Advice on Current Solar System Setup Design

I’m wondering if someone can give advice on my setup. Does everything seem sufficient and safe?


300w Renogy panel.

I have two LiFePo 12 v batteries: wired with a BMS as 2s4p. BMS has a discharge of 60amp and a charge current of 30amp.

I’m using these bus bars: https://www.amazon.com/dp/B0915GWDJ9?psc=1&ref=ppx_yo2ov_dt_b_product_details. I’m getting these tomorrow and can take pictures of everything wired up once I get them.

I have 4/0 wire going from batteries to busbar with an off-switch and a 200 amp windy nation ANL fuse..

I have an AIMS 1000W/2000W surge (seems like it came with 2 gauge wire, but I have some extra 4/0, do you think I should use it here?). I have a 120 amp blue sea 285 series circuit breaker from inverter to busbar.

I have a 40amp mppt Renogy charge controller that will have 6gauge wire going to busbar.

I have 6 gauge wire going from bus bar to 12v blue sea fuse box for 12v appliances. I have a 75 amp circuit breaker (taiwanese) that I got from the person I bought the batteries from, though it seems like I would want something rated for lower amps for my setup, no?

These are my 12v appliances:

-maxx air fan (med: .8 amp; high 2.8 amp)

-water pump (7.5 Amps )

-dometic fridge 80cu (7amp/12 v)

-starter for chinese diesel heater

-led lights (3w)

-mounted fan (1w)

I’m thinking about installing a dc to dc. Renogy 40amp.

Thanks much!

James


Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,814 admin
    edited July 8 #2
    James--My first guess is that a single 300 Watt solar panel is probably not large enough to run your expected loads... However, more details would help.

    For example:

    LiFePO4 batteries... What is the Amp*Hour rating for the bank (12 volt battery bank?)?
    1,000 Watt inverter -- How many hours per day at xxx Watts average (5 hours * 100 Watts = 500 WH)
    Yes, OK to use heavier AWG wire (in general) if you can correctly terminate them (bolted connections, crimp connections, etc.)
    Use this link for max continuous current flow (NEC is on the "conservative" side):
        https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
    For wire runs, need to know the AWG, length, and current to figure voltage drop
        For charger to battery, suggest 0.05 to 0.10 max voltage drop
        For battery to inverter (and other DC loads), max 0.50 voltage drop
        Voltage drop calculator: https://www.calculator.net/voltage-drop-calculator.html
    Max air fan: What speed and how many hours per day/night?
    RV 12 VDC water pump--Generally only run 20 minutes per day or less(?)
    Dometic Fridge: Need to know average power draw over 24 hours (first guess if 50% duty cycle)
    Starter for Diesel heater: Generally there is a fan (or two) for combustion and heated air movement. Can be 8 amps @ 50% duty cycle (cold overnight)
    LED lights: 5 hours a night?
    Mounted Fan: XX hours per day?
    Where will you be and what season(s)? Amount of solar energy per day? Panels mounted flat to roof of RV?
       Weekends in south west in winter?
       Full time off grid towards Canadian boarder during summer?

    Sorry for all the questions... But these need to be answered to design a system that will meet your needs.

    For AC loads, a Kill-a-Watt type meter is nice.
    For DC loads an AmpHour/Watt hour meter is good to have:

    https://www.amazon.com/kill-a-watt-meter/s?k=kill-a-watt+meter
    dc amphour watthour power meter

    Add missing link for DC AH/WH meter:

    https://www.amazon.com/s?k=dc+amphour+watthour+power+meter&ref=nb_sb_noss

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JadesJades Registered Users Posts: 36 ✭✭
    edited July 6 #3
    Thanks for the thoroughness, Bill. Answers are in-line below.

    LiFePO4 batteries... What is the Amp*Hour rating for the bank (12 volt battery bank?)?
    Yes, 12V battery bank. 100 Amp hours each.

    1,000 Watt inverter -- How many hours per day at xxx Watts average (5 hours * 100 Watts = 500 WH)
    Charging a laptop, not really sure what else. Will mostly be operating with 12v for efficiency's sake, ease, and not much needing to run on 120, but want to have it if needed. Might also rent out my van when I'm not using it.

