12V Solar/Battery Q: Can I use glass fuse?

Thread Starter

foolios

Joined Feb 4, 2009
163
I was hoping I could use this switch in-between the 12v 100watt panel and the charge controller:
http://www.menards.com/main/electri...-plug-fuse-safety-switch/p-1773843-c-6435.htm
with this fuse:
http://www.menards.com/main/electri...p-medium-duty-fuse-box-4/p-1449458-c-9543.htm
But I am reading comments about AC breakers being ok to use on low voltage DC circuits and yet there are comments that say they shouldn't.
My question is would it be ok to use an AC fuse on a 12v DC circuit?

One reference:
http://control.com/thread/985805467

Thanks in advance.
 

shteii01

Joined Feb 19, 2010
4,644
Hm... 12 volts, 100 watts... that gives us current of 8.33 amperes. Is the fuse rated for 8.33 amperes?

What the general rule of thumb for the fuses? Should they be rated for twice the current you expect or just 1.5x?
 

alfacliff

Joined Dec 13, 2013
2,458
higher voltage fuses are ok, just dont use lower voltage fuses than the origional. someone used a 120 volt fues out here in a 480 volt motor starter, made a mess, minor explosion.
 

mcgyvr

Joined Oct 15, 2009
5,394
higher voltage fuses are ok, just dont use lower voltage fuses than the origional. someone used a 120 volt fues out here in a 480 volt motor starter, made a mess, minor explosion.
Just to clarify..
The voltage rating on the fuse must be greater than the working voltage of the circuit.. But it still MUST be rated for AC if used on AC and DC if used on DC.

It is not OK to use a 120V AC rated fuse to protect a 12V DC circuit.
 

NorthGuy

Joined Jun 28, 2014
611
You don't need a fuse between controller and the panel. The panel won't produce much more than its rated current even if the controller shorts, so the fuse is completely useless.

As to the switches, most AC switches are also rated up to 24V DC. Or you can buy a dedicated DC switch, e.g. http://www.bluesea.com/
 

alfacliff

Joined Dec 13, 2013
2,458
why is it not proper to use a 120 volt ac fuse on a 12 volt dc circuit? the voltage rating on a fuse is determined by how much voltage it will block when it blows, the current rating is determined by the heat generated by the current flowing through the fuse melting the fuse. a lower volt rated fuse might not open up far enough to break a higher voltage, but a higher voltage fuse will open enough to break a lower volltage.
 

mcgyvr

Joined Oct 15, 2009
5,394
why is it not proper to use a 120 volt ac fuse on a 12 volt dc circuit? the voltage rating on a fuse is determined by how much voltage it will block when it blows, the current rating is determined by the heat generated by the current flowing through the fuse melting the fuse. a lower volt rated fuse might not open up far enough to break a higher voltage, but a higher voltage fuse will open enough to break a lower volltage.
because its NOT rated to break DC currents.
DC is harder to break as it does not cross zero,etc...

In general a dual rated fuse will be rated to 1/2 the AC voltage rating.
Ex.. 600VAC/300VDC

Now will it work.. Probably
But is it "proper" NO its not because its not rated at all for DC no matter what the voltage.
 

Thread Starter

foolios

Joined Feb 4, 2009
163
You don't need a fuse between controller and the panel. The panel won't produce much more than its rated current even if the controller shorts, so the fuse is completely useless.

As to the switches, most AC switches are also rated up to 24V DC. Or you can buy a dedicated DC switch, e.g. http://www.bluesea.com/
Ok, I have that switch. I'll put that switch between the panels and the charge controller.

Do I need to position fuses between the controller and the batteries instead?
Or where would you put the fuses?
 

alfacliff

Joined Dec 13, 2013
2,458
so #12 wire is rated differnt for ac than dc? not in any of my refferences. a fuse is just a peice of metal calibrated to melt in two when a certain current flows through it. an agc3 fuse works just as good on 12 volts as 120. ac or dc. the heat generated to melt the fuse element dosnt change during zero crossing, if it did, fuses would hum. fuse elements respond to average current through them, just like bolometers.
 

