MNPV6 used as DC load center

[ClassicCrazy]

I am helping rewire a friends PV system. The original installer many years ago had used a regular breaker box with AC rated breakers for their 12 volt system . The 12 volts actually comes from a 24v battery via DC to DC step down converter.  The present DC feeds 10 circuits 12 gauge wire only used for DC lighting. The old system had two of these 12 gauge wires on each breaker. 

My plan was to replace this AC box using a MNPV6 combiner box with DC breakers .  I was going to use 20 amp breakers but would it be better to downsize the breakers to 15 amp for added safety ? I also wondered if it is okay to put two or more of these branch circuits on one breaker ? 

I have to do some more planning but I also was thinking of splitting the MNPV6 buss bar and making one side 24 volt and the other side 12 volt . They are replacing their 12 pump with a 24 volt pump so I would like to integrate that in.

Anything I do will make the system better than the hodge podge it is now. There are no inspectors or code involved but of course I want to do this properly !

Thanks for any advice.

I have also read a lot of the posts in this section which offer a lot of good guidance. But there is a lot more to go through - what a great resource. 

[tecnodave]

Classic crazy

I think you should use the MNPV-12 LV to do this. The breaker terminals are not rated to clamp 2 wires. If you need to feed 2 circuits off one breaker you will need to join the wires in an approved wire connector rated for the number and size of wires that you are joining. There is a lot of built in safety in the National Electric Code. 12 Guage wire used in marine applications is rated at 45 amps, true it is slightly heavier and better insulation than normal NM (Romex) used in housing. Don't worry about downsizing the breakers to 15 amps, common Romex will handle 20 amps in a DC circuit easily. The only exception to this is anything that would run continuously. This is not normally found in such a system but if for instance you have a water pump rated for 20 amps that would or could run for more than 2 hours continuously then the wire would need to be down rated to 80% ampacity, i.e. 12 wire is rated 16 amps continous duty but 20 amps for on/off loads with less than 2 hours of continous on time.
I have been living in off grid housing for years using type THHN/THWN wire (normally found in electrical conduit) in "smurftube"  (blue plastic flexible conduit) on 12/24 volt systems for years.
This wire has a much better thermal rating than common Romex or automotive wire as it is rated 90 degrees centigrade.
There are few AC rated panels that withstand the mean arc of DC. The MNPV breakers sold by MidNite are a good deal as most DC breakers are more in the range of $40.00 each. I use some Schnieder electric Multi 9 line breakers but these are also in the $40.00 ballpark and will not be found in any common retail outlets, not even in most electrical supply houses. Square D  QO line is the only common breaker line that I would recommend for this use but only for 12/24 volt as the battery voltage of 48 volt systems will exceed 56 volts at times, they are rated 48 volts DC max. Din rail does have the advantage of more flexible wiring schemes. My newest panel is a MidNite MNPV-12 LV in which I do not use the provided buss bars as the breakers do not feed off a common source. I have multiple battery systems 12/24 volt and also 24 to 12 volt DC-DC converters so I can get tightly regulated voltages for sensitive electronic devices that I do not want to expose to high charge voltages that RE batteries have during equalization. The 150 volt rating of the MNPV line provides a large margin of safety. The other advantage of MNPV breakers is that you can size the breakers to protect the device under load and not just the wiring, for instance I use a 5 amp breaker on 12 Guage wire to protect my 12 volt 48 watt LED/LCD flat screen TV, my feeling is that the $300.00 tv is worth a $12.00 breaker and a dedicated service. Also my electronic medical devices each have a dedicated breaker and service designed to protect that device. AC panels are only to protect the wiring. A very large percentage of home electrical fires are the device plugged in failing, not the service to that device.

MNPV breakers are polarity sensitive so be sure which way that the fault current would be flowing, the line or + side always goes to the higher voltage, in your case to the battery feed.

In my 48 years of doing electricity my experience is that the fail point is not in the wire 98% of the time. It is in the connections. Bad joints, switches not rated for the use, poor quality connectors, cheap breakers, etc.

I do tend to err on the side of safety as I am a commercial electrician and refuse to cut my standards in order to save money. I have seen houses wired with 15 amp wire with 30 amp fuses and have seen the resulting damage, melted insulation, etc. that people have done to save money. I can only ask them "What is your family worth?"   I absolutely refuse to attempt to repair such a damaged system and insist that the only feasible repair is to replace the entire circuit from scratch. I use only the best available, and I sleep very well, thank you.

I have used only the DC rated MN breakers, not the AC rated ones, and can see that they are of a very good quality and what I have seen of Midnites boxes is that they are of very high quality as well. The Carling panel mount breakers offered by MidNite have a stellar reputation in the industrial field and I use those breakers as well to protect my charge controllers, inverters and DC-DC converters.

I hope that you will find this information useful. I want to teach others from my experience so I can free my time to go fishin! Feel free to direct any questions that you want to me and I will help as much as I can.


NOTE: you did not state wire type, if it is not Romex please let me know what type it is that you are using.


