Battery fusing for house bank confusion

Da Nag

Da Nag

Administrator
Staff member
I'm doing some wiring upgrades on the "new" boat, and while not a C-Dory - the issue I'm encountering is somewhat universal. That is, how big of a terminal fuse to put on the house bank?

The current setup has no breaker or fuse within 7" of the positive terminal on the house battery bank, so I'm adding one of these Blue Sea 5191 Terminal Fuse Blocks to the house battery that is connected to the 1-2-All switch:

hN281RC.png


Now...were I only concerned about the house loads, I think the answer would be relatively simple. Calc the total possible load, and plop on a relatively small fuse...probably the smallest offered, the 30A.

However...

Should I forget and leave the 1-2-All on the house bank when starting (or do so intentionally if the starter battery is dead), I've read the starter pulls gobs of amps when engaged - which would easily exceed 30A and blow the fuse.

However, if I size the fuse for this potential load (100A+ perhaps), that seems way too large to protect things when only the house bank is switched on.

Advice?
 
I have this exact same question - on both my C-Dory (if I move the house bank) and my RV (now). Actually for both the fusing and the cable size (for the same reason you state and because fusing and cable size are related). On the boat I can use a "combine" position on the battery switch, and I have a similar function in the RV.

I can figure out all the other wire/cable sizes and thus the fuse sizes, because, as you say, I know what things draw. So I size the wire for that, and then I size the fuse for the wire. But... I'm not sure how to figure out the cable size for when possibly combining banks to start, so hence I can't figure out the fuse size either. I can figure out what the starter draws, and the distance of the run, but.... what about the fact that it is a momentary/short-term load? Does this count? Let's say the RV starter draws 300 amps. That's huge, but it doesn't run for an hour or two in a row like other loads do.

I spoke to Blue Sea and apparently there is an exception in ABYC for starting loads, but if possible I would rather cable/fuse large enough for those loads. But I would like to know how to figure out how large they should be because perhaps if it's not too big I won't need to "use" the exception and can just wire for the real load. (Hmm, maybe I should review the ABYC standards.)

I think it must have something to do with the short term load. Because it's huge but somewhat momentary?

I'm glad you posted as the answer to this question is the last remaining puzzle piece in my planned RV electrical upgrade.
 
Sunbeam":uezjv8ed said:
I spoke to Blue Sea and apparently there is an exception in ABYC for starting loads, but if possible I would rather cable/fuse large enough for those loads.
Click to expand...

As did I just now, and the answer they provided was similar - but the solution they gave me makes sense, and is likely the one I'll put in place unless I hear something better. In short, it's based off this article from their site:

http://assets.bluesea.com/files/resourc ... cerpts.pdf

As shown in the diagram in the article, the battery leads going from both house and start banks to the 1-2-All switch are considered engine starting circuits, so are exempt from protection from ABYC.

For house loads - the feed(s) to these branch circuits from the 1-2-All switch are considered the "source" - and it is from here the ABYC 7" fusing requirement applies. So, installing something like a Blue Sea 7720 within 7" of the switch, appropriately fused for house loads should suffice.

nZpR02o.png


Doing so actually provides an additional benefit. If one leaves the 1-2-All on the start position, the house loads are still protected with the fuse in this location - which would not be the case with a fuse terminal installed on one of the house bank batteries.

With the house loads fully protected with an appropriate fuse, that only leaves the positive leads feeding the 1-2-All from both the start and house banks. While not required by ABYC, if one chose to protect these two locations - I'd assume the aforementioned Blue Sea 5191 Terminal Fuse Blocks could be put in place on both banks, fused sufficiently for starting loads.
 
Correct on starting loads--and in your case the batteries are very close to the diesel engine.

I use a 60 amp breaker on the C Dories, with a second 40 amp on the windlass system (off the starting battery). On the bigger boats, I found that on house banks 60 amps was still fine. But I used separate banks of batteries for inverters. If you are going to run an inverter, then go with the big fuse, which will allow 200 amps or so, depending on the loads. But I would still "fuse" the house loads down the line at 40 to 60 amps. 200 amps would let #12 wire become a fire hazard~

As Bill suggests I do fuse the house load after the "All" terminal, not directly off the house bank. I have never fused a starting load. There can be critical situations where you have to start an engine--and I don't want to have a fuse blow.
 
Back
Top