    Yes, OK to use heavier AWG wire (in general) if you can correctly terminate them (bolted connections, crimp connections, etc.)
    Use this link for max continuous current flow (NEC is on the "conservative" side):
        https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm
    For wire runs, need to know the AWG, length, and current to figure voltage drop
        For charger to battery, suggest 0.05 to 0.10 max voltage drop
        For battery to inverter (and other DC loads), max 0.50 voltage drop
        Voltage drop calculator: https://www.calculator.net/voltage-drop-calculator.html
    I looked at the voltage drop calculator...what do I use for power factor? For load amps, do I put in the average or max amps that I'd use at any one time?

    -I bought a hydraulic crimper, so am able to take care of everything up to 4/0. I'm trying to keep everything close together. PICTURES.
    -For charge controller to battery (charge controller to bus bar to battery) and then 50amp breaker: total wire 1.5 feet (6 awg).
    -For inverter to battery (again, via bus bar) and 125 amp breaker (figure for 1000w/ and 2000w surge, if it breaks, I can just reset, and I won't be using for a lot of things, and if I decide to get a larger inverter down the road, I can get a larger inverter and breaker).

    Max air fan: What speed and how many hours per day/night?
    I'd say medium for five hours/day

    RV 12 VDC water pump--Generally only run 20 minutes per day or less(?)
    Yes, I couldn't see it being used more than 10 minutes a day, if that (if I'm using it). I'm pretty conservative (with energy), but if I'm renting it out, maybe 20 min sounds more accurate)

    Dometic Fridge: Need to know average power draw over 24 hours (first guess if 50% duty cycle)
    From site: "it has an average duty cycle (ie on/off) around 50%"

    Starter for Diesel heater: Generally there is a fan (or two) for combustion and heated air movement. Can be 8 amps @ 50% duty cycle (cold overnight)
    This is what site says: "On startup, it takes approximately 10 amps of power consumption. It takes around 3 minutes to get the burner hot. After the flame is established, our diesel air heaters draw between 0.55 to 0.85 amps, making them very efficient to operate."

    LED lights: 5 hours a night?
    Yeah: 4 or 5 hours.

    Mounted Fan: XX hours per day?
    On avg, 3 hours 

    Where will you be and what season(s)? Amount of solar energy per day? Panels mounted flat to roof of RV?
    I'm currently renting and van is not being used. I'm not sure if I'd live in it full-time, but would like it to be outfitted for that capability. Panel is mounted to roof rack, with a little bit of space in between. (see photo below).

      Weekends in south west in winter?
       Full time off grid towards Canadian boarder during summer?
    Currently in San Diego, and will mostly stay in Cali. I'd like to be prepared for most conditions, but can also purchase extra batteries/solar down the road. Thus, more than anything, I'd like to have the infrastructure (proper wiring) for increases, if I decide to go that route.

    Sorry for all the questions... But these need to be answered to design a system that will meet your needs.
    No need to apologize...appreciate the thoroughness! Also, I'm on the fence about a DC to DC charger, but if I do go with like a Renogy 40amp, that could assist in keeping everything topped off, yes? (Is it both ways, as if you do a lot of local driving, solar would also help to charge the car battery?...that could be a nice feature)

    For AC loads, a Kill-a-Watt type meter is nice.
    For DC loads an AmpHour/Watt hour meter is good to have:
    Will look into these. For DC, is this something you install in the van?







  • BB.BB. Super Moderators, Administrators Posts: 32,814 admin
    OK, comments:

    200 AH @ 12 VDC LiFePO4 battery bank (2x 100 AH @ 12 volt batteries in parallel)

    For an RV, you might want to look at a second/smaller AC inverter (something like 300 Watts)--Most of your loads should be relatively small (laptop, charging drone/etc. batteries, etc.). A smaller inverter will draw around 6-8 Watts Tare Losses (power draw with inverter "just turned on"). Larger inverters can take 10-20+ Watts just turned on.

