Why Over-Current Protection Is Important
Please pay attention to the design guidelines described in this article.
They will save your system and will not put at risk your life or health in case of high amps faulty events in the current circuits.
Get it wrong and you may face many undesired situations starting from fire hazard or dangerous to your health situations.
In an RV solar power system, the corresponding circuits should be protected by all sources of fault currents.
The primary source of fault current in the DC part of the system is the PV solar panel or the solar array.
In the other part of the solar power system, the major sources of such currents are the other active components like charge controller, battery, and inverter.
That’s why the overcurrent protection devices /OCP/ must be implemented in the different segments of the solar system. The main overcurrent protection OCP devices used in the RV and off-grid solar power system are:
– fuses and breakers
-bypassing and blocking diodes
Other devices like junction boxes, combiner boxes, pass-through boxes AC, and DC load centers also act as overcurrent protection devices among many other roles that they play in the solar power system.
The major function of the fuses and breakers is to protect the wires.
The breakers have many advantages over fuses.
They are used to automatically or manually switch on and off the load or the source of the faulty current for many times.
While the fuses are onetime devices- they protect the circuits by blowing out and melting itself under the faulty current, and after that, we must replace them.
When the breaker automatically switches off in case of a faulty event, it can be manually reset and reused once again.
Also, if you want to do repair work, it makes your life easier by allowing you to cut manually off the corresponding segments of the system from the power.
However, this convenience comes with a price: breakers are more expensive, larger in size than fuses.
In contrast, fuses are smaller, less expensive, available for many faulty currents and operating voltages. But they must be replaced after a faulty event and need a fuse holder.
Another thing that is important to mention about fuses is that they are designed for DC and AC voltage correspondingly.
Since the DC and AC current characteristics are different, the triggering and performance behavior of the fuses is not the same.
That’s why DC fuses are the most expensive.
They are the most sophisticated devices compared to AC fuses, so please don’t be tempted to substitute the DC ones for AC fuses.
The combiner box performs three functions: combining several solar panels or strings in parallel, overcurrent protection, and transition to the conduit.
The junction box protects PV panels wire from the environment and has a holder inside for installing bypassing diodes to protect the solar panel from shading.
Usually, a bypass diode is wired in parallel to several connected in series solar cells, thus reducing power losses when they are being shaded.
Typically, the manufacturer installs the junction box on the back of solar panels and set-ups the necessary bypassing diodes inside.
We are going to pay more attention to the sizing of fuses and breakers because they are the most commonly used overprotection devices in RV and small off-grid solar power systems.
We use the fuses or breakers in the following segments of the RV solar power system:
-the DC segment between solar panel and the charge controller,
-the DC segment between charge controller and battery
– the DC segment between the battery and inverter
– the AC segment between inverter and appliances
How To Size Overcurrent Protection Devices
Overcurrent protection devices are sized regarding maximum voltage and current used.
In short, the methodology is as follows.
In the first step, the faulty current of the corresponding segment of the solar power system is calculated.
In the second step, a fuse nameplate value of the current rating is selected.
If the fuse current rating is not readily available, you can choose the next highest rating available.
Then, you have to check if the fuse current rating is lower or equal to the ampacity of the conductor selected.
If not, the conductor size must be increased.
As for the fuse voltage rating, it must be equal to or higher than the highest DC voltage of the system in the DC part of the solar system or equal to or higher than the standard AC voltage of the AC segment of the system.
Before starting the design, let’s recall the parameters of a solar panel essential for protection.
-Voc- open circuit voltage
– Isc – short circuit current of the solar panel.
The other parameters of the solar panel define its ability to generate electric power: :
–Vmp- optimum operating voltage
–Imp- optimum operating current.
–Maximum power at Standard Test Conditions/STC/=Vmp*Imp , i.e this is the nominal power of the solar panel.
The solar parameters change dynamically with the change of solar irradiance and the ambient temperature.
The higher the temperature, the lower the voltage of the panel, and the higher the current.
However, the increase of the current can’t compensate for the decrease of the voltage, and as a result, the generated solar power decreases.
The higher the solar irradiance, the higher the generated solar power.
How To Find The DC Voltage Rating Of The Fuses And Breakers
In the DC part of the PV solar power system, the voltage rating is defined by the higher system voltage.
That is, the solar panel or solar array maximum open-circuit voltage at the lowest ambient temperature Vocmax:
Voc max =1.2*Voc=~1.56*Vmp.
A Basic Rule For Defining the Total Current and Voltage In Case of a Series or Parallel Connection of Several Solar Panels
In the case of several panels connected in series, the maximum current is equal to the maximum current of the standalone panel and the maximum voltage is a sum of the voltages of the standalone panels.
