Wire Gauge and Fuses

[danrclem]

I'm building a small solar system that will hopefully lead me to building a larger one later on and I have some questions. I have four 100W solar panels. I have a 100V 40 amp charger. I have a 700 watt pure sine wave inverter. I have two 12V lead acid batteries that each have an ah rating of 106.

The solar panels are 100W, have 20.4 optimum operating volts and an optimum current of 4.91 amps.

I'm going to run the panels in series so if I'm figuring right it should be 400 watts, 81.6 volts and 4.91 amps. If I use 10 gauge stranded wire that should take me a long way from the panels to the controller, correct? Any idea what the max distance would be? The inverter came with 6 gauge cables so I should be able to use 6 gauge cables across the battery also, correct? The batteries will be ran in parallel because the inverter is for 12 volts only.

Since the panels have an output of 4.91 amps then would a 10 amp fuse be ok for that? The inverter watts divided by 12 volts is 58.33 so would a 60 amp fuse be ok from the inverter to the battery?

Would a 100 amp fuse between the battery and the inverter be ok?

Can somebody point me to some connectors that would work with my solar panels? Are there any particular fuses that would work best with this application?

 

[OffGridForGood]

The solar panels are 100W, have 20.4 optimum operating volts and an optimum current of 4.91 amps.
I'm going to run the panels in series so if I'm figuring right it should be 400 watts, 81.6 volts and 4.91 amps. If I use 10 gauge stranded wire that should take me a long way from the panels to the controller, correct? Any idea what the max distance would be?

You can run this through a DC cable calculator - I used the Omni online DC calculator 80v 6 amps 3% votage drop 10ga = one way distance max 170 feet. (are your PV more than this distance?) "optimum solar voltage" means nothing for wire sizing - you need the VOC voltage x 4 and adjust for coldest day, likely 100v is adequate.

The inverter came with 6 gauge cables so I should be able to use 6 gauge cables across the battery also, correct? The batteries will be ran in parallel because the inverter is for 12 volts only.

Good for shot run up to about 6 feet from battery to inverter. Larger ga cables will have less losses, and stay cooler, be reuseable for larger system later. But 6ga up to 6 feet long will get you started.

I have a 700 watt pure sine wave inverter. I have two 12V lead acid batteries that each have an ah rating of 106.

The two batteries will be connected in parallel - double the current but holding the voltage at 12v. since these are lead-acid most people would recommend to keep them above 50% State of charge (SOC) to prolong the life of the batteries. so 106 Ah each x 2 x 50% = 106 Ah useable.
700W inverter running at 12v = 58Amps however as the battery voltage drops the amps will increase. if you run anything with a significant surge start up load, the amps will be higher than 58 for a short few seconds. We normally recommend Class T fuse between the batteries and the inverter. If it were my set up, I would consider using a 100A Class T fuse. Gives you some room for the expansion you noted also. A 100A Class T fuse is pretty handy later (on a bigger set up) for each battery to a bus bar connection, so re-useable. Blue Sea make a good quality Class T fuse and holder.

Since the panels have an output of 4.91 amps then would a 10 amp fuse be ok for that?

yes, DC rated fuse. I would just use an inline MC-4 fuse holder for this small set up with four panels. A dedicated DC disconnect would be a good idea so working on the system is safe and the PV can then be easily disconnected.

The inverter watts divided by 12 volts is 58.33 so would a 60 amp fuse be ok from the inverter to the battery?

I could be, if you never push that small inverter too hard. 700W is tiny, and it will be easy to over-load such a small inverter ie over 58A is possible.

Would a 100 amp fuse between the battery and the inverter be ok?

That would be my recommendation, especially since you noted future expansion.

Can somebody point me to some connectors that would work with my solar panels? Are there any particular fuses that would work best with this application

PV connections - use MC-4 connectors, and proper PV rated wire where this is exposed to sunlight/weather. My advice is to look on Amazon for a small set of MC-4 connections in a kit with the crimping tool and plastic tools for disconnecting the MC-4's. This will give you everything you need for your small set up, and having the proper crimpers will mean you get proper connections to the wire.

 

[danrclem]

You can run this through a DC cable calculator - I used the Omni online DC calculator 80v 6 amps 3% votage drop 10ga = one way distance max 170 feet. (are your PV more than this distance?) "optimum solar voltage" means nothing for wire sizing - you need the VOC voltage x 4 and adjust for coldest day, likely 100v is adequate.

