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Old 01-28-2014
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Fusing Batteries

I currently have 2 house batteries (flooded G27s) and 1 start battery (flooded G24). In the future I plan to change the house bank to 4 flooded 6v GC's.

In the meantime, I would like to fuse the batteries. Is this the fuse terminal I want?

Amazon.com: Blue Sea Systems 5191 Fuse Block Terminal 30-300 AMP (32561): Sports & Outdoors Amazon.com: Blue Sea Systems 5191 Fuse Block Terminal 30-300 AMP (32561): Sports & Outdoors



I need on of those for each G27, correct? What size fuse do I use with it? Should the start battery be fused similarly?

Thanks
Matt
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Old 01-28-2014
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Re: Fusing Batteries

Matt,

If the G27's are in parallel & close to each other, then one fuse per bank.

What size fuse depends upon your battery bank wiring..

What gauge are your bank wires?

P.S. PM me if you don't have the need battery compartment height for the MRBF terminals. I have an option for side mounting those.
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Old 01-28-2014
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Re: Fusing Batteries

Thanks MS.

I believe I do have the room for the top mount terminal.

I think the battery wiring is #2AWG but I will need to double-check at the boat.

So, I size the fuse for the wire so #2AWG = 200A fuse? (that's from this chart: Wire Sizes and Fuses ) And if I use separate terminals, I'd still use the same size fuse because I am sizing based on the wire. Is that correct?

Since the batteries are within a foot of each other, I will plan on using a single fuse. Should I add a positive buss or can I just put all positive take offs on the outside of the the fuse terminal?

Thanks again!

Matt
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Old 01-28-2014
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Re: Fusing Batteries

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Originally Posted by blowinstink View Post
Thanks MS.

I believe I do have the room for the top mount terminal.

I think the battery wiring is #2AWG but I will need to double-check at the boat.

So, I size the fuse for the wire so #2AWG = 200A fuse? (that's from this chart: Wire Sizes and Fuses ) And if I use separate terminals, I'd still use the same size fuse because I am sizing based on the wire. Is that correct?

Since the batteries are within a foot of each other, I will plan on using a single fuse. Should I add a positive buss or can I just put all positive take offs on the outside of the the fuse terminal?

Thanks again!

Matt
This is the ABYC max ampacity chart for non-bundled wire. You need to know your wire jackets insulation temp rating. The ABYC does allow for sizing the OCP to 150% of this chart, if necessary. I generally recommend 225A as the smallest fuse for starting sailboat AUX engines. 250A is even better and 300A is even better.

A 200A fuse will do it, on most engines, but you run closer to that nuisance blow end of the spectrum...

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Old 01-28-2014
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Re: Fusing Batteries

Unless I missed it in Mainesail's post, you do not need a fuse in the positive conductor from the starting battery.
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Re: Fusing Batteries

I recently re-wired with 105C 00 cabling, and the Blue Sea Systems fuse holders. I used 300A fuses. Now I appreciate that 300A is above the 280A rating for 105C 00 cable in an engine compartment, but I also believe that I am adequately protected from a disastrous short circuit in the system.

Edit : just read the bit about the fuses can be up to 150% of the chart. It's all good then.
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Re: Fusing Batteries

Quote:
Originally Posted by boatpoker View Post
Unless I missed it in Mainesail's post, you do not need a fuse in the positive conductor from the starting battery.
It's not a bad idea though. How likely is a short in the starter or cabling? Likely enough for me to feel happier with a fuse.
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Re: Fusing Batteries

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Originally Posted by boatpoker View Post
Unless I missed it in Mainesail's post, you do not need a fuse in the positive conductor from the starting battery.
If this "not needed" is defined by the ABYC's "exemption" for cranking batteries then it is pretty foolish to abide by that on small sailboat aux engines that are EASILY over current protected.

Sometimes I agree with the ABYC and in this case I strongly disagree with the E-11 on this. Why? Because it is foolishly & needlessly applied to all boats and is very often misinterpreted as you should not fuse a starting bank which could not be further from the truth or intent of that exception... There are many of us on the electrical committees who would love to see that wording changed.

So yes the ABYC has an "exception" to this rule for cranking motor batteries. This exception however is more broad based and written to include for large engines which have massive amounts of starting current being drawn. These engines are very expensive to fuse properly hence the "exception".. Think big sport fishing boats, and our engines are about as far away from that as can be... Small diesel or gas AUX engines on sailboats would be best served fused than unprotected.