    Having 12 VDC loads seems like it saves energy... But using an AC inverter "matched" to your loads does not waste much more power (have the ability to turn inverter on/off to save tare losses when AC power is not needed). 120 VAC outlets (power strip/etc.) are easier to use than a bunch of "cigarret lighter sockets" (larger "plugs", only good for ~10 amps/120 Watts, for me I find them less than reliable at times). Also, 12 VDC solar power can run from 10.5 to 15-16 VDC (discharged to charging). Some 12 VDC adapters really do not do well with that wide range of DC voltages (vs a car that is around 12.5 to 14.0 volts or so).

    There are Anderson Connectors that are used by HAM Radio and others for 12 VDC power--But not really an "off the shelf option" for most DC appliances:

    https://powerwerx.com/anderson-power-powerpole-sb-connectors (for example)

    For Power Factor: Use 1.0 for "DC power" and 0.8 for AC power as a first guess unless you have measured actual current (measured current of X.X amps, use 1.0 PF).

    Use the maximum continuous current you plan on using (i.e., laptop running/charging + drone charging, etc.). You want to have "correct" voltage at the load (i.e, 11.5 volts battery under load, 11.0 volts at load; Or 0.10 or 0.05 max voltage drop when charging your battery bank at XX Amps).

    You can use Amps and Amp*Hours for load calculations--Or Watts and Watt*Hours. Both work... But if you have multiple working voltages (such as 12 VDC and 120 VAC)--Then doing the math in Watts/Watt*Hours is less confusing. For example:

    7.5 Amps * 12 VDC * 1/6 hour (i.e., 10 minutes) per day = 15 WH per day
    30 Watt laptop * 5 hours per day (at 120 VAC or 12 VDC) = 150 WH per day
    3x 5 Watt LED * 5 hours per night = 75 WH per night
    0.8 Amps Fan * 12 VDC * 5 hours per day = 48 WH per day
    etc.And add up total power needs.

    Basically add up all the AC WH loads... Multiply by 1/0.85 for AC inverter losses.
    Add up all DC loads in WH (no "extra losses").

    Once you have your daily loads (AC and DC Watt*Hours)--We can size the solar array and ensure the battery bank will meet your needs.

    Normally, if the RV is driven daily--You probably would charge your House Battery Bank from the alternator (or install a second alternator dedicated for the Li Ion battery bank). If there will be "dry camping" for days/weeks at a time--Then a larger solar array may be needed to keep up with your power needs.

    You can install a DC power meter in your van... And there are Battery Monitors that are designed to keep track of the current into and out of your battery bank. I.e., you have a 200 AH battery bank draw 10 amps * 5 hours = 50 AH and recharge 4 amps * 5 hours from solar = 20 
    AH, then your battery is at 200 AH - 50 AH + 20 AH = 170 AH battery bank state of charge (170/200= 0.85 = 85% state of charge).

    There are Battery Monitors on Amazon, and some very nice monitors from our host. For example:

    https://www.solar-electric.com/victron-energy-bmv-712-smart-battery-monitor.html

    Before you buy equipment--Need to understand your loads/power needs. Then do a bunch of different paper designs and see what meets your needs and costs.

    It is "difficult" to make an "expandable" power system... As your loads increase, your wiring needs to be heavier, a larger battery bank, and possibly even look at changing from 12 volts to 24 volt (or even 48 volt) battery bank. For a 1,200-1,800 Watt load/AC inverter, 12 VDC battery bus can work pretty well. When you are at 2,400 Watts or more, 24 volt battery bus makes more sense (you can keep your wiring AWG from getting too large/heavy--Copper is not cheap. Large cables are not flexible).

    You are jumping in with both feet in off grid/solar power. In general, you really want to conserve energy--Especially for RVs where you don't have much space or too heavy of batteries/etc.).

    So lets take this in steps. Starting with your loads. We can do some quick back of the envelope calculations to see if your expectations 
    "make sense" or if you need to change your loads/power equipment.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JadesJades Registered Users Posts: 36 ✭✭
    edited September 11 #5
    Hi Bill,

    Circling back to this:

    Are you saying buy a second inverter while already having the AIMS 1000, just because cost in energy would be less?

