12 volt wiring question.

[XE3]

My boat was built in 1980 and has an old Raritan Crown II 50 Amp charger. I finally sat down this weekend and traced all the relevant wiring that i will be replacing first including new charger, house batteries, start batteries, 1-2-B switch and Blue Sea ACR, fuses etc... attached is an image of how it is currently wired. i am curious if firstly, the wiring is typical of how it might have been done in 1980 era and second if everything looks relatively safe even though there are no fuses present.

At first glance my inexperienced eyes tell me the second start battery is probably getting undercharged constantly and im wondering is the alternator sending a charge back to the C post on the switch sending charge to whichever bank i have selected?

The cable leading from the Alt to starter is 10 AWG. All the rest of the wiring looks like 4 and 2 AWG. mostly 2 but the long length (20 ft round trip) to the house is 4 AWG

Thanks

 

[Stu Jackson]

OEM 1-2-B Switch Wiring History Alternator/Batteries & "The Basic" 1-2-B Switch BEST Wiring Diagrams

1-2-B Considerations (New 2020 - Rodd finally got around to diagramming what I had done in the above link in 2009 )
1/2/BOTH Switch Considerations

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
1/BOTH/2/OFF Switches Thoughts & Musings

This is a newer primer for boat system wiring design with a thorough digram: Building a Good Foundation (October 2016)
Building a DC Electrical Foundation

 

[Stu Jackson]

the second start battery

It appears you have a 36 foot monohull sailboat. Why two start batteries? And only one house? Perhaps reversed?

 

[Davidasailor26]

i am curious if firstly, the wiring is typical of how it might have been done in 1980 era

Yes, possibly. Who knows what may have been done along the way, but nothing strikes me in the diagram as impossible to be original. Only thing that looks odd is the 2 battery start bank vs 1 battery house bank.

second if everything looks relatively safe even though there are no fuses present.

You’re definitely going to want some fuses. Hard to be safe without that. There’s nothing providing overcurrent protection for any of the wiring.

my inexperienced eyes tell me the second start battery is probably getting undercharged constantly

The two Start batteries are electrically parallel, so whatever charge one gets the other will too, minus some small losses to resistance in the cables. I wouldn’t worry about that relative to some of the other concerns I’d have with the existing setup...

is the alternator sending a charge back to the C post on the switch sending charge to whichever bank i have selected?

Yes, exactly.

The cable leading from the Alt to starter is 10 AWG. All the rest of the wiring looks like 4 and 2 AWG. mostly 2 but the long length (20 ft round trip) to the house is 4 AWG

There are voltage drop calculators that will help you figure out the best gauge based on load and wire length. Look at the loads your panel needs, and charging capacity of your alternator to see if the gauges you have are appropriate.

Now - As for the two battery start bank - that’s pretty unusual. What types of batteries are those, and what is the house battery? Usually you have a big house bank, and a smaller house battery. I’d be a little concerned a previous owner may have swapped the configuration to compensate for other issues starting the engine. In any case, I’d go back to a bigger house bank. The start bank shouldn’t need to be more than a single Group 24 or equivalent.

 

[XE3]

It appears you have a 36 foot monohull sailboat. Why two start batteries? And only one house? Perhaps reversed?

It has a 36 HP 4 cylinder Universal. I honestly forgot what the cranking amps are required for the engine. It s the way it was setup when i bought the boat and didnt understand enough about electrical at the time to change anything. its worked so far even though the house battery is so small. most of my loads are at a dock and are AC which is wired directly to the panel. no inverter or heavy loads on the DC side yet.

the two start batteries are west marine brand...650 cranking amps each if i remember correctly 85 amp hours each i think. house bank is west marine deep cycle 95 amp hour i beleive? plan on replacing this with two 12V 250 AH Trojans.

i can start the engine with either bank although it is instant 1 sec start on the start bank and a couple seconds on the house battery.

 

[mermike]

Fusing is essential. A dead short into one of those cables will make for a lot of unpleasantness. Fix this first.

As David noted, the two batteries marked as start are not correctly wired in parallel, nor is the charger correctly wired to the bank.

10 AWG from the alternator is too small. The correct size depends on the output capacity of the alternator, and the distance between the alternator and battery bank, including both the leg from the alt to the switch and the switch to the battery bank. You want no more than a 3% voltage drop. If you have a 75A alternator and 20 ft RT to the bank, at 100% output (and 14.4 volts) your voltage drop on 10 AWG is around 21%. Your batteries are only seeing 11.4 volts.

Using the formula below, and the above assumptions, I calculated 44,972 circular mils which equates to just over 4 AWG. So the correct size for that wire is 2 AWG as your system stands now. If you increase the size of your alternator to feed those new, larger batteries, you'll need to increase the wire size.

 

[Stu Jackson]

A much easier way to figure wire and fuse size:

Blue Sea Tables Reference - Blue Sea Systems Scroll down to find the PDF download: Protect Your Boat with the Correct Size Wire & Fuse Chart. Handy to keep around.

 

[XE3]

very helpful...thanks

 

[Richard19068]

10 AWG from the alternator is too small. The correct size depends on the output capacity of the alternator, and the distance between the alternator and battery bank, including both the leg from the alt to the switch and the switch to the battery bank. You want no more than a 3% voltage drop. If you have a 75A alternator and 20 ft RT to the bank, at 100% output (and 14.4 volts) your voltage drop on 10 AWG is around 21%. Your batteries are only seeing 11.4 volts.

At risk of hijacking the OP I have a couple followup question to the above.

How do you, or can you, take into account there are two different wire sizes in the same run? For example, it appears the 10AWG wire length in the existing circuit goes from the alternator out to the hot starter post. That 10AWG wire might be only a foot or less in length. The rest of the circuit is made up of 2 or 4AWG. Is there a way to take into account that the 10AWG section is only a foot long?