But when solar panels are connected in parallel, the maximum voltage of the solar array is equal to the maximum voltage of the standalone solar panel.
However, the maximum current is equal to the sum of all standalone panels currents in that case.
So, in the case of N solar panels connected in series/Ns/:
where Voc is the open-circuit voltage of the standalone solar panel, and Isc is the short circuit current of the solar panel. 1.56 is the correction coefficient, taking into account the temperature and solar irradiance influence on solar panel voltage and continuous load as well.
In case of N solar panels connected in parallel/Np/:
How To Find The Current Rating Of The Fuses And Breakers
- How to find the current rating Iocppv of the fuses or breakers for the segment between the solar panels and charge controller.
This current rating is defined by the maximum solar panels current, taking into account temperature and irradiance variations:
where Isc is solar array short circuit maximum current.
Usually, a combiner box is used in this segment of the pv system. It may contain inside fuses or breakers.
Its primary function is to combine several solar panels safely in parallel via a corresponding fuse or breaker.
For that purpose so-called Y combiners are used in the RV system as well.
So, in order to size the combiner box, we must determine:
- The combiner box ampacity and voltage rating
- The ampacity rating of the fuses/breakers through which every solar panel is being connected in parallel to the other ones
If you combine N solar panels in parallel/Np/ in a combiner box, its current ampacity should be equal to 1.56* Np*Isc_solar_panel where Isc_solar_panel designates the short circuit current of the standalone panel. This formula is also valid for a Y combiner used in an RV system.
The combiner box rated voltage should be equal to solar array maximum voltage Voc max, i.e. Vocmax=1.2*Voc
where Voc is the open-circuit voltage of the solar array; when the solar array comprises several standalone solar panels connected in series, its open-circuit voltage is equal to the open-circuit voltage of the standalone solar panel multiplied by the number of the solar panel connected in series.
The current ampacity of the fuse and the breaker via which every standalone solar panel is being connected inside the combiner box before being connected in parallel there is defined by:
where Isc is the short circuit current of the solar panel. 1.56 is the correction coefficient taking into account the temperature, solar irradiance influence on solar panel voltage and continuous load.
Fuses are produced and offered in standard sizes (6, 8, 10, 15, 20, 25, 30, 35,40 amps, etc.),
NEC advises us to select the standard fuse size of the same value as the calculated one or one just above the calculated one.
For example, if the calculated fuse size is 28 amp, we must select 30 amp rated fuse.
By using the largest Wattage solar panels, you can minimize the number of modules needed for achieving the desired solar array wattage.
The National Electric Code generally requires fuses or breakers to protect back-feed from other modules if you parallel more than two modules.
Over two modules, you will need a pair of wires with connectors for each module, wired to a combiner box with breakers or fuses, then properly sized UV resistant wire or wet rated wire in conduit from the combiner to the charge controller.
If up to two solar panels or solar strings are connected in parallel and the cable is rated at least 1.56* Isc no fuse is required.
If up to two solar panels or solar strings are connected in parallel and the cable is rated below 1.56* Isc a fuse connected in series to the panel must be installed.
The fuse ampacity rating /Ifuse/ should be equal to or less than the cable current rating Icable at the maximum ambient temperature.:
2. Sizing the fuses and breakers DC segment between the charge controller and battery
The fuse current rating is defined by the charge controller rated current. The charge controller rated current Icc is calculated by the formula:Icc≥1.56*Isc
Where Isc is the solar solar array short circuit maximum current.
In case of N solar panels connected in parallel the solar array short circuit current is a sum of the current of the standalone panels, i. e Isc=Np*Isc_solar_panel
3. Sizing the fuses and breakers in the DC segment between the battery and inverter:
The fuse current rating Imax is defined as follows:
where Vbatlow- lowest battery or battery string voltage
Pinvwattage – continuous inverter wattage rating
0.9-typical inverter efficiency of 90%.
Usually, the manufacturer of the inverter provides the recommended size of the fuses in the inverter specification, and the best way is to follow their guidelines. You can use the above formulas if such a recommendation is missing.
4. Sizing the AC Disconnect between the inverter and appliances
The ampacity rating of the AC Disconnect Iampdisconect between the inverter and the appliances is defined by inverter AC output current Iinvac multiplied by a coefficient of 1.25:
Iampdisconect= 1.25* Iinvac
Say we have a 2300W continuous power rated inverter per 120V AC. The inverter AC current Invac would be 2300W/120V=19.2A
So in this case Iampdisconect would be 1.25*19.2=24 amps.
A Basic Principle For Wire Selecting And Sizing of The Cables
The maximum cable ampacity must be higher than the maximum circuit current.
