Panel to controller fuse sizing

dchristo

Hi and thanks for the Forum.

I’m getting ready to hook up my first “real” panel of any size, and want to make doubly sure I am sizing the fuses between panel, controller, and controller to batteries correctly. I’ve owned this old inverter since it was new, but have previously only charged it with shore power, or a generator. All the main battery cables are #2AWG and the positive cable between the batteries and Heart is fused correctly. I searched your site for fuse sizing info, and to find out what ISC stands for, (now I know it stands for short circuit current). I plan on using #10AWG  blade type automotive fuse holders for these fuses.

Now, I’m adding the panel and controller below to the system. The guide with the panel gives this formula for sizing the fuses for the positive leads from panel to controller, and controller to batteries:

# of panels in parallel (I have only one), X panel ISC (5.75 amps), X 1.2 

I came up with 6.9 amps.

Does this sound like I am on the right track? I would rather undersize than oversize, as I live in an extreme fire hazard area, and this system is in a shop a mile from my home.

Thanks,

Dan

 

 

Heart Freedom 25 Inverter/charger

(one)Renogy 100 Watt 12 Volt Monocrystalline Solar Panel

WindyNation P20L LCD 20A PWM Solar Panel Regulator Charge Controller

 

Two 6volt golf cart batteries. (Costco)

vtmaps

Welcome to the forum,

If you only have one panel, you really don't need a fuse between it and the controller.  If you put a fuse there, you would size it larger than any current the panel could produce.  What is really nice to have is a circuit breaker, not for safety, but for convenience.  A DC breaker makes a nice PV disconnect switch.  Don't forget that controllers need to be connected to battery before PV, and need to be disconnected from PV before battery.  Therefore it is required (by code) to have a PV disconnect switch.

By the way, fuses/breakers are very important in multi-panel arrays, where you must prevent a group of panels from pushing their combined current backwards through a faulty panel.

THE MOST IMPORTANT fuse/breaker is between the battery and controller.  When controllers fail, they may become a short circuit across the battery.  The fuse/breaker must be smaller than the ampacity of the wires, but larger than the max output of your controller.  Do NOT use automotive blade type fuses for this fuse... it's too important... they have a poor safety record in this application. 

--vtMaps

zoneblue

Yes, you are advised to start the way you mean to continue. Carling or CBi DC breakers. Available right here for only a few bucks. Midnite big baby box or something to keep things tidy.

dchristo

Thank you for the information, and the especially the cautions vtMaps. When it comes to DC power, I am an electrical novice. The controller is rated at 20 amps. How much higher should I size the fuse than that 20amps? I am using 10 AWG wire for panel to controller and controller to battery connections. How do I figure the ampacity of that wire? Is there a guide, formula, or table I can use to determine this?  I have been waiting years to finally hook up a panel to my system and I'm anxious to do it, but not so much that I want to risk a fire! 

 I will throw the blade type fuse holders I ordered in the tool box to be used elsewhere.

Thanks again,

dchristo

Heart Freedom 20 inverter/charger, (1) Renogy 100 watt 12v panel, WindyNation P20L PWM 20 Amp controller, (2) Costco 6volt golf cart batteries, Honda EU2000i, Kubota 6500 Lowboy.                                                                                      

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• P.S.

dchristo

Thank you zoneblue, I will go with DC breakers, as soon as I can figure out/learn what size to use.

oil pan 4

With MPPT charge controller I over fuse by about 150%.

So if its a 5 amp panel I put a 7.5 fuse. Because MPPT controller can be ruined by an accidental short circuit.

If its a more robust PWM controller I use what the manual says or about 200%, which ever one is less. The manual might say to use a max of a 25 amp fuse on a 15 amp controller. But If I only have a 10 amp panel on this 15 amp charge controller I might only put a 20 amp fuse.

Lets say you have a charger with 25 amps going through it and a 25 amp fuse. That fuse will heat up to like 300'F. So you want to go with a fuse higher than the normal current so it doesn't melt the fuse holder during normal operation.

I just use automotive blade type fuses for low voltage.

vtmaps

oil pan 4 said:

Lets say you have a charger with 25 amps going through it and a 25 amp fuse. That fuse will heat up to like 300'F. So you want to go with a fuse higher than the normal current so it doesn't melt the fuse holder during normal operation.

I just use automotive blade type fuses for low voltage.

The main cause of the heat is the poor connections of those blade type fuses.  They are unsafe in this application. 

Some OCPD (over current protection devise = fuse or circuit breaker) are designed to handle their rated current continuously, others will trip/blow at their rated current.  For a charge controller, the OCPD should be 125% of the max current if the OCPD has a continuous rating, or 156% of the max current if the OCPD is not rated for continuous service.