Also, in determining round trip distance, how do you take into account that part of the negative circuit is not wire, but through the engine block? That is, for example, say the positive side is 1 foot of 10AWG and 14 feet of 2AWG and the negative side it say 5 feet of 2AWG from the battery to the engine block. A total of 20 feet of wire plus the block. I know it would be preferred to have a separate wire for the alternator to battery ground rather than through the block but in this case there isn't so how do you calculate for that?

And, what is "DP" in the schematic? is that house loads?

I know these questions reflect my ignorance about wiring. Sorry if they don't make sense. And again, my apologies to the OP if they are too far off topic.

Thanks.

 

[XE3]

At risk of hijacking the OP I have a couple followup question to the above.

How do you, or can you, take into account there are two different wire sizes in the same run? For example, it appears the 10AWG wire length in the existing circuit goes from the alternator out to the hot starter post. That 10AWG wire might be only a foot or less in length. The rest of the circuit is made up of 2 or 4AWG. Is there a way to take into account that the 10AWG section is only a foot long?

Also, in determining round trip distance, how do you take into account that part of the negative circuit is not wire, but through the engine block? That is, for example, say the positive side is 1 foot of 10AWG and 14 feet of 2AWG and the negative side it say 5 feet of 2AWG from the battery to the engine block. A total of 20 feet of wire plus the block. I know it would be preferred to have a separate wire for the alternator to battery ground rather than through the block but in this case there isn't so how do you calculate for that?

And, what is "DP" in the schematic? is that house loads?

I know these questions reflect my ignorance about wiring. Sorry if they don't make sense. And again, my apologies to the OP if they are too far off topic.

Thanks.

DP= DC Distribution Panel. I should have been more clear. Good questions i would be interested in the explanantion as well. The 10 AWG wire is no more than 12" length on my setup and in the interest of completeness, and to correct an earlier error...all my battery cable is mainly 2/0 with the long length to the house battery being 1/0. the alternator is stock/original which if im not mistaken is 50 AMP but havn't been able to confirm.

 

[Davidasailor26]

How do you, or can you, take into account there are two different wire sizes in the same run? For example, it appears the 10AWG wire length in the existing circuit goes from the alternator out to the hot starter post. That 10AWG wire might be only a foot or less in length. The rest of the circuit is made up of 2 or 4AWG. Is there a way to take into account that the 10AWG section is only a foot long?

You would need to add the voltage drop for the two sections of different gauge wire. So if the voltage drop of the foot of 10 AWG is x, and the drop of 10 feet of 4 AWG is y, the total drop is x + y.

I know it would be preferred to have a separate wire for the alternator to battery ground

Yeah, that’d be better, and the drop across the block is probably a bit of a wildcard otherwise.

 

[Stu Jackson]

and the drop across the block is probably a bit of a wildcard otherwise.

No, it's not. The block itself IS "The End." Zero distance. The alternator is grounded via its support bracket to the engine if it doesn't have a separate ground wire from the alternator to somewhere else on the engine/block.

 

[Davidasailor26]

No, it's not. The block itself IS "The End." Zero distance. The alternator is grounded via its support bracket to the engine if it doesn't have a separate ground wire from the alternator to somewhere else on the engine/block.

I respectfully disagree. The negative battery terminal is “the end”, at least from the perspective of measuring the current and voltage that’s charging the battery. There will be some amount of drop between the alternator bracket and wherever the ground cable runs from the battery to the block, then more drop over the ground cable itself.

 

[leo310]

A fuse is to protect the wire not the equipment as most equipment will have its own fuse ie radio, chart plotter so if you have two sizes of wire you would fuse for the smalls size as this would melt/burn out first.

 

[Stu Jackson]

I respectfully disagree. The negative battery terminal is “the end”, at least from the perspective of measuring the current and voltage that’s charging the battery. There will be some amount of drop between the alternator bracket and wherever the ground cable runs from the battery to the block, then more drop over the ground cable itself.

You're right, thanks for pointing that out. We do agree, however, that there's 0 distance to add between the alternator and the block, right? Total circuit length is + from alterenator to batteries, (-) back to block and 0 to add to that (-) side of the circuit.

 

[Davidasailor26]

You're right, thanks for pointing that out. We do agree, however, that there's 0 distance to add between the alternator and the block, right? Total circuit length is + from alterenator to batteries, (-) back to block and 0 to add to that (-) side of the circuit.

Yes, I agree there’s 0 to add for the alternator to block distance. I’m not sure about whether the distance across the block from bracket to ground cable counts for anything. I know Maine recommends running a ground wire from the alt to the negative post on the starter, then to the battery bank from there. But I don’t know if that’s to prevent the engine from creating resistance across the block or just to reduce the number of potential bad connections. The block is a pretty big chunk of metal, so I’m guessing it provides minimal resistance, just the connections.

 

[heritage]

The wire from the alternator to starter will cause an additional 0.07v drop in the charging circuit, if it is 10 gauge copper and 12" long. Assuming it is charging at 35 amps. So I wouldn't worry about that. Though, the latest thinking on charging and starting circuits is to disconnect the short wire between the alternator and starter and connect the alternator to the house bank and the starter to the start battery. An ACR is used to charge the start battery from the house battery. You can run the alternator to a switch to start from the house battery if the start battery dies if you like.

 

[Richard19068]

You would need to add the voltage drop for the two sections of different gauge wire. So if the voltage drop of the foot of 10 AWG is x, and the drop of 10 feet of 4 AWG is y, the total drop is x + y.

Duh. . Sounds almost too simple. Should have been able to suss that out myself. Thanks for pointing that out.

And thanks Stu and David for the conversation about the block resistance. I guess for practical purposes you could forget it.