Battery switch & neg bus bar sizing

[Golden Echo]

Originally Posted by Dadrock33

Hey masca! Welcome to the puddle!!


Rule of Thumb (also helps to alleviate nuances of Murphy's kin & his law): with very few exceptions (but there are some), going bigger pays off in the long run provided that safety and welfare are not "in harm's way" . . . . besides of which - you're talking about $20 at West Marine (I'm sure they're cheaper elsewhere . . maybe). . . . when's the last time you heard someone say "y'know, I've got too much horsepower" . . .

It pays to shop. If you don't need it in a hurry, there is a place in Annapolis that is Mail order and has good prices. PKYS is the name. I needed two small buss bars for an install of a large bilge pump. I ordered 2 for less than the price of one at WM. They also had decent prices on connectors.

Good luck!

[DeepFrz]

Have you read this? Electrical Systems 101

[barnakiel]

I have used Blue Sea bus terminals and I was shell shocked at how wee physically they are - esp the ones with many studs. OK, they are fine for the power rating, but what about the physical size of the studs. Unless your cables are all pretty small.

So, in our project, we opted for their bigger buses. They looked sturdier and were easier to transfer the old cabling to.

b.

[masca]

Originally Posted by Wotname

Before I comment on the entire post, let ask how you are getting the negative cable to the engine block (starter motor)?

Hi. The house battery negative will be linked via a short cable to the start battery negative, which will then go to the neg bus bar (about 0.8m). From there it is direct to the starter motor negative terminal (about 1.3m). All cable will be 2 AWG/34mm2.

[mitiempo]

Originally Posted by masca

....house battery negative will be linked via a short cable to the start battery negative, which will then go to the neg bus bar (about 0.8m including fuse).

Fuses belong in the positive cables, not the negative cables.

[masca]

Originally Posted by mitiempo

Fuses belong in the positive cables, not the negative cables.

Doh! Of course... I do know that... I'm slowly going mad!

[Tasso]

Originally Posted by masca

Hi. The house battery negative will be linked via a short cable to the start battery negative, which will then go to the neg bus bar (about 0.8m). From there it is direct to the starter motor negative terminal (about 1.3m). All cable will be 2 AWG/34mm2.

For high current junctions (eg starter, inverter) I use dedicated wiring as the voltage drop in wires will otherwise cause a slight float in ground voltage. These connections should not touch neg bus bar; use a separate cable for house wiring neg (to bus bar).

If there's room on your battery terminal make connections there. Else a stud block:

https://www.amazon.com/Blue-Sea-Syst...words=blue+sea

close to starter battery...

[Wotname]

Originally Posted by masca

Thanks very much for the further replies!

Should I therefore be putting a 300A fuse close to both batteries (300A so as to enable emergency starting from the house battery)? I would like to use MRBF terminal fuse blocks, but I don't have the necessary additional height above the posts, and I assume that (very carefully) mounting them upside down is considered a bad idea?! The fuses will therefore most likely be mega fuses.

Is it a bad idea to connect the negative battery cables close to the batteries and then bring just a single cable to the neg bus bar? I was planning this just to simplify the cabling. My pos and neg battery cabling will be 2 AWG (approx. 34mm2), which I think should be sufficient. The run from battery terminal to starter motor is just over 2m one way.

I agree that the 250A Maxibus would be better than the 150A neg bus bar I have now and I will upgrade this. I'm sticking with my switch though - sorry! I know that the majority like to use ACR/VSRs, but we want to keep things as simple as possible and we're happy manually switching from #1 (start) to #2 (house) about 20 mins after starting the engine. The switch will otherwise stay on #2 and the both position will never be used other than going via it when switching.

My (non-current carrying) common ground is a glassed-in post just after of the engine, or so I believe?

Originally Posted by masca

Hi. The house battery negative will be linked via a short cable to the start battery negative, which will then go to the neg bus bar (about 0.8m). From there it is direct to the starter motor negative terminal (about 1.3m). All cable will be 2 AWG/34mm2.

Your negative run will work, as others have mention, there are some slightly better ways but whether the benefit to you is important or not, only you can decide.

