Main breaker from combiner and upgrades

garynappi

Currently my system is installed 6 years and is 12 volts supplying my perimeter lighting, yard charging  station with  DC fans, gable attic fan venting and water features but I am looking to upgrade the system to 48 volts to charge my Ebike batteries. In the distant future grid tie may be an option with 6x 400 to 410 w panels (3 strings of two each). 

On a negative DC ground, 12 or 48 volt not earth grounded, not mobile and not (currently) grid tied system, do the main current carrying pair from the combiner to the CC need a two or single pole breaker or is either optional?

Any benefit or downside to either single / double pole implementation? 

Any benefit or downside to earth grounding?  

Any up front considerations should system ever get grid tied? 

On the Ebike charge issue, would a direct connect to the 48 volt bank or DC to DC isolation scheme from the battery bank be best? 

Lastly when / if implemented, my CC's 48 volt night time controlled load output (not the battery charge terminals) will need to have a DC-DC step down for my exterior lighting and water features. My loads are moderate at less then 10a. Any suggestions on a step down / buck I can rail mount?

BB.

Hi Gary, I will attempt to make some useful comments... They may, or may not, apply to what you are trying to do... Feel free to question/correct/etc.

AC/DC/Grounding is a vast and complex subject... Sorry for the long post to skim over the issues you are raising here...

Charging E-Bike batteries... Nominally, 48 VDC probably does not make much of a difference one way or another without knowing exactly what brand/model/electrical interface E-Bike charger you want to use. 

In general, two major choices here. Using 120 VAC (or 230 VAC if in Europe/balance of world), to the E-Bike's charger is usually not difficult. Yes, you may lose 15% of the conversion from XX VDC to 120 VAC... But unless you have some sort of XX VDC to E-Bike battery bank battery bus. Can your E-Bike take "48 VDC" (really from 42 to ~62 VDC DC battery bus voltage) and properly/reliably recharge the E-Bike battery bank? Do you have an XX VDC input E-Bike charger? Or what...

Also, need to be careful with DC powered loads. DC short circuits can be more of a fire issue vs AC short circuits/Arc Faults/properly rated fuses/breakers/switches (basically any DC voltage over ~12 VDC can "sustain arcs" very nicely--And 24 VDC and higher voltage--That is "arc welder" voltage). And with the "relatively" wide voltage of a DC Battery Bank (discharging, resting, charging, etc.)--This can be a wider range of voltage vs what the "typical" load/controller may be designed to accept. You could use a DC to DC buck/boost converter (wide range 48 VDC in--Fixed 48 VDC or whatever is needed out). However, that is not much different (in overall electrical efficiency) vs just using a standard (good quality) AC inverter.

Regarding system grounding... For your needs (portable, not a large system, etc.)--Your DC system can be floating (no grounding) and, electrically, your loads (whatever they are) will be "happy".

With grounded/floating power systems... Some of the major issues in rough order from most important to rarely an issue):

• With floating power systems and multiple branch circuits (i.e., some high current circuits for inverter, E-Bike charging, water pumps, etc.) that have high current draw... Vs other smaller circuits that don't take much current (LED lighting, radio, etc.)--When the circuits are "floating", one breaker/fuse/switch in one leg (such as + leg) works OK. Where you get into issues if if there is a short circuit (to vehicle frame/chassis, plumbing (water, propane, etc.), and other electrical "gear"). What can happen is, for example, your +Battery bus gets shorted to chassis (effectively positive ground) and you have only + wiring breakers... You have a short circuit that can pass 100's of amperes (say large inverter DC bus, or the battery bus itself)... The "first short" does not "hurt anything" and everything runs OK (silent short circuit). However, if that is not detected and fixed, say there is a short circuit in the negative leads to your low power LED lighting (plumbing, chassis ground, metal electrical box, etc.)... Now you can get 100's of amps from the heavy DC battery/iverter buses, and the 5 amp wiring to/from the LED fixture with no "return" breaker will try to carry the 100+ Amp current--And fuse the LED lamp supply wiring and possibly start a fire). For "floating" DC (and AC) circuits, each "branch circuit" should have a double pole breaker (like the double pole breakers in 240 VAC north American panels) so that if there is excessive current in either leg, both breakers are tripped (you can use two fuses--And they will help over current--But a negative fuse blowing can leave the positive leg of the circuit "hot"--And surprise somebody working on the failed circuit--Pairs of breakers that turn off both "floating +/- circuits is "safer" and less surprising if things go wrong).
• If you ground reference one leg of the circuit (typically negative leg in solar/typical vehicle wiring/etc.) then you can get away with single pole breakers/fuses/switches just fine... Because the "negative" leg never gets above (approximately) zero volts with respect to ground
• For telephone circuits, typically they are positive ground to reduce cathodic corrosion of wiring in wet climates/installations. Negative grounding is how most of our "run of the mill" Solar/Vehicle power is designed these days--No reason to "chose" positive grounding except in special circumstances.
• Another major reason for "earth grounding" is for lightning control. If there is a nearby lightning strike, the grounding to local sheet metal/earth ground rod, can provide a safe(r) path for the lightning current (vs going "elsewhere" in your home). Lightning grounding can be a big issue for some folks.
• There can be other reasons for grounding (possibly reducing electrical/radio frequency noise, etc.)--But unless you are a HAM person, want AM/TV reception, etc., there are other "things that need to be done" to help reduce RFI/EMI interference.
• Grounding in vehicle/around plumbing/electrical distribution systems for safety. Earth Grounding (mostly) for lightning control. Floating DC (and AC) power is done with (mostly) smaller system that use power "locally" with respect to battery bank and for portable/non-permanent installations.

