Another beginner question about wiring.

levsmith
levsmith Solar Expert Posts: 56 ✭✭
I'm getting ready to order the wiring for my first system but want to make sure I have one thing right. On the wiring between the charge controller and battery bank, I base the size off of the output of the charge controller and the size or amps of the battery bank does not come into play, correct? I will probably be ordering for the maximum size of CC that i plan to have in the future, So for instance if I plan to have a 60 amp CC i would want probably 6 or 4 gauge?

Also, I'm just starting out with two 6v batteries, would I want the wiring between the batteries the same size as from the battery bank to inverter, or can i go smaller?

I thought this was going to be simple, but boy was I wrong! I cant imagine not having you guys around to ask for help! I would be lost.

Thanks for any help!

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Another beginner question about wiring.
    levsmith wrote: »
    I'm getting ready to order the wiring for my first system but want to make sure I have one thing right. On the wiring between the charge controller and battery bank, I base the size off of the output of the charge controller and the size or amps of the battery bank does not come into play, correct? I will probably be ordering for the maximum size of CC that i plan to have in the future, So for instance if I plan to have a 60 amp CC i would want probably 6 or 4 gauge?

    This is correct. In normal operation the most current these wires will see is the maximum the charge controller puts out; the battery should not be supplying current to the controller. This is why there should be a fuse or breaker on that circuit rated for the wiring; in case something goes wrong like a short inside the controller and the battery starts trying to dump its massive current capacity into it.

    6 AWG is what most of the larger (60 Amp) controllers use. 4 AWG would be used on 80 Amp controllers like the Classic.
    Also, I'm just starting out with two 6v batteries, would I want the wiring between the batteries the same size as from the battery bank to inverter, or can i go smaller?

    Same size: the current through the circuit is the same on negative, positive, and interconnecting leads.
    I thought this was going to be simple, but boy was I wrong! I cant imagine not having you guys around to ask for help! I would be lost.

    Thanks for any help!

    No, it's not simple. Definitely not.

    You're entirely welcome. :D
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Another beginner question about wiring.
    levsmith wrote: »
    On the wiring between the charge controller and battery bank, I base the size off of the output of the charge controller and the size or amps of the battery bank does not come into play, correct? I will probably be ordering for the maximum size of CC that i plan to have in the future, So for instance if I plan to have a 60 amp CC i would want probably 6 or 4 gauge?

    You are correct, but I can still see a problem. First of all read the manual that comes with your controller. It will tell you what size wires and fuses to use.

    You are apparently starting out with a small controller and want to use large wires for a future, larger controller. That is good thinking and planning. The problem I see is that the large wires may not fit onto the smaller controller. Its hardly an insurmountable problem, but if you're far from civilization when you do the installation and you need a few extra parts...

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • levsmith
    levsmith Solar Expert Posts: 56 ✭✭
    Re: Another beginner question about wiring.
    This is correct. In normal operation the most current these wires will see is the maximum the charge controller puts out; the battery should not be supplying current to the controller. This is why there should be a fuse or breaker on that circuit rated for the wiring; in case something goes wrong like a short inside the controller and the battery starts trying to dump its massive current capacity into it.

    6 AWG is what most of the larger (60 Amp) controllers use. 4 AWG would be used on 80 Amp controllers like the Classic.



    Same size: the current through the circuit is the same on negative, positive, and interconnecting leads.



    No, it's not simple. Definitely not.

    You're entirely welcome. :D

    Thank you very much! I might actually be starting to understand some of this stuff. Amazing my simple mind can comprehend this! :D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Another beginner question about wiring.

    If the charge controller is rated for 60 amp maximum output current then:
    • 60 amps * 1.25 NEC derating = 80 amp minimum rated fuse and wire gauge.

    Then you need to look a the wire length... My personal recommendation is a maximum of 0.1 to 0.5 volts maximum drop at 60 amps for a 12 volt battery bank (2x that for 24 volt bank; 4x for a 48 volt bank).

    So, we need to know the length of wiring. Say you have 5 food one way wire run. Using a generic voltage drop calculator:
    • 5 feet at 60 amps and maximum of 0.1 volt drop => 2 awg copper wire at room temp would give 0.11 volt drop

    So, you can see that wire length is critical to a good "low voltage" design.

    Then double check with code as to the actual supported current. NEC is conservative--And pretty safe:

    Wire
    Current Ampacities NEC Table 310-16


    80 amp wire would require somewhere around 4-3 AWG copper wire (note there are insulation choices, ambient temperature derating, and conduit fill factors too).

