Figuring Proper Fuse Size

[Padlin]

Hi folks, looking to install solar and an inverter on a new trailer and am looking for someone review my fuse needs, here's the scenario.

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?

Any other suggeations?

[mena661]

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.

[MEXICOWANDERER]

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.

[CA_Traveler]

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.

[mark_be]

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...

[mena661]

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.

[red31]

Should jumper cables be fused?

[msiminoff]

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

[BFL13]

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.

[mena661]

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.

[CA_Traveler]

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.

[CA_Traveler]

Fuses protect the equipment and the wiring. Wiring can develop a short and you certainly want a fuse to protect it and the equipment. Hence fuses near the battery. And then power gets distributed for other uses. RVs typically have 1 or more distribution panels to protect that wiring and equipment.

[mena661]

BFL13 wrote:
I find it all very confusing

Same here as you've seen many times from me. I do have a fuse on my inverter though. I just don't understand why I need one. It's wiring is #2 to 4/0 to #6 for a 300W inverter. That unit won't ever draw that much current, not without burning itself out in spectacular fashion. Maybe a failure could cause an inverter to run at it's surge rating?? Or maybe the inverter manufacturers are doing a CYA in case someone uses the wrong size wire?? I could understand that. I've seen many times here where the manual says 4/0 and the person installs #2 on a 2000W inverter. Yeah, you'll need a fuse with that install if something goes wrong.

[BFL13]

mena661 wrote:

RJsfishin wrote:
You do not need a fuse in the inverter line. Altho if you lack confidence in your installation being fool proof as far as short circuiting, then maybe "you" do.

I agree with this assessment. The inverter is only going to draw up to its rating and if your wire is enough to handle the rating then you don't need the fuse.

This is a similar argument to Randy saying a current- limited converter doesn't need a catastrophic fuse either. However, most (all?) RV's have a fuse at the battery end of the converter-battery wire.

I find it all very confusing. AFAIK the idea of the fuse is for when something goes wrong, not for normal ops.

The fuse goes close to the battery rule suggests the origin of something going wrong is at the battery end. But the inverter specs all have the fuse size related to the inverter's draw using proper gauge wire, which suggests whatever goes wrong is related to the inverter end.

A car audio store guy in town here told me the amplifier's fuse to be no more than 18" from the battery is actually a law and you can be refused insurance after a car crash with a fire, if they discover there was no fuse. The "thing going wrong" being when the battery is hit in the collision and bad electrical stuff happens.

I don't have the knowledge to sort all this out, but with all the inverter manuals saying to fuse the pos wire close to the battery, I am going to do that, and never mind what RJ says while doing his curmudgeon act 🙂

[SCVJeff]

Alls I know is that it takes 180A to microwave popcorn, and that 125A fuse would spoil the movie. YMMV...