30A connection to 50A service

lazy_q
lazy_q Registered Users Posts: 9
I have avoided purchase of a generator, but may supplement my system in the future to cover the casual use of air conditioning. I live in the southwest but camp quite a bit in the northwest. The solar system that I am planning consists of:
two kyocera 130w panels wired in parallel (may expand over time)
mornigstar –tristar 60Amp charge controller (sized for expansion)
four Trojan T105 6v series-parallel battery bank (may expand over time)
Trimetric 2020
Xantrex prowatt 3000 (msw) invertrer

A 60amp air conditioner disconnect isolates the solar panel output form the charge controller and the same disconnect isolates the output of the charge controller from the battery bank. The inverter has a 400amp fuse. The panels will be wired via #10 gauge cable to a junction box (approx. 5 feet) at the top of the roof. From there I will use #4 gauge cable for the run (approx. 12 feet) to the AC disconnect and the charge controller and #4 to a nearby bus bar. The battery bank will be wired with 2/0 and cabled with 2/0 to the bus bar (about 4 feet). The inverter is within 6 feet of the battery bank and is wired via switch to the bus bar with 2/0 cable. I estimate losses to be minimal.

In the past I have utilized a 30 amp transfer switch, but my new fifth wheel has 50 amp service. I want to avoid the expense of a 50 amp transfer switch and the complication of splitting the 50 amp service. I never use my inverter as a temporary backup for shore power. In other words I use the inverter when shore power is not physically available. I am planning to create a 30amp cord as the output from the inverter. I plan to plug this cord into the 50 amp recepticle on the fifth wheel via a commercial 50amp to 30amp conversion cable. Using this manual method of connection it is impossible to have both shore power and inverter power presented to the AC distribution panel at the same time. So, there is no need for a transfer switch. Obviously both legs of the AC distribution panel are not energized. Likewise all of the AC loads cannot reasonably be serviced by the inverter. In other words I would manually energize only the loads needed, lights, AC recepticles, microwave, and TV/Stereo. Both air conditioners, the internal converter, as well as the electric hot water heater would remain off.

Can I connect the inverter to the fifth wheel using the above scheme?

Sorry for taking the long way around, just trying not to waste your time with incomplete information.
I’ve learned quite a bit from this forum and appreciate your efforts.

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: 30A connection to 50A service

    Depends on the GROUNDS and NEUTRALs

    The shore side, and maybe the coach side, wants to see one leg of the 120Vac tied to ground.

    The inverter may have the battery - tied to ground.

    Usually, the inverter smokes faster then the fuse.

    You need to check this before hand
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • BB.
    BB. Super Moderators, Administrators Posts: 33,422 admin
    Re: 30A connection to 50A service

    I got a little lost in your explanation before figured out you did not have a 60 amp A/C unit in there somewhere. :D

    Working backwards, I could not find a 3,000 watt Xantrex MSW data sheet--But, in general, MSW inverters do not work well if their Negative Battery Lead is grounded and there Neutral/White Lead is grounded on the AC side (usually this is common for North American 120/240 VAC home wiring). Usually, a TSW (True Sine Wave) inverter is required to connect directly to a existing power distribution system that has a Neutral/Safety ground bond.

    Working backwards, is this 3kW inverter a 12 volt or 24 volt input? And what is your battery bank wired for (12 or 24 VDC)?

    Regarding your panels, that is not a lot of electrical power... Just using basic rules of thumbs for solar:

    Two 130 watts (12 volt?) solar panels
    Assume 5 hours of sun per day (sunny weather, clear conditions 6 months or so a year).
    Assume 77% for solar panel / solar charge controller eff.
    Assume 85% inverter efficiency
    Assume 80% battery efficiency (flooded cell)
    Overall efficiency 0.77 * 0.85 * 0.80 = 0.52 system derating

    Total useful 120 VAC power per day based on above:

    2 * 130 Watts * 5 hours per day sun * 0.52 over all eff = 676 Watt*Hours per day

    If you assume that you use 300 watts of load (average) on your inverter:

    676 WH / 300 Watts = 2.25 hours per day

    So, does this meet with your power requirements/expectations?

