How is cable size determined for PV system connected to consumer/distribution board?

Hi all,

Is the cable size for a PV system connecting back to the consumer/distribution board calculated the same way as for any other circuit. Check the load/current that the circuit will be drawing and make sure that the cable's current carying capacity is the same or greater. Then check the voltdrop is within the  permited levels.

In PV system's case, see how much AC and DC current the PV system will be supplying and make sure that the cable current carrying capacity matches it or exceeds it.Check the volt drop in the same way?



Also, if its a grid tied system. Does the current from PV system flow through the incoming DNO's fused cutout ? How do they deal with sites that have large PV and the site's generation current is bigger than the fused cutout size?

  • basically yes, Cable  is sized by thermal and VD considerations. The complication is that the PV generated current flows the wrong way, compared to a traditional load, so the voltage drop is reversed - unsurprisingly the voltage ends up being  higher at the generating end of things. All in verters that have approval must wind down if the terminal voltage gets too high.

    The connection of PV to the grid requires DNO approval, and one reason for this so they can be sure that nothing will be connected that can push so much back into the DNO network that it causes a problem, either of over current, or over voltage.


  • (Obviously also check for ADS / adiabatic etc also as normal)

    BS7671 does not specify anything special wrt. voltage drop (rise) but both for economic reasons and to avoid the inverter(s) disconnecting prematurely on the G98/G99 interface protection the custom is to work to a somewhat reduced limit, i.e. normally 1% AC for domestics, as per the IET Code of Practice.

    Yes, generated electricity not used on site will flow "backwards" through the cutout.

    Any connection in excess of 16A/ph will need prior consent from the DNO per EREC G99. This will include the DNO assessing the capacity of the network, protection etc, require any network upgrade works (including, if necessary, the "company fuse") to be completed at the customer's expense, and result in a site-specific Connection Agreement being signed. If the total generator (inverter AC) capacity exceeds the export limit in the connection agreement - assuming it's allowed by the DNO - there will also need to be an export limiting scheme in accordance with EREC G100. This will need to be noted in the grid application. Clearly this will need to be coordinated.with the company fuse, to avoid such things as causing it to blow following a sudden loss of load...

    You might wish to read the ENA guide to the connection process: