Imput capacity of the Classic 200

[morasolar]

I am in the process of setting up a grid tie 48v solar system. Using 30.6v, 8.43a rated solar modules, how many modules can I string in series and how many strings will the Classic 200 safely handle?   

[TomW]

Your best bet is to use the String Sizing Tool:

http://www.midnitesolar.com/sizingTool/

[vtmaps]

I am in the process of setting up a grid tie 48v solar system. Using 30.6v, 8.43a rated solar modules, how many modules can I string in series and how many strings will the Classic 200 safely handle?   

Welcome to the forum,

After you use the string sizer, you will get a number of panels that you can put in series.  But just because you can use that many in series doesn't man you should.  All of the classics are more efficient and have greater watt-handling capacity with lower string voltages.

With a 48 volt battery and panel Vmp = 30.6 volt, your OPTIMUM configuration will be strings of 3 panels.  Using  Classic 150 you can handle 4800 watts with that configuration.

The only reason to use higher voltage strings is if the distance between the array and the controller is very large.  You will have less power handling capacity with higher voltage strings.  How far is the distance?

--vtMaps

[morasolar]

Thank you all for your response, my distance should be less than 75'.  Isn't higher voltage desirable, to decrease resistance. I was hopping for 4 or 5 strings of 4 modules, but the string sizing tool tells me that with the Classic 200, 3 strings of 4 modules is the right configuration for a hot Florida roof.

[Westbranch]

The closer the string Voltage is to the bank voltage, the less power that is lost to heat in the CC due to down conversion of ie 91V +- TO 48V WILL NOT GENERATE AS MUCH HEAT  as 120V or 150V will.

[vtmaps]

The closer the string Voltage is to the bank voltage, the less power that is lost to heat in the CC due to down conversion of ie 91V +- TO 48V WILL NOT GENERATE AS MUCH HEAT  as 120V or 150V will.

Quite right.  To explain further: A higher voltage means a bit less line loss in the cable, but more loss in the controller.  The higher voltage very often means that you are trading heat production in the cable for heat production in the controller.

Suppose you had 15 panels (Vmp = 30.6 Imp = 8.43) which is an array size of 3870 watts.  Consider the numbers for 75 ft (one way) of #6 copper cable.  I will give the numbers for 5 strings of three panels (string voltage of 91.7999) and 3 strings of five panels (string voltage of 153 volts). 

With Vmp of 91.8 the Classic 150 can handle over 4800 watts.  The voltage drop will be 2.75%.  At full nameplate power (3870 watts, seldom achieved, and only briefly) the power loss in the #6 cable will be 107.4 watts.  Realize that the power lost in the cable goes as the square of the power that the controller is handling, so at more typical power levels the power lost in the cable is much, much less.  This is a good conservative design... your peak power is almost 1000 watts below what a Classic 150 can handle.

Consider what happens with the same array configured with a Vmp = 153.  The same #6 cable has a 1.0% voltage drop and at full nameplate power (3870 watts, seldom achieved, and only briefly) the power lost in the cable is 38.7 watts.  But what about the controller?  You will need a less efficient Classic 200 to handle the higher voltage.  The Classic 200 with a 153 volt input voltage is rated to handle just a bit below 3800 watts.  Your array is more than it can handle... not good conservative design.  The reason is efficiency.  Midnite has not produced efficiency curves for the Classic, but the Midnite engineers have indicated on these forums that the curves are probably similar to the Outback efficiency curves (no surprise... the cofounders of Midnite cofounded Outback). 

Getting back on topic, the Outback and probably the Classic are about 2% more efficient at the lower string voltage.  At full power, 2% of a 3870 watt array is 77.4 watts.  That's more than the difference in cable loss between the two voltages.  And the 2% efficiency difference is throughout most of the power range, where as the cable loss decreases as the square of the input power decreasing.  That means the advantage shifts much more in favor of the low voltage at the more typical power levels that an array runs at.

As I wrote in the first paragraph of this post: The higher voltage very often means that you are trading heat production in the cable for heat production in the controller.  If it makes you crazy to lose power in the cable, just use #4 cable instead of #6.

Getting back to conservative design, the reason that the Classic is rated to handle so much less power at higher voltages is the 2% hit to efficiency.  Try putting a 77 watt lightbulb inside a metal box the size of a classic... it gets very hot!  At higher voltages the controller has to be derated to handle all the extra heat.

Here's a quote from Robin Gudgel, co-founder of Outback and Midnite:
http://midniteforum.com/index.php?topic=324.msg1869#msg1869

Max power for the Classic 250 is somewhere between 3000 and 3500 watts depending on your system configuration.
<snip>
I would not put more than 3000 watts on the Classic 250 myself. It will just run hot. That is never a good idea to run the controller flat out at its maximum day after day.

--vtMaps

EDIT: I changed 91.eight) to 91.7999) because this forum turns the "point eight)" into some sort of smiley.

here's what happens if I type 91.eight) as all numbers:
91.

[dgd]

Vtmaps,

That was a good detailed explanation 

Sort of reminded me of when the MX60 appeared and the option was there to get it up to 70 amps output. Then it did heat up.
Various solutions for this heat dispersion were interesting, from the fitting of a small cross flow extraction fan to one of the wire openings to removing the bottom cover and rigging a boxer fan to push air around.
I just used some 10mm thick aluminum plate between the MX60 and plywood wall which seemed to disperse excess heat.
Have not seen the C150 get hot enough to consider this

dgd

[mike90045]

....I just used some 10mm thick aluminum plate between the MX60 and plywood wall which seemed to disperse excess heat.
Have not seen the C150 get hot enough to consider

I think I'm going to get me some of that stuff and sandwich it into my wall mount.  Save the fans that run all day.

[Westbranch]

have a machinist or welder make it like this (bottom view)
     _______
__|             |__


where the heat source is on the top solid line and the feet are attached to the wall line and it will vent naturally when mounted vertically on the wall

[mike90045]

I was thinking of a U channel, with fins about 2" outboard from the Classic:

|                      |
|___________|   easier to bend, and more free air contact, more convection.

[Westbranch]

Morning Mike, looks like we are the same thing, so just to be clear here is a bit more of what I meant


             _   Classic__
       ___|                   |__  < Angle Alum.
   
Wall---------------------------------------