You only have basic requirements for your electrical system and therefore I believe complex solutions are not needed. The cost benefit ratio is just not there.

What is more important IMO, is doing everything with quality components and good workmanship. While not universally true, a neat attractive installation of wires, terminal, clamps etc usually results in a good, maintenance free and safe system.

[Wotname]

Originally Posted by mitiempo

Fuses belong in the positive cables, not the negative cables.

I am not disagreeing with you but I do wish to add that fuses work just as well in the negative cable as the positive. I'm sure you know this so I adding this post for the benefit of other less savvy electrical readers

[mitiempo]

Originally Posted by Wotname

I am not disagreeing with you but I do wish to add that fuses work just as well in the negative cable as the positive. I'm sure you know this so I adding this post for the benefit of other less savvy electrical readers

Fuses in positive cable only per ABYC.

Lets say you do put the fuse in the negative cable and a wire chafes through near the engine causing the fuse to blow. You still have an un-fused positive in an area where the negative (engine block etc.) is exposed.

[Wotname]

Originally Posted by mitiempo

Fuses in positive cable only per ABYC.

Lets say you do put the fuse in the negative cable and a wire chafes through near the engine causing the fuse to blow. You still have an un-fused positive in an area where the negative (engine block etc.) is exposed.

Let try to understand this....
The fuse in the -ve cable has blown and then the +ve cable that is exposed to the engine block chaffs though. The engine block is no longer at -ve potential because it is disconnected from the -ve post of the battery because the fuse is blown. The current path somehow (by magic?) continues though the blown fuse in the -ve cable to the battery post. .

Around here, once the fuse is blown, the circuit is open circuit and thus current no longer can flow.

I understand the ABYC requirements but they (IMO) due to convention. There also circuits that require fuses in both +ve and -ve feeds but they are beyond the scope of this thread.

[mitiempo]

Originally Posted by Wotname

Let try to understand this....
The fuse in the -ve cable has blown and then the +ve cable that is exposed to the engine block chaffs though. The engine block is no longer at -ve potential because it is disconnected from the -ve post of the battery because the fuse is blown. The current path somehow (by magic?) continues though the blown fuse in the -ve cable to the battery post. .

Around here, once the fuse is blown, the circuit is open circuit and thus current no longer can flow.

There is more than one negative connection or item in many engine spaces. Dedicated alt negative for one, most any electrical motor - pump for example, fuel or otherwise - is case grounded. Not guaranteed they are powered by the same battery bank.


On paper yes you can fuse the negative, but in practice it has its issues. It would be flagged as wrong by any surveyor I know of as well.

[masca]

Thanks again to all who posted!

[exMaggieDrum]

Originally Posted by Wotname

Welcome aboard CF, masca. Nice to see the first post is an electrical question

Yes, the battery selector switch is suitable.

The bus bar is only just big enough (IMO) but this can depend on exactly how it is wired.

Given your stated loads, the only high current will be the starter motor and as that is for such short periods, the bus bar will be OK but I rather see the 250A bus bar (Blue Sea 2127).

This is exactly what I would say as well.

[NYSail]

Originally Posted by S/V Alchemy

You're getting some good advice here. For further advantage, I would suggest sending all charge sources to the house battery, and then running an ACR/Echo Charger to the start. That's worked well for me in the past. The idea is that your house bank keeps the start bank very close to full charge all the time. Of course, you don't switch back to house until the engine's stopped to preserve the alternator's diodes.

You may find this post in my refit blog of interest, and the same tags indicate similar, related topics: https://alchemy2009.blogspot.ca/2016/09/wired-and-perspired.html

I am doing a bit of an electrical upgrade with my new to me beneteau 423...... European wiring is crazy. Anyway currently there are a large bundle of wires on the negative point on the engine, so I intend to add a meg bus bar adjacent to this, take most if not all negs off engine grounding point and put the neatly onto bus. Then run a single cable to engine. In your diagram I do not see a wire leading back to engine from negative bus. Is it just assumed in your diagram?

Thanks!
Greg