The maximum cable ampacity must be taken considering maximum working ambient temperature; that is, cables are usually rated at an ambient temperature of 30C.
The higher the ambient temperature, the lower the cable’s maximum current rating.
The cable size must take into account the voltage drops as well- the higher the diameter of the cable ( the lower AWG gauge), the lower the voltage drop.
Our goal is to minimize the voltage drop and make it as low as possible.
However, the reduction of the voltage drop requires the usage of a cable with a higher diameter, which in turn makes our system more expensive.
Therefore, choosing the right cable diameter for the corresponding segment of our PV system is always being a tradeoff to be made between our budget and the optimal cable size.
That is why it is recommended the total voltage drop of the system be in the vicinity of 4-5%.
Though, the voltage drop between the charge controller and the battery should be specially calculated for this segment to be a maximum of 1% and even lower.
Otherwise, the charge controller would not be able to define the battery voltage properly, and the controller will apply an incorrect battery charging mode.
For example, a 2% voltage drop over the charge controller to battery wire at a fully charged lead-acid battery would be as high as 0.02*14.4V=0.29V.
You can use our solar wire calculator to select your wire.
Practical Example Of Overcurrent Protection Devices Sizing In A Typical RV Solar Power System
Let’s apply the above-mentioned overcurrent protection guidelines on the following RV system:
Solar panels parameters:
- Sizing the DC segment between the solar panel and the charge controller.
1.1 Sizing the fuses F1, F2, F3 connected in series of each solar panel
Let’s begin with sizing the conductor wire coming into the combiner box.
The wire must sustain at least the maximum circuit current: 1.56*Isc=1.56*13.3=21A
Lets define the fuse current amp rating Ifuse of F1,F2 and F3:
The fuse size selected must be equal to the calculated value or the next larger standard rating
We choose 25A rated fuse.
The wire ampacity rating must be higher than the maximum circuit current and equal to or higher than the fuse rating.
This rating corresponds to the conductor size of at least of 12 AWG wire. The 12AWG type UF would sustain 25A current at 60C.
However, the cable with a higher diameter might be needed to achieve the desired lower voltage drop over the cable installed between the solar panel and the combiner box.
You may use our free cable calculator here to calculate it:
In case of a cable of higher diameter cable, the fuse ampacity rating stays the same as being chosen, namely 25A.
Let’s calculate the fuse DC voltage rating Vdc:
We choose a standard fuse of 32 VDC
Since solar panels are connected in parallel, this value of 28Vdc is the maximum voltage of the solar power system.
The NEC standard recommends that all DC protection devices be rated at least at the maximum DC voltage of the system.
Therefore, all fuses and breakers at the DC part of the system should be rated at least of 28V or higher.
1.2 Calculating the fuse rating of the fuse F4 after combining the three solar panels in parallel.
Let’s begin with sizing the conductor wire coming out of combiner box.
The wire ampacity must sustain at least the maximum circuit current and with three solar panels connected in parallel, would be at least: 1.56*3* Isc=1.56*3*13.3=62A
Lets define the fuse current amp rating Ifuse:
We choose a fuse with a standard ampacity rating of 70A and a cable of 4AWG type UF capable of providing 70A at 60C.
Once again, we might need a cable of a higher diameter to achieve the desired voltage drop. However, the fuse size must remain the same: 70A.
1.3 Sizing the fuse F5 between the charge controller and the battery.
The charge controller rated current should be at least equal to the maximum solar array current. We have already calculated its value: 1.56*3* Isc=1.56*3*13.3=62A
Let’s say we have selected a 65A rated charge controller.
We choose a fuse with a standard ampacity rating of 70A for F5 and a cable with a diameter ensuring a 1% voltage drop between the charge controller and the battery.
1.4 Sizing the fuse F6 between the battery and the inverter.
Let’s say we are using a 700W continuous power inverter.
(you may use our free solar power system calculator to design your complete system)
The lowest battery voltage for a lead-acid battery bank of 12V would be around 10V.
Therefore, the maximum continuous current that a load could withdraw from the battery is:
= 1.25*700/(0.9*10)=97 A
We choose a standard fuse of 100 A for 32VDC.
So, the cable between the battery and the inverter must withstand at least 100A at 60C.
1.5 Sizing the fuse F7 between the inverter and AC load
The fuse size is equal to 1.25*Iac,
where Iac is the alternating current.
Since we have a 700W inverter for 120VAC, the AC current would be 720/120= 5.83A
So, the fuse size is 1.25*5.83=7.3A
We select a fuse of 10A AC for 120VAC.
However, when sizing the fuses at the input and the output of the inverter there is another consideration to be kept in mind.
