30 ft of 4/0 copper welding cable length from batteries to inverter?

Prince Of Darkness said:

Voltage drop in battery-to-inverters is not the only conideration, when using extremely long cables for this DC. connection.

There is a Dynamic performance factor as well. This results from the impedance that these long cables present to the currents in these cables. This impedance can result in poor inverter performance with large changes in this current, and this Z, can affect the dynamic stability of the inverter.

Twisting the battery cables together, running them in the same conduit, and probably using metal conduit for the cables could help.

I am not an expert, and probably my attempt at the description above, is not that clear, but something to think about.

FWIW, IMO, and so on, Luke

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CoolWill said:

If you can, go to Graybar instead of Amazon or Windy Nation. It's a good bit cheaper.

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pollenface said:

Really depends on the load

15k load @ 85% efficiency over 30ft of 4/0 = 390 amps @ 48v = 2.384% voltage drop
7.5k load @ @ 85% efficiency over 30ft of 4/0 = 195 amps @ 48v = 1.192% voltage drop

*using Voltage Drop Calculator Pro on my android phone

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RCinFLA said:

At 12kW you will have a maximum of about 275 amps of battery current.

30 ft pair of 4/0 with 275 amps will have about 0.9 vdc voltage drop. For 48v system that is satisfactory.

Make sure you tape the battery cables together in parallel to reduce series inductance of battery cables to reduce voltage ringing on inverter DC input that can be damaging to inverter.

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Tomthumb62 said:

FYI, batterycablesUSA.com is my current favorite seller. Great quality, flat rate shipping I think $5 and great prices. They don’t sell on Amazon or eBay, so their overhead costs are lower. Shipping from east to west coast has averaged 5-6 days for me. You can select your required terminal lugs or bare ends. They sell by the foot but you can select by the inch to get perfect length cables. Excellent marine grade heat shrink and solid hydraulic crimps on the lugs.

Edit to say that if you have the tools to make your own crimps, it might be cheaper to buy a 100’ roll but it will be same color for positive and negative, so be sure to install a foot of appropriate colored heat shrink on the ends and maybe every few feet a couple inches to mark which is which. NOT a place you want to hook up reverse polarity!

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Tomthumb62 said:

@3boysatbeach i think this is your answer. I didn’t run the maths because I didn’t know your loads, but 1.5% voltage drop is excellent. 3% or less is the goal.

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RCinFLA said:

3% rule used for higher AC voltage wiring should not be used for lower voltage battery cables. There are two factors to be concerned about. One is cable heating per foot and other is voltage drop.

Inverters regulate AC output voltage so for given AC load, as inverter DC input voltage drops, due to cable loss, the inverter draws more DC current to maintain regulated AC output power. As inverter DC input voltage drops and inverter draws more DC current the conversion efficiency of inverter degrades.

This compounding combination is a downward spiraling situation. Lower DC voltage -> more DC current. More DC current -> lower inverter efficiency -> more cable voltage drop, which requires even more DC current by inverter.

Acceptable cable heating depends on wire gauge and insulation tolerance to heat. Large gauge wire has greater surface area to dissipate heat. (Heating of cables is power loss).

For 275 amps through 30' pair of 4/0 cables there will be about 8 watts per pair foot of cabling yielding a total cable heating of 239 watts (cable loss). 0/4 cable at 8 watts per pair foot will get quite warm to touch.

I would recommend Excelene insulation. Stay away from cheap vinyl insulated cables. At this amount of possible heating, vinyl will become soft and malleable. Any pressure point can indent enough to pierce through to copper wire. Many people find this out using cheap vinyl insulated wire to circuit breakers which can get quite hot transferring a lot of heat to circuit breaker terminals then into wiring ends.

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