    Re a system being "expandable": I see what you're saying and it is a bit contrary to spend more upfront on beefier wiring, but I have quite a bit of different size wiring, so cost isn't a huge issue here: unless, I were to be able to sell it, but figure I'll just use what I need for this purpose. I have 4/0, 2/0, 2awg, and 6awg. I do see what you're saying with regard to flexibility of large wire  (4/0 is suer unwieldy and a behemoth to deal with)

    Yes, agreed, conserve as much as possible.


    Here are the calculations:

    lights: .6 amps (3watts) * 5 hours  = 15WH * 4 = 60WH

    led strip: 3.5 watts * 4 hours = 14WH

    maxx fan: med .8 amps (9.6 watts) * 5 hours = 50WH

    fridge: 2.2amp/h * 12   = 25WH (seems too low)

    water pump:  7.5 Amps (90 watts) * 20min = 30WH

    regular mounted 12v fan: 12 watts * 6 = 72WH

    heater: .85amps (10.2 watts) * 6 hours = 61.2 WH (only somewhere in winter)

    12v sockets: iphone = 5 amps or 60 WH


    Total: ~373 WH


    Inverter:

    laptop 30W * 2.8 hours = 100WH

    maybe blender 1000w * 1min = 16WH

    makita battery charger 5amp/h = 60WH * really only occasionally.


    Total 116 WH

    If we multiply by 1/.85, then it comes to 136.


    Footnote 1:



    Footnote 2:


    https://expeditionportal.com/forum/threads/dometic-cf-80-fridge-mini-review.47755/



    Company specs for the inverter give a 100amp fuse and 8awg wire. This seems small. I see what you're saying about it being difficult to build an expandable system, because you need the wiring in place. 4/0 seems too thick, but I could use something like 2/0 wire (it's a very small distance, probably about a foot).

    https://theinverterstore.com/product/1000-watt-pure-sine-inverter/?gclid=Cj0KCQjwjvaYBhDlARIsAO8PkE0srzr9Vq_LX2AkGt9TGojTTGNyzmQm_OUn_rkZ-YXH0uxM_v5RM6gaAmKgEALw_wcB

    AIMS specs: https://www.aimscorp.net/1000-watt-pure-sine-power-inverter-kit.html


    Fusing and circuit breakers are spec'd to the wire and not to the device being energized, correct? So, if you go with heavier wire, you would go with a smaller fuse? (Theoretically speaking, if the wire were large enough, you wouldn't need a fuse/breaker is that correct?). 

    I'm also wondering why different type fuses equate to different amperages? (ANL needs larger fuse than a MIDI) If I'm using circuit breakers, which amperage should I use to multiply by 1.25? https://www.bluesea.com/support/articles/Circuit_Protection/1441/Part_2%3A_Select_a_Fuse_and_Fuse_Holder_For_Your_DC_Product_Installation

    When BlueSea refers to bundled vs single wire, "bundled" is just the standard type of copper "stranded" wire that one should be using, or is this something different?

  • littleharbor2littleharbor2 Solar Expert Posts: 1,756 ✭✭✭✭✭
    edited September 12 #6
    "fridge: 2.2amp/h * 12   = 25WH (seems too low)"


    You need to multiply this times the voltage ie; 120 vac. = 3000wh + 3kw

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.BB. Super Moderators, Administrators Posts: 32,814 admin
    James "Jade" has some questions... First a little math talk:
    I will go to that discussion now... I guess you are asking about the overall power discussion, not just the last post:
    "fridge: 2.2amp/h * 12   = 25WH (seems too low)"

    You need to multiply this times the voltage ie; 120 vac. = 3000wh + 3kw
    The last line should be = 3,000 WH = 3 kWH (conversion of k=1,000 x or :"kilo"). The plus (+) sign is a typo.

    There is the confusion of Amp and Amp*Hours vs Watts and Watt*Hours... Watt (and WH) are "complete units" which tells the whole information about power (a rate like Miles per hour) and energy (an amount like miles driven).

    In the "olden days" where, for example, everything on a boat or camper was running at 12 volts (12 volt lights, fans, pumps, radios, etc.) you could stay with Amps and Amp*hours... Generator was rated in Amps and battery bank was rated in Amp*Hours with the assumption that "everything" was running at 12 VDC.