I quote our good friend Nigel Calder here:

"The net result is that nowadays, electrical shorts are probably the number-one cause of fires on boats.

There is simply no excuse for not protecting all high-current circuits , including the cranking circuit."
Nigel Calder Cruising Handbook


The ABYC also refuses to address the vast majority of boats out there that are fed by a 1/2/BOTH switch where either bank can be house or start with the flip of a switch and NEITHER bank is really considered a "cranking battery". In these circumstances one would be very wise to protect both banks.

Relying on the ABYC exemption in this case is a cop-out and can be flat out unsafe....


Here is a good case study of where this foolishness fails us:

This was a junior sailing program chase boat used at our club. It was a 15' Whaler with a Honda OB motor.

STOOPID #1 - Because the battery on this boat was not more than 800CCA it was exempted from needing a battery switch. D'oh.. Okay if the ABYC says so.....

STOOPID #2 This battery was ABYC exempted from over current protection because it is a "cranking battery".. D'oh......

STOOPID #3 Because of the failings of the ABYC standards in this instance three 8 year olds and a sailing instructor were almost burned up.... Safe..?

A simple $30.00 fuse or battery kill switch could have prevented this yet this boat was ABYC compliant....

Yes a single battery less than 800CCA (exempted from a battery switch) and one that "cranked" a motor (exempted from over current protection at the battery) caused all this....


So yes I often exceed ABYC standards in my wiring because the standards are often quite insufficient or too broadly applied.
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Last edited by Maine Sail; 01-28-2014 at 07:25 PM.
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Old 04-04-2014
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Re: Fusing Batteries

Hi Folks,

Working on a rewire of my 1977 Santana. Came across this discussion, along with Maine Sail's musings on the 1/2/B switch. I have a follow on question that I can't seem to find much info on in regards to the MRBF terminal fuses.

Specifically, I am looking to setup as a use switch, with alternator leading back to the house bank, then using a battery relay handle keeping the reserve battery topped off.

My question is this: If I setup with terminal fuses, likely 125A at the battery to the 1 and 2 inputs on the switch, can I with a 2 position terminal fuse also have the Alternator directly wired to the House battery with a second Terminal fuse (I think in 75A range)? I see that blue sea carries 2 fuse terminal fuse blocks, but there's little documentation on them and I'm unclear if they are independent of each other, although that would seem to be the intent.

I expect highest amp draw is going to be the starter which according to spec will draw 75A (rated .9KW), and my alternator is 35A output. Engine is a Westerbeke 12b Two Diesel.

If this is possible, then I'm thinking I'd attach the Blue Sea mini ACR to the 1 and 2 terminals back of the 1/B/2 switch. Am I on the right track?
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Re: Fusing Batteries

Quote:
Originally Posted by tatracy View Post
Hi Folks,

Working on a rewire of my 1977 Santana. Came across this discussion, along with Maine Sail's musings on the 1/2/B switch. I have a follow on question that I can't seem to find much info on in regards to the MRBF terminal fuses.

Specifically, I am looking to setup as a use switch, with alternator leading back to the house bank, then using a battery relay handle keeping the reserve battery topped off.

My question is this: If I setup with terminal fuses, likely 125A at the battery to the 1 and 2 inputs on the switch, can I with a 2 position terminal fuse also have the Alternator directly wired to the House battery with a second Terminal fuse (I think in 75A range)? I see that blue sea carries 2 fuse terminal fuse blocks, but there's little documentation on them and I'm unclear if they are independent of each other, although that would seem to be the intent.

I expect highest amp draw is going to be the starter which according to spec will draw 75A (rated .9KW), and my alternator is 35A output. Engine is a Westerbeke 12b Two Diesel.

If this is possible, then I'm thinking I'd attach the Blue Sea mini ACR to the 1 and 2 terminals back of the 1/B/2 switch. Am I on the right track?
Two post will be fine for alt and starting loads. I would suggest 200A for each bank as a min and 75A on a 35A alt will be fine. Ignore starter motor "specs" as there is no standard to accurately rate them into "starting loads"...

An Echo charger may be easier on the back of the switch as the wires are only 10GA....
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Last edited by Maine Sail; 04-04-2014 at 05:18 PM.
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