Need help checking my math for wiring new system

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).

My advice:
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

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?

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