Tecnodave

[ClassicCrazy]

Thanks for the detailed answer Dave !  You have pretty much answered most of my questions. Yes the present installation is using Romex wire which looks to be good quality so that shouldn't be an issue from what you told me. 

There is one pump circuit that I did not see any fuse or breaker on. It is a 12 volt slow pump that is on 10 gauge wire. It sounds like I should put a smaller breaker on this one - so a 20 amp breaker should do that job for that one according to what you said about down rating the breaker.

Could I twist and solder two of the Romex circuits together along with one other 12 gauge wire to feed the breaker with that one wire ? I would of course keep this junction in the breaker box and insulate it well. There is not going to be much load on any of the circuits since they are being  used  for LED lighting that won't add up to more than an amp or two . 

I have a related question about 12 volt wiring . From my beginnings in solar back when everything was 12 volts we were always supposed to be very concerned about any oxidized connections causing some resistance and voltage drop. So soldering was always the thing to do when possible to keep this from happening. But now I have one of those inexpensive Harbor Freight hydraulic crimpers and the ring terminals I have put on with it I doubt any solder would ever get in there even if you tried cause the wire is smashed so tight in there. Do you think joining two wires together using a hydraulic crimper in some kind of compression terminal would just as good for low voltage as soldering ? 

Also curious about what your choice for terminals are for higher current DC ? In the ham radio world they adopted using Anderson power poles . They are okay but fussy about getting the wire crimped properly in if you try to use larger wire. If you solder it  also then that can make it hard to slip in the housing. The smaller  Anderson power poles when mated don't lock together though they do tend to stay together. And if the terminal didn't click in the housing quite square and is cocked a bit it can make a poor connection.   I have read about round terminals that the RC hobby people use and might get some of these to try out. They look like they make much better contact than Anderson and are rated for a lot higher amps.  Have you ever seen or used those ?  I was just trying to find a link to show you what I was talking about and found this very good review on a lot of DC connectors https://sites.google.com/site/tjinguytech/reviews/rc-connectors

Your setup sounds very well engineered !  That is the nice thing about doing your own so you can tailor it to suit your own needs.

I have also seen some electrical and other trade type work that was done so shoddy it is scary ! 

[vtmaps]

Do you think joining two wires together using a hydraulic crimper in some kind of compression terminal would just as good for low voltage as soldering ? 

Better.  A good crimper creates an airtight space.   Solder can make a stranded cable more brittle... the copper gets little stress cracks right at the edge of the solder.

--vtMaps

[ClassicCrazy]

Better.  A good crimper creates an airtight space.   Solder can make a stranded cable more brittle... the copper gets little stress cracks right at the edge of the solder.

--vtMaps
[/quote]

yes I have seen that brittle effect and wires breaking in that area in the past.

[tecnodave]

ClassicCrazy,
I use a hydraulic crimpers but of a much better quality than those at harbor freight bet those should do as well.  On terminals , don't use the cheap ones as they will fail in time, Home Depot sells Buchanan brand terminals rebranded as GB Gardner Bender brand that are of excellent quality. I use only Brundy, Ideal, ETC, and Buchanan brands as these are of industrial quality, nickel plated pure copper. The ones found at kragans, o'reilly, radio shack, auto zone are made in China and are of suspect quality. They are cheaper but you get what you pay for, I have had many fail. As to oxidizing you will get this anytime you use dissimilar metals in an dielectric environment, the simple cure for this is to use Ideal brand No-Alox.  No-Alox can be found at Home Depot and any electrical supply house, about $10. / 4 oz. bottle, enough to do 10 electrical panels everywhere.  This compound is meant for joining aluminum wire to copper wire. I have found it to prevent oxide corrosion in extreme conditions including agricultural applications where the wiring is exposed to acidic conditions, pesticides, fungal sprays and our deadly salt fog here in the Pajaro Valley. It also prevents the screws on breakers from seizing. As a commercial electrician I am never without it as it prevents call backs.

If you have to join multiple strands of Romex I would suggest tightly twisting the wires right handed, cut ends flush, coat with No-Alox and use Ideal brand "wing"nuts, for stranded wire the knurled variety are better as they grip better on the fine strands.

Hope this helps

Tecno

[tecnodave]

Classic Crazy,

I missed the question about Anderson Power Poles, I like them, they are universal in industrial fork lifts for the main battery packs and battery chargers where the current is in the order of 300 amps.
I have seen them fail due to improper termination of the wires into the terminals, on these I do use solder but not the more common tin/lead variety but I use "silver solder" , incorrectly named as it is really silver braze alloy. This is now required in water piping as there is no lead and it is much stronger. Spotless cleaning it the trick here as any solder or braze depends on clean metals. On my battery connections I use Anchor brand marine wire as it is nickel plated pure copper silver brazed to nickel or tin plated connectors. I use glue lined heat shrinks to insulate. I use Au-15 silver braze alloy or the stuff sold at Home Depot for household plumbing, no lead, and a MAPP gas torch with a tiny flame, with wet rags to control temperature on wire insulation, I use 350 amp power poles for my emergency battery disconnects and never had a problem with them. I also use standard arc welder connectors, plugs and sockets for the same use but is possible to get to a situation of reversed connections.

