DC circuit breaker

Nabiru3 · July 2018

I have another question, I have 2 (300w panels producing around 10A) connected together in series. I am really confused about the size of the circuit breaker I should use between the panels and my mppt charge controller.
It says on the back of each panel that the maximum series fuse rating is 20A, so should I use a 20A DC circuit breaker ?.
Thank you guys alot you have been very helpful to me

mcgivor · July 2018

The rating of the breaker would be to protect the conductors, if the maximum specifications call for a 20A but the panels only produce ~8.5A then it's OK to use a breaker of lesser value, say 10 or 16A, common outside the US or ,15A common in the US, but never exceed the recommended maximum. The rationale is if something were to happen with the controller, there is a possibility battery current, which is of extremely high potential could back feed into the panels, putting the conductors at risk. This brings into question the polarity, some breakers are polarized, the positive side should be connected to the source of highest potential, the battery, it is counterintuitive because the source is the array but the batteries out trump the panels every time. Keep asking questions, there are many answers, mine may not always be correct, which is the purpose of discussion, we often correct one another.

BB. · July 2018

Yes. That is correct.

However, as long as the wire size is at least 12 AWG from array to charge controller, you do not need any breaker at all (only need breaker(s) with 3 or more parallel strings, one breaker per string).

Many people still choose to put in breakers so that they have an easy on/off switch for when they service the charge controller system.

And you do need a breaker on the output of the charge controller to battery + cable connection (breaker should be near the battery bank, the source of "high current" if there is a short circuit).

Also, when wiring up the solar charge controller, connect (turn on) the battey connection first, then the solar panel connection(s). The charge controller takes its power from the battery connection and this make sure the controller boots up correctly.

-Bill

Dave Angelini · July 2018

I would say that the rating on the back of the panel is for a string of panels and required when more than 2 strings are used. The battery current is already protected by using the circuit protection that the charge controller specifies on its battery connection.

mcgivor · July 2018

Why would two or three strings in parallel need overcurrent protection as opposed to one, is this because the breaker/fuse applied to the output of the charge controller would protect the conductors of two or three strings in parallel in the event of a fail short condition of the controller? If this were the case, the assumption would be that the overcurrent protection downstream of the controller would be to protect the controller, which is not the purpose of overcurrent protection, the purpose is to protect the conductors.

My apologies to @Nabiru3 for the sidetrack, but useful information will be gained, there is no argument here, just discussion, from which we may all learn something.

Dave Angelini · July 2018

The reason we circuit protect more than 2 strings is so you do not start a wildfire near my house from a defective solar panel.

I know in Thailand that the odds are low but.....

Dave Angelini · July 2018

I do not use any protection for this as I always use high voltage and never more than 2 strings to get around 6 KW of solar.
Just a 3 dollar junction box for the 2 strings and a knock-out for an SPD or 2. A disconnect outside the structure is required in most cases and is good to have.

mcgivor · July 2018

Actually I was suggesting even one string should be protected, even less chance of a wildfire, which is a threat during the dry season, I back burn a perimeter of 50 meters or more when the wind is low, however I digress.

Dave Angelini · July 2018

There is not enough energy in less than 3 strings to cause this issue. It is NEC code and was tested at Sandia Labs by the solar code guru John Wiles back in the 1980's. Pretty old code.

About 10% of my business is with electricians and so I get how hard it is to know all of this stuff.

I would love to be able to back-burn. We use Echo line trimmers out 200 feet around the house. PITA !

BB. · July 2018

Isc~11 amps for your panels... 2x 11a=22a (two strings feeding into shorted string #3) would exceed the 20 amp maximum breaker/fusing for current through a single panel

It is a close call for 3 parallel strings (~22 amps Isc vs 20 amp fusing)--But that is how most solar panels Isc/Ifuse work out. It is to stop other panels from overheating/causing fire in a single shorted panel/string.

-Bill

blue · July 2018

I have a 60 amp mppt controller running 6awg wire 8ft. with a 60 amp breaker to the battery. It keeps tripping. I'm at 24 volts with 8_ 12 volt batteries and 1500 watts of panels. It ran perfect for 6 months. Now I have to throw a regular size towel over a panel to keep it from tripping. The controller is supposed to handle 1600 watts at 24 volts. What can I do to keep it from tripping without a towel?

blue · July 2018

Also wanted to add I tried a new 60 amp breaker with the same result. I have never seen the panels put out over 45 amps.

westbranch · July 2018

tell us your location, is it cold there right now? Cold temps will increase the PV output.

blue · July 2018

Central Florida

blue · July 2018

90 degrees

blue · July 2018

Would it help to go to 4awg wire? Can I go to 80 amp breaker? It is what the controller is sending to the batteries. I don't believe I am going up to 60 amp from the Controller. The panels don't come close to that. I checked ground. It is 8foot pole and tight. All fittings are tight. Just out of the blue starts tripping. A towel over a panel and a half fixes the problem.

Estragon · July 2018

Is it a DC rated breaker? If so, what voltage rating?

Is it polarized (with one terminal marked '+' or 'line')? If so, which side (battery or controller) is connected to the breaker terminal so marked?

Estragon · July 2018

Also, if I'm reading it right, you have 4 x24v strings of batteries in parallel. Do you have fuses on each string?

BB. · July 2018

Just to be clear. 45 amps from array to controller or 45 amps from controller to battery maximum current.

What is the battery bank voltage range when charging?

It is possible for an mppt controller and 1,500 Watt array to output 60 amps into a battery bank in cool weather, edge of cloud effects, and a battery bank under 24 volts when charging.

Under typical peak current conditions, 41-45 amps into the battery bank does sound correct.

