Lazy Daze Owners' Group

I suspect they chose the fuse based on the wiring they installed for that circuit.  Most automotive fuses are there to protect the wiring in the vehicle.

From: Andy Baird
 To: lifewithalazydazerv@yahoogroups.com
 Sent: Saturday, February 14, 2009 8:38 PM
 Subject: [LD] Re: Why did LD limit the charging rate to 40 amps?

"you're right, Andy, in theory."

And in practice. :-) This isn't hair-splitting, it's simply the way electricity works. There's no such thing as a fuse that protects the wire on one side of it but not on the other side. When a fuse blows due to excess current (and remember, current is by definition the same at all points in a series circuit), the entire circuit is de-energized, preventing overheating and fire.

Andy,
 Respectfully, that's not my understanding. Consider a fuse installed some distance from the battery in a chassis ground (negative) system like our LDs. Let's say the fuse is 2 feet away from the battery and wired in series with the positive battery terminal. That fuse will protect only that portion of the circuit (wire etc) downstream or  further away from the battery than is the fuse.
 e.g. if the wire one foot from our battery should chafe through and touch chassis ground, there will potentially be a catastrophic failure (much spitzen sparkin!) and the fuse will not blow or protect from this. Current will flow from the negative battery terminal through the chassis (ground), to the chafed shorted wire and then back to the battery positive terminal completing the circuit. Note that this current surge will not go through the fuse.
 It should be obvious why it is considered good practice to install the fuse panel, or a main, larger fuse as close to the battery as practicable.

all the best,
 bumper (ex Navy Electronics Tech, licensed electrician in CA - - - though I sure wouldn't want to live in a house I wired.) Yonder

 Sam, our '83 has two of these mounted on the firewall, one protecting the run from the battery to the isolator, the other from the battery to the converter panel.

So, it appears to me that it does not make a lot of sense to upgrade to a higher capacity alternator if one were to keep the same one wire circuit from the isolator to the battery.  Am I missing something?

Yes. If what you have currently is a 45A alternator, that must supply about 25A or so to the headlights on brights, plus all the running lights, plus ignition, plus charge a chassis battery possibly depleted from a cold morning start, plus supply any 12V accesories in the coach you are running enroute, including 15-20A to the fridge.... AND charge a coach battery you may have depleted in several days camping. I also am led to believe you may soon be more than doubling the capacity of your current coach battery. All this really calls for a higher charge capacity.

Most of us who have done upgrades have also replaced some of the wiring and those circuit breakers. Since I no longer have a coach battery up front, I also upgraded the wiring to the new location, replacing the circuit breakers with a binding post, and installing a pair of 100A fuses in the new location.

Steve

Sam An important fact to remember is that automotive alternators only produce their rated output when the alternator is cold. As they warm up, the output can significantly decrease.

A few years ago, I pulled a 63-amp Delco alternator off a co-

MH and put it on the alternator-testing machine at work.
It produced 60-amps cold but then dropped down to 45-amps after running under load for ten minutes.
45-amps is not enough output to provide much charging to the batteries when the engine, dash A/C and headlights are operating.
Ford got it right when it started installing 130-amp alternators in the E350/450 cutaway chassis.

Larry