Figuring Proper Fuse Size

I fail to see how a device designed to "accept" 30 amps can magically accept 10 times that but you guys seem to have this all figured out. Since I don't, I'm going to just do it my way.

Knowing the barn door got left open and all the critters strayed I have no idea if Glowing Butterfly Dragon inverters have catastrophic fusible links inside them. Has anyone taken apart one of these junk, err, strike that, economical inverters and mapped out the circuitry to see if they have some form of catastrophic over-current protection?

An unlimited-fire-Chernobyl within an inverter can frying wiring and produce noxious fumes that would knock out a Hippo.

mena661 wrote:

CA Traveler wrote:
Fuses protect the equipment and the wiring. Wiring can develop a short and you certainly want a fuse to protect it and the equipment.

Fusing is for protecting wire. Regardless, this does not make any sense (at least not to me) if you have properly sized wire. In a 300W inverter, the max it draws is 30 amps from the batteries (maybe it can draw up to its surge rating IDK). If I have wiring that can accommodate 100-380 amps (this is my setup), how is a short going to do anything to the wire? If the equipment shorts internally, it has fuses and protection circuits to handle that. Not to mention, shorts ALWAYS lead to an open and usually in a short period (pun) of time because internal circuitry is not designed to run over its design parameters. I'd be more worried about the equipment catching fire than wiring melting.

What you're missing is that shorts are abnormal and don't obey any rules or preconceived notions. Inverters may/may not have any fuses to protect the heavy DC draw electronics and they don't need to because the fuse should be near the battery. But if it does have a fuse what if the short occurs between it's fuse and the terminal lug? You don't want the wiring to overheat and cause a fire or cause the battery to overheat and be destroyed or cause a fire.

red31 wrote:
Should jumper cables be fused?

If you see smoke coming from your jumper cables, would you disconnect them? Same difference, it's only a manual "fuse" in this case...

BFL13 wrote:
A while ago we had some threads about ISTR Xantrex saying in their manuals that you had to fuse to the battery shorting ??

I remember that sort of and someone explained very well why there should be a main battery fuse. My "inverter fuse" is actually near the batteries and is an ANL 60 amp. It was initially sized for the converter. If I get that 2000W inverter then I'll change the fuse to allow for that.

Should jumper cables be fused?

Padlin wrote:
dual 6V batteries, 230A. 1500w Inverter, 125A. Solar Controller, 30A. Converter, 55A
Connecting the battery - via 2awg to a 500A shunt to feed a Trimetric Battery Monitor
Battery + via 2awg to a Main fuse.
Main fuse to 3 way power distribution block.
Fuse for inverter leg 150A, 4awg
Fuse for converter leg 60A, 8awg
Fuse for solar controller leg 40A. 8awg
Question, what size should the Main fuse be? do I even need a Main fuse?

Hi Bob & Deb,

Fuses are always sized to protect the wire... and sometimes the power source.

- Yes, you need a main fuse! Between the battery and distribution block! For the 2ga wire use a fuse that is rated 200A or less, preferably more like 175A (note that 2ga wire has an ampacity of ~210A). Mount this fuse as close as possible to the battery + terminal, and absolutely no more than 18" away. I chose to mount mine directly to my batteries (see photo below).

- The shunt for your trimetric will be installed in the negative wire and as such it does not need to have a dedicated fuse.

- 4ga wire is too small for your 1500W inverter (IMO, so is 2ga)! Have you considered a dedicated run from the battery to the inverter? Anyway, since you stated that you are using 2ga from the battery to the distro' block then I suggest you use 2ga to the inverter as well. Keep these wires (both + & -) as short as possible. Personally I would select a fuse for the inverter with a lower rating than the one at the battery so that if my inverter caused a fuse to blow it wouldn't affect my other circuits. IF you use the 4ga wire then the fuse must be rated less than 160A.

- For the 8ga wire to the converter, use a fuse rated at 70A or less. Note that IMO 8ga is small for a 55A converter.

- For the 8ga to the solar controller , use a fuse rated at 70A or less.

Hope this helps,
-Mark

A while ago we had some threads about ISTR Xantrex saying in their manuals that you had to fuse to the battery shorting ?? That led to questions of how much was that and where to find the battery spec that told you that, and whatever it was would be a ridiculously large amount.

All that soon faded away and we got back to the usual idea of having a fuse big enough to handle normal current and to blow if current got higher instead of the wire melting.

Yes, good point on the battery to buss wire being able to handle the total amps of all the branch wires to that buss.

CA Traveler wrote:
Fuses protect the equipment and the wiring. Wiring can develop a short and you certainly want a fuse to protect it and the equipment.

Fusing is for protecting wire. Regardless, this does not make any sense (at least not to me) if you have properly sized wire. In a 300W inverter, the max it draws is 30 amps from the batteries (maybe it can draw up to its surge rating IDK). If I have wiring that can accommodate 100-380 amps (this is my setup), how is a short going to do anything to the wire? If the equipment shorts internally, it has fuses and protection circuits to handle that. Not to mention, shorts ALWAYS lead to an open and usually in a short period (pun) of time because internal circuitry is not designed to run over its design parameters. I'd be more worried about the equipment catching fire than wiring melting.

smkettner wrote:

Padlin wrote:
Question, what size should the Main fuse be? do I even need a Main fuse?

Let the inverter fuse also be the main fuse. I would probably go 150 or 200 amp.
You will not have much if any inverter surge issue in an RV.

The main fuse needs to protect the inverter draw plus all other draws (lights etc). Most likely 200A will handle the loads, perhaps 150A also.