Circuit breakers and current control

[miles]

Hi to all...
I need to find some circuit breakers for my system but the only ones I can find are thermally operated that is they seem to trip when they get warm not when the circuit goes over current (or stays way under the rated current of the breaker)...

I have 8 panels equating to 1.96 KW (8x245W) with maximum current flow of around 50A at 24 Volts (highest current flow measured to date is 48A)...

These go through a 60A circuit breaker (http://www.ebay.co.uk/itm/3604997338...84.m1439.l2649 ) and then into my charge controller which is a 60A MPPT and then through another 60A breaker same as before on to a 24V 900Ah gel traction battery...

I then have my load connected to the batteries through a similar 40A breaker (load never exceeds 30A).

So what seems to happen is on a really sunny day everything starts fine until the incoming current gets near to the 40A mark at which point the breakers start tripping even though I am 100% positive the Amps are no where near the rated breaker amperage. I think the breakers contain a bimetallic strip so when they get warm they trip, not really a great design but I like the breakers because they allow me to isolate parts of my circuit to perform changes etc but also give protection from shorts.

My question is what alternatives are there that do what I need that aren't over expensive?

[Sunking]

You have done two things wrong.

1. For the breakers between panels and controller the conductor (and breakers) have to be sized to 150% of the rated current since it is considered continuous. Most breakers are deigned to only operate at 80% of their rated capacity continuously.
2. You misconfigured your panels using a MPPT controller. Sounds like you wired them all in parallel 8 x 1. You completely defeated the purpose of a MPPT controller. They should be wired 2 x 4 which doubles the voltage with 1/2 the current.

Simple solution. rewire your panels.

[Naptown]

You have done two things wrong.

1. For the breakers between panels and controller the conductor (and breakers) have to be sized to 150% of the rated current since it is considered continuous. Most breakers are deigned to only operate at 80% of their rated capacity continuously.
2. You misconfigured your panels using a MPPT controller. Sounds like you wired them all in parallel 8 x 1. You completely defeated the purpose of a MPPT controller. They should be wired 2 x 4 which doubles the voltage with 1/2 the current.

Simple solution. rewire your panels.

That breaker is probably not rated for the voltage it would see if wired in series. even at 48V it could be pushing it as they are really designed for a 12V application.

[miles]

You have done two things wrong.

1. For the breakers between panels and controller the conductor (and breakers) have to be sized to 150% of the rated current since it is considered continuous. Most breakers are deigned to only operate at 80% of their rated capacity continuously.
2. You misconfigured your panels using a MPPT controller. Sounds like you wired them all in parallel 8 x 1. You completely defeated the purpose of a MPPT controller. They should be wired 2 x 4 which doubles the voltage with 1/2 the current.

Simple solution. rewire your panels.

Yes I have my panels all in parallel. The OC voltage of my panels is 35V, max voltage into my controller is 55V therefore if I wire my panels 2 in series then 4 parallel this gives me 70V the voltage will be too high for my controller? Also running 24V batteries (have 3 x 24V in parallel) so again 70V coming into controller will this be too high?

Everything seems to work really well, previously I had fixed fuses so if I wanted to disconnect I had to undo nuts and bolts so I figured the circuit breakers were the way to go but then they started to trip at lower than rated currents.....

So you are saying that if I have 48A peak coming in I should have 72A breaker (or nearest I guess 80A) and for my load of 30A I should use 45A?

I think I understand what your saying ref the panels but I have bought the controller & inverters now so if I rewire I would need to replace all this stuff? and also have 3 batteries so cant double up to 48V and no room for battery #4 either!

[Mike90250]

you will have to research your charge controller's max input voltage, but decent ones should be able to handle 70 or so volts.

Re-wiring would reduce your amps to the point where the breaker won't give you false trips, but even at 35V, you are likely beyond the design voltage of the breaker, 70V would be way beyond, and I'm pretty sure it will fail.

With 4 in parallel, you will need a (even with the 8 in parallel) Combiner Box, and that should have 4 breakers, at the size spec'ed on the panels (most panels have a breaker size on their label.)

[miles]

My controller has a label stating 55V max input voltage so doubling up is not the way forward me thinks unless I want bonfire night to come early this year!...

So considering the first reply am I right in saying if you have the correct controller you could run 70V into it and connect 24V battery or am I misunderstanding? Is there a limit to the difference in PV voltage and battery voltage out?

[Sunking]

My controller has a label stating 55V max input voltage so doubling up is not the way forward me thinks unless I want bonfire night to come early this year!...

So considering the first reply am I right in saying if you have the correct controller you could run 70V into it and connect 24V battery or am I misunderstanding? Is there a limit to the difference in PV voltage and battery voltage out?

