Upgrading system - more panels - fusing

westbranch · July 2008

I am about to add 2 more 120 w Mitsubishi panels to the current 2 that I have and also am changing out the BluseSky CC for an MX 60.

My questions are about the Fuse / CB needed at the battery.

The panel specs call for a minimum 15 A fuse (per panel?)

My BS2000e manual stated a 30 a fuse was needed as close as possible to the battery.

the MX 60 manual states a fuse is needed but no recommendation as to size... however the wiring diagram shows an 80 A max breaker (in a 24 v susstem)

Am I safe in leaving the 30 amp fuse in place with 4 panels (Isc =4 x 7.36 A = 29.44 A) it seems a bit close to the edge, premature blown fuse? should I be using a 60 a fuse (4 x MIts. 15 a recomm.)? or should I be at 60 A x 1.25 = 75 a ???

Also, am I correct in assuming that this fuse does double duty: it protects the battery from over charging and also it protects the downstream applications from a short circuit thus preventing a fire.

thanks for any comments.

Eric

mike95490 · July 2008

Re: Upgrading system - more panels - fusing

westbranch wrote: »

Also, am I correct in assuming that this fuse does double duty: it protects the battery from over charging and also it protects the downstream applications from a short circuit thus preventing a fire.

The fuse only protects from over-current, either going in or out. It does not regulate the charge of the battery.

The fuse needs to be sized to protect the wires, too small of wire, and they burn before the fuse can blow. There are charts/tables to properly size wires and fuses to your loads

BB. · July 2008

Re: Upgrading system - more panels - fusing

And the reason for multiple fuses is to protect multiple sources of energy...

For example, the fuse/breaker near the battery protects things that may short from getting too much current from the battery...

With the solar panels, you have several issues... One is to protect the panels+wiring from shorts in the controller being powered from the battery...

The other protection is used when you have, typically, more than two parallel strings of solar panels...

The panels themselves can short. But if there is only one or two strings, there is not enough current/energy present to overheat the wiring or panels and cause a fire... However, when you have three or more parallel strings (does not matter how many series panels there are), if one panel/wire run shorts, then the other 2+ strings can output all of their current into the one shorted panel harness/or internal panel short--and cause a fire.

So, you should find a "series protection fuse" rating for your panels. And when you have more than two parallel strings--each single string should have 1 fuse/breaker of the rating, in the + or - panel lead--to protect that lead from internal shorts.

-Bill

niel · July 2008

Re: Upgrading system - more panels - fusing

a 30amp fuse at the battery will be too low for 4 120w pvs no matter how you wire them because the controller will be outputting at 12v. assuming about 7amps each pv and add mppt action could boost the current over 30amps without much effort and blow the current 30amp fuse. you are correct to go to a 60amp fuse or circuit breaker between the battery and controller.
pv to controller fuses bb has covered and you could wire the pvs all in series making it 1 string or go with 2 strings of 2. this avoids a needed upgrade to wiring too as the current is staying low.

BB. · July 2008

Re: Upgrading system - more panels - fusing

Sizing fuses/breakers becomes quite an interesting ordeal...

You have a "current" that you want to move from A to B. The wire that moves the current is, generally, only allowed to operate up to 80% of capacity... Meaning that if you want to move 30 amps, you need to size the wire to 30a/0.80=37.5 amp rated wire... So, then you need approximately the next even size of wire up to move 40 amps and a 40 amp fuse/breaker.

This also allows some "headroom" for running breakers and fuses--generally, they rated to "break" around the amperage listed... So, you don't want a 29 amp current running through a 30 amp fuse/breaker--it will probably trip in the very near future.

And, if you put the wire in conduit--then you have heating effects from other wires in the conduit, ambient temperatures, types of insulation, etc....

And, you may have ~30 amps of current at 12 volts--but if you system is 24 volts, then the current will be roughly 1/2 or 15 amps...

You can really pull your hair out trying to get this stuff right and safe--not always the same thing.

-Bill

westbranch · July 2008

Re: Upgrading system - more panels - fusing

thanks guys, you confirmed my suspicions.Not at home right now so will try to verbally describe what I envision the end install to look like..

due to losses due to converting 24 to 12 v I hope to stay with 12 v. wiring changes should not be an issue as I 'over wired' to start with. short run from panels to CC.

I plan on a combiner block, + lead to be fused before combining, so each panel will be protected from back feeding. 12 v ~30 A Isc

Should the - also be fused?

the combined line will be thru a single throw fuse box (~ 40 a) then to the MX 60. The + & - are a very short run, will have to be resized. will use the 'sizing table sticky'. MX handles a max of 2 ga

Pos to battery. In line fuse at least 40 A

+ & - Power lines then go thru another single throw fuse box (MASTER POWER SWITCH) to supply domestic uses. All circuits 12 v <5 A and each on a separate fuse.

This is a safety 'ALL POWER OFF' that the wife wanted installed in case some one broke in and left lights on and killed the battery.

Inverter to be added later.

I think that is everthing, hope this clarifies what i am thinking about.

Eric

BB. · July 2008

Re: Upgrading system - more panels - fusing

If you are ground referencing the negative lead (which you probably are), then you should not put any fuses/breakers in the return lead...

Think of it this way--when you "trip a breaker/fuse", you want the circuit to be dead to anything it may touch...

If you put a fuse in the negative lead--you could pop a fuse, and the + side would be still hot with respect to your safety grounds... So, you could still have further "accidents" with shorting to earth ground (or other - leads).

-Bill

niel · July 2008

Re: Upgrading system - more panels - fusing

to be clear eric, you could put a 40a fuse or 30a fuse or even 10a if you like. i am one who believes you can fuse with a lower current value as long as the desired current doesn't trip the fuse (30a and 10a fuses would trip in your case) and you are still protecting the wires from a short circuit from the battery. take for example the spec they put onto your 120w pvs of 7.36a isc. now they also spec 15a for the fuse and odds are that pv will never output 10a, but they go that extra a anyway. i forgot exactly what the nec says, but if memory serves correctly i believe jim mentioned they do that 25% twice and then round up to the next value available. in any case that fuse value should be the largest value allowed for that said pv and not necessarilly the mandatory or only value for i see nothing wrong with putting a 10a fuse in there if one wants to.

westbranch · July 2008

Re: Upgrading system - more panels - fusing

Hi guys, I have just been investigating inverters and fusing. A bit confused, pardon the pun...

In the MX 60 manual there is a wiring diagram and it shows a Circuit breaker in the + lead to the inverter, value unspecified...

In the Exeltech manual for the 600w model it shows direct connect for the inverter.

To be safe a CB would be good, but of what value?
In a 12 v system, by my calcs 600w would need a 62.6 A fuse ( 600 / 12 * 1.25) minimum, =~70 a (next upward size).

comments?

Eric

Upgrading system - more panels - fusing

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