--vtMaps

BB.

In general, fuses and breakers should be 1.25x larger than the maximum continuous current you expect to run at. If you run 10 amps through a 10 amp breaker/fuse it may trip in minutes to hours to days.

Wire size, here are a couple handy charts to start with. NEC is much more conservative than the typical Marine ratings for wires (note that AWG wire gauges used in NEC are very slightly larger that SAE wire gauges used in Marine wire ratings--Not a big deal, but can be confusing if you are looking at the details).

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
http://www.boatus.com/boattech/articles/choosing-cables-and-terminals.asp

Note that Marine allows 10 AWG to be ~51-60 amps maximum, and NEC allows ~30-40 amps maximum.

Note that NEC also derates for how many wires in a conduit (fill factor) and other reasons.

For Solar, we typically use larger gauge wire because we need low voltage drop--For 12 volt systems, we can only use ~0.5 volt drop for rated current drop. And it also reduces wire heating/wasted energy (generally a 1% to 3% maximum voltage drop).

-Bill

vtmaps

dchristo said:

Heart Freedom 20 inverter/charger, (1) Renogy 100 watt 12v panel, WindyNation P20L PWM 20 Amp controller, (2) Costco 6volt golf cart batteries, Honda EU2000i

I assume that you realize that your solar panel cannot support two golf cart batteries.  You will need to run the generator anytime you intend to fully charge those batteries.

dchristo said:

The controller is rated at 20 amps. How much higher should I size the fuse than that 20amps? I am using 10 AWG wire for panel to controller and controller to battery connections. How do I figure the ampacity of that wire? Is there a guide, formula, or table I can use to determine this?                                    

Wire is rated for ampacity... search for "ampacity chart"... there are dozens to choose from.  There are some nuances... the ampacity is a function of ambient temperature, location (in open air, or crowded in conduit), the temperature rating and thermal characteristics of the insulation, etc.

Under most circumstances 10 AWG cable is safe for 30 amps, but a 25 amp breaker (continuous rated) or a 30 amp breaker would be appropriate for a 20 amp controller.   Of course, you can get by with much lower rated breakers as long as you only have one panel.

How long are your cables?  You are working with some low voltages and cannot afford much voltage drop.  You may need heavier cables, not for ampacity, but for voltage drop.

-vtMaps

dchristo

I appreciate everyone who has taken the time to chime in on my post in an effort to help me install this system more safely! Obviously, I'm doing this on a shoestring. I purchased a very inexpensive controller, that had good reviews on Amazon. The golf cart batteries I just purchased were to replace the very old Trojans I had, and I miss their performance comparatively. These new batteries were half the cost of new Trojans or equiv., and while not the same quality, do not seem to be half bad. The 100w PV specs show "optimum operating current" as 5.29 amps, so I'm going to order a 7 or 8 amp breaker (if I can find one) as a switch between PV and controller.

I'm using 10 AWG wire for that, and from the 20 amp controller to batteries.  10 AWG is rated at 30 amps, so I am going to order a 25 amp breaker for controller to battery run.  

If any of this sounds unsafe (or just a bad idea) to anyone, I would welcome your comments.

dchristo

Heart Freedom 20 inverter/charger, (1) Renogy 100 watt 12v panel, WindyNation P20L PWM 20 Amp controller, (2) Costco 6volt golf cart batteries, Honda EU2000i, Kubota 6500 Lowboy.     

dchristo

vtMaps, I was hoping the 100w PV I have would at least maintain the golf cart batteries, and I was counting on topping them off, and equalizing them with a generator. I hope to add more panels as finances allow.

The 10 AWG cable runs will be as short as I can make them. 12 feet to and from PV/controller and 6 feet from controller to batteries. Trying to make the most with what I have.

Thanks again!

oil pan 4

Using a 25 amp breaker or fuse on a system that will see 20 amps continuously all depends on what kind of breaker or fuse you use. But NEC does not recommend running a breaker at more than 80% load for more than 2 or 3 hours. Running 20 amps on a 25 amp breaker for hours on end is on the raged edge of what they recommend. So use a 30 amp.

There are blade fuses that are 30 years old in my old Camaro and their connection is just fine.

I can just check them with my flir to see if they are over heating. I already know if you try to use a 25 amp auto blade fuse on a circuit that carries 25 amps, the fuse will be cooking at about 330'F. A 25 amp auto blade fuse even at 50% rated load can still heat up to 200'F if ambient temperatures are nice and hot. I would have to say that blade fuses are not rated for continuous use at rated load. This is the reason I use them at 75% of their rated load or less.