# True of Fusing system for solar panels

Registered Users Posts: 13 ✭✭
Hello, I m installing 4 solars in parallel, Isc is 9,26x4=37,04A. So on other forums i saw they are using calculation where they multiplying Amperage with 1,56 which in my case should be 37,04x1,56=56,78A. My question is ,should i then have 60A or 50A fuse? I understood that Amperage of circuit should be higher than rating of Fuse in order to protect circuit. Is this why 1.56 is used ,0,56 represent that excess current?

• Registered Users Posts: 4,494 ✭✭✭✭✭
The fuse/breaker (OCD) is there to protect to protect the wiring generally from a short circuit fault, so size depends on the size of wire being protected. The derating of the OCD may be needed to prevent overheating in normal operation. Note that breakers should be DC rated.

Your panels should have a maximum OCD size (often 15a) spec. Because you have 4 in parallel, a short in one could result in the combined current from the remaining 3, so you should have OCDs on the positive of each panel. This is normally done in a combiner box.

Wire size from combiner to controller is generally based on current and distance in order to keep voltage drop reasonable, and it should work out that the wire size is large relative to the required ampacity of the wire for normal operating current, so the breaker can also be large relative to normal operating current (ie derating probably isn't an issue in a well designed system).
Off-grid.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
• Registered Users Posts: 13 ✭✭
Ok, so what you actually suggest for this set up? If i have 15A OCD on each panel, does than need one more fuse after combiner box?
Short answer, if the main wire from your solar array is rated to carry 4xIsc>37.04amps.

And rated for 4x1.25*Imp-array--You are OK with a (37.04 amp * 1.25=) 46.3 amps ~ 50 amp branch circuit and breaker. If you have a building inspector that just applies the numbers--1.56x 37.04 amps may be required no matter what.

In any case since your main cable rating is > the Isc-array rating... No "extra" breaker/fuse is needed.

More or less, you want the main cable to carry all the "reasonable" amount of current you would expect the array to output.

Imp and Vmp are close to "ideal" marketing numbers (25C/75F)... Solar panels run much hotter under full sun and Vmp is much lower (~80-90% of Vmp-std). Note that Imp does rise a small amount when the panels get hot (maybe 1-2% or so), but we normally ignore that portion.

-Bill

Just to copy my last post on pretty much the same subject:
With one breaker per string (parallel connected solar panel(s)), you will not need another breaker on the main run back to the charge controller (solar panel input).

The basic current requirements for NEC (there are more details in the NEC manual), is here:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Using the table as a minimum wire gauge (based on insulation type--Other items like fill in conduit, ambient temperature like on a sunny roof, can derate.

For solar power (arrays, solar charge controllers, AC inverter DC inputs, AC and DC connections for AC to DC battery charger), I suggest a minimum of 1.25x the maximum continuous current (for example, a 20 amp current * 1.25 derate = 25 amp minimum branch circuit rating--both wire and fuse/breaker rating). NEC ratings are for typical house hold and light industrial power needs. When you have a circuit that is running many hours per day, you need the 1.25x up rating for fuses or breakers--Your system will be much more reliable (no false fuse/breaker trips) and last longer (wiring gets hot and cooks insulation, binding screws get hot and ruin wire ends/breaker screws, etc.).

At a minimum, I suggest that you do two calculations. One based on voltage drop using a calculator (for solar arrays, typically 1% to 3% drop at Vmp-array voltage--For the charge controller to battery bank, I suggest 0.05 to 0.10 volt drop for 12 volts, 0.1 to 0.2 for 24 volt, and 0.2 to 0.4 volt drop for 48 volts).

For DC bus drops (to lights, DC input to AC inverter, etc.), I suggest 0.5 volt max drop for 12 volt bank (1.0 volt for 24 volt bank, 2.0 volt drop for 48 volt bank).

http://www.calculator.net/voltage-drop-calculator.html (one way distance for wiring)

The 1.56x solar derating you see--I think that is double derating (1.25x NEC derating * 1.25x solar = 1.5625x derating for variable solar harvest). We do not do that in industry (the double derating). The solar mfg. should already have the (for example) 60 amps "backed into" their specifications (60a/1.56=38.4 amps "solar" max current). There are various reasons why I think this is overkill (keeping the 1.25x derating for wiring+breakers--Very useful/good).

There is the Isc rating of panels--That is (roughly) the maximum current you see from the panels ever (short circuited). Double check that the Isc of all the panels added up is less than the rating of the single cable from the array (combiner box) to the solar charge controller input. Usually, you will see that one of the other calculations (wire diameter for voltage drop as an example) already is bigger than n*Isc of your parallel array. If the cable is larger diameter than the Isc-array rated current, then you do not need the separate "master breaker" on the solar charger panel input.

However, you still need the breaker/fuse on the solar charger output to the battery bank (located near the battery to stop short circuit current from battery to "any" wiring running off the battery bus)--sized for the wire/branch circuit rating or downsized to the charge controller rating (i.e, 60 amp controller * 1.25 NEC derating = 75 Amp minimum, round up to 80 Amp branch circuit+breaker rating).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 197 ✭✭✭
A fuse doesn't do much more than provide a disconnect and degrade system performance.  You can not get more than the short circuit rating of the panels and the likelihood of getting that are remote. A larger size fuse has the advantage of lower voltage loss. With multiple panels individual fuses do have a purpose. Multiple panels could feed total current into a single shorted panel and does serve a purpose fusing each panel. As far as wire exploding, even a #22 wire can handle like 20A if memory serves me.
• Registered Users Posts: 13 ✭✭
BB. said:
Short answer, if the main wire from your solar array is rated to carry 4xIsc>37.04amps.