Thread Starter

foolios

Joined Feb 4, 2009
163
so #12 wire is rated differnt for ac than dc? not in any of my refferences. a fuse is just a peice of metal calibrated to melt in two when a certain current flows through it. an agc3 fuse works just as good on 12 volts as 120. ac or dc. the heat generated to melt the fuse element dosnt change during zero crossing, if it did, fuses would hum. fuse elements respond to average current through them, just like bolometers.
Most charts I've seen for AC circuits state that I can use 10AWG wire up to 25-30 amps.
But this chart for 12V DC circuits says I need 8AWG for 25-30 amps.
http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html
 

NorthGuy

Joined Jun 28, 2014
611
Do I need to position fuses between the controller and the batteries instead?
Absolutely. If the controller fails short it'll destroy the battery very quickly. Fuse or DC breaker will protect against this. Breaker might be better because it replaces both the fuse and the disconnect.
 

alfacliff

Joined Dec 13, 2013
2,458
foolios, that chart is for lengths of wire and size of wire. since fuses arent 25 feet long, they wont have as much voltage drop at 20 amps as a piece of 10 guage wire has. and the chart is for completely different subjece, voltage drop per length of wire. of course 12 volts cant take as much drop as 120, but thats the same for ac as dc.
 

Thread Starter

foolios

Joined Feb 4, 2009
163
Absolutely. If the controller fails short it'll destroy the battery very quickly. Fuse or DC breaker will protect against this. Breaker might be better because it replaces both the fuse and the disconnect.
Ok, that's what I was wanting to do, have a switch to turn off the panels before I unhook a battery so as to avoid damaging the charge controller. I was thinking a fused switch would be even better.

I'm kinda confused now with options. But what I really need to know is if that switch with old style glass fuse is ok for using on this solar circuit. I'm gonna reread the posts. I got off on a tangent about wire thickness and amperage and that's not my concern right now.
I just want to know if that switch will work. I do have the other switch but I like the idea of the fuses built into the one I posted about.
 

mcgyvr

Joined Oct 15, 2009
5,394
so #12 wire is rated differnt for ac than dc? not in any of my refferences. a fuse is just a peice of metal calibrated to melt in two when a certain current flows through it. an agc3 fuse works just as good on 12 volts as 120. ac or dc. the heat generated to melt the fuse element dosnt change during zero crossing, if it did, fuses would hum. fuse elements respond to average current through them, just like bolometers.
Well that depends on the freq of the AC and if the skin effect comes into play
But in general the industry uses the same tables for AC (50/60hz) and DC as far as wire ampacity is concerned.

BUT regardless..
I'm from the professional world which is dictated by standards/ratings/approvals.
If I went into UL with a product intended for DC operation and protected by an AC rated fuse it would immediately be rejected.
Hence why its not "proper"..

DC fuse and circuit breakers are typically also designed differently from AC devices. With AC there is essentially no arc on each half cycle.
In general the DC rated circuit breakers are larger as more arc suppression (blow out coils,etc..)/contact material,etc... is built into them again because its harder to interrupt DC current flow than AC.


Out of curiosity.. You did/don't work for Nasa do you.. You sound just like a guy I talked to over there once that said he couldn't use this fuse we provided him with because it said 300VDC on it and he was only using it in a 12VDC circuit.. After my explanation as to the what/why he said "Thats not how it works in my book" and hung up the phone on me.
 
Last edited:
so #12 wire is rated differnt for ac than dc? not in any of my refferences. a fuse is just a peice of metal calibrated to melt in two when a certain current flows through it. an agc3 fuse works just as good on 12 volts as 120. ac or dc. the heat generated to melt the fuse element dosnt change during zero crossing, if it did, fuses would hum. fuse elements respond to average current through them, just like bolometers.
It is rated different for an reason. Could be politics.

Either way who the hell are you to argue?

You need to learn how to just follow instructions.
 

mcgyvr

Joined Oct 15, 2009
5,394
Ok, I am seeing the difference after watching this video:
https://www.youtube.com/watch?v=Zez2r1RPpWY

Is it possible a glass fuse rated for DC exists for this switch?
http://www.menards.com/main/electrical/rough-electrical/load-centers/safety-switches-disconnects/30-amp-plug-fuse-safety-switch/p-1773843-c-6435.htm
I would be so grateful if there was one. I bought the switch thinking it was gonna work.
I have not seen screw in DC rated fuses.. I'll check my bussmann book though.
(edit.. nope... none of the screw in plug fuses carry dc ratings)
 
Out of curiosity.. You did/don't work for Nasa do you.. You sound just like a guy I talked to over there once that said he couldn't use this fuse we provided him with because it said 300VDC on it and he was only using it in a 12VDC circuit.. After my explanation as to the what/why he said "Thats not how it works in my book" and hung up the phone on me.
He works for Nasa redefining all the rules based on opinion. God help us.
 
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