Size of DC ground wire question

[rom]

If there is one "series cable" then it"s 400A, if there are 3 "series cables" then it's 133A. Is it called respectively 3P/2S and 2P/3S ? I always forget that part ...

[s/v Jedi]

I know the OP thinks he has all the answers but here’s what he -should- do:

- confirm if your alternator has a DC negative cable and if it has, wire it to the same bolt on the block that gets the 4/0 negative cable. The cables on the alternator should be rated for the alternator output, not for any other cable size in use. Example: my 225A alternators have 4/0 cable, smaller alternators should get smaller cables (their contacts are designed for smaller terminals).

- there should not be any extra DC negative cables attached to the block.

- the positive and negative cables that run to the engine block should be sized for the biggest load which normally is the starter motor, not the alternator. The engine owners manual will recommend proper cable size. I can’t believe 4/0 is required.

- off-topic but before investing in LiFePO4 batteries, I recommend to invest in a proper inverter/charger

[gscriba]

Originally Posted by s/v Jedi

I know the OP thinks he has all the answers but here’s what he -should- do:

- confirm if your alternator has a DC negative cable and if it has, wire it to the same bolt on the block that gets the 4/0 negative cable. The cables on the alternator should be rated for the alternator output, not for any other cable size in use. Example: my 225A alternators have 4/0 cable, smaller alternators should get smaller cables (their contacts are designed for smaller terminals).

- there should not be any extra DC negative cables attached to the block.

- the positive and negative cables that run to the engine block should be sized for the biggest load which normally is the starter motor, not the alternator. The engine owners manual will recommend proper cable size. I can’t believe 4/0 is required.

- off-topic but before investing in LiFePO4 batteries, I recommend to invest in a proper inverter/charger

No where close to having "all" the answers. Slowly and carefully picking the ripe berries found among all the thorns.

Planning to upgrade to an approx 200A alternator (not sure which one yet) but I want my DC neg alt cable to run to the DC negative bus, which will go to my shunt then terminate at the house negative post. I'll think more about your suggestion; perhaps it would work better than my current plan. I'll look again.

Not sure about your last recommendation. I've already invested in LiFePO4 batteries and the Xantrex (though it would not have been my pick) was installed by the previous owner and has worked great. Specs and functionality are ideal for me and I was quite surprised to find the advanced custom charge settings are excellent. No reason I can see to swap it out at this point.

[s/v Jedi]

Originally Posted by gscriba

No where close to having "all" the answers. Slowly and carefully picking the ripe berries found among all the thorns.

Planning to upgrade to an approx 200A alternator (not sure which one yet) but I want my DC neg alt cable to run to the DC negative bus, which will go to my shunt then terminate at the house negative post. I'll think more about your suggestion; perhaps it would work better than my current plan. I'll look again.

Not sure about your last recommendation. I've already invested in LiFePO4 batteries and the Xantrex (though it would not have been my pick) was installed by the previous owner and has worked great. Specs and functionality are ideal for me and I was quite surprised to find the advanced custom charge settings are excellent. No reason I can see to swap it out at this point.

Yes for a 200A alternator I would install 4/0 as well. Just use the schematic from the installation guide, which will show one DC negative cable from engine to busbar and one DC positive cable as well. Do not add more cabling. If your boat has a bonding system, then any bonding wires connected to the engine, should be connected at the same bolt that the DC negative is connected to. Do not create a bonding system when there isn’t one now.
If you divert from the manufacturer schematic, you run the risk of creating ground loops and currents through secondary wires, or at least a sub-optimal, higher resistance setup.

On your inverter/charger: keep fingers crossed but do know that Xantrex units have damaged batteries in the past. Most break down pretty quick before they harm the batteries (can’t deal with the high charge rates of LFP). It has cost me $4k to learn that lesson in the past but you may have a newer redesigned model that is better, I haven’t kept up with the brand but never heard of good changes yet...

[john61ct]

What specific Xantrex device do you have?

[Wotname]

FWIW, I think s/v Jedi's posts (below) are on the money and are a good starting point for any re-design of the DC negative wiring. However I can't speak to the merits or otherwise of the Xantrex units.