And rated for 4x1.25*Imp-array--You are OK with a (37.04 amp * 1.25=) 46.3 amps ~ 50 amp branch circuit and breaker. If you have a building inspector that just applies the numbers--1.56x 37.04 amps may be required no matter what.

In any case since your main cable rating is > the Isc-array rating... No "extra" breaker/fuse is needed.

More or less, you want the main cable to carry all the "reasonable" amount of current you would expect the array to output.

Imp and Vmp are close to "ideal" marketing numbers (25C/75F)... Solar panels run much hotter under full sun and Vmp is much lower (~80-90% of Vmp-std). Note that Imp does rise a small amount when the panels get hot (maybe 1-2% or so), but we normally ignore that portion.

-Bill

Just to copy my last post on pretty much the same subject:
With one breaker per string (parallel connected solar panel(s)), you will not need another breaker on the main run back to the charge controller (solar panel input).

The basic current requirements for NEC (there are more details in the NEC manual), is here:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

Using the table as a minimum wire gauge (based on insulation type--Other items like fill in conduit, ambient temperature like on a sunny roof, can derate.

For solar power (arrays, solar charge controllers, AC inverter DC inputs, AC and DC connections for AC to DC battery charger), I suggest a minimum of 1.25x the maximum continuous current (for example, a 20 amp current * 1.25 derate = 25 amp minimum branch circuit rating--both wire and fuse/breaker rating). NEC ratings are for typical house hold and light industrial power needs. When you have a circuit that is running many hours per day, you need the 1.25x up rating for fuses or breakers--Your system will be much more reliable (no false fuse/breaker trips) and last longer (wiring gets hot and cooks insulation, binding screws get hot and ruin wire ends/breaker screws, etc.).

At a minimum, I suggest that you do two calculations. One based on voltage drop using a calculator (for solar arrays, typically 1% to 3% drop at Vmp-array voltage--For the charge controller to battery bank, I suggest 0.05 to 0.10 volt drop for 12 volts, 0.1 to 0.2 for 24 volt, and 0.2 to 0.4 volt drop for 48 volts).

For DC bus drops (to lights, DC input to AC inverter, etc.), I suggest 0.5 volt max drop for 12 volt bank (1.0 volt for 24 volt bank, 2.0 volt drop for 48 volt bank).

http://www.calculator.net/voltage-drop-calculator.html (one way distance for wiring)

The 1.56x solar derating you see--I think that is double derating (1.25x NEC derating * 1.25x solar = 1.5625x derating for variable solar harvest). We do not do that in industry (the double derating). The solar mfg. should already have the (for example) 60 amps "backed into" their specifications (60a/1.56=38.4 amps "solar" max current). There are various reasons why I think this is overkill (keeping the 1.25x derating for wiring+breakers--Very useful/good).

There is the Isc rating of panels--That is (roughly) the maximum current you see from the panels ever (short circuited). Double check that the Isc of all the panels added up is less than the rating of the single cable from the array (combiner box) to the solar charge controller input. Usually, you will see that one of the other calculations (wire diameter for voltage drop as an example) already is bigger than n*Isc of your parallel array. If the cable is larger diameter than the Isc-array rated current, then you do not need the separate "master breaker" on the solar charger panel input.

However, you still need the breaker/fuse on the solar charger output to the battery bank (located near the battery to stop short circuit current from battery to "any" wiring running off the battery bus)--sized for the wire/branch circuit rating or downsized to the charge controller rating (i.e, 60 amp controller * 1.25 NEC derating = 75 Amp minimum, round up to 80 Amp branch circuit+breaker rating).

-Bill
Thank you Bill,
I read throughout your text and i draw simplified version, is this in short what you mean ? If main PV array cable can carry current than is fine. No need extra fuse,only optional DC circuit breaker. Anyway does parallel connection necessary requires in line fuses as i draw?
You wrote "And rated for 4x1.25*Imp-array--You are OK with a (37.04 amp * 1.25=) 46.3 amps ~ 50 amp branch circuit and breaker. If you have a building inspector that just applies the numbers--1.56x 37.04 amps may be required no matter what"  Is this mean in line fuses are not needed?
• Solar Expert Posts: 3,814 ✭✭✭✭✭✭
edited February 2018 #7
Your drawing is incorrect and would never work, what's needed is a fuse in each positive, downstream of all the fuses is where they are all combined in parallel or tied together. The way outlined in your drawing all 3 fuses except the one on the left would be carrying too much current, the extreme right one would be the first to blow, if not all 3, this would mean 0 output. Remember a fuse is not a means of disconnect a suitable DC rated circuit breaker rated for total current  in the hone run would be required to protect those conductors, as well as providing a means of disconnect.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Registered Users Posts: 13 ✭✭
mcgivor said:
Your drawing is incorrect and would never work, what's needed is a fuse in each positive, downstream of all the fuses is where they are all combined in parallel or tied together. The way outlined in your drawing all 3 fuses except the one on the left would be carrying too much current, the extreme right one would be the first to blow, if not all 3, this would mean 0 output. Remember a fuse is not a means of disconnect a suitable DC rated circuit breaker rated for total current  in the hone run would be required to protect those conductors, as well as providing a means of disconnect.

You mean something like this?
• Solar Expert Posts: 3,814 ✭✭✭✭✭✭
edited February 2018 #9
That's the way, just remember if using fuses a means of disconnect, preferably at the controller is a useful addition.
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.