Ok you messed up from the start getting an inferior charge controller. All the quality MPPT controllers can take up to 150 Voc input and run a 12, 24, or 48 volt battery. The new lines coming out can take up to 600 volts input on 12, 24, 48,....120 volt battery.

There are three reasons to use MPPT.

1. Is the one you missed initially was to use high voltage input to lower current and use smaller less expensive wiring.

[miles]

i wanted to keep cost down as much as possible and some of the controllers are VERY expensive... you are quite correct it is a cheap controller... My setup works ok but of course theres always a better way it just depends on how much you want to spend! I'm not totally dependant on my panels they are just to save a bit on the bills so don't want to spend what ive saved already!

still not decided that i want to make too many changes, the whole point is to save money at the end of the day, if my controller went pop then maybe id rewire as part of the replacement process now that ive proved the system saves me money but the 24 volt system suits me atm. just want to get some trip switches so i can isolate and also have protection without having fuses and switches.

[Sunking]

I'm not totally dependant on my panels they are just to save a bit on the bills so don't want to spend what ive saved already!.

Well you are going to find out in th elong run anything you take off-grid is going to cost you 5 to 10 times more than buying it from the POCO. It is impossible to save any money with off-grid. Over the life of the system at today's cost will cost you 50 to 75 cents per Kwh the rest of your life with constant battery replacement. Where as you can buy it for 8 to 15 cents on the grid.

[inetdog]

i wanted to keep cost down as much as possible and some of the controllers are VERY expensive... you are quite correct it is a cheap controller... My setup works ok but of course theres always a better way it just depends on how much you want to spend! I'm not totally dependant on my panels they are just to save a bit on the bills so don't want to spend what ive saved already!

still not decided that i want to make too many changes, the whole point is to save money at the end of the day, if my controller went pop then maybe id rewire as part of the replacement process now that ive proved the system saves me money but the 24 volt system suits me atm. just want to get some trip switches so i can isolate and also have protection without having fuses and switches.

You are correct that a breaker which is in a hot area and/or is restricted from getting rid of heat (insulated box for example) will trip at a lower current than one at or near room temperature.

Is the breaker box out in the sun, like on the roof? That is a bad idea unless it is shaded and away from the roof surface where the air temperature is higher.

Does the breaker get hot? If it is hot to the touch while operating, you have probably got ventilation problems OR you may not have properly tightened the wire connections or may be using too small a wire. Heat from the connections can raise the breaker temperature causing it to trip at a lower voltage.

Is your breaker rated for DC? If not, that is not by itself a reason for it to trip at a lower current, but at the same time repeated trips under DC load may have damaged the contacts so that they overheat from normal current. You can check this by accurately measuring the voltage across the breaker when it is closed and at full current.

[Sunking]

Eaton makes a full line of magnetic DC breakers in voltages up to 600.

[miles]

the breakers are in my garage which is currently around 16 degrees celcius, yes the breaker is getting warm end even hot to the touch, I am using 4mm square wire, the wire does get warm at max current but definitely not hot maybe I need thicker wire? not sure what the maximum current through this wire should be? Anyone?

My panels bring in a max of 48A @ 24Volts. yes the breaker is designed for DC voltage and the connections are heavy duty and done up tight, maybe I need to put soldered crimps on the ends of the wires then put into the breakers.....

also taking into account previous comments I have a couple of 100A breakers on their way, hopefully this will cure the issue.

[Mike90250]

If your wire is getting warm, it is at least 2 sizes too small. wires should never get warm. They need to be cold, or there is no margin if there is a fault and the wire will burn before the breaker pops

[Sunking]

I am using 4mm square wire, the wire does get warm at max current but definitely not hot maybe I need thicker wire?

Then you have two big problems.

1. 4 mm wire is only rated for 25 amps.
2. If the 4 mm wire is more than 10 feet long you have excessive voltage drop aka power losses.

Low voltage systems are designed to have no more than 2% voltage drop. You wire is not even close to being large enough.

[miles]

Total power coming in form my panels is 50A so I have them in 2 banks of 4 (in parallel), then each bank of 4 is connected to my garage (8 or 9 meters run) by 4mm wire, so therefore each run of wire will see max of 25A...

however when I get into my garage I have combined both wires into 4mm wire (oops) I thin I will double this up too! saying that though the wire only gets a bit warm perhaps 40 degrees or so, not hot at all because the lengths are very short (a few inches to connect things together mostly)...

I think I need to do a re-model and upgrade...

the project was about finding out if it was a viable way of saving money and so far the project cost is around £2500, and since I connected my panels I have saved between £50 & £80 per month! therefore I have calculated the panels etc will be paid for within 3 or 4 years! then free electricity for maybe 15 years thereafter?