Inverter cabling question

[Helmsman]

Ok, I spoke with a very knowledgeable tech with ProMariner. Here is what he said.

Confirmed that the cable length from the isolator to the inverter bank should be doubled to get total length. He also told me that the size can be derived from an ABYC chart such as Bud provided. That is a big relief, because when I called this fall they told me it was limited to the 12 feet in their chart.

He stated that AGM batteries (which I have) require a higher charge (in the mid 14's) than regular "lead acid" batteries (in the 13's)/ That is why they use the 3% loss chart for cabling. So, I will need to change out the cables from the isolator to the start batteries also, I would guess.

He also said the voltage reading I had of 15.0 volts (isolator to inverter battery switch) is not possible. The output can never be higher than the input voltage which at its highest was 14.87. That means that the multimeter I have may not be sufficient quality for testing.

So, during my down time, I will replace the cables with 2/0 or 3/0 depending on what the length calls for. Thanks for the help and I would welcome any other suggestions or thoughts.

[km1125]

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Thanks Bud. The cable from the isolator to the battery bank battery switch is 13.66 feet. Then about 6 feet through the batteries to the inverter negative feed. Then about 4 foot From the inverter positive through the fuse to the switch to the positive battery connection. So I would say about 20 to 22 feet. There is also a properly sized ground cable that goes to the starboard engine from the inverter but I don’t think that counts, does it?

You don't really care about the wires from the inverter bank to the inverter (for charging purposes), just from the engine to the isolator and then to the batteries.

If the ground cable from the starboard engine to the inverter (did you mean "inverter battery"?) is the only connection then that must be taken into account in the charging circuit just like the positive cable. What size is it?

[km1125]

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He also said the voltage reading I had of 15.0 volts (isolator to inverter battery switch) is not possible. The output can never be higher than the input voltage which at its highest was 14.87. That means that the multimeter I have may not be sufficient quality for testing.

-OR- there was some voltage issue on the NEGATIVE side where the meter was connected, so it wasn't really measuring "0VDC", which throws off any of the readings the meter is trying to get from the (+) lead.

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So, during my down time, I will replace the cables with 2/0 or 3/0 depending on what the length calls for. Thanks for the help and I would welcome any other suggestions or thoughts.

see my earlier post about cable sizing relative to battery charging

[Helmsman]

Source of the post Well, that's definitely a FET-type isolator, so that's good news.

Here's a thought. Move the ground wire on the unit over to the negative terminal on your inverter bank.

Also, the instructions don't really cover much of the applications where you have two alternators. They just say that the dual alternators "connect to ALT1 and ALT2" and that the only other requirement seems to be that "ALT1 and BATT1" need to come from the same engine. Does that mean "ALT2 and BATT2" need to come from the same engine? Does that also mean in the charging sequence that it always starts with BATT1 and then goes to BATT2 and then to BATT3 and then to BATT4? Could you put your inverter bank on the BATT2 terminal so it gets the second charging sequence?

If you're struggling with the decision between going with 2/0 and 3/0, just remember that while the smaller gauge might work, the larger cable will ALWAYS result in more overall current flow and quicker charging of the batteries if there is available current at the source. The batteries only charge because of a DIFFERENCE in voltage between the source and the battery. If the battery is already at 11.7 volts and the charging source is at 13.6, then it's really only a 1.9 VDC difference that's making the battery charge AT ALL. Increasing this difference by just 1 volt is nearly a 50% difference, and every little bit helps.

I agree the directions are not clear for 2-4. The Way the charging sequence works is the following: Alt 1 and Eng 1 have to be on the same engine. Alt 2 and eng 2 have to be on the other engine. The Proiso first charges the two engines. After they show current at 13.3 volts, the isolator goes to the other two banks and charges them. I am pretty certain that the charge to the other two banks is based upon "who" needs it, one or the other, or both.

I added another post which describes what the ProIso tech told me. It addresses wire size, and you are spot on. He said the difference between what "used to be" on my boat and what the new isolator does is with AGM batteries (I switched to them a year or so ago) the charge they "like" is in the mid 14's while the "water added" batteries are "ok" with 13.5. So I will go with the larger cable size for the inverter, and look at the others also.

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Thanks Bud. The cable from the isolator to the battery bank battery switch is 13.66 feet. Then about 6 feet through the batteries to the inverter negative feed. Then about 4 foot From the inverter positive through the fuse to the switch to the positive battery connection. So I would say about 20 to 22 feet. There is also a properly sized ground cable that goes to the starboard engine from the inverter but I don’t think that counts, does it?

