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Old 06-05-2017, 04:43   #1
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Battery switch & neg bus bar sizing

Hi. I'm about to embark on a rewire as part of a refit on a 'new' to me boat. I've been reading and reading , but would still be very grateful for a couple of reassurances (or, of course, otherwise!) regarding the required size of a 1-2-Both battery selector switch and main negative bus bar, please... Our setup is very much KISS, as follows:

- Beta 25 with single 70A alternator
- 75Ahr starter battery (not fused, as per Beta manual)
- 225Ahr house battery (100A mega fuse)
- Low house loads via Blue Sea CB panels (basic navigation gear, LED lights, 12V charging of phone/tablet, no fridge or electric windlass... that's about it really)
- Solar charging to be added at a later date

I intend to use a Blue Sea m-Series battery selector switch (6007) with 300A continuous rating, and a Blue Sea 150A bus bar. Are these both big enough for my needs?

Thank you!
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Old 06-05-2017, 04:55   #2
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Re: Battery switch & neg bus bar sizing

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).
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Old 06-05-2017, 05:29   #3
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Re: Battery switch & neg bus bar sizing

Thanks Wotname!

Quote:
The bus bar is only just big enough (IMO) but this can depend on exactly how it is wired.
I had intended for the bus bar to connect everything - battery negative (single, as I intend to connect the starter and house negatives in the battery locker), starter motor negative, negatives from two small DPs, plus those from all of the individual loads (by my calculation, less than 25A combined if all are in use). Will this be okay?
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Old 06-05-2017, 05:47   #4
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Re: Battery switch & neg bus bar sizing

Oh, there'll also be a connection to the common ground point, but I assume that shouldn't make any difference!
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Old 06-05-2017, 07:41   #5
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Re: Battery switch & neg bus bar sizing

I'd also go up to the next size on basic infrastructure like a busbar, be able to add devices later, even temporary ones without needing to upgrade.

Unless the difference really hurts financially.
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Old 06-05-2017, 08:30   #6
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Re: Battery switch & neg bus bar sizing

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" . . .
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Old 06-05-2017, 09:20   #7
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Re: Battery switch & neg bus bar sizing

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
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Old 06-05-2017, 09:21   #8
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Re: Battery switch & neg bus bar sizing

If you have a start battery and a house battery then you don't use a 1-2-all switch.

You use the 7650 kit

You could use the 7649 if you rewire the alt direct to the house bank.

I would get the 250a bus

And fuse the start battery with a big mrbf fuse.
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Old 06-05-2017, 10:07   #9
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Re: Battery switch & neg bus bar sizing

Thanks very much for the replies. Looks like it's the larger bus bar - I'm sure I can re-purpose the other one elsewhere at some point anyway! Will also take on board the other advice, and will definitely read the blog S/V Alchemy. Thanks again.
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Old 06-05-2017, 14:52   #10
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Re: Battery switch & neg bus bar sizing

Quote:
Originally Posted by masca View Post
Thanks Wotname!



I had intended for the bus bar to connect everything - [B]battery negative (single, [/B]as I intend to connect the starter and house negatives in the battery locker), starter motor negative, negatives from two small DPs, plus those from all of the individual loads (by my calculation, less than 25A combined if all are in use). Will this be okay?
Quote:
Originally Posted by masca View Post
Oh, there'll also be a connection to the common ground point, but I assume that shouldn't make any difference!
Quote:
Originally Posted by masca View Post
Thanks very much for the replies. Looks like it's the larger bus bar - I'm sure I can re-purpose the other one elsewhere at some point anyway! Will also take on board the other advice, and will definitely read the blog S/V Alchemy. Thanks again.
Whoa there...
Your existing bus bar may well be okay given this new information.

I had based my first opinion thinking you where using the bus bar as your common ground point and all the battery negatives, starter motor negative etc where going to it.

Without seeing a schematic drawing of your proposal, it appears your 150A bus will suffice.

As for the battery switch, there are many variations, (some of which mentioned by others above) and some fine nuances between the options. You will have to make your own choice but IMO, there is nothing wrong with your plan although there are better (more expensive) solutions offering (IMO) marginal benefits.
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Old 06-05-2017, 15:09   #11
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Re: Battery switch & neg bus bar sizing

Starter jumping from House is a very short-term load, so maybe OK, but 300A like the switch better.

Don't rely on the chassis for return of big loads, pos/neg should come back to the source on same length and gauge wires.
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Old 06-05-2017, 15:09   #12
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Re: Battery switch & neg bus bar sizing

for the main negative bus bar ( all the heavy cables) I make my own using 2 neg dist posts and a piece os 1/4" x 1"brass bar. that way I can customize the length I need. This way I can have 4 1/0 awg lugs on it plus room for the BMS shunt and a few extra spots for future loads. I have individual negative bus bars for things like all the cabin lights and all the USB chargers.
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Old 07-05-2017, 00:55   #13
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Re: Battery switch & neg bus bar sizing

Quote:
Originally Posted by masca View Post
- 75Ahr starter battery (not fused, as per Beta manual)
I don't understand this. I would fuse both batteries. Using large enough wire there is no reason not to fuse a start battery. With a 1/2/both switch ABYC requires it.

The negatives should go to a busbar from each battery. From there there should be one wire to the engine - preferably the starter negative. The panel negative as well as any other such as bilge pumps should also go to this bus. A larger bus (250 amp) would be my choice.

I agree with smac that there are better switch choices - the Blue Seas 7650 dual circuit with ACR is a good option.
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Old 07-05-2017, 01:31   #14
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Re: Battery switch & neg bus bar sizing

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?
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Old 07-05-2017, 02:00   #15
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Re: Battery switch & neg bus bar sizing

Quote:
Originally Posted by masca View Post
......
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.
.......
Before I comment on the entire post, let ask how you are getting the negative cable to the engine block (starter motor)?
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