Nerdy electrical circut protection question

[JonnyBoats]

I was reading the Bluesea whitepaper "Technical Brief - DC Circuit Protection" http://bluesea.com/files/resources/tech ... ection.pdf which I found quite useful.

The paper references the ABYC standards for the placement of circuit protection in the DC main circuit (ABYC E-11). The basic rule seems to be the "7 inch rule" with certain exceptions. In particular:

If the conductor is connected directly to a battery and is contained throughout its entire distance in a sheath or enclosure
such as a conduit, junction box, control box, or enclosed panel, the overcurrent protection shall be placed as close as
practicable to the battery, but not to exceed 72 inches (1.83m).

They further state:

In any case, fuses, circuit breakers, and switches should not be installed in battery compartments because of the risk of
corrosion coupled with the potential presence of explosive gasses.

I would like to understand best practices for constructing a battery box to hold 4 golf cart batteries in series/paralell (12 volt system) and provide main circut protection for this house bank. (The starting battery is seperate). It would seem nearly impossible to mount a circut breaker outside of the battery box and keep the length of the wire to the battery less than seven inches.

Also why does the ABYC permit the lenght to increase when conduit is used?

[catamount]

They further state:

In any case, fuses, circuit breakers, and switches should not be installed in battery compartments because of the risk of
corrosion coupled with the potential presence of explosive gasses.

Interestingly, Blue Seas sells terminal post fuses intended for mounting directly to the battery!

I think maybe the 7" starts at the exit from the battery box. In any case, the positive post you actually take your power from might not be that far from the edge of the battery box.

I believe the reason the length to the fuse can be extended if the cable is in conduit is because the conduit will provide protection that limits the possibility for the cable to become chafed or cut into in such a way that a short circuit could form.

Why not mount your fuse block to the outside of your battery box (assuming you're building a plywood box to hold your batteries) just where the cable exits the box? That's my plan!

Regards,

Tim A.

[Hirilondë]

Any distance your battery wires travel before fusing is unprotected. Unprotected wires directly connected to a battery are very dangerous. A direct short to a battery wire can draw mega-amps. If these wires are inside a conduit they are safer in that damaging the insulation, and shorting out is harder to do. I have 30 amp in line fuses on each battery while the wire is still in the battery box (30 amps is less than my wire is rated for). This protects the wire to my electrical panel. Actually, it is sufficient to protect all my wiring, but all circuits have a breaker as well that is less than 30 amp yet still sufficient for all demands placed on them.

My battery fuses are in holders similar to "H" on this page:

http://www.westmarine.com/webapp/wcs/st ... _sku=38914

[Ceasar Choppy]

Interestingly, Blue Seas sells terminal post fuses intended for mounting directly to the battery!

Tim, I have a picture of these, but help me visualize how these work. I've ever seen these before. Are these for batteries with male terminal posts? My Lifelines have threaded recepticals that take a 5/16 bolt. wondering if I can use these fuses.

EDIT: OK, I looked it up and found this. Nevermind, now I understand. These look pretty cool.

[Quetzalsailor]

I wish I'd read this thread before I did all my battery work.

I moved the batts from an acid-ruined wood box over the fuel tank in the keel to a new polyethylene box in each lazarette. I reused all the runs of multi-aught wire. The heavy wire is serving a 100 amp Balmar and the starter motor. Relatively light wire serves the DC panel. I've never understood the way the disconnect switches are wired or why I have two sort of in sequence; I cookbooked the system back together (tsk-tsk!).

If I put inline fuses close to the batts, then the gremlins that keep me up at night will subside (on that issue at least). There are inline fuses near the Balmar (6' of unprotected wire to each bank).

I like the elegance and neatness of the post-top fuses but surely that's a lousy place for them from an acid-damage standpoint? Is it the cat's meow to have both the + and the - conductors fused? I've read it somewhere, but the logic escaped me. My '83 Volvo's starter motor power wire was unfused; when it shorted to the block it caused an instant total failure of the electrical system and a great deal of heat and smoke accompanied by luck (the car did not burn and I coasted off the Schuylkill without being hit).

[Ceasar Choppy]

What I don't like about these fuse blocks, however, is that they step down a 3/8" terminal stud on the battery, to 1/4". Although I said I have 5/16" studs, I have 3/8"... and the terminals on my battery cable are 3/8" which means I would have to replace this terminal.