# fusing between controller and battery

Great forum! Thanks for having me over. First of many questions: what (and how) is the formula used for determining rating or size of breaker/fuse between controller and batteries, batteries and inverter (not grid tied)

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: fusing between controller and battery

Assuming you use a PV controller that limits output current and a breaker that's rated for continuous duty, the controller-to-battery breaker's ampacity spec can be the same as the controller's rated output current. [Ref: NEC 690.8(B)(2)]

The battery-to-inverter breaker spec is a bit more complicated. Assuming a 1,000 W inverter, a minimum input voltage of 11 V, and 91% inverter efficiency, the maximum current would be 1,000 W / (11 V x 91%) = 100 A, and the design current would be 100 A x 125% = 125 A. Assuming a breaker rated for 100% continuous duty, you'd need a breaker rated at 125 A. [Ref: NEC 690.8(A)(3) and 690.8(B)(1)]

You'll need DC rated breakers for both applications.

HTH,
Jim / crewzer
Re: fusing between controller and battery

The battery-to-inverter breaker spec is a bit more complicated. Assuming a 1,000 W inverter, a minimum input voltage of 11 V, and 91% inverter efficiency, the maximum current would be 1,000 W / (11 V x 91%) = 100 A, and the design current would be 100 A x 125% = 125 A. Assuming a breaker rated for 100% continuous duty, you'd need a breaker rated at 125 A. [Ref: NEC 690.8(A)(3) and 690.8(B)(1)]

Is this between the battery and the inverter? the inverter is a small Statpower with a couple of AC receptacles; I will just plug in a couple of items like a light and small charger for cel & laptop. Thanks
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: fusing between controller and battery

The formula should work for any inverter. What model/size is the Statpower inverter?

Regards,
Jim / crewzer
• Registered Users Posts: 10 ✭✭✭
Re: fusing between controller and battery

Hello, sorry for butting in.
I have been also folowing this thread & have a couple of questions.
My inverter has the following specs-
1500W Continuous
1700W 30 Minutes
2300W 3 Minutes
3000W 5 Seconds
Which figure out of the above do I use for your formula? For the surge of 3000w could I use a slow blow fuse for the worst case scenario?
Thanks - Leigh
Re: fusing between controller and battery

A slow blow fuse it typically intended for motor starting... Anything over 10 seconds is probably beyond a slow-blow application.

So, several things to think about. The fuse/breaker/etc. is intended to prevent fires. So, foremost, you size the fuse/breaker for the size of the wire you are using. You can always use a smaller fuse, but never a larger fuse. For many solar systems with long runs you will have larger gauge wire to reduce voltage drop, but you would still use the 125% rated current of your design for the fuse/breaker.

As a side note, you want to make sure that the wires can carry enough current to blow the fuse. In the case of long runs, it is possible that the voltage drop is enough that even with a dead short, there is not enough current to blow the fuse/breaker (and the wires would simply cook/melt as the short circuit current is not enough to blow the fuse).

Lastly, design for the load(s) you expect. You can run your 1,500 watt inverter for small loads (washer, lights, etc.) and never need the surge current. So fusing based on 1,500 watt output is just fine. However, perhaps you are running a deep well water pump with very large starting loads. There, you would want to size it for 3,000 watts (plus efficiency, plus 125% rated wire and fusing). And, with DC on your inverter, you would want to rate for 3kWatt (heavy, short wires) because, unlike the typical AC motor which can withstand a voltage sag of 10-20% (120 VAC would be a 12-24 volt drop, 240 VAC would be twice that) while starting with 4x+ load--your inverter cannot. If, for example, you had a 24 volt battery bus for your inverter, if the voltage sagged more than ~15% or so--at 24 VDC, that is only a 3-4 VDC drop--not the 12-24 volt drop that an AC system could withstand (the inverter will simply shut down when the voltage drops much below 21 VDC).

