# Need help checking my math for wiring new system

Registered Users Posts: 8
Hi all!
Could someone tell me if this sounds right? We are setting up our 24 volt system, we have 8 Sharp 224w 36.6VOC 8.33A panels in two strings of four. Which in my calculations, each string is 146V and 8.33A so paralleled together would be 146v and 17.66A. We plan to run #8 awg wire 120 feet in conduit to a Tristar MPPT-45. My math says the voltage drop is less than 2%. Does this sound correct? Will this all work?
Also, do we need a combiner box when paralleling 2 strings? Where do we put a fuse if not?
The controller, inverter, shut-offs and such have all been put together and wired by our local solar shop. So we are set there.
The batteries, well the batteries are not perfect but crazy cheap. We got 6 12 volt 90Ahr gel batteries for 30 bucks a piece. They were back up batteries for a tech company. @ years old never discharged more than 5%. Everyone says you kill your first set of batteries, at least with these we didn't spend much.
Our electricity needs are modest, having lived off-grid using a generator for 6 years. 9 months out of the year. Rainy season we are not here very often. I imagine though, that we will find all kinds of things we want to plug in when we are able to do so.

• Solar Expert Posts: 3,738 ✭✭✭✭
Re: Need help checking my math for wiring new system
mitzilani wrote: »
Hi all!
Could someone tell me if this sounds right? We are setting up our 24 volt system, we have 8 Sharp 224w 36.6VOC 8.33A panels in two strings of four. Which in my calculations, each string is 146V and 8.33A so paralleled together would be 146v and 17.66A. We plan to run #8 awg wire 120 feet in conduit to a Tristar MPPT-45. My math says the voltage drop is less than 2%. Does this sound correct? Will this all work?
Also, do we need a combiner box when paralleling 2 strings? Where do we put a fuse if not?

First, the combiner. No you do not need one if you only have two parallel strings. You do need a fuse or circuit breaker between the panels and the controller. That said, you will need a combiner box because your scheme to use only two parallel strings is probably not viable.

There are a number of problems... the Voc of four panels in series is too high for the morningstar controller. If you must have such high voltage (4 panels in series), you could use a Midnite Classic 200 or 250 volt controller.

Next problem... The Amp rating of the morningstar and other MPPT controllers is the OUTPUT to the batteries. If your array is pushing 1800 watts into your controller, your controller is outputting almost 75 amps at 24 volts. 75 amps is way over the capacity of a 45 amp controller.

You could use a Midnite classic 200 or 250 controller. Your array would be near the capacity of that controller and the controller would be running very very hot because the controller is less efficient at down converting a high panel voltage to 24 volts than a lower panel voltage to 24 volts.

The advantage of high voltage panels, as you realize, is that you can reduce line loss on a 120 ft cable. (By the way, when you calculate the voltage drop of the cable, you should use the Vmp, not the Voc. Thus your voltage drop is higher than you calculate).

If you want to stick with 24 volt system, you should buy a Midnite classic 150 or an Outback FM80, configure your panels in four strings of 2, and use very heavy cable (#2 gauge copper, or equiv aluminum) between the combiner and the controller.

If you want to go to 48 volt system, you will need another panel so that you can have three strings of three. You will then need #4 or #6 cable and you will be able to use an MPPT controller rated at 45 amps.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 1,218 ✭✭✭✭
Re: Need help checking my math for wiring new system

vtmaps covered it well. The thing about wiring loss to the array is that you have to compare that to controller losses to keep things in perspective. Here we have 60ft each way, 3 strings of 2, and with 6awg the cable losses were much less than the losses you incur in the controller by running it at a higher voltage. This isnt an overly easy thing to calculate. But you can see how i did it in the link my sig. One of these days im going to develop a model for this based in the published FM80 power curves.

Also Morningstar dont have the Voc tolerance that midnite has. Remember that in cold weather that 145v could go quite a bit higher.
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar

• Registered Users Posts: 8
Re: Need help checking my math for wiring new system

Thanks, I'm pretty new at all this as you can see. I had thought 48 volts would be the way to go but I was over-ruled. We do have another panel. So if we do go 48volts the 45 amp controller would not be outputing too much for the batteries? But then we already have the 24 volt inverter.
So to make it work for a 24 volt system we'd need a new controller and a bunch of #2 wire and to make it work for a 48 volt system we'd need a new inverter. Oh damn.
Got to talk to the guys at the solar shop, seems to me they made a mistake. Right?
• Solar Expert Posts: 1,925 ✭✭
Re: Need help checking my math for wiring new system
mitzilani wrote: »
to make it work for a 48 volt system we'd need a new inverter.

You can sell your old inverter and recover part of the cost of a new one.
• Solar Expert Posts: 1,218 ✭✭✭✭
Re: Need help checking my math for wiring new system

Sounds like the shop sold you two bits of gear that dont work together? Those guys are sometimes real busy, and they arent really being paid to design your system. But you ought to be able to exchange one of them, right?

As to which one, youll need to redo the numbers for both scenarios to compare them. Starting with the Vmp of your panels?
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar

• Registered Users Posts: 8
Re: Need help checking my math for wiring new system

Alright, now I'm thoroughly confused. First, the Vmp of the panels is 29.28
We went back to the shop and they suggested the outback fm 80. (They were actually being paid to design our system, since they wired together the panel with components they recommended) But be that as it may, I'm still being overruled on switching to 48volts. So selling the inverter is not happening right now. So working with what we have...
Would we really need #2 wire to run 120 ft from the panels to the controller? Why four strings of 2? Why not 2 strings of 4?
Too many variables!
• Solar Expert Posts: 3,738 ✭✭✭✭
Re: Need help checking my math for wiring new system
mitzilani wrote: »
Would we really need #2 wire to run 120 ft from the panels to the controller? Why four strings of 2? Why not 2 strings of 4?

2 strings of 4 will have a Voc that is too high for the FM80. period.

As far as the cable is concerned, you propose to move high current (33 amps) with low voltage (60 volts) through 120 ft of copper. If you use #2 copper you are looking at a 2.2% voltage drop. If you use #4 copper you will have a 3.5% voltage drop. It might save you some dollars to use #0 aluminum cable.

Another option: Xantrex makes an expensive charge controller that can handle very high voltages... with that controller you could put the 8 panels in series and use #14 copper cable. Morningstar has announced (but not shipping yet) a similar high voltage controller.

--vtMaps
4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
• Solar Expert Posts: 1,218 ✭✭✭✭
Re: Need help checking my math for wiring new system

Can you built a nice wee shed under the array, put the inverter there?
1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
http://zoneblue.org/cms/page.php?view=off-grid-solar