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Size of cable for solar system.


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Voltage? If 12V then 4 O, as in four zero guage, is not unreasonable.

I use 2 O within the battery bank, four 8D deep cycle AGM at 225AH, and 4 O through fuse and switch to inverter on plus side and 4 O to ground through current shunt.  My system is "24V" which means we operate a float voltage of 27 V.  Magnum 4000 is the inverter/charger with solar worth about 25amps at max charge.

When these batteries fail then it is some kind of lithium battery for us. 

There calculation sites for cable sizing based on current and length of the cable run. Remember the longest run defines the cable diameter.

Bill

 

 

 

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Bob, I'm NOT a Solar Expert but a long retired power distribution design engineer so for what its worth here is my "opinion":

1) 2000/12 = 166 amps EVEN IF YOU NEVER ACTUALLY USE THAT MUCH.. not accounting for inefficiency and heat losses

2) When I practiced (pursuant to the NEC at that time) to size a conductor after we calculated the "Maximum Continuous Current" we specified the conductor have an ampacity of no less then 125% MCC and 1.25 x 166 = 207 Amps. I cant say from here your MCC, its likely less then 166 

3)  FIRST I would recommend NO LESS THEN 2/0 Copper even though its unlikely you ever draw that full rated amount.

4) SECOND I would compute the line voltage drop based on current and wire size and distance and if necessary (or to be on the safe conservative side to reduce voltage drop) upgrade to even 4/0. I don't like over 3% drop better yet 1% on the conservative side.

NOTICE you don't have to take my word for it I'm long retired so in order to draw your own conclusions here are some Blue Sea Systems and other references relevant to wire size and voltage drop etc.

Part 1: Choosing the Correct Wire Size for a DC Circuit - Blue Sea Systems

dc cable ampacity chart - Bing

Voltage Drop Calculator

SUMMARY even if smaller could get you by, if you use 200 Amps (which I doubt ) at 8 feet to be safe and conservative  MY CHOICE would be  NO LESS THEN 2/0 and if you want to reduce line voltage drop even more, upgrade if you like

PS DO NOT forget overcurrent protection at the source YIKES the potential current and heat this batteries could pump into a short circuit !!!!!!!!!!!!!!!!!!!!!!

PS AN EXAMPLE for illustration NOT recommendation:

Ifffff you were drawing 166 Amps FROM your batteries TO your Inverter ,,,,,,,,,, used 2/0 Copper Wire,,,,,,,,,,,Distance was 10 feet:

VOLTAGE DROP (Per calculator not me) would be 0.26 Volts, or 2.16%  

Ifffffffffffff you were drawing 166 Amps and used 4/0 Wire

VOLTAGE DROP 0.16 Volts, 1.36% 

I have given you the tools now its YOUR choice

 

John T Again NOT a solar expert see what they have to say, but regardless Id use NO LESS then 2/0 or bigger to further reduce line voltage drop 

Edited by oldjohnt
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My first thought without researching voltage drop and doing it right as oldjohnt suggested is at least 4/0.  Most of the better brand name inverters provide a surge capability and I design for that.  Even with 4/0 eight feet is a long run and there are some $$$ involved.  My 2000 watt inverter is 16" from my batteries and has a 3000 watt surge capability.  I used 4/0 as the price difference was not significant.  The voltage sag when using an inverter on 4 golf cart batteries IS significant and there will be little room for additional voltage loss.

Edited by Randyretired
Clarity
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This is one of those cases where the cost difference for say 20 feet of 2/0 versus 4/0 isn't all that much in comparison to the cost of the Inverter, so to be on the safe and conservative side and reduce voltage drop even further, even if 2/0 (My absolute MINIMUM recommendation) were to suffice dependent on actual current and cable length, there's no harm in using 4/0 which will reduce Voltage Drop IE Bigger is Better !!!!!!!!!!! Its his choice we have done all we can for him at this point. The actual continuous current and max short term surge current info is needed to refine this further.

Again, 166 Amps,,,2/0 Copper Wire,,,Distance of 10 feet:

VOLTAGE DROP  0.26 Volts, or 2.16%  

166 Amps using 4/0 Wire

VOLTAGE DROP 0.16 Volts, 1.36% 

IF the calculator I used is correct ???