    Or you can use a Boating Standard--Much less conservative, but still popular:

    ABYC Wiring
    Standards


    They would allow 8-6 AWG wire minimum (exposed wire has better cooling, so smaller awg can work). Note that boating standard uses SAE Gauge (I think), which is very slightly smaller than American Wire Gauge (NEC tables)....

    Remember, that normal house wiring is hardly ever pushed to its limits (your 15 amp outlet usually only has a couple of amps of load; your 200 amp main panel may only have 10 amps on average going though it, etc.)...

    However, as poster Chris Olson has shown... When you are running battery banks with solar and generator power--It is very possible to operate at 100% of rated power for hours (or even days) at a time--Even equipment that is running at rated current can overheat when operated for many hours per day (especially on hot summer days).... Commercial generators will have both a "standby" and "prime mover" ratings--With the Prime Mover at ~80% of the standby rating (Prime rating is ability to supply power for hours-days-weeks at rated power).

    I would high suggest that you be conservative in your wiring/application choices when dealing with the "battery bus" side of your power design... The AC output side of your power distribution--Following code or even ABYC standards is fine... Charging and Discharging the battery bank with hours of high current operation--Be conservative.

    Head spinning yet?

    -Bill

    PS: It is simple--But all of these "simple rules" must be addressed/respected at the same time... Forget one just of them when designing/building a system--Oops. :cry:
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Another beginner question about wiring.

    Bill;

    Your post in another thread about excessive derating is somewhat prophetic here. :D

    NEC wants the power output on controllers derated, making a 60 Amp a 45 Amp (some will say "no derating", btw) so then you end up with 4 AWG wire on a 60 Amp controller handling 45 Amps. :p

    I argue with their application of "continuous use" for controllers as they do not see full current all the '3 hour' time (nor should they).

    BTW, it is as I said where 60 Amp controllers take 6 AWG. There is no point in trying to "choke down" 4 AWG so it will fit in the terminals. The power demands should not be peak all the time and the wire lengths should not be significant enough to cause V-drop concerns. 6 AWG can actually take 80+ Amps for long enough that the circuit protection should trip before the wire fries.

    NEC deratings are sometimes greatly over-rated. :p:D
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Another beginner question about wiring.

    Marc,

    I would tend towards the 4 awg and 80 amp ratings... Even if it required trimming a few wires off the cable to fit into the charge controller connection or splicing the last foot of 4 awg wire with 6 awg to get it into the terminal.

    We tend to derate the arrays to by 1/0.77 = 1.3x larger array... Which means that the charge controllers are much more likely to see ~100% of rated current for many hours per day.

    That is an example of where we up rate the solar array to make up for "not very practical marketing numbers" to meet the actual needs--And that now means the 0.80 or 1.25 NEC derating factors are more important.

    If the array is not "optimized" to maximum "cost effective" support size, but smaller--Then up rating the cabling to the controller is not as important.

    Remember that J. Wiles does this on the other side--He wants Isc of the solar array to be derated by 1.25*1.25 or by 1.56 ... His own double derating on top of what the charge controller mfg. has already (in theory) derated so they can list maximum continuous supported current (vs "marketing" otuput).

    My head is spinning.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Another beginner question about wiring.

    Bill;

    The worst case would be hybrid GT inverter systems where the controllers are expected to supply peak capacity for as long as possible in order to maximize power to the grid/loads. That really would be continuous (we hope) maximum power output: more then 3 hours. Definitely want to maximize wire size for that use.

    In the off-grid world, however, this would not be typical operating conditions.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Another beginner question about wiring.

    I tend to look at worst case conditions.

    If you just had a two day storm and now you have a cool/clear day of full sun with 50% state of charge on your battery banks and a bunch of appliances to run around the home... May take 1+ days of full sun to recharge the bank (and perhaps generator if several days+ of no sun)--You will be running at 100% of available power for many hours.

    Or if you run your battery bank at 50-80% state of charge cycling--The charging sources will be at 100% of output during theses. Or you may be running your opportunity loads (water heater)... All a much more load intensive set of power requirements than the "typical" home on grid power.

    Anyway--My reasoning for making conservative choices--Different people will have differing needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Another beginner question about wiring.

    Well normally we "size up" charge controllers to begin with: if the expected maximum current is 50 Amps you'd be using a 60 Amp controller, so there's a degree of derating right there. And depending on how much you over-size the array there won't actually be full power available all day as the panel production will start low, ramp up as insolation improves, then taper off. It isn't like an AC based charger that can be switched on a full current come 7:00 AM. :D

    At least I haven't seen one yet that is so over-paneled. Doesn't mean it isn't possible. :p