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Old 10-09-2016, 06:39 PM   #11
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You are exactly right. My first picture created confusion.

Heavier black wire goes from POS to inline fuse to my inverter, red wire goes to the WFCO control center.
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Old 10-09-2016, 08:42 PM   #12
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Myron's second photo shows the connection between batteries: positive of the left battery to negative of the right battery, making the two 6-volt batteries into one 12-volt battery. This is the only way to use 6-volt batteries in a 12-volt RV, and is installed by Escape. This cable needs to be able to handle all of the current passing through the batteries, so I would rather see it at least as heavy gauge as the inverter cables. It looks like it was sized for just the converter (WFCO), not the inverter as well.

Myron, is the negative connection of your inverter made through the frame? I don't see a cable from the negative battery terminal to the inverter, only the light green cable to the frame. If the only connection for the inverter to the battery negative is via the cable from battery to frame, this cable to the frame needs to have just as much capacity as the positive cable, and the thinner it is the more power is being lost in resistance in the cable. Usually a large inverter has its own cables direct to the battery terminals for both positive (as shown here) and negative.

For practical purposes, electrical current moves only in complete circuits, and every part of the circuit handles the same amount of current... so every part is a link in a chain and the weakest link limits the capacity of the chain.
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Old 10-10-2016, 01:41 AM   #13
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Yes Brian, to the frame with the same heavy gauge (black) wire.
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Old 10-10-2016, 02:28 AM   #14
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Originally Posted by Brian B-P View Post
Myron, is the negative connection of your inverter made through the frame?
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Originally Posted by MyronL View Post
Yes Brian, to the frame with the same heavy gauge (black) wire.
So then this is the situation:
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If the only connection for the inverter to the battery negative is via the cable from battery to frame, this cable to the frame needs to have just as much capacity as the positive cable, and the thinner it is the more power is being lost in resistance in the cable.
When the inverter uses a hundred amps (for example) that big red positive cable carries 100 amps from battery to inverter (fine), the heavy black cable carries that 100 amp current from inverter to frame (fine)... and that smaller green cable takes the same 100 amp current from frame to batter (probably not thick enough), and the cable between the batteries carries the same 100 amp current to complete the circuit (also doesn't look as heavy as the red inverter cable).
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Old 10-10-2016, 12:47 PM   #15
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So then this is the situation:

When the inverter uses a hundred amps (for example) that big red positive cable carries 100 amps from battery to inverter (fine), the heavy black cable carries that 100 amp current from inverter to frame (fine)... and that smaller green cable takes the same 100 amp current from frame to batter (probably not thick enough), and the cable between the batteries carries the same 100 amp current to complete the circuit (also doesn't look as heavy as the red inverter cable).
Agreed. Not criticizing Myron's install, but just want to confirm it is safe and reliable and I'm learning for when I want to install my own inverter. Based on information from Myron's second photo I will assume the inverter is a Xantrex ProWatt SW 600 for purposes of conversation (only one of that size in their current lineup). Rated at 60 amps DC input current at full load. These units have both a chassis ground connection and a direct negative battery connection. The whole key to the best inverter performance is minimizing voltage drop so a direct connection from the inverter to the battery negative with proper conductor size is really the correct way to wire it. Obviously it works for Myron, but at full load could potentially present an issue. It would be interesting to know what the voltage is across the batteries and how that compares to what voltage is read on the inverter. This would be an indicator of any high resistance connections. At a minimum, I agree the green wire from the battery negative to the frame is likely too small. It may be ok to remain as-is if the large gauge neg line from the inverter were brought directly to the battery as described.
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Old 10-10-2016, 01:57 PM   #16
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Just looked at my Xantrex Prowatt SW Sine Wave Inverter owner's guide.

On page 10, Table 1, Voltage Drop Per Foot of DC Cable, they like a best wire size of AWG 0, which has a voltage drop of 0.0060 per foot. The wire I'm using is AWG 4, with a voltage drop of 0.0152 per foot.

Estimate my distance from POS to Inverter to be 5 ft max. From Inverter to frame, 2 ft max. If my math is correct that's a voltage drop of 0.0760v and 0.0304v, respectively.

No way I'm using AWG 0, thinking that's wire thick and stiff enough for holding up a small suspension bridge or connecting to the electric chair at Sing Sing.
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Old 10-10-2016, 04:07 PM   #17
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Just looked at my Xantrex Prowatt SW Sine Wave Inverter owner's guide.

On page 10, Table 1, Voltage Drop Per Foot of DC Cable, they like a best wire size of AWG 0, which has a voltage drop of 0.0060 per foot. The wire I'm using is AWG 4, with a voltage drop of 0.0152 per foot.

Estimate my distance from POS to Inverter to be 5 ft max. From Inverter to frame, 2 ft max. If my math is correct that's a voltage drop of 0.0760v and 0.0304v, respectively.

No way I'm using AWG 0, thinking that's wire thick and stiff enough for holding up a small suspension bridge or connecting to the electric chair at Sing Sing.
(These values from the Owner's Guide for the Xantrex PROwatt SW Sine Wave Inverter are for 60 amps of current)
That makes sense, but you also need to include the much smaller-gauge green wire from the frame to the negative battery post (at least a couple of feet of perhaps 8 gauge), plus the wire between the batteries (a foot or two of perhaps 6 gauge). Although the two big cables only cause about 0.1 volts of voltage loss, the other cables will add significantly to that.

I'm not suggesting 0-gauge cable, but I would suggest for anyone building a similar configuration:
  • inverter negative cable directly to the battery negative terminal, or battery negative to frame cable larger gauge than inverter cables
  • cable between the batteries at least 4-gauge
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Old 10-10-2016, 06:16 PM   #18
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"...but you also need to include the much smaller-gauge green wire from the frame to the negative battery post ",

Naa... don't think so. I'll never come remotely close to drawing 60 amps in my trailer.
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Old 10-10-2016, 09:06 PM   #19
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"...but you also need to include the much smaller-gauge green wire from the frame to the negative battery post ",

Naa... don't think so. I'll never come remotely close to drawing 60 amps in my trailer.
Okay, so calculate the drop at the lower voltage, and include all of the wire. By the way, why put in a 600 watt inverter if you're never going to come remotely close to using 600 watts of AC power from it?
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Old 10-10-2016, 10:54 PM   #20
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MyronL, referring back to my post #4, I think you should connect + of one battery to - of second battery with jumper cables. That should satisfy everything.
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