John T

Edited by oldjohnt
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2 hours ago, Jack Mayer said:

It is a simple answer. 4/0. Do not use less.

Absolutely no harm in using bigger cable which reduces line voltage drop AMEN I often practiced that as an engineer even if a Gauge less satisfied all requirements and any applicable codes and especially if the cost increase was negligible... Of course, the actual current (166 Amps for a continuous 2000 Watts at 12 Volts), distance (think he said 8 feet ???) and any manufacturers specification (I dont have that) contributes to the necessary minimum wire size, I advise NEVER to go less then what they say but probably no harm in going bigger.........  

John T Yall take care now and be safe

Edited by oldjohnt
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17 hours ago, Randyretired said:

With inverter inefficiencies and a surge rating of 3300 the current could easily go up around 300 amps. 

Hey there Randy, GOOD POINT That's right if he were to pull 300 Amps at 12 volts that's 3600 Watts and correlates to 350 MCM (subject to time of course) conductors N0W WERE TALKING SOME HUGE COPPER LOL I had jobs where I specified 350 MCM or even 500 but that's about the biggest I had to deal with in my practice. My electricians hated it soooooooooo hard to deal with pull and bend grrrrrrrrrrrr

HOWEVER I  1) Don't have his Inverter specs (but I don't see it drawing 300 amps for very long) to know its surge capacity (current and maximum time duration) orrrrrrrr 2) For how long such a huge amount of current he might need would last and 3) Best I recall subject to current and duration when I practiced the NEC we weren't required to size conductors for a maximum short term surge but we had to compute the "Maximum Continuous Current" and size the conductors to have a minimum ampacity of 125% greater.

REGARDLESS I'm sticking with my opinion if he has a 2000 Watt rated Inverter located 8 feet from his battery the ABSOLUTE MINIMUM wire size I would recommend would be 2/0 but to reduce Line Voltage Drop and to be conservative and allow such things as short term high current surges 4/0 is even better !!!!!    While 350 MCM would certainly greatly reduce Line Voltage Drop and is rated for 300 Amps, yet due to his batteries and Inverters Maximum Surge capacity and subject to time, it could be "overkill", hey he's free to use it if he so chooses, but just like 2/0 would be my ABSOLUTE MINIMUM I doubt Id go much over 4/0 unless the specs and reality (extreme long distance or high currents) called for it WHICH I SERIOUSLY DOUBT I suspect 4/0 would handle any short term high current surge without degrading.... Hard to say absent real specs and currents but I'm giving the best advice given the limited specs and data available 

 PS For 200 Amps (2000 Watts/12 Volts= 167 Amps + 10% = 184 Amps) Blue Sea Systems for "Critical" (3% while less is better) recommends 2/0 for up to 20 foot cable runs and 4/0 for 25 to 40 feet, so for 8 feet and 184 Amps (not withstanding surges) we are all pretty well in the ball park and in good company as far as what we recommend. YAYYYYYY

PS FYI If the Voltage Drop Calculator I used is correct (no warranty) 

184 Amps (2000 Watts/12Volts  x 1.1) with 8 feet cable: Not withstanding short surges

2/0  Cable,,,Voltage Drop = 0.23 Volts or 1.91%

4/0 Cable,,,Voltage Drop = 0.14 Volts or 1.2% (would save 0.09 Volts) 

dc cable ampacity chart - Bing

Its fun and a pleasure sparky chatting with you gentlemen THANK YOU  I think we have pretty well beat this horse to death lol but Ive enjoyed it and hope it helps

Best wishes and God Bless

John T  Long retired n rusty power distribution engineer so NO warranty, consult the code and experts where fire and life safety are concerned NOT me ............

 

Edited by oldjohnt
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3 hours ago, bobsallyh said:

I picked up 4/0 for the inverter on Friday.

Bob, thanks for the update, WISE CHOICE, it never hurts to go bigger then necessary (or bigger then what Blue Sea Systems suggests) as it reduces voltage drop, reference my numbers above (based on an online chart no warranty) above showing if you were pulling 184 amps (likely not that much that often??) you would save 0.09 volts in cable line loss versus if you used 2/0...... 

John T

Edited by oldjohnt
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