Push it or no?

cupcake
cupcake Solar Expert Posts: 254 ✭✭✭
I got 3 panels with an 8.43 short circuit amp rating each...
I got 10 guage wire going to the controller which can handle 33amps

SO 8.43 X 3 = 25.29

So Im good here...

HOWEVER!

If I add ONE MORE panel (4 total) the total short circuit amps would be 33.72 (10awg wire can only handle 33amps)

So should I do thicker wire OR will the 'real world' amps of this setup never really get to 33.72??

Thanks...

--cuppycaxe


~1.5Kw PV in parallel
Morningstar MPPT-60 controllers (2) in parallel
3 Trojan tr-1275's in parallel 450ah total
Samlex 2,000 watt 12-volt inverter hardwired


Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    The NEC requires wiring and fuses to be 1.25x max continuous expected current:

    33..72 * 1.25 = 42.15 amps rated wire/fusing/breakers

    NEC also requires an additional 1.25x derating for solar--Or 1.25*1.25=1.56x derating (allow additional power for edge of sun, high altitude, etc.).

    1.56 is a lot of derating, and would seem to be excessive. Check the manual(s) for your devices and see what they recommend.

    And you have to look at the voltage drop too... Adding more current gives you more voltage drop--Which can be an issue itself (and frequently why we have much heavier wire than would normally be found in a standard 120/240 VAC power system).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cupcake
    cupcake Solar Expert Posts: 254 ✭✭✭
    BB. wrote: »
    The NEC requires wiring and fuses to be 1.25x max continuous expected current:

    33..72 * 1.25 = 42.15 amps rated wire/fusing/breakers

    NEC also requires an additional 1.25x derating for solar--Or 1.25*1.25=1.56x derating (allow additional power for edge of sun, high altitude, etc.).

    1.56 is a lot of derating, and would seem to be excessive. Check the manual(s) for your devices and see what they recommend.

    And you have to look at the voltage drop too... Adding more current gives you more voltage drop--Which can be an issue itself (and frequently why we have much heavier wire than would normally be found in a standard 120/240 VAC power system).

    -Bill



    So if it where YOU, what would YOU DO??


    --cake
    ~1.5Kw PV in parallel
    Morningstar MPPT-60 controllers (2) in parallel
    3 Trojan tr-1275's in parallel 450ah total
    Samlex 2,000 watt 12-volt inverter hardwired


  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    CC, as you are aware the main risk is in an under rated wire or device that can not survive an over current event...and thus starts a fire... your decision.... not mine or Bills... its your home.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • cupcake
    cupcake Solar Expert Posts: 254 ✭✭✭
    westbranch wrote: »
    CC, as you are aware the main risk is in an under rated wire or device that can not survive an over current event...and thus starts a fire... your decision.... not mine or Bills... its your home.


    I understand all that, I'm asking for a hypothetical opion of what Bill would do --- I am fully insured, don't worry...
    ~1.5Kw PV in parallel
    Morningstar MPPT-60 controllers (2) in parallel
    3 Trojan tr-1275's in parallel 450ah total
    Samlex 2,000 watt 12-volt inverter hardwired


  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    For your system, the Controller to Battery Bank, 60 amp MPPT controller, 60 amps * 1.25 = 75 amps ~ 80 amp branch wiring/circuit breaker/fuse branch circuit.

    For the input, your series fuse rating for the panels is probably 15 amps--So at the most, you need 4x15 amp = 60 amp rated wiring (and circuit breaker if you wish). I don't know the Vmp/Imp of your panels, but you should probably have:

    Imp*1.25 = minimum rated branch circuit wiring

    To:

    Isc*1.25= another maximum rating needed
    Imp*1.56 = maximum needed to satisfied NEC (or 60 amps, which ever is smaller).

    There is NEC which talks about derating charge controller input:

    60 amp rating / 1.25 NE derating = 48 amp maximum input current--That I don't believe is needed (personal opinion--If the mfg. of the controller did their job, the controller should be rated for 60 amp input current and not need a derating).

    Note that you can choose to run two panels in series (assuming Vmp-array is OK with the charge controller). This would let you use much smaller AWG wiring for the solar array... The down side, is that the MPPT charge controller can be quite a bit less efficient with high Vmp-array and low Vbatt (i.e., Vmp-array>>~30 volts and Vbatt~12 volts). That is your choice.

    Of course, you can use NEC wiring specs (temperature, conduit fill, etc. deratings and different insulation options)... Or you can even use Marine specs. which are much less "conservative" than NEC. (you can get even into more details... Marine "SAE" wiring gauge is a slightly smaller diameter than NEC/NRTL spec'ed gauge).

    In the end, as Westbranch says--This is your choice. I like to be conservative. Solar/Generator Battery charging is maximum current for hours at a time--Not many of us really use that kind of maximum current loads/ratings in our day to day residential/commercial life. Being a bit on the conservative side usually will cause you less headaches down the road.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    cupcake wrote: »
    ...... I am fully insured, don't worry...

    Uh, no. You are fully insured for accidents. Under-spec wire is an installer error, and the installer will pay from own pocket.

    I'd use a voltage drop calculator and run wire that is 2% or less loss for your possible 4 panel array.