Does that sort of help?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: fusing between controller and battery

Apologize for delay in reply.Difficult weekend with wildfires.
The panel is SX20U, SS-10L-12V Sunsaver controller & Statpower portawattz 600 inverter. Morningstar recommends maximum wire size of #10 wire which I have; Statpower recommends wire size larger than #6, I have some #2. For a battery I will probably buy a 42AH gelcel. Wire runs will be very short since it will all go into a carrybox. This will be for a light and wall warts to charge radio, celphone, laptop etc. no motors. Thanks for the good practical information
Re: fusing between controller and battery

OCCAM,

Is this a single 20 watt solar panel? At full sun, you would not expect more than 2 amps or so at 12 VDC. So, unless you have a long run of wire from panel to battery-- 10 awg wire is way overkill.

Also, how much power are you expecting to use from your system... Assuming a good summertime average of 5 hours (fires=LA?), that is:

5hours * 20 watts * 80% lead acid battery eff * 80% inverter eff. = 64 watt*hours per day (or running your 600 watt inverter at full power for about 6 minutes per day).

For your battery, if you assume that it is 12 VDC and your only run it down to 50% capacity (to prolong battery life):

42AH *12 VDC * 50% * 80% inverter eff = 201 watt*hours (running 600 watt inverter for 20 minutes per charge).

To recharge the battery after 50% discharge:

42AH * 12 VDC * 50% / 20 watts * 5H * 80% = 4 days with solar panel (best case--could take longer)

Not saying that your system will not work or is not a good idea--Just trying to make sure you understand how much power is available from your system for use. And the above calculations are pretty optimistic--Solar panels putout less power as they get hot, drawing 50-60 amps for 20 minutes straight from a 42AH battery is not going to happen, etc...

So, if your expectations are to run a small laptop for a couple hours per day and maybe a small CFL lamp and/or recharging a cell phone--probably will work fine. If you are expecting more--maybe not.

The wire gauges are fine for larger systems--but seem to be larger than needed for powering a small standby power source (for example, planning on not taking anymore than 60 watts continuous from your system is probably more realistic). Now you are talking about 5 amps maximum instead of 50-60 amps.

And, with solar, you should look at conservation--is a laptop/cell phone 12vdc adapter more efficient than using a 600 watt (modified?) sinewave inverter plus wallwarts/powerbricks which down converter back to low voltage DC.

Is this helping?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: fusing between controller and battery

Occam,

You might want to check the inverter's manual for a fuse recommendation. Xantrex acquired Statpower several years ago, and they now market the ProWatt 600. The recommended maximum fuse for that inverter is an 80 A model. See: http://www.xantrex.com/web/id/415/docserve.asp for specifics.

This sounds about right to me: 125% x (600 W / 90% peak efficiency) / 10 V minimum = 83 A.

If, however, you don't expect you load to exceed, say, 300 W, then you might want to consider a 40 A fuse (or DC rated breaker).

HTH,
Jim / crewzer

P.S. And, make sure you pay attention to Bill's excellent analysis and advice.
Re: fusing between controller and battery
crewzer wrote:
P.S. And, make sure you pay attention to Bill's excellent analysis and advice.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: fusing between controller and battery

Can this be right?

From BB's post; "As a side note, you want to make sure that the wires can carry enough current to blow the fuse. In the case of long runs, it is possible that the voltage drop is enough that even with a dead short, there is not enough current to blow the fuse/breaker (and the wires would simply cook/melt as the short circuit current is not enough to blow the fuse)."

If there is not enough energy to blow the fuse how would it be possible that there is enough energy to melt the insulation on the wire?
• Solar Expert Posts: 8,657 ✭✭✭✭✭
Re: fusing between controller and battery

Consider your circuit, where the wires have some fixed resistance. Remove the loads, short the wires, you get 130 amps flowing. you have a 150 A fuse. 130A @ 12V = 1500W Those wires are going convert that 1500W into heat, like a large room heater. They are covered with plastic insulation. The heat can't escape. the current still flows, and the plastic melts and chars, the wood frame members the wires run thru (studs, whatever) heat and char. Fire. 130 amps, and you had a 150A fuse. Uh Oh.

Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

Re: fusing between controller and battery

TC,

Yes, this is right... But the problem depends on the energy source too. Shorting solar panels at the end of a long run of, properly designed, wires is not going to be a problem because the solar panels hardly generate any more current when shorted.

