Sizing battery-to-inverter conductor & breaker

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Sizing battery-to-inverter conductor & breaker

Postby Tom D on Fri Jan 04, 2013 7:22 pm

Would you please help me size the battery-inverter cable for a DC coupled battery backup system? I learned wire, OCPD and disconnect sizing in SEI training for most parts of the system, but not specifically the battery to inverter part.
The method I'm taking is in 3 steps.
Step 1 was to size for ampacity. With a VFX3648 inverter I used the 3600 VA continuous power rating divided by .9 inverter efficiency and 44V resulting in 90.9A. From this I selected a 100A rated breaker for the disconnect/OCPD. 90.9A X the 1.25 continuous power factor and divided by a 0.91 temperature correction factor gave an adjusted ampacity requirement of 124.9A. From NEC Table 310.17 I chose THWN-2, which might not be the right conductor type, and found 4 AWGCu would be sufficient.
Step 2 was to require a minimum size of 2/0 AWG Cu based on NEC 690.74.
Step 3 was to select the size needed to keep voltage drop below the amount that would keep the inverter from shutting off due to low voltage during a surge. However in that calculation, I don't know what voltage drop would result in the inverter shutting off. Could you help me find that quantity? Also, I don't know whether to choose the Peak Maximum Output Current (70A AC), or the Surge (6000VA), 5 second (5000VA) or 30 minute (4000VA) AC Overload Capability for estimating the current.
What does OutBack recommend?
Thanks very much.
Tom D
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Re: Sizing battery-to-inverter conductor & breaker

Postby vtmaps on Sat Jan 05, 2013 2:53 am

Welcome to the forum. I think all of your questions are answered in the VFX installation manual (downloadable from Outback).

Not only do they specify wire sizes, but also torque for tightening the connections.

--vtMaps
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Re: Sizing battery-to-inverter conductor & breaker

Postby roks on Thu Jan 24, 2013 3:36 am

Hi All,

For VFX3024E, manual recommends 4/0 for battery cable, but my battery bank is only 400AH (4x200 or 2x400ah, 12v).

What's the minimum size cable I could use?

Thanks.
roks
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My RE system: VFX3024E
Mate
Battery: 200Ah + RTS
Mains AC for charging

Re: Sizing battery-to-inverter conductor & breaker

Postby SteveHiggins on Thu Jan 24, 2013 7:08 am

Yes because that 3500 watt inverter can surge to 2-2.5 times its rated load... not just 3500 watts. Thats why they suggest 4/0 cable.
Steve Higgins
Technical Services Director- Surrette Battery Company

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Re: Sizing battery-to-inverter conductor & breaker

Postby Gordon-Loomberah on Sun Jan 27, 2013 6:15 pm

Tom, does the inverter not have its own cables already?
I use a 4kW cont. (12kW surge) 24V Latronics inverter, which comes with 75mm^2 cables. My battery to bus bars (via BCU and 500A contactor) connections are all with 150mm^2 (3 X 50mm^2) cables, to avoid volt drop under high loads when welding etc. At the moment I have a 160A fuse on the positive side, but will soon be replacing that with 3 * 63A fuses.
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Re: Sizing battery-to-inverter conductor & breaker

Postby roks on Thu Jan 31, 2013 9:26 am

Is it not possible to reduce the input/output AC amps with the Mate. Then use a smaller DC cable (1/0 or 2/0) and smaller breaker 175A maybe.
Its just that I wont be using the invert to its full load ever...most extreme 50% and its tough to find a 4/0 cable.
roks
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Posts: 16
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Location: Bangladesh
My RE system: VFX3024E
Mate
Battery: 200Ah + RTS
Mains AC for charging

Re: Sizing battery-to-inverter conductor & breaker

Postby gww on Thu Jan 31, 2013 4:03 pm

Arizona wind and sun store has 4/0 ten foot cables for about $100 before shipping.
Hope this helps
gww
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Re: Sizing battery-to-inverter conductor & breaker

Postby roks on Mon Feb 04, 2013 3:18 am

I am in the UK, not sure if they would deliver or if it would be economical.

I've been looking at cables here in the UK, a 95mm2 (684/0.04) is rated 500Amp or 70mm2 (999/0.30) rated at 485Amp.

Can someone tell why I cant use the 70mm2? Am I missing something?

Thanks
roks
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Mains AC for charging


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