Good for shot run up to about 6 feet from battery to inverter. Larger ga cables will have less losses, and stay cooler, be reuseable for larger system later. But 6ga up to 6 feet long will get you started.

The two batteries will be connected in parallel - double the current but holding the voltage at 12v. since these are lead-acid most people would recommend to keep them above 50% State of charge (SOC) to prolong the life of the batteries. so 106 Ah each x 2 x 50% = 106 Ah useable.
700W inverter running at 12v = 58Amps however as the battery voltage drops the amps will increase. if you run anything with a significant surge start up load, the amps will be higher than 58 for a short few seconds. We normally recommend Class T fuse between the batteries and the inverter. If it were my set up, I would consider using a 100A Class T fuse. Gives you some room for the expansion you noted also. A 100A Class T fuse is pretty handy later (on a bigger set up) for each battery to a bus bar connection, so re-useable. Blue Sea make a good quality Class T fuse and holder.


yes, DC rated fuse. I would just use an inline MC-4 fuse holder for this small set up with four panels. A dedicated DC disconnect would be a good idea so working on the system is safe and the PV can then be easily disconnected.

I could be, if you never push that small inverter too hard. 700W is tiny, and it will be easy to over-load such a small inverter ie over 58A is possible.

That would be my recommendation, especially since you noted future expansion.

PV connections - use MC-4 connectors, and proper PV rated wire where this is exposed to sunlight/weather. My advice is to look on Amazon for a small set of MC-4 connections in a kit with the crimping tool and plastic tools for disconnecting the MC-4's. This will give you everything you need for your small set up, and having the proper crimpers will mean you get proper connections to the wire.

Thank you for this information. I just wanted to know for sure what would or wouldn't work. I haven't figured out where I'm going to locate the PV but it should be a lot less than 170 ft. and maybe less than 100 ft.

If I build a larger system later, I'm planning on it being hybrid but I may be able to reuse some of what I have now.

Here's another question I'm wondering about. If during a power outage would it be ok to throw my main breaker from my grid power and throw every other breaker in the box with the exception of maybe two 15 amp circuits. I know the electric company probably wouldn't like it since they may have people working on the lines but I would throw the main. Thoughts?

 

[Dagoth Ur Does Solar]

Here's another question I'm wondering about. If during a power outage would it be ok to throw my main breaker from my grid power and throw every other breaker in the box with the exception of maybe two 15 amp circuits.

Definitely against code everywhere, and probably criminal in some places.

This is what a mechanical interlock is for, to prevent the possibility of the main breaker being on while the generator input is on.

 

[danrclem]

I was thinking that it wouldn't be allowed and I can see why. I didn't know about an interlock but I'll check it out.

 

[OffGridForGood]

...Or wire in a Critical Loads Panel - put your fridge, freezer bathroom lights etc on the CLP so you have an easy way to power up a few things you need.

 

[danrclem]

I'm getting ready to do some more ordering tomorrow and have a question. I have some solid 10 awg pump wire. Would it be suitable to use this from the panels to the charger and would I be able to crimp the solid wire when using MC-4 connectors?

 

[OffGridForGood]

I have some solid 10 awg pump wire. Would it be suitable to use this from the panels to the charger

No, you need PV rated cable for Solar panels to the charge controller

and would I be able to crimp the solid wire when using MC-4 connectors?

No, MC-4 connectors need stranded wire for the crimping process to grip the wire properly.

PV connections - use MC-4 connectors, and proper PV rated wire where this is exposed to sunlight/weather. My advice is to look on Amazon for a small set of MC-4 connections in a kit with the crimping tool and plastic tools for disconnecting the MC-4's. This will give you everything you need for your small set up, and having the proper crimpers will mean you get proper connections to the wire.

From post #2

 

[danrclem]

I'll have a fuse in between the battery and inverter so do I need a fuse in between the controller and the battery. If I do, what amp fuse do I need. It's a 40 amp controller. A 40 amp controller so a 40 amp fuse?

Thank you very much for your help.

 

[OffGridForGood]

There are various points of view on if a fuse is needed between the SCC and the battery, based on the idea that a 40A charger can't exceed 40A so long as the wire is sized above 40A you shouldn't need a fuse here. That all said, I am part of the 'shit can happen so be prepared' crowd so I would use a fuse. If the charger is 40A, a #8 AWG cable (or larger) and fuse rated for 40A makes sense. I assume the cable length is not very long in this case.

 

[danrclem]

The distance between the charger and battery is probably only going to be around three feet (five at max) so I'll go with a 40A fuse.

I'm working on finding some PV wire right now.

Thanks again.