    With "modern" mixed voltage systems (DC side may be 12/24/48 VDC) and there is a 120 (or 120/240 or 230 VAC) side too. I like to convert everything to Watts and Watt*hours and then convert back to A and AH when needed...

    For example, say you have a 1,200 Watt load (say induction cook-top) that runs at 120 VAC, and you have a 12 volt battery bank.

    Power = Voltage * Current
    Current = Power / Current (and a bunch of other related electrical equations)

    To convert:
    Current = Power / Current = 1,200 Watts / 120 VAC = 10 Amps @ 120 VAC 
    Power DC = Power AC / inverter efficiency = 1,200 Watts / 0.85 typical AC inverter eff =  1,412 Watts @ 12 VDC
    1,412 Watts DC load / 12 VDC = 118 Amps @ 12 VDC

    The energy used--Say 20 minutes which is 1/3 of an hour:
    energy used AC side: 1,200 Watts * 1/3 hour = 400 WH of AC load cooking (energy used on AC side)
    energy used DC side: 1,412 Watts DC load / 12 VDC =  118 Amps @ 12 VDC
    AH used DC side: 118 Amps * 1/3 hour = 39 AH @ 12 volts taken from battery bank
    If 400 AH @ 12 VDC battery bank: 39 AH load / 400 AH battery bank = 0.0975 = ~10% of battery capacity used

    Regarding post #5... Remember Watts (and amps) is a "RATE" like miles per hour (Watts and Amps is a rate of energy/current usage per unit time). Watt*Hours (and AH) is an amount..., like total miles driven (total energy or AH used).

    In Physics, Watts, Amps, etc. are "per seconds" numbers... In energy usage, use use "per hour" to keep numbers a "reasonable" size (3,600 Seconds in 1 hour).

    Here are the calculations: The "details look ugly"--But follow each step by step, and it is pretty easy and clear.

    lights: .6 amps (3watts) * 5 hours  = 15WH * 4 = 60WH

    Lights: 0.6 amps * 12 volts * 5 hours = 36 WH per lamp (for 5 hours of usage)

    Lights: 4 lamps * 0.6 amps * 12 volts * 5 hours =  144 WH of lamp usage (over 4 lights for 5 hours per evening).

    Note that 0.6 amp * 12 volts = 7.2 Watts per lamp (rate of energy usage)... You might wish to look at lower wattage lights. If you are using "focused beam" lamps, that is a fairly bright lamp. Also--Another FYI--The human eye is a "logarithmic" photo receptor device... More or less, we have a very difficult time "seeing" the difference between a 6 Watt Lamp and a 3 Watt Lamp--You could probably use 1/2 the size bulbs and see very little difference in most applications--10x difference (i.e., 6 vs 0.6 Watts) is a "night vs day" difference--You would hardly notice 0.6 watt lamp vs a 6.0 watt lamp (all about energy conservation here).

    led strip: 3.5 watts * 4 hours = 14WH

    Assuming LED strip light is 12 volts:

    P=V*I

    I=P/V= 3.5 Watts / 12 VDC = 0.29 Amp

    3.5 Watts * 4 hours = 14 WH

    0.29 Amps * 4 hours = 3.48 AH @ 12 volt battery bus (just to show how math works)

    maxx fan: med .8 amps (9.6 watts) * 5 hours = 50WH

    MaxxFan: 0.8 Amps (medium) * 12 volts * 5 Hours = 48 VDC @ 12 volts

    (I always like to show Watts / Current / Voltage and not assume... Like the LED strip lights--There are both AC and DC versions of these guys.

    fridge: 2.2amp/h * 12   = 25WH (seems too low)

    Note Amp/Hour (and Watts/Hour) is not a "valid" unit here--Amps and Watts are already a Rate... A/H and W/H would be the same thing as saying Miles per hour per hour (in physics, that is actually "rate of acceleration"--Not "useful here).

