Basic advise on fuses

raymate · June 2015

Hi,

New here.

Just setting up a basic system for my shed, nothing fancy 2 x 25w 12v panels going into a morning star charge controller. Going out to a very small sealed lead acid battery 9-18 amp (not got that yet) then my load will be LED lights and a 5v USB charger.

My question is about fuses, I would assume fast blow glass fuses are best but again I'm not sure. I plan on using 10 awg cable and was going to put in 3 or 5 amp inline fuses on every positive cable in the system.

Do the fuses needed to be rated for 12v system or can I use any 3 amp fast fuse, I seen people use car blade fuses are they OK, should I use a circuit breaker in a system this small.

Many thanks

mike95490 · June 2015

car blade fuses are rated for 32V I think, but on a 12V system, they should be OK. 10 ga wire can use a 30A fuse, if the wire is 90C rated.

BB. · June 2015

Normally, you fuse/breaker to protect the wiring... 10 AWG wiring can easily carry ~30 amps (using NEC tables).

If you have a 12 volt 9-18 AH battery--Realistically, you would expect it to support around 1-2 amps of continuous load (~8 hour discharge rate) maximum--Unless you need high amounts of current for short periods of time (like running a small pump, electric drill, etc.).

It is very good to use heavy gauge copper wiring, espeicailly if you are going to be sending 12 volts (low voltage) DC power around. On average, you can have about 0.5 volts of drop in your DC wiring and still have things work relatively well. Sending current longer distance at 12 volts becomes difficult (need very heavy copper cable to keep voltage drop low).

For a small system you are looking at--Glass fuses or even automotive fuses can work fine (I am not real thrilled with automotive fuse holders, they are not really built well and have been known to over heat even when operated well below rated current).

You may want to go with circuit breakers--Many can be use as an on/off switch--And save you the need of wiring in another switch.

A lot of times, you can find surplus breakers in local surplus electrical/electronic stores or even online:

http://www.electronicsurplus.com/circuit-protection/circuit-breaker

But, with your smallish battery--A single fuse and some on/off switches is probably good enough for your needs.

-Bill

raymate · June 2015

thank you for all the replies, some great info.

Still not fully understanding the fuses, I will probably use some fast blow glass type fuses either 3 amp or 5 amp, only as I have seen some nice inline holders for them. That will be plenty for my application.

But I can only find ones rated at 250v does that matter or are they not going to work.

Forgot to say my charge controller is a 15amp ProStar so I would think it will shut things down before my 10 wag gets into a problem.

I'm also using the load out feature on the ProStar I'm not running any 12v accessories direct from the battery.

Should I ground this system, is it as easy as putting a long copper rod into the ground?

Brlux · June 2015

Perhaps try something like this.

http://www.amazon.com/OLS-PSZACCEPS051H-6-Way-Illuminated-Blade/dp/B00QMTAZ1W/ref=pd_sim_263_6?ie=UTF8&refRID=1JDXW3BKMGQYHVA1PCET

BB. · June 2015

Fuses have all sorts of ratings, Maximum interrupt voltage, AC or DC, Maximum Amps Interrupted, trip time, etc.The basic requirement is that the fuse is rated over your working voltage (in this case DC).

AC current is "easier to break" for a fuse/breaker, and the AC working voltages/interrupt current is usually higher than the DC ratings.

Your 50 Watts of solar panels have a working current of (roughly):

50 watts / 17.5 volts Vmp = 2.85 amps

The Isc is probably around:

2.85 amps Imp * ~1.25 = 3.57 amps Isc (short circuit current)

And the minimum fuse/wiring rating should be (per NEC):

1.25 * 3.57 Amps Isc = 4.64 Amps rating or ~5 Amps fuse/breaker/branch circuit wiring minimum for your 50 Watt panels.

Your 10 AWG wiring is good for ~30 amps pretty easily.

Fuses/breaker are not really there to protect your loads, they are there to protect your wiring (not devices)... If you wired everything on the load side with a minimum of 14 AWG copper wire (good for ~15 amp rated current), you really only need the one fuse to the battery bank (and you could use use a 20 amp fuse on your Prostar--if you ever decided to add more solar panels/larger battery bank).

If you have small wiring (such as going to an LED fixture, radio, etc.), then you fuse fuse for the size wire going to that specific load.

-Bill

mike95490 · June 2015

BB. wrote: »

.....Fuses/breaker are not really there to protect your loads, they are there to protect your devices.....

I disagree, solid state devices are used to protect the fuses. Ask anyone of hundreds of users that open up their inverters to replace a "fuse" and the fuses are fine, and all the FET's are blown.

BB. · June 2015

Oops... Intended to type fuses/breakers protect your wiring.

And as Mike typed--The silicon devices pop much faster than any fuse/breaker can. And over voltage failures for silicon are not even protected by breakers--You need surge protectors in front of your loads for that--Which do not always do a great job either.

-Bill

Wheelman55 · June 2015

So in the OP's situation...should the breaker/fuse be put as close to the battery as possible to protect the 10 g wire?

BB. · June 2015

Yes, that is the "standard"--But the fuses/breakers as close as practical to the source of high current (I think I have seen 18 inches as being a marine standard).

The idea is to have as little "unprotected wiring" as possible.

-Bill

Dave Angelini · June 2015

mike95490 wrote: »

I disagree, solid state devices are used to protect the fuses. Ask anyone of hundreds of users that open up their inverters to replace a "fuse" and the fuses are fine, and all the FET's are blown.

I am knocking on wood as I write but I have never had an XW with blown anything. I am guessing about 40 of them and still counting. Agree with your thought though.
I am not counting lightning and even with SPD's and great grounding it is not something you can reason with. Prayer is best.

raymate · June 2015

Thanks for all your replies, I just ordered a midnite baby box and 4 breakers from wind & sun 5,7,10 amps and small blade fuse box from amazon with assorted blade fuses.

Basic advise on fuses

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