With respect to possible bringing utility power on-site (grid tied?). Best bet is to follow the national electric code (or whatever is used in your area). That way you don't have to rip everything out and start over (i.e., Main panels, breakers, grounding, UL listed AC inverter/etc.).

One person here had to go so far as to even put in a Utility Meter Socket (for a pure off grid cabin/home--Probably zero chance of utility power in his location). But it provided something that the local inspector(s) needed to understand and anchor the inspection to.

Also, following NEC and doing "required" inspections" / code compliance / etc. -- That may be an issue with home/dwelling/business property insurance. if (heaven forbid) there is a fire (damage, injury, etc.), you don't want to give the claims people anything that they could use to disallow your claim) (and, of course, you want "best practices" for safe wiring/plumbing/etc. for your place).

If you just have a "power shed" and (for example) a separate RV/Trailer on-site--Then the code issues may moot.

Regarding DC Isolation--There is really no "isolation" as we think of vs an AC isolation transformer in an AC inverter (PSW/TSW type). All wiring from the solar array to charge controller to battery bank to DC loads with "typical" non-isolated buck DC to DC converters are all common "copper wire" connections. For example, lightning (or other) surge current will just follow all the DC wiring (from array to loads).

There are isolated DC to DC converters--But why not just bite teh bullet and use standard isolated DC to 120/240 VAC (TSW/PSW) inverter(s). A typical AC isolation transformer (should be) tested at the factory for at least 1,800 VAC of isolation (basically 600 VAC rated tested at 3x rated voltage). Using surge suppressors if lightning is an issue..

Other reasons for 120/240 VAC--You don't have the "wide" DC bus voltage (42 to 62 VDC)--You have a regulated 120/240 VAC instead.

Also, at higher voltages you are using less current (Power=Voltage*Current)... At ~3x the voltage, you have 1/3rd the current (smaller wiring, less voltage drop) and are using standard 120/240 VAC plugs/wiring/breakers/panels... And you can power almost anything in the USA with 120/240 VAC -- Or get small USB/battery converters as needed.

And you have less issues with Arc Faults (broken wiring connections, "soft short circuits", etc.) as AC arcs tend to be better at self extinguishing vs DC arcs:
https://www.youtube.com/watch?v=Zez2r1RPpWY
Yes, you can get DC to DC converters for Wide Range DC 48 VDC to 24 or 12 volts or even 5 volts (USB)... But why not stick with 120/240 VAC as needed and use off the shelf products (power supplies) instead. No special 24/12 VDC plugs, etc.

In general these days--Good Quality AC input power supplies are pretty rugged, supply isolation, and are just about as efficient as doing everything in "DC"... Decades ago, AC system efficiency was not a (government or customer) requirement. These days, with a little looking around (and a handy Kill-a-Watt meter), you can find some pretty darn efficient AC power systems (and they are probably cheaper too).

If you were designing a system that ran on a car, or larger truck/boat/etc.--Then maybe 12 or 24 VDC makes sense for powering most loads (lots of 12 VDC native equipment, fair amount of 24 VDC truck/marine equipment). However, for use around home/etc., just sticking with 120 VAC power is usually a good way to go... And with 120 VAC safety gear (panels, breakers, GFI outlets, etc.)--You can have a pretty safe system (i.e., issues of 120 VAC around ponds/water features or a simple 24 VDC transformer for off the shelf systems)--It usually makes economic sense too.

For example a 120 VAC GFI outlet (Ground Fault Interrupter in North America) is quite cheap at the home center. There is really no 48 VDC GFI equivalent device out there. 120 VAC GFI is "electrically" very easy/cheap to design and manufacture (plus the huge volumes used today in homes). DC GFI is actually much more difficult and expensive--And is not designed to protects against shock/electrocution.

