MC Cable length of run

Dr.Prepper
Dr.Prepper Registered Users Posts: 14
Question about cable diameter and length. I am running cable from my panels(2 215 watt panels) to my shed which will house my charge controller and batteries. The run is about 50 feet total. The mc cable coming off of the panels(the short tails) are 12 gauge diameter. Would I be doing any good but running a thicker gauge wire(say 10g or 8g) from the panel to to the charge controller since the panel only starts with a 12 gauge wire? Will I experience much loss with a 12 gauge wire? Thanks in advance.

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: MC Cable length of run

    Well here's the thing. You're talking about power loss through Voltage drop, and there's a couple of ways to deal with it.

    The first is to use the right size wire. Your 215 Watt panels probably have a Vmp around 30 and an Imp of about 7 (depends on the actual panel) so 14 Amps for the array. Voltage drop depends on both the Voltage and the current. With those specs across 50 feet to keep the V-drop below 3% you end up using 6 AWG wire.

    The second issue is the Voltage itself, as two panels can be wired in parallel or in series. Series wiring doubles the Voltage, halves the current, and reduces the V-drop. So the array then goes to 60 Volts @ 7 Amps and you can now transmit the same amount of power (V*A) over the same 50' using 12 AWG. The difficulty here is you have to run an MPPT type controller to convert the higher Voltage into current at the battery Voltage. You may need to use one anyway if the panels are of the "odd" spec I mentioned (a 24 Volt system would use panels with a Vmp around 35).

    As far as splicing between wire sizes goes to accommodate Voltage drop, that will work: you can have your MC4 pigtails (normally 10 AWG) connected to larger wire to make the long distance run without suffering so much V-drop and then switch to a size that the connection terminals of the controller can accommodate (if necessary).

    You can not do this with current: the smallest wire size in the circuit has to be able to handle the full current as that will be the same throughout.
  • Dr.Prepper
    Dr.Prepper Registered Users Posts: 14
    Re: MC Cable length of run

    Thanks for the reply, here are some specs on the panels
    imp 7.43 amps
    vmp 29 volts
    isc 8.12 amps
    voc 35.6 volts
    I was thinking that wiring in parallel would give me more charging amps to my battery so I was planning on that. I could wire in series though if I can get a better result. I would like to get a MPPT controller but the expense has me leaning toward something like a Tristar ts-45.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: MC Cable length of run
    Dr.Prepper wrote: »
    Thanks for the reply, here are some specs on the panels
    imp 7.43 amps
    vmp 29 volts
    isc 8.12 amps
    voc 35.6 volts
    I was thinking that wiring in parallel would give me more charging amps to my battery so I was planning on that. I could wire in series though if I can get a better result. I would like to get a MPPT controller but the expense has me leaning toward something like a Tristar ts-45.

    The panel specs are pretty much what I expected, which means you have a problem. Without an MPPT controller you have either too much Voltage for a 12 Volt system (which should have Vmp around 17-18) or not enough for a 24 Volt system (which should have Vmp around 35-36).

    So with a PWM type controller what you would get is the current from the panels (7.43 * 2 = 14.86 max) but at the Voltage of the battery. On 12 Volts that means your 430 Watts of panel will operate like 260 Watts of panel at the "correct" Vmp would. This is a significant loss of power and can only be made up for by using an MPPT type controller.

    On a 24 Volt system the Vmp of 29 is not sufficient to fully charge the batteries (Absorb is typically 28.8) due to V-drop from panel heating and wire loss.
  • Dr.Prepper
    Dr.Prepper Registered Users Posts: 14
    Re: MC Cable length of run

    MPPT it is then. What do you think about the Tristar ts-mppt-45? The Outback FM60 also looks to be very good. Using a MPPT I should be able to run in series and turn that voltage in to amps at the battery so I could use a smaller wire from the panels to the controller? Thanks
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: MC Cable length of run
    Dr.Prepper wrote: »
    MPPT it is then. What do you think about the Tristar ts-mppt-45? The Outback FM60 also looks to be very good.

    Of the two the Outback is better. $515 http://www.solar-electric.com/oupofl60mpso.html
    The TriStar is cheaper @ $409 http://www.solar-electric.com/motr45ampmps.html but that doesn't include the display which is another $90.

    What you (and so many others) really need is the yet-to-be-released Rogue 3024 or MidNite "Kid" - which are both 30 Amp MPPT controllers. We're all still waiting on their availability.
    Using a MPPT I should be able to run in series and turn that voltage in to amps at the battery so I could use a smaller wire from the panels to the controller? Thanks

    Yes, as the power would be transmitted as Voltage more so than current. With the panels in parallel you'd be look at 30 Volts @ 15 Amps over 50' and 8 AWG minimum, but you can also put the panels in series for 60 Volts @ 7.5 Amps over 50' and use 12 AWG.

    On a 12 Volt system you'd get roughly 28 Amps peak current for charging with either way (as opposed to about half that relying on panel current only through a PWM controller). There would be slightly lower controller efficiency with the higher array Voltage, but not enough to worry about.
  • Dr.Prepper
    Dr.Prepper Registered Users Posts: 14
    Re: MC Cable length of run

    I may just get the Midnite Classic 150, seems to be about the best available plus I can add more panels with no problem. I really appreciate the advice.
  • WNY Dave
    WNY Dave Registered Users Posts: 23 ✭✭
    Re: MC Cable length of run

    I just put in a FM60 Controller with slightly higher specs as yours, and 230 Watt each, running the standard 12 AWG cables about 50 feet. 24V battery bank.
    (Voc 37. Vpm 30.3, Isc 8.25, Ipm 7.6)

    I have the 2 panels wired In Series and here is what I was getting on my controller the other day around noon just after I had them running for about an hour.

    Attachment not found.
  • Dr.Prepper
    Dr.Prepper Registered Users Posts: 14
    Re: MC Cable length of run

    Dave,
    That looks really good! Thank you for posting that. Using a 24 volt system do you use a step down converter or something if you want to run 12 volt items like lights?
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: MC Cable length of run
    Dr.Prepper wrote: »
    Dave,
    That looks really good! Thank you for posting that. Using a 24 volt system do you use a step down converter or something if you want to run 12 volt items like lights?

    Generally (and of course there are exceptions) avoid DC lights and appliances. There are good sine wave inverters in all sizes and input voltages. If you have DC wiring you will need much thicker wires, DC rated circuit breakers, DC rated light switches, and you will pay more for your lights and appliances. In theory a DC light or appliance can be more efficient than an AC light or appliance, and there will be some power loss in the inverter, but low voltage DC systems have their own inefficiencies, mostly due to losses in the wiring.

    Example: I have purchased the most efficient bathroom vent fan on the market. It is powered by 120 volt AC, but the fan motor is a DC motor... there is a small "wall wart" type of power supply built in to power the motor. Power supplies are so cheap and efficient these days that it may be better to wire your house for AC and then create DC where you need it (if you really need it at all).

    Another problem with DC systems is the voltage swings. Your battery voltage varies (charging and discharging) and this may play havoc with your DC lights and appliances.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i