Fuses for Solar panels

[T1 Terry]

It is advisable to connect the solar regulator direct to the battery with a high capacity fuse, one that is inlikely to blow unless there is a major short circuit between the regulator and battery. If you want to disconnect this cable from the battery at any time you need to disconnect the solar from the regulator first, a switch or relay or circuit breaker works well for this task.
The reasons behind this:
The solar can produce up to 21v in a 12v system, if you break the path to the battery that 21v is now being switch on and off very quickly by the regulator as it sees 21v, turns it off then v0v, turns it on, then 21v and so one, this very quickly stuffs a regulator so the solar needs to be turned off first, then the battery disconnected.
A fuse in the solar wiring is a bit like a rear pocket in a singlet, useless and more trouble for no value.
The only current in the cable between the solar panels and the regulator is geberated by the solar panels. A fuse needs to be at least 10% greater value than the highest load it will carry, this is so it doesn't fail due to overheating. So if the panels produce 20 amps and the fuse is 25 amps then 20 amps will never cause the fuse to blow, so why waste the effort of putting it in there. If you fitted a 20 amp fuse it would continually fail due to overheating makinf the system prone to failure, why add something that will continually cause a failure?
If you had a big system and wired each panel individually to the controller then yes, a fuse would work. Each cable has a 25 amp fuse, the multiple cables can pass more than 25 amps so if one cable goes dead short near the panel the combined panel output can not flow back up that cable and cause damage or fire, the 25 amp fuse would blow.

T1 Terry

[GordMay]

The required (ABYC) Ampere Interrupting Capacity (AIC, or Interrupt Rating) of a breaker depends upon the total Cold Cranking Amps (CCA) and Voltage of the connected battery bank, as follows:
< 650 CCA requires 1500 AIC at 12 - 24 Volt
651 - 1100 CCA = 3000 AIC @ 12/24V
> 1100 CCA = 5000 AIC @ 12/24V

[Dsanduril]

Steve,

I'm sure you'll get a variety of opinions that differ from mine. In a one or two panel installation there isn't much point in putting fuses in the individual lines, they won't ever do anything. There is a point in putting in a fuse as close to the battery (or charging bus) connection as possible.

The panels are "current limited devices" even within the meaning and scope of the electrical code. A 100W panel has a short circuit current (Isc) of about 6.5A, and an operating current of about 5-6A. Depending on controller and battery conditions it would not be unusual to operate near Isc some of the time. So no point in having a fuse for a single panel. When you put two in parallel, if there is a short in one panel the current from that panel will flow to the short, and the current from the other panel will flow to the other side of the short, but at no location will the current exceed a single Isc, so, again, no point in a fuse. That all falls apart with three or more panels, so this case only applies to one or two panels.

The battery, on the other hand, can supply huge amounts of current. So, if you get a short anywhere in the solar system it is the flow from the battery that needs to be prevented. Since your two panels could in theory put 16-17A into your batteries I would put something between a 20A and 30A fuse (depending on availability) into the charging line where it connects to the battery (or the charging bus). The key with these types of fuses is they have to be as close to the battery as possible. If a short occurs in the wire between the battery and the fuse the fuse won't do anything.

Hope that makes sense, if it doesn't please say so and I'll try to put together a sketch and post it.

[Captain Bill]

I agree with everythin Dsanduril said. I have a 30 amp fuse between my panels and the controller. I have 4 x 75watt panels connected in parallel. The primary funcrion of the fuse is tp prevent battery amperage from getting to my panels/wiring should something short to ground in the pannel assembly.

Also, having the fuse there allows you to pull it and disconnect the panels from the controller before you open the case should you have to work on it. I have one between the controller and the battery as well for just that purpose.

[Andina Marie]

Why would you use 4 gauge for connecting a 200 watt solar panel? 10 gauge would be ample.

The panels don't need a fuse, they couldn't create enough current to blow a fuse. A fuse between the battery and the controller would protect the wiring from battery current flowing into a failed controller.

[noelex 77]

Originally Posted by Andina Marie

Why would you use 4 gauge for connecting a 200 watt solar panel? 10 gauge would be ample.

The panels don't need a fuse, they couldn't create enough current to blow a fuse. A fuse between the battery and the controller would protect the wiring from battery current flowing into a failed controller.