    Regarding basic wiring... Normally, you do not need a switch between the Solar Array and the Charge Controller. If you have 3 or more panel strings in parallel, then you should have a series protection fuse/breaker per string (value from panel data sheet).

    Between the charger and the battery bank, a fuse is fine. You can use a switch/breaker if you wish.

    Between battery bank and loads. One fuse or breaker per load wire--rated to protect the wiring (example, 14 awg wire to somewhere. Fuse/breaker near battery rated at 15 amps). Fuses/Breakers always have to be sized to the wire gauge or fuse can be smaller (long wire runs need heavier copper to keep voltage drop down... Say 15 amp load on 10 AWG wire (wire would be OK with ~30 amp fuse).

    For your inverter, there is the fuse/breaker for protection. And there is the Switch to turn off the inverter to prevent it running down your battery bank when AC power is not needed.

    In general, you want to use as small as practical inverter for your AC loads. I.e., do not use a 3,000 watt inverter to run a 60 Watt laptop computer + a couple of CFL/LED lamps.

    And, large inverters require large amounts of DC current for proper operation... For example, if this is a 3,000 watt inverter at 12 volts DC. Assume 85% efficient and 10.5 volts minimum voltage:

    P=V*I; I=P/V

    I= 3,000 watts * 1/10.5 volt min voltage * 1/0.85 efficient = 336 Amps

    Usually, I like to recommend that people keep the nominal DC current to around 100 amps maximum... If the current is above this, the wiring, fuses, breakers and such need to be very large and tend to be expensive.

    So, for 12 VDC, roughly 1,200 watts would be a handy maximum for the inverter power rating:

    I= 3,000 watts * 1/10.5 volt min voltage * 1/0.85 efficient = 130 Amps

    Also, you need to look at the battery bank total rated AmpHour capacity. Normally, for heavy loads, you probably don't want to be much more than C/10 or C/5 for continuous loads. Lead Acid batteries will get hot and are much less efficient at higher current draws.

    We can talk about the rest of your installation too--But this is probably enough confusion from me for the moment. ;)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazy_q
    lazy_q Registered Users Posts: 9
    Re: 30A connection to 50A service

    Sorry for the confusion. Thanks for the help/education.

    Try this URL for Xantrex it is a prowatt 3000
    http://www.xantrex.com/web/id/15/p/49/pt/8/product.asp

    It is a 12v input inverter. My battery bank is wired for 12 volt output.
    I have 4 - 6volt Trojan T-105 batteries. Two pairs (2 batteries wired in series) and the pairs are wired in parallel. The pos and neg cables. to the loads are taken from opposite pairs.

    As far as meeting my expectations – I have been reasonably satisfied in the past with fewer watts of solar and less adequate charge controllers, and with cheaper batteries that were less capable. So with that said, your estimate is lower than I expected, but expansion of panels and battery bank are a possibility, so its an adequate starting point.

    As far as neutrals and grounds with regard to the inverter. The chassis of the inverter was always grounded to the trailer. The bare wire in the AC cable is attached to the green wire from the inverter. Black & white (hot and neutral) are wired the same as the ac cable. I have wired the cable coming from the inverter directly to the 120 volt umbilical coming from the 30 amp fifth wheel that I have previously owned. Hopefully I was not risking life and limb. After being exposed to the forum for a few weeks I decided to be safe rather than sorry and so I am asking far more experienced folks like yourself if I should do something different in general or specifically because I am now plugging the inverter output into a 50 Amp service receptacle, albeit through a 50A to 30A converter. My assumption was that this was no different than using a 30 amp pedestal at a campground. I now see that it could be different because of the handling of the ground wire.