[tecnodave]

CC

I will throw in one more item, a tool that I find indispensable.....the infrared heat gun.....they spot bad connections, hot breakers, weak frayed wires where there is no obvious damage. When I open an faulty electrical panel my first tool in is the infrared heat gun, it points to the fail mode very quickly, they are cheap now but I bought the Fluke high end one as I am a tool junkie and technologist. I can spot bad connections in utility wires 15 feet in the air from the ground! Indispensable in my trade, they are now under $40.00,  much less than I paid, but as with all tools, they pay for themselves, now if I could only convince the lady friend of that. Lol , a $1200. Hammer??   A Bosch roto hammer!  Never did convince her of that one!

[ClassicCrazy]

I am surprised you don't have a thermal camera for your collection.  A recent article in Home Power showed how to use them to diagnose faulty solar panels too.  And if you had a certain model  $1000 Flir you can make it into an $8000 Flir with a little tweaking shown in this link   http://hackaday.com/2013/11/04/manufacturer-crippled-flir-e4-thermal-camera-hacked-to-perform-as-high-end-model

Yeah I have used some of that anti-oxidant compound before mainly in the screw type compression fittings. Probably wouldn't hurt to put it on all battery connections and terminals.

The marine wire is nice - I spent extra to have my 4/0 battery cables made by a marine place and they also used some of that good heat shrink. I should get some of that to have around.

/

[mtdoc]

I am surprised you don't have a thermal camera for your collection.  A recent article in Home Power showed how to use them to diagnose faulty solar panels too.  And if you had a certain model  $1000 Flir you can make it into an $8000 Flir with a little tweaking shown in this link   http://hackaday.com/2013/11/04/manufacturer-crippled-flir-e4-thermal-camera-hacked-to-perform-as-high-end-model

It just so happens that I got an early Xmas present yesterday.  A Flir E4 now "upgraded" to an E8 via Mike's hack. Lots of discussion of this over at the EEV blog.

I'll gets lots of good use out of this.  Just today I used it to identify an overheating IC on a misbehaving function generator I was testing.

And guess which Classic had it's array still in the sun when I took this shot:

[Westbranch]

what does it show for energy leakage out of windows?  or a human body?
Makes me think about Airport screening...

[mtdoc]

what does it show for energy leakage out of windows?  or a human body?
Makes me think about Airport screening...

Well because it's pretty cold outside now (though not quite as cold as where you are!) my windows are pretty uniformly cold. Human skin temperature varies more. 

It works by measuring IR radiation and there are several factors which can impact the imaging  like differences in  thermal emissitvity, reflection, etc -   but I'm just now trying to learn more about thermography.

It is pretty cool to be able to see in the dark with it though...

[tecnodave]

I did evaluate an flir from Fluke when they were new but they were $8000 then and I don't need a picture that bad, my $200 Fluke infrared thermometer does a good job showing me hotspots. I get in there , fix the problem, get paid and move on.

These days I mostly rewire whole houses from scratch for house flipping crews, I do not work for consumers, only professionals in the field, tearing out outdated electrical panels and new complete re wires on 30-50 year old houses so not that much diagnostics, patching junk is not that profitable.
My other cash cow is the local Watsonville Strawberry farmers, they have 30-100 horsepower water pumps and a farmer with no water will gladly pay $125/ hour and more if you can get there now! Live 480 3 phase 100 amp service.....no big deal.....can't wait 5 days for Pigs,Goats, & Elephants (our local nickname for PG&E, Pacific Gas & Electricity the worst utility in the nation) to turn off power, the farmer is going to lose $250,000 waiting for the utility. On a hot day, water pump down I will take home $1000 by 2:00pm so why mess with fixing outlets, etc. Been doing this far too long, have retired twice from industry , communications & computers, used to work for mega corporations, now I am a one guy shop, only hire labor when needed to avoid government rules and regulations, they will tax you out of business at every turn if possible. My rule for dealing with the government: Treat them like mushrooms, feed them @*#& and keep them in the dark!......my real joy.....gone fishin!  25+ years in industry and another 20 in construction I wanna quit but who is gonna support our politicians? Can't convince them to get a real job!  enough ranting...on to the joys of life...tinkering solar....off grid life!..off the governments radar!

[tecnodave]

mtdoc

Great shot of your classics lit up,  just curious, how much power to get them that lit up? Looks like you are getting good heat output there! Need it tonight though 31 degrees here in Santa Cruz county just 6 miles inland, canada has come for a visit!

[ClassicCrazy]

mtdoc - what is so warm in the top left of the photo ?  Sure would be nice to get one of those thermal imaging camera's !  Where did you get yours from ?

techno dave - glad you share your experience online here