Bill

mike95490 · July 2018

Some breakers are thermal activated, and some are magnetic. Thermal breakers can "cook off" and trip with a long steady load.

Try the magnetic DC solar purpose breakers from Midnight Solar.

panel mt & DIN rail:

http://www.midnitesolar.com/productPhoto.php?product_ID=197&productCatName=Breakers&productCat_ID=16&sortOrder=13&act=p

http://www.midnitesolar.com/productPhoto.php?product_ID=183&productCatName=Breakers&productCat_ID=16&sortOrder=18&act=p

#6 wire is marginal for 60A, you will have some voltage drop across it.

#4 wire is ok for 70A

You can use larger wire and still use a 60A breaker, but if you use a larger breaker, you must use wire rated for the breaker amps

mcgivor · July 2018

Thermal circuit breakers will heat up under load, especially when operating near their rating, poor air circulation, mounting sideways, high ambient temperatures along with the quality of the breaker itself may cause premature tipping. Can you elaborate on the make, model, trip curve or better yet a link to a website, here is some information regarding this http://bdbreakers.com/breakertypes.php

blue · July 2018

Thanks so much for your responses. It so nice to find such a great forum. I have ground based panels, have to move soon. Both breakers are Asdomo 12-24 dc fuse inverter 60 amp. The first one had lugs to connect the second one is inline, just stick in the wire. I noticed it said 1-50amp although it is a 60amp breaker. I cant find if it is thermal or magnetic. My highest voltage on the battery side reached 31.75. I remember seeing the panels up to 1300 watts. I did not keep records on the amps. I have epsolar Et6415bnd 60amp mppt controller. I have the wireless remote reader.

blue · July 2018

I have 4 x24v strings of batteries in parallel. I do not have fuses. I have not heard that is necessary. The breaker does not have a plus or minus on it. My panels are at 25 degrees facing south. Clear sun from 10 to 4.

blue · July 2018

For the midnight breaker I would have to buy the box too and they look way too expensive.

blue · July 2018

I read that I should have a 60 amp breaker between the 60 mppt controller and the batteries. Why does the controller determine the size of the breaker? It would seem like the breaker would protect the batteries and they should determine the size of the breaker. Should I go to 4awg wire and a 70 amp breaker?

mcgivor · July 2018

The Asdomo circuit breakers are automotive type components, not the best choice for the application as they tend not to handle continuous high loads very well. Over current protection OCP is particularly important when batteries are involved, when multiple strings of batteries are used, it is advisable to put protection for each string, to protect against the other strings feeding into a string which could develop an internal short. The controller doesn't determine the breaker size, the conductors do, a 60A can be used with a larger gauge, just because a larger conductor is used doesn't mean a larger breaker is required as it will still be protected, the larger conductors will help reduce voltage drop. Do you have OCP from the batteries to the inverter? Circuit breakers may seem expensive but liability will be considerably more expensive in the event of a fire, more so if lives are at stake, do it the correct way and be safe.

To assist in the design, it's best to describe your system in detail, or better yet provide a drawing, fragments of information lead to confusion as there is no continuity.

BB. · July 2018

I like to suggest that NEC derating of 80% for continuous current circuits (thinks that can run for an hour or more at rated current). Battery chargers can run for upwards of 5 hours (or more) when you have a deeply discharged battery bank. So, if you have a 60 amp capable charge controller (and solar array), then the suggested branch circuit wiring+breaker rating would be:

60 amps * 1/0.80 = 75 amps ~ 80 amp branch circuit rating

Also, with MPPT controllers, the battery bank voltage also changes the amount of current from the MPPT controller... For example:

1,500 Watts * 1.0 derating (near perfect conditions) * 1/29.0 volts charging = 51.7 amps (battery ~80-90% state of charge)
1,500 Watts * 1.0 derating (near perfect conditions) * 1/24.0 volts charging = 62.5 amps (battery less than ~50% state of charge)

A 60 amp MPPT controller will limit its output to ~60 amps maximum (design of charge controller).

-Bill

Estragon · July 2018

It might be worth checking the voltages of each battery individually while charging to see if any are much higher or lower than others.

As you may already know, the controller should be booted fully with battery power only before it sees any pv voltage. Some controllers can boot up improperly if booted with pv voltage first. This likely isn't the source of the problem, but if the breaker on the battery circuit is opening, it could be an added problem.

mike95490 · July 2018

If the midnight breakers are too expensive ($20 ea) and having to buy a box for them (I'd linked to panel mount and DIN rail mount) is too much, then just keep on resetting the breakers as needed.

Generally, the more often a breaker "Trips" it wears and trips at lower and lower thresholds.

Constant DC charging loads are quite hard on cables and breakers.

BB. · July 2018

You need to fix... Eventually something will fail from the breakers tripping (basically surge current, high voltage spikes, arcing of breaker/switch contacts, electronics that fail from surges, etc.).

-Bill

blue · July 2018

I have a absolute anl 60amp fuse. I am going to put that in with a dc shutoff and check the results. I already have one. I will get back with the results. I did check batteries with a hydrometer. They are all fairly close in the good range.I bought them 7 months ago. I will look into fusing them. I need to read more about it. Thanks. I only have a shutoff between my batteries and 2500 watt pure sinewave inverter. I guess that is a problem. Any Idea what amp fuse I should have there? Batteries are 12v 114ah. What fuse should I have between each battery pair? I did know about booting with battery first. My worry was when battery breaker trips and solar stays on. No problems so far the controller just goes blank. I shut it off then reboot battery then turn on solar. My batteries, controller and inverter are all in a small room I built away from the house. Thanks so much for your help.

DC circuit breaker

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