Both DC cables need to be the same length?

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stevetull
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Posts: 91
Joined: Tue Feb 24, 2015 6:40 am
My RE system: FlexPower Two:
2 VFX3648, X240, 2 FM80, Mate 3s, FnDC, Hub 10
One array facing south at 30 degree tilt
One array facing southwest at 10 degree tilt
Total of 8kW
16 Fullriver AGM DC400-6 = 830Ah @ 48v
OpticRE: Paige cabin
Location: Near Bastrop Texas
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Both DC cables need to be the same length?

Post by stevetull »

Is there any truth to the notion that the DC cables of a pair need to be exactly the same length? I built a Flex 2 system a couple years ago. We're doing some upgrades with our system and I'm putting four new 2/0 inverter cables. I was going to make them almost as short as possible, but I just heard that they really ought of be all the same length. Is this the case?

Thanks.

-Steve
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Kurt Lundquist
Alpha Energy Engineering
Posts: 1447
Joined: Tue May 31, 2011 7:23 am
My RE system: VFX3524, Mate3, 200Ah 24V AGM Battery Bank
Location: Arlington, WA

Re: Both DC cables need to be the same length?

Post by Kurt Lundquist »

With parallel strings of cycling batteries you should try to make sure the parallel conductor runs have similar resistance to help balance the charge on both bank. If one string has higher resistance then it will be chronically undercharged.

Here is an article that I found linked in this forum years ago: http://www.smartgauge.co.uk/batt_con.html

Thanks,

-Kurt
Kurt Lundquist
Renewable Energy Project Engineer
http://alphaenergy.us/
stevetull
Forum Virtuoso
Posts: 91
Joined: Tue Feb 24, 2015 6:40 am
My RE system: FlexPower Two:
2 VFX3648, X240, 2 FM80, Mate 3s, FnDC, Hub 10
One array facing south at 30 degree tilt
One array facing southwest at 10 degree tilt
Total of 8kW
16 Fullriver AGM DC400-6 = 830Ah @ 48v
OpticRE: Paige cabin
Location: Near Bastrop Texas
Contact:

Re: Both DC cables need to be the same length?

Post by stevetull »

Thank you Kurt. I guess I'm still unclear.
I'm about to build new 2/0 cables that go from each inverter terminal to it's respective positive or negative BBUS inside the DC enclosure. I measure that a 28" red and a 24" black cable work the best for the upper inverter, and a 28" red and a 30" black would work the best for the lower inverter. From there on out to the battery bank I have two 2/0 96" cables.
Would you think the cables inside the FW need to be the same length or is this close enough?

Thank you.

-Steve
User avatar
Kurt Lundquist
Alpha Energy Engineering
Posts: 1447
Joined: Tue May 31, 2011 7:23 am
My RE system: VFX3524, Mate3, 200Ah 24V AGM Battery Bank
Location: Arlington, WA

Re: Both DC cables need to be the same length?

Post by Kurt Lundquist »

stevetull wrote:I measure that a 28" red and a 24" black cable work the best for the upper inverter, and a 28" red and a 30" black would work the best for the lower inverter.
If it were my system I would go with 28" red and 30" black for both.
stevetull wrote:From there on out to the battery bank I have two 2/0 96" cables.
Do you mean you have 2 red and 2 black or just 2 total?

-Kurt
Kurt Lundquist
Renewable Energy Project Engineer
http://alphaenergy.us/
stevetull
Forum Virtuoso
Posts: 91
Joined: Tue Feb 24, 2015 6:40 am
My RE system: FlexPower Two:
2 VFX3648, X240, 2 FM80, Mate 3s, FnDC, Hub 10
One array facing south at 30 degree tilt
One array facing southwest at 10 degree tilt
Total of 8kW
16 Fullriver AGM DC400-6 = 830Ah @ 48v
OpticRE: Paige cabin
Location: Near Bastrop Texas
Contact:

Re: Both DC cables need to be the same length?

Post by stevetull »

Roger that on the inverter cable lengths.

And yes, two total. One red and one black 96" cable to the battery bank.

Thanks again.

-Steve
User avatar
Kurt Lundquist
Alpha Energy Engineering
Posts: 1447
Joined: Tue May 31, 2011 7:23 am
My RE system: VFX3524, Mate3, 200Ah 24V AGM Battery Bank
Location: Arlington, WA

Re: Both DC cables need to be the same length?

Post by Kurt Lundquist »

stevetull wrote:And yes, two total. One red and one black 96" cable to the battery bank.
If the cable run is in free air I think it should technically be 4/0 copper minimum. If it is in conduit then then 4/0 isn't enough. I'm assuming that you have 2 x 175A breakers for each inverter so that gives a total of 350A of over current protection. I haven't seen 2/0 rated above 300A so it would be a good idea to double up the 96" run. At ~<$100 USD I'd say it is a reasonable investment in safety. Also the double run will minimize your voltage drop which should increase your system efficiency.

-Kurt
Kurt Lundquist
Renewable Energy Project Engineer
http://alphaenergy.us/
stevetull
Forum Virtuoso
Posts: 91
Joined: Tue Feb 24, 2015 6:40 am
My RE system: FlexPower Two:
2 VFX3648, X240, 2 FM80, Mate 3s, FnDC, Hub 10
One array facing south at 30 degree tilt
One array facing southwest at 10 degree tilt
Total of 8kW
16 Fullriver AGM DC400-6 = 830Ah @ 48v
OpticRE: Paige cabin
Location: Near Bastrop Texas
Contact:

Re: Both DC cables need to be the same length?

Post by stevetull »

I have one 175 amp breaker for the positive to each inverter. So that's two 175 amp breakers total for the two inverters. The two 2/0 cables, (red and black), are in free air (partially behind the batteries) but are routed through a piece of 2 1/2" PVC for about 12" directly below the DC enclosure.

Added: If I get new 4/0 battery cables, I need 96" for the positive cable to reach all the way to the far left bottom battery, and I suppose I would have to go with a 96" negative cable even though a 48" cable would reach to the far right upper battery.

Edited again: I don't know what I was thinking. I could use a positive cable running to the far left top battery and a negative cable running to the far right lower battery. Then each cable could be 6 feet long. In fact if 2/0 would then be adequate I could lop off 2' off each and re-lug.

Edited for the last time, I promise: I could connect the upper string to the lower string this way instead. Move the 36" black interconnect cable to the far left and the positive one to the far right. Then the positive cable could only be 48" to the DC enclosure BBUS, but the negative cable would still need to be 72" to reach to the far left upper battery. I wonder if that would allow me to use my existing 2/0 cables (cut shorter of course). Does it matter which cable is shorter positive/negative?

I'm sorry for all the questions and thank you so much for your time.

What do you think? Thank you Kurt.



-Steve
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