solar fuses

[Muddydogs]

What fuses are you guys using to connect you solar panels to controller and controller to battery's?

[Salvo]

To be more exact, instead of "no fuse needed" change to:

A fuse will never blow during a shorting condition.

CA Traveler wrote:

The Bussmann link shows fuses for arrays. Common RV examples:
1. Single or series panels - no fuse needed.
2. 2 panels in parallel - no fuse needed
3. 2x2 series/parallel panels - no fuse needed
4. More than 2 panels in parallel - fuse each parallel connection including when there is more than 1 panel in each series connection

[time2roll]

When the Isc of the array exceeds a panel fuse rating is when each parallel string needs a fuse. Generally that is 3 or more panels in parallel.

[CA_Traveler]

BF13, Yes

The Bussmann link shows fuses for arrays. Common RV examples:
1. Single or series panels - no fuse needed.
2. 2 panels in parallel - no fuse needed
3. 2x2 series/parallel panels - no fuse needed
4. More than 2 panels in parallel - fuse each parallel connection including when there is more than 1 panel in each series connection

For panel definitions: String is a series connection and array is a parallel connection.

[BFL13]

mena661 wrote:

BFL13 wrote:


To clarify you need a fuse on each string for more than three strings in parallel

http://www1.cooperbussmann.com/pdf/9df1f7ec-8c62-4210-8cf8-9504927394f0.pdf

Thanks BFL.

The panel will have a "series fuse" size given in its specs. Can't remember how that fits in with all this. ISTR it is the size for the string's fuse when you do need to fuse the string?

[mena661]

BFL13 wrote:


To clarify you need a fuse on each string for more than three strings in parallel

http://www1.cooperbussmann.com/pdf/9df1f7ec-8c62-4210-8cf8-9504927394f0.pdf

Thanks BFL.

[CA_Traveler]

Bottom line is if the wires from each panel can handle the total current from all the panels, most likely they can, there is no reason to use fuses

Fuses for each panel for 3 or more parallel panels is required. The fuse protects both the panel and it's wiring. The wiring may/may not handle the current and a shorted panel might not. Fuses are about safety first and not the additional IR losses.

[time2roll]

Muddydogs wrote:
Ken
What happens as both the controller and battery are both energy sources?

If the supply cannot exceed the wire ampacity no fuse is needed. So a 30 amp controller that passes through 10 amps of solar power wired with #10 needs no fuse because no harm from a dead short. However the battery can supply more power and so there should be a fuse close to the battery.

[LScamper]

Reprint from 4/10/2014

RE: Proper Fusing Of Three Parallel Panels

Trying to make sense of panel fusing. A fuse is a current protection device. A fuse at the output of a panel is to protect the wires from the fuse to whatever it is connected to, mainly the controller. An array is the parallel connection of two or more panels (strings). A string is the series connection of two or more cells. A panel can be say 30 cells or 90 cells. A string can be one panel of say 90 cells or three panels of 30 cells in series, same exact thing! Say a panel is rated at 10A short circuit current and the fuses are 15A. So what happens if there is a short directly at a panel before the fuse? If there is only one panel the panel output goes to zero and the controller gets no input. The fuse will not blow, no current through it. If there are two panels in parallel and one shorts directly at a panel the output the voltage goes to zero and the controller gets no input. The short circuit current supplied by the other panel, 10A, now goes through both fuses, the one on the good panel and the one on the shorted panel. Neither fuse blows because they are rated to handle the current from one panel. The wires are safe because they are rated to handle at least the short circuit current from one panel. If there are three panels in parallel and one shorts directly at a panel the output voltage goes to zero and the controller gets no input. The short circuit current supplied by the other two panels is added, now 20A total, and flows through each of the good panel’s fuses, 10A each, and through the shorted panel fuse. This fuse now gets two times its normal current, 20A, and should blow from over current. When the fuse blows the voltage goes up at the controller and will still charge the battery from the two good panels. If the wire is rated to handle the current from the two good panels no fuse would be required. This would be the case in most of the installations talked about on the forum; most use much larger wire than needed to keep voltage drop at a minimum. This brings us to a reason not to use fuses at the panel, voltage drop! In order for a fuse to blow it has to dissipate some power to heat it to the melting point. This means that the fuse must have resistance. A 7A fuse resistance is about .013 Ohms and a 10A fuse resistance is about .008 Ohms. A 10-gauge wire has a resistance of about .001 Ohm per foot. So adding a 7A fuse is like adding about 13 feet of 10-gauge wire from the panel and a 10A fuse is like adding about 8 feet of 10-gauge wire. Plus there are all the connections that add resistance and could become loose or not a make good connection. Bottom line is if the wires from each panel can handle the total current from all the panels, most likely they can, there is no reason to use fuses and they introduce voltage drop plus added connections.

[pianotuna]

Hi,

On my converter to battery, there are automatic circuit breakers at both ends and reverse polarity fuses in the actual converter.

Fuses are needed close to the battery. Remember--they used to weld using batteries.

Muddydogs wrote:
On my trailer near as I can tell the main lines off the battery to the converter are not fused.

[KJINTF]

Fuse or switchable breaker on both the Panel side and the Battery side

As Tech Writer and others stated protection is only one of the reasons I always install both. Another is the simplicity of system isolation, flip a switch or two and the controller is completely isolated. Morningstar MPPT controllers require a hard reboot (power disconnect) after firmware downloads but typically not configuration changes. A switch with a fuse in line or a switchable breaker makes it very easy.

[Muddydogs]

Ok great info. Thanks guys.

[ken_white]

Muddydogs wrote:
Ken
What happens as both the controller and battery are both energy sources?

The controller is an energy source for the battery and needs a fuse sized according to its output current and cable size.

The battery acts as an energy source for the DC panel and other DC loads so it needs a fuse at its output to these loads and the fuse is sized according to the cable size.

The converter acts as a source to the DC panel, loads, and battery so it needs a fuse sized according to its output current and cable size.

Each of these fuses protect excessive energy from being passed through a cable that isn't designed to handle the heat that is generated from a shorted fault condition.

EDIT: Oh and the reason the solar panels under some conditions do not need to be fused is because they act as a constant current source which means a finite amount of current is generated.

So, if the cable can handle this finite, short circuit current indefinitely without overheating, a fuse sized for this cable would never open anyway.

[Muddydogs]

Ken
What happens as both the controller and battery are both energy sources?

[BFL13]

mena661 wrote:

2oldman wrote:
Fuses are not required for series PV installations, as mine is. Go ahead, tear me apart.

NEC only requires fuses when you have 3 or more parallel strings. That's a minimum of 6 panels wired series/parallel. Anyone here have that kind of setup? When I get my solar setup, there will be no fuses on the panels to controller side. A hint on why: solar panels are designed to be shorted. You're not protecting those, you are protecting the wire and only when you have X number of panels connected will you ever stand a chance of going over a wires current limit.

To clarify you need a fuse on each string for more than three strings in parallel

http://www1.cooperbussmann.com/pdf/9df1f7ec-8c62-4210-8cf8-9504927394f0.pdf