Wiring needs to sized so it both will carry the maximum current and not induce significant voltage drop. The latter condition is normally what dictates most wire sizes on a boat.
200w solar panels normally need a much larger wire than 10 gauge, but it needs to be calculated for each situation based on the distance and expected current.

If we use a current of say 12A (panels in parallel) 10 gauge wire will give more than a 3% voltage drop once the panels are more than 2.2m (7 feet) from the batteries.
Some people feel a 5% voltage loss is acceptable particularly for solar close to the maximum current, but this still leaves 10 gauge too thin for most 200w installations.

[Andina Marie]

Nolex, that would be true for a source like a battery with a very low source impedance driving a load. A drop in the cable from a fixed voltage can be significant due to less voltage = less power at the destination.

However solar cells are a high impedance CURRENT source so if there is a small 3% drop in voltage along the cable, the voltage output of the solar cell goes up 3% with virtually zero reduction in current. It is CURRENT that charges a battery, not voltage. The current through the lighter gauge wire will be virtually the same with a 10 gauge wire.

You can verify this by measuring the open circuit voltage of the solar cell, it is considerably higher than the 12 volt battery. If it were a low impedance source like a battery hundreds of amps would flow when connected to a 12 volt battery but it is a current source, not a voltage source so as soon as you apply a load the voltage drops to the voltage of the load.

Even if you short the cell out, it is not a voltage source like a battery that would blow fuses or burn cables, it is a current source and the current through the short will still be limited to a safe level.

[noelex 77]

With an MPPT controller the % voltage loss in the wiring is almost directly reflected by the same reduction in output.

With a non MPPT controller the situation is more complex. The voltage reduction normally has very little effect. The I/V curve is such that that small voltage reductions have very little effect on the current. However when the Vmp falls below the battery voltage, due to shade etc there is a very dramatic reduction in output. The effect of the voltage loss is therefore quite variable, but its average effect is similar to the MPPT controller.

If you use the maximum current of the solar panels when doing the voltage drop calculations the average voltage (and therefore power) drop will be lower as the solar panel output drops at lower light levels, but as most of the power is produced when the current is high the power loss is still significant.

A simple way to look at voltage loss in the wiring is to imagine that for each 0.5v lost in the wiring you have lost one of the 36 solar cells on each panel.

[Dsanduril]

Originally Posted by noelex 77

With an MPPT controller the % voltage loss in the wiring is almost directly reflected by the same reduction in output.

+1

With an MPPT controller energy lost heating your wires is genuinely lost. With a PWM controller under many conditions voltage loss will cause you to slide up the I/V curve slightly and you will lose some input power, but not what you would calculate from the voltage drop. This is only because under these conditions you have excess power available at the panels that you are not managing to get into the batteries.

For the OP, 4AWG wire is more than sufficient. I'd probably put in 6 or 8 myself depending on distance, but if you have 4 then use it.

[NorthPacific]

Firstly a real thanks for you guys taking the time to explain the science.

Used 4 gauge, as per voltage drop, (explained to me as the pressure needed to force amps to fully charge batteries) Also that the distance between sensor and battery. very important. Also I may expand number of panels. The extra few dollars for 4 gauge is much less than paying for an mppt controller that, again advice says, is not needed unless you have a 400w or bigger system.

From my limited experience, I have found that when it comes to wiring Inverter, windlass, Wind gen and now solar, bigger tends to be safer. I hope. I need to digest this information tonight. Thanks again for the feed back.

[CharlieJ]

This year's issue of the ABYC Standards E-11 has revised the AIC/CCA chart to add Ahr rating as another determining factor for AIC. This is especially useful when determining OCPD AIC for house banks and has been missing from the E-11 before this revision.

140 Ahr: 1500 AIC main; 750 AIC branch
141-255 Ahr: 3000 AIC main; 1500 AIC branch
256-500 Ahr: 5000 AIC main; 2500 AIC branch
>500 Ahr:

11.10.1.2.3 For batteries or battery banks with a CCA rating greater than 2200 CCA, or 500 amp hours, battery overcurrent protection shall have a minimum ampere interrupting capacity (AIC) rating at least as great as the battery
manufacturer’s short circuit rating or 100 times the battery’s nominal amp hour (Ah) rating. (Emphasis added.)