    Once again, thank you for taking the time to educate me.
    Regards,
    ron
  • BB.
    BB. Super Moderators, Administrators Posts: 33,422 admin
    Re: 30A connection to 50A service

    Not a problem... I just was laughing at my confusion. :D

    From the Xantrex 3000 manual:
    On Page 36 of the PDF File:

    The neutral (common) conductor of the inverter AC output circuit is connected to the Chassis ground. Therefor, when the chassis is connected to the ground, the neutral conductor is also grounded. The conforms to NEC requirements...

    So that looks fine.

    And from Figure 3-5, they show the battery DC bus being Chassis grounded too.

    So, there appears to be no issue with connecting the inverter Neutral Output to the common vehicle chassis ground.

    So, yes, you should be able to connect your RV umbilical to the output of the inverter (ensuring that undesired loads are turned of to preserve battery power).

    If you where to load the inverter to 2,500 watts, from the Xantrex spec. sheet, it would drain your 4xT105's in about 1.3 hours... That is pretty near C/1 which is a pretty heavy continuous load for a Lead Acid battery. Of course, we recommend that you only discharge to 50% capacity... So time on battery at 2,500 watts should be about 1/2 that.

    Realistically, if you limited your power to around 600 watts (~6 hours to dead / 2 = 3 hours)--that would be a comfortable maximum (excluding surges). 300 watts = 6 hours to 50% etc. (note that the lower the usage, the batteries become more efficient and you can get a longer runtime).

    Of course, if you limit your power to 600 watts, a smaller inverter may be more appropriate/desirable. Running a microwave for 5 minutes on the large inverter to make popcorn is your choice (of course).

    A quick re-reading of your first post does not raise any flags for me at the moment. But--I am reading on my netbook with 10" screen at the moment--so I may have missed something.

    Keep the Solar Charge Controller close to the batteries (for accurate voltage measurement by the charger) and I would recommend a remoter battery temperature sensor option for the charge controller (charge controllers tend to run hotter than the battery bank and will adjust the battery charging voltage a bit low).

    A very nice tool for your RV and Home is a Kill-A-Watt meter... You can measure average loads and total power used (kWH). The KaW meter will give you a much better handle on your loads and how much life you can expect from your battery bank.

    Note: solar panels should be facing the sun (tilted to latitude)--but that is variable. If you are in the south traveling during summer, flat mounted panels are not bad. If you are in the north in the winter--panel tilt is much more important.

    You can use the PV Watts website to predict your power generation. Use 1kW for solar panels (and multiply by 0.260 for your 260 watts of solar panels) and use a derating factor of 0.52 (assuming off grid, batteries, and inverter). The answers will be in kWhrs per month, but you can divide by 30 to get kWH per day (1,000 watts = 1.0 kW).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazy_q
    lazy_q Registered Users Posts: 9
    Re: 30A connection to 50A service

    Bill,
    I do have a temp. sensor. I've ordered a trimetric, but I like the idea of the kill a watt meter - thanks. My CC and inverter are both within 4-5 feet. Only way to get closer would mean that I would have to put them in the bay with the battery bank. I wanted to avoid that because the batteries are wet cell and even though I have vented battery boxes I just did not want to take a chance. AGM’s are too expensive (for me) for this application. Most of my use is in the summer and I am to lazy to tilt the panels and then lower them to travel, so I will have to suffer with the flat mount.

    You mention that a smaller inverter might be more efficient. How do I compare two inverters say a 1200 watt vs. the 3000 watt. Peak efficiency on the one I have is 90%, but it is also 90% on the smaller prowatt models. No load current is actually higher on the 1750 vs. the 3000, but I don't think that is what you mean. I foolishly thought that the reason you went to larger inverters was for higher continuous loads and because of heavy starting requirements, like when an AC turns on. I never realized that a smaller inverter supporting the same load would be more efficient than a larger inverter, given that they are both of the same technology generation, my inverter is 10 years old so I am sure that there have been huge improvements. The beauty of electronics is that everything gets 4 times better and costs ½ as much year over year.

    Thanks for all the other information. I wish I could return the favor, but I will have to settle for thanking you twice.

    Look forward to your reply.