However, say you over-sized the wiring because of losses and put the solar controller out with the panels. And, you fused for the size of the wire instead of that solar controller. Now when there is a short near the controller, the batteries can supply 10's of thousands of amps into the wire--but the wire's resistance prevents high current flow and does not immediately blow the fuse.

This not normally a big issue--but it sometimes catches people unaware when they install the safety ground wire. Code normally does allow you to use smaller gauge ground wires than the main run (smaller gauge wire without insulation will not heat too badly in the presence of a momentary short). However if the wire run (or ground run) is too long, the ground wire may not be able to supply a 'good enough" short to trip the breaker or fuse.

When designing power systems with multiple power sources (panels and batteries) long wire runs and very high current sources (batteries), you want to be very careful with safety.

For a couple of examples;

People run small gauge current shunt sense wires to remote meters/devices. These wires can be very small gauge because they don't carry any current to speak of--but they are connected to a battery bus with 10's of thousands of amps available. Shunt wires should have either a resistor or fuse in each lead at the shunt connection to protect against shorts anywhere else downstream of the shunt.

DC current and batteries are very difficult to protect against high current (see arc welders). DC current wants to keep arcing (even at 12 VDC) and batteries (even 1 or 2 car batteries) can supply enough current to weld the contacts on the standard home style AC circuit breaker. Using the right kind of breaker/fuse is very important too.

-Bill

PS: Jim, that was an unintended post from me when I quoted you--I was going to suggest you talk to my wife and tell her that my advice is sometimes OK--but I (thought) I chickened out during the post... :-P

PPS: Also, remember that the wires may not be strung in open air--but instead may be buried (earth is a relatively poor conductor of heat), may be packed in conduit with lots of other wire under load, and/or may be in a wall or ceiling stuffed with insulation--all preventing that excess heat from escaping.
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Re: fusing between controller and battery

Great discussion and food for thought. The bits and pieces that I now have were acquired, some scrounge, the wire free, from construction dumpster. I see your points pretty clearly BB, the little panel really would only need a very small battery, 42ah is overkill. I had considered charging the battery up from a charger first, then using the panel to recharge it slowly after some use. I'll get a large panel later as I can afford it. Crewzer, I found the spec sheets for the controller and the inverter. The controller specs rated PV input 10 amps;the inverter manual is very informative and suggests a BUSS ANL-80 fuse (you were right, thanks). The BUSS ANL-80 and fuseblock 4164 are pricey...
I have available a SquareD QO24L70RBCP 70A. Can I use a 10A breaker for the panel? I'll use a 40A or maybe 70A breaker for the inverter (I'll never need to get the max out of the battery) The items that I'll run will be a compact fluorescent, weather radio, a couple of wall warts etc. very little juice.
To sum up my doubts or questions at this point in the discussion: with the SquareD breakers, I know that they are primarily for AC and suitable for DC, but, must they be derated for DC application? or is the stated rating the same whether AC or DC? as far as wire size, I've read the lively discussion on the RV wiring viz wire size. If cost and handling (wirepath) are no object, then is it best practice to go for largest guage possible? Is a solid conductor preferrable to twisted or braid?
PS (fires=F LA)
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: fusing between controller and battery

Occam,

The QO breaker is primarily an AC breaker, but it's also rated for 40 ADC 48 VDC. That info is buried in the Square D literature. I don't know the DC AIR (amps interrupt capcity) for those breakers. I seem to recall that the NEC minimum is 20,000 A.

You're probably OK using the 70 A breaker, but it's ultimately your decision. FWIW, I used a 60 A QO breaker with my old 600 W inverter for several years with no problems... however, I never ran the inverter at full rated load.

I seem to recall that I used #2 wire (19 starnd UL-listed from Home Depot) from the batteries to my inverter, which was ~6 feet each way.

HTH,
Jim / crewzer
Re: fusing between controller and battery

Thanks Crewzer! this real world info is very useful. I am really interested in the best placement and size etc. of the fuses or breakers and grounding for solar as I learn. I have already spent a great deal of my time replacing expensive burnt out amps,power supplies, cable meltdowns etc. We are a lightning,surge, and other electrical phenomena hotspot here in fiery Fl.
• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: fusing between controller and battery
PS: Jim, that was an unintended post from me when I quoted you--I was going to suggest you talk to my wife and tell her that my advice is sometimes OK--but I (thought) I chickened out during the post...
Bill,

ROTFLMAO!!