Originally Posted by s/v Jedi

I know the OP thinks he has all the answers but here’s what he -should- do:

- confirm if your alternator has a DC negative cable and if it has, wire it to the same bolt on the block that gets the 4/0 negative cable. The cables on the alternator should be rated for the alternator output, not for any other cable size in use. Example: my 225A alternators have 4/0 cable, smaller alternators should get smaller cables (their contacts are designed for smaller terminals).

- there should not be any extra DC negative cables attached to the block.

- the positive and negative cables that run to the engine block should be sized for the biggest load which normally is the starter motor, not the alternator. The engine owners manual will recommend proper cable size. I can’t believe 4/0 is required.

- off-topic but before investing in LiFePO4 batteries, I recommend to invest in a proper inverter/charger

Originally Posted by s/v Jedi

Yes for a 200A alternator I would install 4/0 as well. Just use the schematic from the installation guide, which will show one DC negative cable from engine to busbar and one DC positive cable as well. Do not add more cabling. If your boat has a bonding system, then any bonding wires connected to the engine, should be connected at the same bolt that the DC negative is connected to. Do not create a bonding system when there isn’t one now.
If you divert from the manufacturer schematic, you run the risk of creating ground loops and currents through secondary wires, or at least a sub-optimal, higher resistance setup.

On your inverter/charger: keep fingers crossed but do know that Xantrex units have damaged batteries in the past. Most break down pretty quick before they harm the batteries (can’t deal with the high charge rates of LFP). It has cost me $4k to learn that lesson in the past but you may have a newer redesigned model that is better, I haven’t kept up with the brand but never heard of good changes yet...

[Wotname]

Originally Posted by john61ct

What specific Xantrex device do you have?

Xantrex SW3012 (ref post #4).

[gscriba]

Noted re: Xantrex. I'll dig deeper.
Thanks again.

[rom]

Originally Posted by s/v Jedi

Yes for a 200A alternator I would install 4/0 as well.

Don't you care for the round trip distance from the alternator to the battery ? A 50mm² cable is good enough to carry 200A in a engine room. For a 10 meter round trip and 200A alt I went with 70mm²; that's about 4% voltage drop or 100W of power dissipated in the cable at max output. Good enough for that kind of application. Bigger cables are such a PITA.

[rbk]

Originally Posted by rom

Don't you care for the round trip distance from the alternator to the battery ? A 50mm² cable is good enough to carry 200A in a engine room. For a 10 meter round trip and 200A alt I went with 70mm²; that's about 4% voltage drop or 100W of power dissipated in the cable at max output. Good enough for that kind of application. Bigger cables are such a PITA.

X2 ^^ size it appropriately.

[Pelagic]

Originally Posted by rom

Don't you care for the round trip distance from the alternator to the battery ? A 50mm² cable is good enough to carry 200A in a engine room. For a 10 meter round trip and 200A alt I went with 70mm²; that's about 4% voltage drop or 100W of power dissipated in the cable at max output. Good enough for that kind of application. Bigger cables are such a PITA.

Are you basing this on a 12v system or 24v system for Start and House?

[s/v Jedi]

Originally Posted by rom

Don't you care for the round trip distance from the alternator to the battery ? A 50mm² cable is good enough to carry 200A in a engine room. For a 10 meter round trip and 200A alt I went with 70mm²; that's about 4% voltage drop or 100W of power dissipated in the cable at max output. Good enough for that kind of application. Bigger cables are such a PITA.

I’m sure you will be fine. In the US, a 3% maximum drop is used as a guideline. I have runs of double 4/0 aboard (so 4 cables, two negative and two positive) in order to do with 12V where most boats go to 24V.

[rom]

Originally Posted by Pelagic

Are you basing this on a 12v system or 24v system for Start and House?

Seriously Pelagic ? You know that current limitation (e.g. 200A) in a cable is not related to voltage. If your cable is rated 200A and you're using 12V, the good news is that switching to 24V will allow you to carry twice the power (P=VxI), still rated 200A.

[Pelagic]

I'm pretty dumb when it comes to figuring that out..[emoji20] ......thanks!

[gscriba]

Originally Posted by Pelagic

I'm pretty dumb when it comes to figuring that out..[emoji20] ......thanks!

I can relate. Most of us have an area of expertise and when we have to ask what may be "simple" questions (for some) it is truly humbling.
I appreciate the help on this thread.