You don't really care about the wires from the inverter bank to the inverter (for charging purposes), just from the engine to the isolator and then to the batteries.

If the ground cable from the starboard engine to the inverter (did you mean "inverter battery"?) is the only connection then that must be taken into account in the charging circuit just like the positive cable. What size is it?

Source of the post

Source of the post

Thanks Bud. The cable from the isolator to the battery bank battery switch is 13.66 feet. Then about 6 feet through the batteries to the inverter negative feed. Then about 4 foot From the inverter positive through the fuse to the switch to the positive battery connection. So I would say about 20 to 22 feet. There is also a properly sized ground cable that goes to the starboard engine from the inverter but I don’t think that counts, does it?

You don't really care about the wires from the inverter bank to the inverter (for charging purposes), just from the engine to the isolator and then to the batteries.

If the ground cable from the starboard engine to the inverter (did you mean "inverter battery"?) is the only connection then that must be taken into account in the charging circuit just like the positive cable. What size is it?

That is the ground cable attached to the outside of the inverter case which is used to ground to the engine. it is 3/0 and is a protective ground.

[km1125]

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That is the ground cable attached to the outside of the inverter case which is used to ground to the engine. it is 3/0 and is a protective ground.

I think I need a diagram to look at to really understand this. If that's the "protective ground" I'm thinking of (as in a 'safety ground" for a generator or inverter) then it's really oversized. Not that it's a bad thing, but just doesn't need to be that large. Typically a #6 would be more than suitable.

However, if that's carrying any of the inverter load or charging load, then it gets more complicated. Is there a dedicated large (2/0 or more) cable from the inverter to the negative post on the inverter battery? Is there also a large (#2 or larger) cable from the inverter battery negative to the boat's negative buss? (is there a "boat negative buss" on this vessel?)

[Helmsman]

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Source of the post
That is the ground cable attached to the outside of the inverter case which is used to ground to the engine. it is 3/0 and is a protective ground.

I think I need a diagram to look at to really understand this. If that's the "protective ground" I'm thinking of (as in a 'safety ground" for a generator or inverter) then it's really oversized. Not that it's a bad thing, but just doesn't need to be that large. Typically a #6 would be more than suitable.

However, if that's carrying any of the inverter load or charging load, then it gets more complicated. Is there a dedicated large (2/0 or more) cable from the inverter to the negative post on the inverter battery? Is there also a large (#2 or larger) cable from the inverter battery negative to the boat's negative buss? (is there a "boat negative buss" on this vessel?)

The ground cable to the engine has to be no more than one size smaller than the battery cables used for the battery setup you have. I have a 12 volt 380 amp battery setup with a 2000 watt inverter. The cables installed are designed to handle the amperage. The case ground cable HAS TO BE DESIGNED to handle the max DC load. it is a common misconception, and from I understand, the most common mistake to use an undersized wire for the ground fault cable. I said 3/0. It may be a 2/0 if that is what the batteries are set up with. I cant remember off hand. Appears to be 5/8" thick.

Here is a pretty good representation which shows the wiring for an inverter and is the way mine is setup. What is missing is the charge cable from the isolator which on my setup is connected to the battery on/off switch.

Inverter-wiring-simple.jpg

[km1125]

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The ground cable to the engine has to be no more than one size smaller than the battery cables used for the battery setup you have. I have a 12 volt 220 amp battery setup with a 2000 watt inverter. The cables installed are designed to handle the amperage. The case ground cable HAS TO BE DESIGNED to handle the max DC load. it is a common misconception, and from I understand, the most common mistake to use an undersized wire for the ground fault cable. I said 3/0. It may be a 2/0 if that is what the batteries are set up with. I cant remember off hand. Appears to be 5/8" thick.

Here is a pretty good representation which shows the wiring for an inverter and is the way mine is setup. What is missing is the charge cable from the isolator which on my setup is connected to the battery on/off switch.

Interesting. This must be a newer change to the ABYC rules. My Heart Interface inverter does not have that as a requirement. I don't see it on the Mastervolt inverters either but do see it on the Xantrex. It's interesting that on the Magnum inverters the requirement is just for a 8AWG ground, except on boats where it must be "one size smaller than the DC feed". There also appears to be a (rather new?) ABYC requirement that the case cannot be connected to the negative supply side... I wonder why that is? (considering all the other things that ARE like that including starters and alternators).

Like I said earlier, there's nothing "wrong" with it being larger, but I didn't think there was a requirement for it to be that large.

