MNDC 175 Plus and connecting DC load off the bat bank

[rosiakr]

Basic wiring question. I'm working on my first PV project and have the MNDC175 Plus disconnect box - see MNDC175Plus.JPG attachment for my box prior to wiring. BTW - I added the neg. bus bar on the shunt just like they have it in the manual for MidNite 150 Classic CC on page 26 - that's where my PV- and BAT- (from Classic) in will connect to.

I will have an inverter, so I guess the set up is pretty standard. I chose the Plus disconnect box for the extra DIN rail in the upper left where I want to put a breaker for my DC load (bypassing the inverter). That breaker is currently not shown in MNDC175Plus.JPG. DC load aside - the rest of my connections will resemble what the 2nd attachment shows - MNDC_wiringExample.png (which comes straight from the DC disconnect's manual).

My question is - how do I add the DC load wiring to all of this? I know I have to somehow go from the batteries terminals to my future DC load breaker. I'd rather not run another set of 2/0 cables - I don't think I have enough room.
Thank you in advance.

-Rich

[ClassicCrazy]

For DC you would take a positive wire from the bottom lug of that big inverter breaker to the bottom of your DC load breaker. The negative of your DC would connect up to the shunt  negative buss bar  - opposite from  where the battery connects. You will only have one cable going from shunt to battery - all other negative connections will be on the other side of the shunt at your buss bar.

So no  you don't need more big cables from the battery for your DC loads. The battery terminals are effectively moved into the box at the inverter breaker and shunt.

Not sure which breakers you are using but in the example photo there is room for a DC breaker to bottom right below the big inverter breaker. There is room for 5 Din type breakers and three of the panel mount type. So one breaker is for PV input to controller, one is for PV output to battery , and then the others could be used for DC loads - but if you have a box with room for more breakers then you could put them there too. Look at the schematic for the box - it is a very good  schematic so you can see where everything hooks up. Ignore the stuff in the outline for ground fault protector because if you are using the Classic you don't need one.

Are you going to put a Whizbang on  ? You will find it easier to install on the shunt  before you do too much wiring .

Larry

[rosiakr]

Larry,

Thank you. After reading your response it makes perfect sense 

How do I know which DC breaker should I pick for the load? I think I'll have 4 AWG pos and neg wires going to that breaker and from there on maybe #10 AWG or even smaller? The load will be lights, office computers/printers and fans.

Thank you.

[ClassicCrazy]

You would size the breaker for the wire you are using.You will  have to determine your loads and distance and figure out the voltage drops.  You can put a smaller breaker on larger wire , but can't go bigger than it is rated for.  I don't know if you said what voltage your DC loads will be but that will be in your voltage drop calculations.

So if you had 10 gauge wire you would have to use 30 amp breaker max or smaller. 

Larry

[dgd]

Here are some photos of an MNDC250 with 250A breaker and three carling type DC breakers, 100A,80A  wired to two Classics and 60A to other DC loads.
Major difference to pics above is that the shunt has been repositioned to lower part of box and wired to a brass bolt busbar so that cables are connected using copper ring lugs and nuts/lock washers.
There is a +ve cable from lower 250A breaker connector to a copper bar that commons the three carling breakers. The red cables to these breakers are #4, the main red from battery bank is #0. It goes out of MNDC to a master lever disconnect and 400A T class fuse.
The black #000 (70mm2) goes from shunt direct to bank.
Note the WBjr attached below the shunt.

dgd

[ClassicCrazy]

Thanks for the photo dgd -
Never thought of tying those three breakers together like that

Larry

[dgd]

I just had too many problems with the din rail mounting breakers in the MNDC. Mostly because couldn't get #4 multistrand copper into the breaker terminal and never trusted it was tight enough.
It was getting weird having to retighten that grub screw every week or so and just getting at it with bent scredriver was a hassle. But mostly it was the very flexible #4 that defeated me.
Carlings with their posts and nuts and ring lugs on the #4 was the proper way.
I was also not smart enough to deal with the + and - markings on the din breakers as opposites were needed for connecting Classics as opposed to connecting a load so could not use a common bar over one end of din breakers in a row as my getting older brain would get confused as which in the row was down for out and up for in (my brain hurts just writing this).
Dgd

[ClassicCrazy]

I just had too many problems with the din rail mounting breakers in the MNDC. Mostly because couldn't get #4 multistrand copper into the breaker terminal and never trusted it was tight enough.
It was getting weird having to retighten that grub screw every week or so and just getting at it with bent scredriver was a hassle. But mostly it was the very flexible #4 that defeated me.
Carlings with their posts and nuts and ring lugs on the #4 was the proper way.
I was also not smart enough to deal with the + and - markings on the din breakers as opposites were needed for connecting Classics as opposed to connecting a load so could not use a common bar over one end of din breakers in a row as my getting older brain would get confused as which in the row was down for out and up for in (my brain hurts just writing this).
Dgd

I know what you mean - those Din rail breakers can be a pain to get the wires in . Seems like torquing on the screw on those will split the breaker apart too unless you hold it which you can't do too well when it is in place.

Larry

[rosiakr]

When I originally created this post, by DC load I envisioned wires going from my MNDC to several "outlets" where I would be able to plug in some computers/phones/lamps/network routers. Since these devices don't really need AC current I wanted to avoid the loss of power when going through an inverter.
After researching the topic a little bit I realized this is not as straight forward. I have 24 V battery bank and devices in my office are rated to different voltages (19.5 VDC, 48 VDC). And I think the only DC outlet/receptacle available is the car DC socket, like for cigarette lighter. Then maybe I would use one of those little 160 W car inverters which plugs in to a car round outlet and then has an AC outlet, but wouldn't my ampacity from battery bank be too high? Those little car inverters are rated at 15 A max. I think using inverters is what I was hoping to avoid anyways...

Also these devices like computers or phones have wall warts which transform from AC to DC. I think I'm becoming more confused the more I think about it 

Not even sure if what I'm trying to do is possible or practical and I'd appreciate any guidance. Thank you!

-Rich

[ClassicCrazy]

You will have to decide on some type of DC plug that isn't like an AC outlet.
The ham radio community decided to use 30 amp type Anderson power poles for their universal DC plugs on equipment.
I don't really like those after making my own power pole distribution block. So now I use XT60 RC type plugs on things . They are inexpensive , polarized, and easy to install. If you want an outlet type on one end not sure if there are those commercially available in XT60 but maybe . Or make your own.

You can buy inexpensive boost or buck DC to DC converters which are pretty inexpensive depending on the current you need to run. I have been putting them on lots of things now and just bought 10 step down for under a buck each. There are boost that will also take voltage up.
One example here - 4 to 35v input and adjust the output
http://www.ebay.com/itm/140716238408?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

So if the wall wart is under 2 amps you can cut off the cord and hook up one of these adjusted to the correct voltage .

There are also boost converters but pay attention to the currents available on them because if it is called 3 amp you might see in the specs it is really 2 amp continuous and that may not be enough for something like your laptop computer running while charging battery in it.

Yeah you don't want lots of those little car inverters - if you were to look at the idle current draw on them you would be wasting a lot of power.  If you start adding up all the looses in voltage drop on DC circuits, ineffiencies in DC - DC converters, etc you might just come to the conclusion that it is better to just have one inverter running and AC circuits going where you need them.

Larry

[schroew]

I did something like this for wall wart devices. I used a rig runner and a step down converter. I also used the usb buddy to take the 12V down to 5v for blink camera system.