12-Volt Fusing Questions

[soused moose]

I could use some help here if anyone’s got the time.



I’m trying to figure out a few things concerning our 12-volt wiring system…

What I’m doing is using 12/2-shielded marine 12-volt wire in 2 long runs (approx. 30 ft each side) that will be the main 12-volt feed line for our lights.



Anywhere there’s going to be a light, I branched off a line (from the main trunk line) for it.

The lights are all standard RV types, and they’ll be approx. 8 of them on each side.

Here’s what I’m trying to figure out…each of the lights are approx. 2 amps…So…

Do I multiply 2x8= 16 amps to come up with the right sized fuse at the distribution box?



Also, Is it smart to put an inline fuse at each light, for added safety? Since the light uses 2 amps …what sized fuse would be the right one to use here? A 2-amp one?



Thanks for any help you can give us on this,

Michael

[Vern1]

Greetings,

As for the size fuse required, I am not sure on that one.

As for location, I would say the fuse should be at the beginning of the line next to the power supply.

That way, if you have a short along the line, the fuse will blow and not do a slow meltdown of the whole wire.

[lapeer20m]

not only that, but having all the fuses in one location makes it much easier to replace a blown fuse. You don't have to go digging aroud in service panels trying to remember 10 years down the road where the fuse is located for a particular light.

[Les Lampman]

Hi Michael,



I would agree about not installing fuses all over the place; mostly you want to end up with one really good 12-volt fuse block (maybe two if you need that many circuits) located where you have easy access. My favorite fuse block is the Blue Sea Systems #5025 (6-circuit) or #5026 (12-circuit) ATC/ATO Fuse block. For AGC glass fuses I like the #5025 (6-circuit) model (they don't make a 12-circuit AGC block). Blue Sea Systems is here. Their products are high quality, have covers to protect the fuses and have space on the covers for circuit labels. The glass fuse models have an ejector lever which pops the fuse up; that really helps rather than trying to get your fingers in there to grab a slippery fuse.



I noticed in the photo of your power panel area that you've got a a Blue Sea Systems switch panel. This is also a good way to go on the lights since each switch has its own easily accessed fuse. You could split up the lighting circuits onto a couple (or more switches); perhaps one switch for left side lights and one for right side lights (or foward lights and aft lights; or whatever meets your needs). For your 16-amp max requirement on your light circuit I'd suggest a 20-amp fuse since there are no "starting" loads on lights; any problem anywhere in the lighting circuit is going to pull more than 20 amps and will blow the fuse. You don't need to fuse each light.



So...if you're going to use a fuse block (and the ATC/ATO style fuses are easiest to deal with) you want to run the main feed wire to it sized to handle the maximum possible load of all the fuses on the block. This feed line should be fused at or near the battery (or bus bar...see below) to protect the feed wire in case of a major malfunction (an in-line circuit breaker works well here and allows you to shut power down to the fuse block if necessary; like the Blue Sea 7100 series suface mount breakers here). Then split up your lighting circuits the way you want on the fuse block (left, right, fore, aft, etc).



If you're going to use a fused switch panel (like the Blue Seas System panel) it's really pretty much the same except you usually pull power off a bus that's fed from the batteries since you don't want to keep running a wire for every switch panel you add back to the battery. In fact, I usually install one healthy bus bar (fed with a large cable from the battery and fused) inside near my power center to pull power off of when using a fuse block or a switch panel; that way you can use smaller wire to feed your fuse blocks and switch panels and again, you don't have to keep running wires to the battery.

[soused moose]

I like our Blue Seas distribution panel, quality piece of equipment … expensive though (around $95.00 here in Sitka)



I’ve been chatting about circuits & fusing with the Folks over on the MAK Bus Conversion Board for a couple of days.



It’s hard to weed out what’s really correct to do, informationally speaking versus just a bunch of techno gobblygook over there sometimes.

It’s a great Resource, with some really nice folks who want to help…but there’s definitely a bunch of folks there who get too wrapped up in what they think is the only correct way to go about a conversion. They’re big on RV ‘Code’ installations …unfortunately there is No such thing as a set in Stone RV code. All it is …is an organization that arbitrarily set standards for RV parts & installations that’s 6 years behind the times …they’re not a regulatory agency. They’re really just Toadies for the RV Building industry.

So…it’s hard to ask a question that doesn’t receive a well ‘this is the only way to go about it or you’re doing it wrong answer.