    (note - spelling errors are result of new forum software intercepting my spell checker. Fie on bad software)

    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 ,

  • Denman
    Denman Solar Expert Posts: 42 ✭✭
    Personally I always go with bigger wire when ever it's close. Two reasons. 1) I work for a utility company and I've been to way too many house fires because folks wouldn't pay the extra few bucks for big enough wire, then the house catches fire they are always very sad when sitting there watching their life and history go up in smoke. 2) When I use larger wire and later decide I need to add load for new, higher wattage equipment I then do not have to replace the original slightly cheaper wire with newer, bigger, more expensive wire. If I go with the bigger wire in the first place it costs a little more but runs cooler with somewhat less resistance to current flow and I don't have to do stuff twice.

    Not doing things twice is priceless.
  • cupcake
    cupcake Solar Expert Posts: 254 ✭✭✭
    mike95490 wrote: »

    Uh, no. You are fully insured for accidents. Under-spec wire is an installer error, and the installer will pay from own pocket.

    I'd use a voltage drop calculator and run wire that is 2% or less loss for your possible 4 panel array.

    (note - spelling errors are result of new forum software intercepting my spell checker. Fie on bad software)


    I am FULLY insured - accidents, installer errors, warfare, and acts of nature. THANK YOU.
    ~1.5Kw PV in parallel
    Morningstar MPPT-60 controllers (2) in parallel
    3 Trojan tr-1275's in parallel 450ah total
    Samlex 2,000 watt 12-volt inverter hardwired


  • cupcake
    cupcake Solar Expert Posts: 254 ✭✭✭
    Denman wrote: »
    Personally I always go with bigger wire when ever it's close. Two reasons. 1) I work for a utility company and I've been to way too many house fires because folks wouldn't pay the extra few bucks for big enough wire, then the house catches fire they are always very sad when sitting there watching their life and history go up in smoke. 2) When I use larger wire and later decide I need to add load for new, higher wattage equipment I then do not have to replace the original slightly cheaper wire with newer, bigger, more expensive wire. If I go with the bigger wire in the first place it costs a little more but runs cooler with somewhat less resistance to current flow and I don't have to do stuff twice.

    Not doing things twice is priceless.




    OK - I'll go for LARGER wire then...

    Thanks for all your inputs...

    Cakey-Cakes
    ~1.5Kw PV in parallel
    Morningstar MPPT-60 controllers (2) in parallel
    3 Trojan tr-1275's in parallel 450ah total
    Samlex 2,000 watt 12-volt inverter hardwired


  • Ethan Brush
    Ethan Brush Solar Expert Posts: 235 ✭✭
    cupcake wrote: »
    I got 3 panels with an 8.43 short circuit amp rating each...
    I got 10 guage wire going to the controller which can handle 33amps

    SO 8.43 X 3 = 25.29

    So Im good here...

    HOWEVER!

    If I add ONE MORE panel (4 total) the total short circuit amps would be 33.72 (10awg wire can only handle 33amps)

    So should I do thicker wire OR will the 'real world' amps of this setup never really get to 33.72??

    Thanks...

    --cuppycaxe



    where are you getting your ampacity of 33 amps for 10AWG?? all other issues mentioned by others aside, including that you are skipping the 20% derating for continuous loads and the additional 20% solar derating, if we want to get hypothetical we can talk about the ampacity of the 10 gauge. according to NEC 310.16, the ampacity of #10 copper is 35 amps using the 75 degree column and 40 amps using the 90 degree column. Also note that #14 is not 15 amps and #12 is not 20 amps. so why then does everyone say #14 15 amps, #12 20 amps, #10 30 amps? That comes from 240.4 which gives an additional derating for these small conductors in most applications. So the insulation of #10 copper can actually take 40 amps. Then there is the debate about the termination temperature rating which the NEC would normally require to be the 75 degree column ampacity, even though the lug itself is likely rated 90 degrees. Im not saying ignore the NEC or that these deratings are not necessarily, or that I would do it - just giving you the hypothetical.....
  • Alaska Man
    Alaska Man Solar Expert Posts: 252 ✭✭
    I have 30 amps running through 10 gauge everywhere it is needed, tied to a 30 amp main disconnect in the distribution panel and I have NO insurance. To each their own.

    I also have 20 amp breakers tied to 12ga. I consider this a safe installation, but that is my opinion and may not be that of the local Electrical Inspector. The good thing about where I live is that there is no local electrical inspector, just like there is no building inspector.

    If you build it and it burns down or falls on your head it's nobodies fault but your own out here.

    Seeing that you are so well Insured, you might want to run it by the Insurance Company and get them to sign off on it. It sure would suck if the worst happened and they said, "So Sorry"........ Because they will do everything they can to NOT pay a claim.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Interestingly, there is a "*" next to the NEC ratings for 14/12/10 AWG wiring:

    http://lugsdirect.com/WireCurrentAmp...ble-301-16.htm
    The load current rating and the overcurrent protection for conductor types shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.

    I have never found the reason the NEC "hard coded" the maximum current rating vs the "calculated" current rating for these 3x AWG sizes.

    So, technically, 20 amps to 25 amps should be fine based on NEC tables (depending on insulation type and ambient temperatures)--15 amps is a pretty large derating.

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