also think a 3000 watt inverter for a 12 volt system is not a good idea. With voltage sag on a low battery and high wattage with an inverter that has a 10 volt cutoff, that would pull 300 amps + 15% efficiency losses = 345 amps. That does not take into account surge. So here is the NEC chart I use to choose wire size based off ampacity:

1633469140063.png
So to me, that is some mighty thick copper wire for that 345 amps.
 

sunshine

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Apr 24, 2020
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515
Using cable that is way oversize makes jobs more difficult to string and steals money from the beer fund.
A undersize cable would also save the inverter from overloads. The low voltage cut off reacts faster than the over temp one.
 

chrisski

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Aug 14, 2020
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2,705
A undersize cable would also save the inverter from overloads. The low voltage cut off reacts faster than the over temp one.
I'm not sure udersizing a cable for an inverter is a best practice.
 

sunshine

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Apr 24, 2020
Messages
515
I'm not sure udersizing a cable for an inverter is a best practice.
look at the cable as a resettable fuse. How undersized depends on the user selecting between inverter life and just how many low voltage cut offs one can stand.
.,,,and you thought that the skinny cable supplied with the Chinese inverter was just them skimping on costs!
 

nogridhome

NoGridHome.com
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May 13, 2020
Messages
48
Location
Chapel Hill, N.C.
I arrived at 8" jumpers and leads that I fabricated. As you can see, they can be too short. Electrons are unhappy and might get stuck in sharp corners. I use the 250 Amp type T marine fuse as I felt it was the best device available. Build your banks as shown so you can add batteries if needed. Just add another parallel row.
 

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