    If we keep the numbers and units clean/used correctly, the math is easier to understand and catch errors... For example:

    Incorrect: 2.2 Amps/H * 24 hours = 52.8 Amps (a rate--As the 1/H * H cancel each other out)

    Correct (in AH): 2.2 Amps * 24 hours = 52.8 Amp*Hours

    In Watts (rate of energy usage): 2.2 amps * 12 volts (battery bus voltage?) = 26.4 Watts

    In Watt*hours (total energy usage per day): 2.2 Amps * 12 volts * 24 Hours per day = 633.6 WH per day @ 12 volts

    Now--Assuming the fridge runs a 50% duty cycle (such as runs for 15 minutes and is off for 15 minutes 24 hours per day):

    Fridge: 2.2 AH * 0.50 duty cycle * 24 hours = 26.4 AH @ 12 volt battery bus estimated daily usage

    Fridge: 2.2 AH * 12 volts * 0.50 duty cycle * 24 hours = 316 AH per day @ 12 VDC bus

    316 WH per day for an RV fridge is probably a good estimated energy usage--Remember that in Hot weather, the duty cycle can climb to near 1.0 (i.e., 95F inside RV, poor air circulation around fridge, etc.)... Hot days probably has lots of sun--If you have solar panels, there is an advantage to "sunny days" too.

    water pump:  7.5 Amps (90 watts) * 20min = 30WH

    Water Pump: 7.5 Amps * 12 volts * 1/3 hour per day = 30 WH per day

    Water Pump: 7.5 Amps * 1/3 hour per day = 2.5 Amp*hours per day @ 12 volts

    regular mounted 12v fan: 12 watts * 6 = 72WH (Mixed equation... can be confusing)

    Fan: 12 Watts * 6 hours per day = 72 Watt*Hours per day @ 12 volts

    Fan: 1 Amp * 12 volts = 12 Watts

    Fan: 1 Amp * 6 hours per day = 6 Amp*Hours

    heater: .85amps (10.2 watts) * 6 hours = 61.2 WH (only somewhere in winter)

    Regarding heater... Assume 10 Amps @ 3 minutes cycle (0.05 Hours). And 0.85 Amps for "running"--Both 12 volts.

    Say one cycle is 10 minutes on, and 10 minutes off over night (12 hours). 50% duty cycle of 20 minutes or 3x per hour:

    Each cycle Starting: 10 amps * 0.05 hours * 12 volts = 6 WH per cycle "starting"

    Each 20 minute Run: 0.85 Amps * 12 volts * 0.5 duty cycle * 20 minutes * 1/60 minutes per hour = 1.7 WH per 20 minute cycle

    Over night starting: 6 WH per cycle starting * 3 cycles per hour * 12 hours = 216 WH per night "starting"

    Over night running: 1.7 WH per cycle * 3 cycles per hour * 12 hours = 61.2 WH per night per night "running"

    Over night total: 216 WH Starting + 61.2 WH running = 277.2 WH per night total

    Overnight toral: 277.2 WH per night / 12 volts = 23.1 Amp*Hours @ 12 hours @ 12 volts

    12v sockets: iphone = 5 amps or 60 WH

    Phone Charging: 12 volts * 5 amps * 1 hour = 60 Watt*Hours per day(?)

    There are lots of meters these days that measure energy usage.. For 120 VAC, the "kill-a-watt" type meters:

    https://www.amazon.com/kill-a-watt-meter/s?k=kill-a-watt+meter

    And there are lots of DC AH/WH meters too:

    https://www.amazon.com/s?k=DC+Amp*hour+Watt*hour+meter

    And if you want a separate Battery Monitor/Meter (many modern integrated systems include battery monitors):

    https://www.amazon.com/s?k=battery+monitor&sprefix=battery+mon

    The nice thing about AH/WH meters is you simply plug in your loads an come back 12/24/whatever hours later and read the AH/WH total used...

    There are lots of nice AC+DC Current clamp meters these days too... And they work very well for debugging and estimating energy usage--But because you need to know the duty cycle of many loads--Current Clamp Meters are not always the best tool for measuring AH/WH/Energy usage... A couple "clamp meters" that are "good enough" for our debugging needs:

    https://www.amazon.com/UNI-T-Digital-Handheld-Resistance-Capacitance/dp/B0188WD1NE (inexpensive)

    https://www.amazon.com/Auto-Ranging-Resistance-Klein-Tools-CL800/dp/B019CY4FB4 (mid-priced)

    With RV/Solar/Off Grid power--It is all about the details...