8-)
Jim / crewzer

• Solar Expert Posts: 10,300 ✭✭✭✭
Re: fusing between controller and battery

bill,
your wife doesn't seem to appreciate or listen to you very much then as i think you have been giving lots of good advice. if she doesn't listen to you then why should or would she listen to us? don't feel bad as my wife doesn't listen to me either, but she's quick to point a finger at me when if she'd have listened it would have turned out better than the way it did, yet i'm blamed anyway. :roll:
Re: fusing between controller and battery

After considering all of this good advice I reviewed the manuals and specs again for detail. I decided not to go with the SquareD breaker which came close (but not quite) to manufacturer suggested rating. I looked around and "discovered" the "Blue Sea Systems" range of goodies. I ordered up ANL block & fuse and ATO fuseholder and fuse. I am really impressed with quality and price. They manufacture some other very interesting 12V components. Comments by other users?
• Solar Expert Posts: 8,657 ✭✭✭✭✭
Re: fusing between controller and battery
occam wrote:
After considering all of this good advice I reviewed the manuals and specs again for detail. I decided not to go with the SquareD breaker which came close (but not quite) to manufacturer suggested rating. I looked around and "discovered" the "Blue Sea Systems" range of goodies. I ordered up ANL block & fuse and ATO fuseholder and fuse. I am really impressed with quality and price. They manufacture some other very interesting 12V components. Comments by other users?
http://bluesea.com/category/5/21/productline/129

The ANL fuse block does not hold the ATO fuse, AFIK.

ATO = 30A, 32V

ANL = 35 -750A 32V
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
Re: fusing between controller and battery
I looked around and "discovered" the "Blue Sea Systems" range of goodies. I ordered up ANL block & fuse and ATO fuseholder and fuse. I am really impressed with quality and price. They manufacture some other very interesting 12V components. Comments by other users?

Occam,

looks to me like you ordered an "ANL block and fuse" and a separate "ATO fuseholder and fuse(s)". While I have no direct experience with these particular items, I've used some other Blue Seas products (example #1 and example #2) and I found them to be well built and reasonably priced. I'd agree that Blue Sea makes useful items for 12 VDC and 24 VDC systems. Considering they're intended for the salt water environment, I'd expect the products to be fairly rugged.

You may want to consider which safety listings are important to you. For example, while these products meet US Coast Guard safety standards, I don't believe they're UL listed.

Good Luck!
Jim / crewzer
Re: fusing between controller and battery

You're right Jim, I did buy both separately. I read the article on the Amish community and their ready acceptance and use of solar power. Evidently many of their homes are already wired for 12vdc and they can easily just "drop in" the solar power components as an upgrade. The article has started me thinking of all the uses of 12vdc and the many low consumption items that are available, interesting. The marine products seem well suited for 12vdc distribution and definitely ruggedized.
• Registered Users, Solar Expert Posts: 58 ✭✭✭✭
Re: fusing between controller and battery

This is all good information to help size the fuse, but somewhat misses the point. The fuse or breakers that are used in these circuits are meant to protect the cables, not the equipment. So if you already know the size wire you are going to use, match the correct breaker to it. That isn't always easy to determine either due to different temp ratings, conduit or not, free air conductor quantities etc. Charge control wires to the breaker are typically very short. No conduit, so the chart to use is single conductor in free air You can use 6AWG wire (THHN) and our 63 amp breaker for any controller up to 60 amps. Even if you have a 10 amp controller, this configuration meets code. Using smaller wire and smaller breakers also works though sometimes it is easier to just go large for future expansion.
The fuses in the inverter and charge controller circuit will not protect the electronics what soever. These electronics are all internally protected. If the breaker trips in a properly designed system, the electronics is usually already broken and is now a direct short. This is when the breaker needs to react and the wire needs to be large enough to carry the current until the breaker trips. Even a shorted 10 amp controller will trip a 63 amp breaker immediately, because it is essentually shorting out the battery bank.
Robin Gudgel
MidNite solar