fuse needed between MPPT and solar panels in case of mppt malfunction/short?

[rillweed]

Let's assume I have 1 panel (200W, 20V, 10A) connected to an MPPT with a disconnect switch. This setup has no need for a breaker since the panel is a limited power source but what happens if the MPPT malfunctions and causes a short and backfeeds from the battery? Is a fuse needed for that scenario to protect the solar wire? I have read quite a few threads on not needing a breaker for small systems in series but I haven't seen anyone mention this

I am using a victron mppt and I know they have some internal fuses but I don't know that they will prevent this. The victron install manual for the 100/30 actually recommends using a fuse between the mppt and the panels. Is it in case of malfunction or are they generalizing?

 

[timselectric]

Let's assume I have 1 panel (200W, 20V, 10A) connected to an MPPT with a disconnect switch. This setup has no need for a breaker since the panel is a limited power source but what happens if the MPPT malfunctions and causes a short and backfeeds from the battery? Is a fuse needed for that scenario to protect the solar wire? I have read quite a few threads on not needing a breaker for small systems in series but I haven't seen anyone mention this

I am using a victron mppt and I know they have some internal fuses but I don't know that they will prevent this. The victron install manual for the 100/30 actually recommends using a fuse between the mppt and the panels. Is it in case of malfunction or are they generalizing? View attachment 253073

There should be OCP (Over Current Protection) between the SCC (Solar Charge Controller) and the battery. This protects the SCC and panels , from the battery.

 

[rillweed]

Any idea why the victron diagram shows fuses in both locations?

 

[timselectric]

Any idea why the victron diagram shows fuses in both locations?

Covering all bases.
They don't know what you might connect.

 

[rillweed]

So just to clarify there isn't a situation where the mppt could short and not blow the battery fuse or back feed in some other way?

I just want to make sure I'm not sending too much current through the solar wires since those will be a smaller gauge than my mppt to battery due do the lower voltage from the battery

 

[timselectric]

So just to clarify there isn't a situation where the mppt could short and not blow the battery fuse or back feed in some other way?

I just want to make sure I'm not sending too much current through the solar wires since those will be a smaller gauge than my mppt to battery due do the lower voltage from the battery

Anything is possible. But it would take a lot of things going wrong to cause an issue.

 

[timselectric]

Anything is possible. But it would take a lot of things going wrong to cause an issue.

To clarify what I mean.
You would have to have a fuse on the battery side that wouldn't blow, and the SCC would have to internally short between the input and output sides, and the PV string/ or panel would have to have a short.

 

[robbob2112]

The wire should be sized to 156% of the expected current. This is large enough that even a dead short across them won't overheat them if they stay that way all day. No fuse needed, but it is allowed. Two strings parallel, same thing if the. wires are 156% they can stay shorted all day, no fuse required but is allowed. When your get to 3 strings in parallel there is enough current that a short in one set of panels could melt things in that string and cause a fire.

There is a max fuse amps listed on the back of the panels that cannot be exceeded without damaging wiring and runs in the panel.

 

[Zwy]

I always fuse a wire or have a breaker in the circuit.

I've seen enough melted insulation on DC circuits in 37 years of repairing DC systems. On my arrays, I use an IMO for a disconnect but have breakers in the box. I wired the shop array the same as I wired my house array shown here. The only difference is I used a grounding busbar for all the grounds.

 

[Suijkerbuijk]

There should be OCP (Over Current Protection) between the SCC (Solar Charge Controller) and the battery. This protects the SCC and panels , from the battery.

No not
A fuse is not for protecting a scc or other stuf.
A fuse is use to protect the cable.
So that your cable not become a fuse..
That is the reason a fuse is use

For the rest a fuse on a solarpanels side is thare so the cable do not burn out
A solar panel do not stop if thare is a broken cable.
Really a solar panel you have treat it like a battery .
Both do not stop if thare is short circuit

 

[timselectric]

No not
A fuse is not for protecting a scc or other stuf.
A fuse is use to protect the cable.
So that your cable not become a fuse..
That is the reason a fuse is use

OCP can serve more than one purpose.
You shouldn't limit your thinking.