How much power you want to use (DC or AC) is also a good guide for what voltage battery bank you want to run... A good starting point is:

• 12 VDC == 1,200 to 1,800 Watt max system power
• 24 VDC == 2,400 to 3,600 Watt max system power
• 48 VDC == over ~2,400/3,600 Watts system power

More or less, the above helps you reduce copper wiring costs (and heavy cable that does not bend well)--Power=Voltage*Current. A 1,200 Watt @ 12 volt system is nominally moving 100 Amps around...

Yes, you can find >3,000 Watt @ 12 VDC AC inverters out there--But if you don't need 12 volts (i.e, not in an RV where much of the DC loads are 12 volts for pumps, lighting, electronics, etc.)--Pick the battery bus voltage based on the power and available hardware to do what you need (AC inverters, chargers, controller, loads, etc.).

I am not sure I have answered your questions... But I hope I have at least given you some ideas. If you are sending power any distance... 12 VDC is horrible (high currents, high voltage drop, heavy copper wiring). And 120 VAC is 1/10th the the current and much better for sending "significant" power more than a couple dozen feet (@ 12 volts). 24 and 48 VDC is "better" (higher voltage/lower current)--But I would still just bite the bullet and go with 120 VAC for most of your power needs.

Keep the DC side reasonably small (meet your needs). Keeps costs/maintenance issues smaller. The higher voltage battery buses, the more issues you have finding the "right" breakers, switches, etc... DC breakers/switches are physically larger/heavier than similarly rated AC switches/breakers.

If you are going to get into remote on/off, more than a few branch circuits... The 120 VAC standard stuff is just gong to be less of an issue (cheaper parts, UL rated for application, design to meet "code" and insurance requirements, and in some ways "safer" than "high" voltage/high current DC circuits).

For solar arrays--As they get larger and if you have shade issues (need to put panels "in the sun" away from the home/power shed/trees and such)--Running the array at 100 VDC (typical max for many solar controllers--And upwards of 400 VDC Vmp for "high voltage" solar charge controllers)--You can run your "high voltage" DC circuit(s) as need there--And stay with the 12/24/48 VDC typical off grid/Hybrid AC inverter/GT/OG systems...

If you have to "up size" your system to use AC inverters/AC loads--You may just need to upsize your solar array and battery bank by 20% or so vs a "very efficient" DC only power system... In the big picture--20% more solar panels and batteries are not usually major cost/design issues.

Your thoughts?

-Bill

garynappi

Wow Bill, that's more a few mouth fulls :-) and yes you did answer my questions. Thanks! 

From where I am with the 12 volt parts of the system (which I'd like to keep) are for the most part more practical to leave as is. 

"Any benefit or downside to either single / double pole implementation? " 

As it is, all of my in and out breakers are already double pole, my Q was more of a clarification of the issue. 

"Any benefit or downside to earth grounding?" 

Your point is well taken. Even though south Florida has a substantial number of lightning events yearly, I had not considered it much as my  panels are not currently in a very exposed area (south facing wall mounted below the roof peak) but the new additions will be so I'll plan on tying everything to a ground rod.

"On the Ebike charge issue, would a direct connect to the 48 volt bank or DC to DC isolation scheme from the battery bank be best? " 

The charger that comes with the Ebike is like any other small battery charger, AC in, DC out with a 2 pole plug like there is on say a modem or WiFi / network switch. I would think that daytime scavenging charge power directly from the battery bank would be OK, but I guess a small inverter to the AC side of the Ebike battery charger makes more sense.

The CC's max load is 20A, and at night my exterior lighting load (attic fan, water pumps etc. are all off) is very low, on the order of less than 4A, so with an inverter on the CC load to the Ebike battery AC input, I could let it do its thing over night. Good idea! 

I have several 12v inverters from 100w, 1000w to 4000w which I used on my boat for the Christmas boat parade down here so pressing at least one of them into service is probably a good idea. 

Now when / if electric sport motorcycles get the range I want, for sure I'd use an inverter to charge it... but alas, that's  a long way out. 

Since the proposed upgrade to 6 more panels will be mounted on top of a pergola (which is still in the planning stage from the builder) , the 6 panels I planned for will be near the max due to the limitation I'll have with the size of the pergola roof. Once it's installed, I may be able to install two more panels but until the pergola is in place and I have all of the other issues fleshed out I won't know for sure. 

My builder said that the 400 watt panels (at 77" wide) may not work, and I may have to drop the physical size down to 65" to get them to fit. That would mean dropping the panel wattage to or below 300 watts each, but if I can squeeze two more panels up there I'd not be losing anything in the long run.