On your boat, Is there also a connection from the negative terminal on the inverter battery back to the engine or some other "negative buss"?

[bud37]

I think part of the reason for the rules, is the tremendous amount of amps that can be run thru the wires in a short. Depending on the fusing there could be a fire before the fuse goes.

The other, in my fuzzy understanding is they want all the short energy going thru a short direct path directly to ground. If it runs thru a buss, all the wire sizes change to smaller.

I wrestled with this recently with a new charger install and a bow thruster battery bank to get charged....it took a lot of reading and phone calls to get it right. It seems that just when you think you got it figured, someone comes along with another perspective. Even the engineers have some disagreement.

I think Nate that once you get the cable sizes right you will be on the right track. Myself I was one volt low and changed out the cable sizes and got to where the isolator would work.

[Helmsman]

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The ground cable to the engine has to be no more than one size smaller than the battery cables used for the battery setup you have. I have a 12 volt 220 amp battery setup with a 2000 watt inverter. The cables installed are designed to handle the amperage. The case ground cable HAS TO BE DESIGNED to handle the max DC load. it is a common misconception, and from I understand, the most common mistake to use an undersized wire for the ground fault cable. I said 3/0. It may be a 2/0 if that is what the batteries are set up with. I cant remember off hand. Appears to be 5/8" thick.

Here is a pretty good representation which shows the wiring for an inverter and is the way mine is setup. What is missing is the charge cable from the isolator which on my setup is connected to the battery on/off switch.

Interesting. This must be a newer change to the ABYC rules. My Heart Interface inverter does not have that as a requirement. I don't see it on the Mastervolt inverters either but do see it on the Xantrex. It's interesting that on the Magnum inverters the requirement is just for a 8AWG ground, except on boats where it must be "one size smaller than the DC feed". There also appears to be a (rather new?) ABYC requirement that the case cannot be connected to the negative supply side... I wonder why that is? (considering all the other things that ARE like that including starters and alternators).

Like I said earlier, there's nothing "wrong" with it being larger, but I didn't think there was a requirement for it to be that large.

On your boat, Is there also a connection from the negative terminal on the inverter battery back to the engine or some other "negative buss"?

No there isn’t a connection to a negative buss. Just the case ground. So it is a self contained unit as it shows in the drawing.

[km1125]

Source of the post

Source of the post

Source of the post
The ground cable to the engine has to be no more than one size smaller than the battery cables used for the battery setup you have. I have a 12 volt 220 amp battery setup with a 2000 watt inverter. The cables installed are designed to handle the amperage. The case ground cable HAS TO BE DESIGNED to handle the max DC load. it is a common misconception, and from I understand, the most common mistake to use an undersized wire for the ground fault cable. I said 3/0. It may be a 2/0 if that is what the batteries are set up with. I cant remember off hand. Appears to be 5/8" thick.

Here is a pretty good representation which shows the wiring for an inverter and is the way mine is setup. What is missing is the charge cable from the isolator which on my setup is connected to the battery on/off switch.

Interesting. This must be a newer change to the ABYC rules. My Heart Interface inverter does not have that as a requirement. I don't see it on the Mastervolt inverters either but do see it on the Xantrex. It's interesting that on the Magnum inverters the requirement is just for a 8AWG ground, except on boats where it must be "one size smaller than the DC feed". There also appears to be a (rather new?) ABYC requirement that the case cannot be connected to the negative supply side... I wonder why that is? (considering all the other things that ARE like that including starters and alternators).

Like I said earlier, there's nothing "wrong" with it being larger, but I didn't think there was a requirement for it to be that large.

On your boat, Is there also a connection from the negative terminal on the inverter battery back to the engine or some other "negative buss"?

No there isn’t a connection to a negative buss. Just the case ground. So it is a self contained unit as it shows in the drawing.

Well, that could be an issue then, to your charging circuit. That means your charging circuit has to go from the engine, to the negative buss, then to the case ground on the inverter then back to the inverter battery via the negative cable. That's a circuitous path, when it should be direct from the negative buss (which essentially should be the engine) to the inverter battery negative post. This will be complicated if there is ANY current draw from the inverter, where all bets would be off on how much voltage drop would be on the negative cable from the inverter battery to the inverter. Any voltage drop on that portion of the circuit would impact the charging capability from either alternator.

EDIT: Actually, rethinking this, it could even be a bigger issue. Since there appears to be an ABYC requirement that says " the case cannot be connected to the negative supply side", there might be NO legitimate negative connection between your charging sources (engines) and the inverter battery bank. Is this accomplished some other way?