 

[GXMnow]

With a single panel or series string, fuses are rarely required, but it certainly does not hurt to have them. With parallel strings, most systems require fuses to properly protect the wiring and the traces in the solar panels.

I also recommend having surge suppressors on the lines from the solar panels to the charge controller. If the panels are raised up, they could act as a lightning rod. So even with a single panel, having a fuse and then a surge suppressor to ground can trap a bad surge and hopefully pop the fuse before the surge suppressor fails and lets the voltage spike fry the charge controller.

 

[rillweed]

The wire should be sized to 156% of the expected current. This is large enough that even a dead short across them won't overheat them if they stay that way all day. No fuse needed, but it is allowed. Two strings parallel, same thing if the. wires are 156% they can stay shorted all day, no fuse required but is allowed. When your get to 3 strings in parallel there is enough current that a short in one set of panels could melt things in that string and cause a fire.

There is a max fuse amps listed on the back of the panels that cannot be exceeded without damaging wiring and runs in the panel.

Where does 156% come from?

 

[rillweed]

With a single panel or series string, fuses are rarely required, but it certainly does not hurt to have them. With parallel strings, most systems require fuses to properly protect the wiring and the traces in the solar panels.

I also recommend having surge suppressors on the lines from the solar panels to the charge controller. If the panels are raised up, they could act as a lightning rod. So even with a single panel, having a fuse and then a surge suppressor to ground can trap a bad surge and hopefully pop the fuse before the surge suppressor fails and lets the voltage spike fry the charge controller.

If lightning strikes my van I think I have bigger problems

 

[robbob2112]

That comes from the NEC.

From the AI - summary is amps * 1.25 * 1.25 = max current with safety factors.



The 156% figure for PV wires stems from a requirement in the **National Electrical Code (NEC) 2023** (and in earlier versions) in **Article 690.8(A)(1) and (B)(1)**. It explains how to size conductors in PV systems to handle potential increases in current due to solar irradiance and environmental conditions.

Here’s a breakdown of the NEC requirements that lead to the 156% value:

1. **Continuous Current Adjustment (125%)**:
NEC 690.8(A)(1) states that the maximum current in a PV system is determined by multiplying the short-circuit current (**Isc**) of the module by **125%** to account for the continuous nature of PV power production. This adjustment ensures that conductors can handle prolonged currents during operation.

2. **Additional Safety Factor (125%)**:
NEC 690.8(B)(1) applies an additional **125% multiplier** for continuous current requirements. This second factor accounts for potential unexpected current increases due to environmental factors, such as high irradiance or cooler temperatures, which can lead to higher current output.

The result is a combined adjustment factor:

1.25 * 1.25 = 156%


### Application
When determining the required ampacity of conductors in a PV system, NEC requires multiplying the Isc by **156%** to ensure the wire can carry sufficient current even in worst-case scenarios.

 

[rillweed]

To be clear I'm concerned about the scenario of an mppt malfunction. I know it's not needed for my simple example of one panel. Is there a scenario where a fuse is needed here? It doesn't seem like it

 

[robbob2112]

There is a failure mode where the mppt shorts the PV inputs to the battery wires. In that case there could be enough juice to melt the wires and a fuse would most likely prevent it. A breaker between the mppt and battery should be present and should also trip in case the mppt becomes a chunk of conductive material.

Many mppt include a replaceable fuse, but not all.

 

[timselectric]

To be clear I'm concerned about the scenario of an mppt malfunction. I know it's not needed for my simple example of one panel. Is there a scenario where a fuse is needed here? It doesn't seem like it

No, it's not needed or required.
But if it helps you sleep at night, having one doesn't hurt anything.