What type of cable from an inverter?

  • #13956 Reply

    jules

    I am shortly to move into a new build and would like to have solar panels fitted to the south facing rear. As the walls have yet to be completed I intend to ask the builder to lay a cable from the loft through to the junction box in readiness. What size cable would you recommend and is there anything else I should consider before the house is completed

  • #14468 Reply

    Norman

    The standard cable is 4mm 2 twin and earth.

  • #14557 Reply

    Norman

    Standard practice is to site the inverter in the attic as near to the panels as possible.

    There are two reasons for this:

    1. To keep the DC cable run as short as possible. (it’s high voltage and small amounts of power can be lost in the run to the inverter)
    2. Space. Due to the size of the inverter the attic is best placed rather than within the house.

    Attics get very hot in the summer so your inverter may have a fan which kicks in to stop overheating. As your roof faces South West it will get the sun late in the evening so the noise may affect your child. Maybe check this with your installer.

    If you have an internal garage this could be suitable. The inverter could be wall mounted onto a fire proof MDF board.

  • #16706 Reply

    Stuart

    Volts drop and maximum cable run for a 4 KW solar PV system
    AC side.

    Figures from BS7671 : 2008
    Incoming Voltage single phase 230v
    4 KW Solar PV System.
    4000 watt / 230 v = 17.39 Amps
    Maximum Volts Drop = 1% = 2.3 Volts

    Current Carrying Capacity Table 4D2A reference Method B enclosed in conduit.

    1.5 mm = 16.5 amps (not acceptable as below the 17.39 amps required)
    2.5 mm = 23 amps (maximum length 7.34 meters but not recommended by SMA)
    4.0 mm = 30 amps (maximum length 12 meters)
    6.0mm = 38 amps (maximum length 18.11 meters)
    Volts drop Table
    2.5 mm = 18 mV/A/m
    4.0 mm = 11 mV/A/m
    6.0 mm = 7.3 mV/A/m
    Example if we want to use a 2.5mm Cable this is how we work out the volts drop.
    Amps x Length x Volts Drop /1000 = 2.3 volts (maximum volts drop)
    17.39 x Length x 18 /1000 = 2.3
    17.39 x 18 x Length = 1000 x 2.3
    313.02 x Length = 2300
    2300/313.02 = Length
    Length = 7.34 Maximum Length in Meters

    This is why we use 4.0mm or even 6.0mm cable from the mains to the inverter.

    Using the same working the maximum length for 4.0mm is 12 meters and for 6.0mm is 18.11 meters.

    You need a 6.0mm cable running from the mains DB to the inverter to comply to BS 7671.






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