They say that I can’t just run 2 lines (12-gauge wire) with branches off it for the 12-volt lights (one each side of the bus). I did this in our last conversion and never had a single problem. No wires got hot, and we didn’t have any brown outs.

They’re saying I would need a separate wire (circuit) with no more than 2-3 12-volt lights attached.

I don’t know if this really right or not? I was going to attach branches for 8 lights on each main trunk line that I’m installing on each side. The lights all together (per side) would draw 15 amps if they were all used at once …but I seriously doubt we’d have them all turned on at the same time. And even if I did turn them all on at once, I’m not sure this would hurt anything.



If they’re right I’ll have to pick up another 3 circuit Blue Seas ($60.00) and wait for a month till they get restocked with the wiring I’m using. I bought out they’re whole stock of 300 feet of the stuff. Living on an Island in the middle of nowhere sucks sometimes.





Well, anyway …here’s how were going to send power to our distribution panel (s).

I’m coming off the battery bank with a heavy-duty primary cable (+) that will go to a Perko marine quality on/off switch …these are big honking units with very good shielded connector posts. They’re wonderful for shutting down your 12-volt system quickly if you have any problems …plus if you’re running any devices like wired-in CO & propane detectors that are drawing power all the time, it’s good to have a system that will shut everything down if you leave the bus for a few days (no hidden battery draining).



On the ‘out’ side of the switch I’ll run a short section of primary cable to a marine Power post that will be screwed down inside our Power panel area…from the power post there will be the lead to our Distribution Box (this line will have an in-line 45 amp fuse on it to protect the panel).

For the (-) side I’ll run primary cable to another power post that will have a smaller gauge wire leading to the negative bus bar.

We did the same thing on our last bus ‘Home’, worked great.

[soused moose]

Here's an update on my trying to figure out what to do problem They’re really hard to get in touch with, because they're always off somewhere wiring up some commercial fishing boat here in Sitka…but I was able to contact the guys who actually know something about this.



It turns out I’ll have No problems with the system I described above. They said as long as I don’t exceed the load rating on the wire I’d be fine …so there it is. Whewww



I kind of knew that already …but it’s funny how you can start second guessing yourself in the search for the perfect Bus Conversion.



Someone said the really appreciate this board the other day …I’d like to second that motion. It is a great bunch of folks.

A particular thank you to Les …you helped an Old Dog learn a few new tricks.

[Les Lampman]

Hi Michael,



You got to talk to the guys up there before I could answer you and I'm glad they made you feel good about your wiring. I was essentially going to tell you the same thing. In fact, the factory installed overhead interior roof lights in my Blue Bird are wired exactly like that and so are the overhead lights in almost every boat I work on regardless of the builder.



The only thing I would add (and I'd be the first to admit leaning to the conservative side) is in an in-line fuse or breaker for the primary wire running from the battery to the Perko battery switch; it should be as close to the battery as possible. The thing is that fuses and breakers are in a circuit to protect the wiring, not the load; the wiring is being protected to keep it from getting so hot that it starts a fire, especially when it's running within walls and such. The idea is the same here...all those fuses in the fuse block and switch panels can (and will) protect the wiring that runs out to the things attached to them but they can't protect the primary wire upstream that supplies the power. If the fuse block had a major failure or there was a direct short in the primary wire (connection coming loose or the insulation rubbing through, etc) nothing would stop the primary wire from taking as much power as the battery could supply; that would be enough to completely melt the insluation and cause havoc (in fact, it's enough to weld the wire to a metal chassis and ignite any combustibles in the area).



As an alternative, since you're putting a Perko switch in, you could put an in-line fuse or breaker between the Perko switch and the Power Post (as close to the Perko switch as you can conveniently get it); this will cover almost all the situations the fuse/breaker at the battery would and may be easier to install. The only piece of wire you wouldn't protect in this installation is the one from the battery to the switch when the switch is in the "Off" position; depending on that wire's length and route that may be an acceptable situation. If that wire is very long or runs through walls or is otherwise susceptible to damage I'd really, really want a fuse/breaker at the battery.



In addition to circuit breakers there are also Maxi plug-in fuses available; they look like big ATO/ATC car type fuses. It's not as convenient as resetting a breaker but since you shouldn't ever have to change it (or at least not very often) this probably isn't a big consideration and this may be less expensive and easier to install than an in-line circuit breaker (since this is typically a one-time deal I'd still probably choose the breaker). Depending on your total load you may exceed the largest Maxi fuse and need to go with a breaker or T-class fuse.