    Regards,
    ron
  • BB.
    BB. Super Moderators, Administrators Posts: 33,422 admin
    Re: 30A connection to 50A service

    Ron,

    It is my pleasure--and it sounds like you have a goodly amount of experience in the RV/DC side of things--we can always use another pair of fingers to help with answers when I get in a bit over my head ...blub...blub... ;)

    Anyway, back to your questions...

    With inverters you have lots of variables with them... There are standby losses, idling losses, I^2 * R losses, and such... I will start with this recent thread:

    Inverter Efficiency

    Basically, you need to look at the inverter's specifications vs where you will be operating them. And you need to look at the extremes of the operation too (lots of standby/idling operation and such). Standby losses can exceed device power at times.

    You are correct, the no load losses on inverters do not always seem to track well with size. So reading the specifications (and sometimes doing the measurements) is required.

    Another issue with large inverters that I find is that many are owned by people new to solar power and off-grid battery power. Many times we see a 1,500 watt inverter setup with a 45 watt panel and a battery that would fit a motorcycle.

    So, I tend to push back and try to get them to look realistically at the loads they expect to support. Usually expectation do not match up well with reality.

    Also, there is a real issue with MSW (Modified Square/Sine Wave) inverters. Their output is not ideal and probably 10-20% of devices can have problems with them. Basically, their square wave like output can confuse timers, and overheat motors and some small electrical/electronic appliances. And when running things like motors, about 20% of the output energy of the inverter goes up as waste heat in the typical AC (as in alternating current) motor. A couple of Inverter FAQ's for you to read:

    All About Inverters
    Choosing an inverter for water pumping

    Granted TSW (True Sine Wave) Inverters are way more expensive than MSW--but for reliable and efficient operation of random equipment--the TSW inverters are a usually much better choice.

    However, we are a practical bunch here too (i.e., cheap!). A recommendation often made is to get a >1kW MSW inverter to run the well pump, microwave, and stuff.... And get a small TSW to run the computer and sensitive home electronics (possibly AV, wall warts for cell phone charging, etc.).

    Towards that end, the Morning Star 300 watt TSW 12 volt inverter is a really nice unit. Also has a very low power search and standby mode too.

    Just an FYI--the links I am posting are to our host's web store. Everyone here is volunteering their time and has no financial interests with NAWS (except our Admin/Host Windsun). You are welcome to purchase from NAWS or any supplier of your choice.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • lazy_q
    lazy_q Registered Users Posts: 9
    Re: 30A connection to 50A service

    Bill,

    Your the best- thanks.
    I assume that you can run multiple inverters from the same battery bank, but you have to separate the AC loads. I will start to investigate the smaller TSW inverters – great trade-off—price vs realistic load requirement. Most of my usage is lower power electronics (laptop, TV/DVD), with the exception of the microwave and the by now famous hair dryer. Now that you have encouraged me to do the math its obvious that I am not using as much as I thought I was, because the batteries would be in a death spiral. So I assume that I had a heavy load once in a while and light loads most of the time and the batteries must have recovered enough during the day to make it through the night and if the next day was a light load then the batteries might fully recover.
    As far as NAWS goes I really appreciate what they have done and value the moderators time and energy. I have learned a lot from the users as well. Just so you know I have a medium/large (for me) size order in with NAWS right now just waiting for batteries to arrive before I pick it up. I found them by chance, prices were relatively good and they were helpful, but the forum is a huge plus.

    Thanks again for the education.

    Regards,
    ron
  • BB.
    BB. Super Moderators, Administrators Posts: 33,422 admin
    Re: 30A connection to 50A service

    Ron,

    Again, you are very welcome... We aim to educate (and learn ourselves).

    Yes, you can attach multiple inverters, chargers, etc. to the battery bank. Normally, each wire run off the batteries should have their own, appropriate fuse/breaker.

    A few inverters can be "stacked"--but for the most part those are larger inverters so it is probably not useful for you.

    I am sure Windsun will be happy to hear about your order... He spends a lot of time on the back end keeping thing running and paying the hosting bills.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset