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Discussion Starter #1
For a 2300W inverter ( https://www.amazon.com/gp/product/B002AMPGE6/ref=oh_aui_search_detailpage?ie=UTF8&psc=1 )

I have the inverter only about 2' from the battery (super short wire runs) and would love to use some existing 6 GA cable (13mmSQUARE). I know that usually folks opt for size 2, or 0, or 0/2 cable.

Since my wire runs are this short, will 6GA work to get the adequate supply to the inverter? The inverter is separated from the battery via a 250A fuse.

Is the calculation of needed amps for a 12v system as simple as: W = VxA So: 2300 = 12x(NeededAmps) So: Needed Amps = 192A?

Thanks!

 

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Hi,
I think the calculation is correct, but it does not include the efficiency of the inverter. That is, when the inverter is putting out 2300 watts on its output side, and if its 90% efficient, then its input wattage will be (2300 watts) / (0.9 efic) = 2555 watts, and your calc for amperage would be (2555 watts) / (12 volts) = 213 amps.

Wire sizing is a 2 step deal. The current in the wire cannot exceed the ampacity rating for the wire and the voltage drop along the wire should not exceed about 2%. The table you are using only accounts for voltage drop. The 213 amps would exceed the ampacity rating of #6 wire by quite a bit, and I would not use it -- the wire is likely to overheat.


BlueSea has a calculator you can use that accounts for both ampacity of the wire and voltage drop.
http://circuitwizard.bluesea.com/

So, if you go into the calculator with 12 volts, 213 amps, insulation rated for 90C (most common), total length of conductor 2+2 = 4 ft, and 2% for voltage drop, the calculator comes up with AWG 0.
This is what I'd use.

I'd also take careful look at the user manual for the inverter. In my experience, they usually want a wire that is even larger than what the calculator gives. If it does recommend a larger wire, I'd go with that.

You will want a fuse or breaker near the battery end of this wire.

edit: Not to beat this to death, but when you run the calculator at the link above, after you press Calculate then click the button that says "Explain Results". As it explains, the wire gage for ampacity (maxium allowed current for a wire) is AWG 0, while the acceptable wire gage for voltage drop is AWG6. So, AWG 0 is required to keep the wire from overheating even though AWG 6 would be OK for voltage drop.

On another page on the BlueSea site, they say that currents in excess of the 150% of the maximum ampacity that the insulation will melt off the wire. AWG 6 has a maximum ampacity of about 100 amps, so at 230 amps you will be well over the current at which the insulation will melt.

I promise -- no more edits :)

OK, one more. You mention using a 250 amp fuse on a #6 wire -- this is just way to large breaker or fuse to use on a #6 wire -- it may provide some protection to the wire in the case of a hard short to ground, but provides essentially no protection for overloads.

Gary
 

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You need a bigger wire because of voltage drop when the high current is carried and the resultant heat generated. High current/power to the inverter is often continuous for many minutes or longer. This is a different situation than wire size needed when two batteries are combined. In this case the surge is only for a few seconds to up to a minute then the current drops dramatically and wire heating is minimal.
 

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Discussion Starter #4
Thanks guys!

Does both the Pos and Neg wire need to be run at the same gauge? (IE will the inverter be limited by the capacity of the smaller wire if they aren't identical?) For some reason I had an impression that the Neg/ground could be smaller with no ill effect.

And in addition to the normal Pos/Neg terminals on the inverter, there's a small "ground" terminal, clearly made for a much smaller gauge wire. The inverter runs fine without this being connected. Does it serve a meaningful purpose?
 

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Thanks guys!

Does both the Pos and Neg wire need to be run at the same gauge? (IE will the inverter be limited by the capacity of the smaller wire if they aren't identical?) For some reason I had an impression that the Neg/ground could be smaller with no ill effect.

And in addition to the normal Pos/Neg terminals on the inverter, there's a small "ground" terminal, clearly made for a much smaller gauge wire. The inverter runs fine without this being connected. Does it serve a meaningful purpose?
Hi,
The negative wire carries the same current as the positive wire, so it needs to be the same gage as the positive wire.

The small ground is a safety ground -- it only comes into play if there is some kind of failure like an internal short of the hot wire to the case of the inverter, which would make the case hot. It can be grounded to the chassis or taken back to the battery area and grounded there, but, for safety reasons it should be grounded even though the inverter works without it.

Gary
 

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Discussion Starter #6
For the purpose of calculating wire run length off inverter:

Do I just measure the total distance (Pos&Neg) between Inverter and Aux battery, which it's connected to? Or do I need to include the additional distance to the van body/chassis ground connection? (standard spot in front of the driver seat)

My Aux battery is under the driver seat, and inverter is just behind the driver seat.

A Pos wire from vehicle battery (through an isolator) and Negative/ground wire from the vehicle chassis are both run under the floor and driver seat to the house battery as well.
 

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Hi,
Its just the length of the wires from the house battery to the inverter.

But, as you say, for the BlueSea calculator its the length of the pos wire plus the length of the neg wire. Some of the calculators and tables use the one way distance, but the BlueSea one wants the total length of the two wires.

Gary
 

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Out of curiosity I went and read to see what the mfg recommended for one of their 2000w inverters

(This one--- https://www.renogy.com/renogy-2000w-12v-off-grid-pure-sine-wave-battery-inverter-w-cables/ ).

If a mfgr supplies cables is it a safe bet to just use them rather than calculating them as above and using something different (bigger)? And why is there four cables and not just two (pos and neg)?
Hi,
A 2000 watt inverter is going draw about (2000 watts) /(12 volts)(0.9 efic) = 185 amps.

The maximum current for a #4 wire is about 95 amps, so two #4 s in parallel would be good for about 190 amps, so seems OK to me. Why they want to use two #4 s rather than one larger wire I don't know, but maybe it will make the installation easier as the #4 wires will be easier to bend into the shape you need than a single larger wire would be?

If the manufacturer of the inverter wants a larger wire than the wire table or BlueSea Circuit Wizard, then I'd go with the manufacturers recommendation -- it may be the inverter does not tolerate voltage drop on the supply lines well, or that they are concerned about surge loads, or who knows? But, if they recommend a smaller wire than the wire table or calculator, I'd go with the table wire -- no harm is going to come from having a wire that is a bit to large.

Wire table: https://www.bluesea.com/resources/1437
Calculator: http://circuitwizard.bluesea.com/

Gary
 
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