need dual battery cables if I only have 1 string?

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need dual battery cables if I only have 1 string?

Postby Megunticook on Thu Nov 30, 2017 12:49 pm

Just finishing up wiring my FPR-8048A. The manual calls for dual 00 AWG cables from the battery bank to the inverter. A guy I spoke with on the phone at Outback says since I only have one bank of batteries (8 6v Rolls S-550, 428 AH), I'm fine with just one pair of cables. Can anyone please confirm that this is correct? The documentation is pretty clear about needing two pairs, but the rep. I spoke to says that assumes you have 2 strings of batteries in parallel.

Thanks.
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Re: need dual battery cables if I only have 1 string?

Postby klundquist on Thu Nov 30, 2017 1:13 pm

The FPR-8048A has 2 x 175A breakers. That means you need a conductor that can continuously support 350A of current. Also voltage drop is a consideration if you have a long wire run from the batteries to the inverter. Off the top of my head I think the OutBack recommendation of 2 x 2/0 is for about a 10' or less run. At less than 10' a single 4/0 conductor in free air should be able to get the job done however you should make sure that whatever conductor size you use is acceptable per local codes. If the S-550 has an LT style terminal maybe you can double lug on it (one lug on each side of the terminal). I'd consult with Rolls to make sure that is acceptable.

-Kurt
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Re: need dual battery cables if I only have 1 string?

Postby Megunticook on Thu Nov 30, 2017 9:06 pm

So what the Outback rep. told me on the phone is wrong? I need to double up on those battery cables then? And I need to double up on the short cables connecting the batteries together in series as well? Plus a second fuse block? (I was going to install a 200amp class T fuse on the positive cable).

This does not reassure me about the quality of Outback tech. support if they told me the wrong thing here.

The cables are 8' long and run in 2" metal conduit from the battery box to the inverter. I sure hope I don't have to install a second run of conduit!
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Re: need dual battery cables if I only have 1 string?

Postby blackswan555 on Thu Nov 30, 2017 11:15 pm

Sorry to throw a spanner in the works but I would go a different route,
single 000 cable & a 250amp fuse/breaker on the battery end & link across breakers inverter end,
Why ?
The inverter is 8000/48 = 166 amps continuous and a reasonably easily obtainable surge of 12000/48= 250amps which the fuse should handle, The cable is rated just above so the fuse would protect that in the event of eg a dead short inverter end, The other benefit of going up a size would be less voltage drop,

Tim,
To add, I know you are not intending to use that much power with your size batteries, But it is possible so you have to size for it :smile: In the event of a dead short they could deliver into the 10s of thousands of amps, Thats what the fuse looks after,
My comments are based on my experience and research, They are not endorsed or checked by Outback.I am an independent British electrician living in Spain, So please take this into account when reading /acting on my post`s.
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Re: need dual battery cables if I only have 1 string?

Postby Kent Osterberg on Fri Dec 01, 2017 4:01 pm

Outback recommends the doubled 2/0 cable because it has lower resistance and inductance than single 4/0 cable.

Four 2/0 cables will physically fit in 2" conduit within the allowed 40% fill, but passing the cable lugs through the 2" conduit will be a challenge. Unfortunately, four cables in conduit only carry 80% of the current in NEC table 310.15(B)(16), so four 2/0 cables in conduit only have an an ampacity of 140 amps (for 75°C rated wire) or 156 amps (for 90°C rated wire). However, 140 amps is sufficient ampacity to run the inverter to its maximum surge rating: 2 x 140A x 48V x 90% efficiency = 13 kW. Since the inverters can't sustain that amount of power, the cables cannot be overloaded and the 175-amp breakers still provide fault protection if the inverter fails. For 90°C rated wire, this arrangement complies with the NEC rules on breaker selection because the next available breaker size above 156 amps is the 175 amp breaker.
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Re: need dual battery cables if I only have 1 string?

Postby Megunticook on Thu Dec 07, 2017 7:14 pm

I was able to pull the 4 cables through the 2" EMT without much trouble--did a careful job taping them tightly together and pre-lubed the conduit before the pull. Pretty short run with just one 90 degree elbow.

Thanks for the advice, everyone. Cables are rated for 105°C incidentally, so I should be all set.

CEO came by this morning and the inspection went well--looks like we're good to go. I just need to hook up the batteries, program the MATE3, and we can start cranking out the watts!
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