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,814 admin
    Regarding these questions:

    Fusing and circuit breakers are spec'd to the wire and not to the device being energized, correct? So, if you go with heavier wire, you would go with a smaller fuse? (Theoretically speaking, if the wire were large enough, you wouldn't need a fuse/breaker is that correct?). 

    I'm also wondering why different type fuses equate to different amperages? (ANL needs larger fuse than a MIDI) If I'm using circuit breakers, which amperage should I use to multiply by 1.25? https://www.bluesea.com/support/articles/Circuit_Protection/1441/Part_2%3A_Select_a_Fuse_and_Fuse_Holder_For_Your_DC_Product_Installation

    When BlueSea refers to bundled vs single wire, "bundled" is just the standard type of copper "stranded" wire that one should be using, or is this something different?

    Fusing/Breakers are to protect the wiring... So Fuses/Breakers can be at rated wire current, or smaller (in 12 VDC systems, we typically use heavier wire to keep the voltage drop down on longer cable runs).

    In older cars, the did not use fuses in the Battery Cables... If there was a short circuit in the starter motor, the battery would go dead before the cables melted....

    Today, most vehicles (that I have seen) use a fusible link from the battery to the rest of car electrical systems... The short piece of cable is rated to burn through if there is too much current (and not start a fire, hopefully).

    With today's large battery banks for Solar/Off Grid/RV power systems--It is always a better idea to use fuse/circuit breakers everywhere to prevent the large battery bank from frying cables if there is a short.

    Regarding fuse/breaker ratings... For the most part, people are not going to operate their AC inverter at 100% rated power for hours on end... usually, they run at 50% of less of capacity--And only surge to 100% (to even 200% for many good quality inverters) for a few seconds to start an induction motor/well pump/compressor/etc... So designing wiring (and fuses/breakers) to 100% of planned max current (for seconds/minutes) is generally OK.

    Where you may wish to design for 100% continuous load is usually for battery charging... If you have a 120 Amp charger, the battery bank can run for many hours to charge from 50% or less state of charge back over 80% or to near 100% SoC (i.e., lithium batteries which tend to absorb max rated current until nearly full).

    The NEC 80%/1.25x contnuous current derating--Most North American fuses/breakers are designed to blow at 100%+ of rated current (may take minutes or hours), and not blow at 80% or less of rated current... Say 120 Amp charging branch circuit current:
    • 120 Amps * 1/0.80 (or 1.25x) NEC continuous current derating = 150 Amp suggested minimum derated Branch Circuit current
    Here is a thread that talks about various "fuse/breaker" rating properties:

    https://forum.solar-electric.com/discussion/353232/oversized-wire-and-breaker

    "Bundled wire/cable"... This is taking two or more wires and bundle tying or running in conduit. The more wires you run in a bundle, the more heat is generated and the less exposed surface are for cooling. One wire in "free air" will self cool much better than a bundled/conduit group of cables...

    The NEC code book has lots of charts and tables that offer derating guidance for wiring.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • JadesJades Registered Users Posts: 36 ✭✭
    Thanks for this, Bill!
  • BB.BB. Super Moderators, Administrators Posts: 32,814 admin
    You are very welcome James.

    Regarding using an AC+DC current clamp meter... For AC measurements, the meter does not need zeroing. For DC measurements, you need to zero the meter first before making accurate DC current measurements (current off, or no wires in clamp). DC current sensors tend to drift over time (minutes more more?).

    Also note that AC meters do not have a +/- direction for "energy flow". For DC current, there is a +/- direction. It is sort of confusing as the polarity of the current flips if the wire direction is flipped in the clamp. Also, you get into definition issues... Is discharging a battery "-" current flow or "+" current flow.

    To get more comfortable with using a DC current  clamp meter, I clipped on Battery Cable, Alternator cable, etc. and played with the car (motor off, head lights on, motor running and charging battery, etc.).

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