Besides the continuous power, the inverter has another power rating: the surge power.
This is the power that the inverter can sustain from 3 to 15 seconds and up to 15 minutes.
The surge power could be up to 3-5 times higher than the continuous power.
Though, the most common value of this power is about two times higher than the continuous one.
The surge power is useful for starting a pump, vacuum cleaner, or any other device containing a motor.
If you plan to use the surge power, the inverter’s output and input fuses must be resized accordingly.
Let’s do it.
Let’s assume that the inverter has a surge power rating of 1400W. The inverter’s input fuse F6 would be:
We choose a standard fuse of 200A for 32VDC.
The corresponding wire must sustain at least 200A.
The inverter output fuse would be:
We choose the standard fuse size of 15A at 120V.
Blocking And Bypassing Diodes
What is a bypassing diode used for
The bypassing diode is used to mitigate the negative impact of shading on the solar panel or solar array performance.
When a solar cell or a solar panel has been shaded, the resistance of the corresponding cell or solar panel increases highly.
The shaded device ability to generate solar power decreases.
What is more, the shaded cell or solar panel acts as a load to the unshaded cells or solar panels.
Because of the thermal effect of the current flowing through shaded devices, the part of generated electrical power by illuminated devices dissipates in the form of heat via shaded ones.
The heat increases the internal temperature of the shaded cell or panel significantly.
Also, in case of a deep shading, the corresponding shaded cell or a panel may become reverse biased, and this way, the whole generated power by the good cells or panels dissipates via shaded ones.
As a result, the overheating of the reverse-biased device increases dramatically, so it may meltdown, i.e., phenomena like glass cracking, solder melting, etc. could be observed.
The so-called hot spot is being formed.
To avoid forming a hot spot, a bypassing diode connected in parallel of the device is added, as shown in the picture below.
In case of a deep shading, when the device becomes reverse biased, the current flows via the lower resistance of the forward-biased bypassing diode, thus completely circumventing the shaded cell or panel.
Thanks to the bypass diode, the power lost is lower than in case of not using one. But most importantly, the shaded device is saved from being melted down.
Every solar panel contains several bypassing diodes, which are usually incorporated in the junction box.
For example, a solar panel for 12V nominal voltage typically comprises 36 solar cells connected in series, and it has most commonly at least two bypass diodes in the junction box, i.e., a diode per every 18 cells.
How to select the bypassing diode
The bypassing diode/ D1,D2,D3/ is sized to sustain the 1.56 times the short circuit current of all solar panels connected in parallel or series.
Its maximum reverse voltage must be at least 1.2 times higher than the maximum voltage of the solar array.
Fast switching Schottky diodes are preferred to lower speed silicone ones.
What is a blocking diode used for
The blocking diode/D4/ prevents the battery from discharging during the night.
It is most commonly used in a simple low power solar power system without a charge controller.
In these types of systems, a solar panel or the solar array is directly connected to the battery.
There is no need for a blocking diode when the charge controller is used because it fulfills the function of a blocking diode, among many other useful ones.
Also, the blocking diode could be deployed to prevent the power from the illuminated panels to be dissipated as heat in the shaded panel as depicted in the picture below.
When one or more solar panels are shaded, their blocking diodes/D1,D2,D3/ switch them off from the circuit because the corresponding diode becomes reverse-biased, and its internal resistance increases to non-conductive values.
The major drawback of this implementation is that we are losing more power than in the case of using a separate charge controller per panel, as described in our article mixing solar panels: how to squeeze more solar power from different solar panels.
However, this is a cheap and straightforward solution than deploying a separate charge controller per panel.
It is most commonly applied in boats where frequent and sudden shading of the panels happens because of changing tack.
How to select the blocking diode
The blocking diode is sized to sustain the 1.56 times the short circuit current of all solar panels connected in parallel or series.
Its maximum reverse voltage must be at least 1.2 times higher than the maximum voltage of the solar array.
Another important consideration is that the diode must have a lower forward biased voltage to avoid voltage losses.
The silicon-based diodes have a forward voltage of 0.6-0.7V and low switching speed.
Whereas a Schottky diode has a fast switching speed and lowest forward biased voltage of 0.15-0.45V.
Therefore, the usage of the Schottky diode is recommended for a blocking diode.
- Pop MSE, Lacho, Dimi Avram MSE. 2019. “Top 40 Costly Mistakes Solar Newbies Make: Your Smart Guide to Solar Powered Home and Business”, Digital Publishing, Amazon Kindle Edition
2.Pop MSE, Lacho, Dimi Avram MSE. 2019. The New Simple And Practical Solar Component Guide, Digital Publishing, Amazon Kindle Edition
Recommended additional literature:
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