Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • Replies 11 replies
  • Subscribers 35 subscribers
  • Views 132 views
  • Users 0 members are here
Options
You may also be interested in

What is the logic for solar sub boards avoiding overload of main board bus bar

Alan B

Just looking at an installation now where there's a single phase 100 amp supply, high load and solar.

Initially I thought guidance says if I put solar on a seperate board I avoid going over 100 amps for the main switch and busbar.
But then thinking about it if I put the solar on a sperate board and it's feeding the house rather than the grid all I achieve is focusing the extra current flow on the main switch and one end of the bus bar. I guess whoever came up with guidance assumed the solar would be feeding the grid or there was more than one board taking the load, seems like a poor assumption.
It makes more sense if the limitation is heat dissipated by devices, 15W or so per RCBO or MCB at full load, which from memory at least is different to the guidance i have watched or seen in the past. I can also put the solar at the far end of the bus bar and distribute the devices running closer to max load.

If it's to do with heat dissipation I am down to working out how many devices are running at a significant proportion of max load, heat dissipated and max thermal load on the enclosure, data that's probably not available. 


To me it's more important to distribute the load over more than one board. 

I know there are a few boards that are rated at 125A, so far from what I have looked at a cheap make I don't trust or an expensive makes that tend to over inflate costs of doing things like upgrading to type A RCD's and I avoid in principle because of this.My hope is that Fusebox say it's ok with their 125A main switch,they don't have any instructions or data sheet available online

Parents

  • If you are going to have two separate CUs then for this to work, you need at least three fuses, one of which will be the DNO fuse, arranged in a Y.

    If we assume that the DNO supply comes in at the bottom of the Y then you will have a (in this case) 100A fuse there.  You will also need a 100A fuse from the centre of the Y to the main CU (say at the top left) and you will need a fuse, rated at a maximum of (100-max solar current) for the second CU going to the top right of the Y.  The wiring between the fuses will need to be rated at the sum of the DNO fuse and the solar inverter rating.

    Basically, each CU needs to be on its own fuse (just install a standard switch fuse before each CU).

    Note that the wiring between the fuses will include the service head to meter and meter onwards tails, there may be those who would argue that these need to be protected, in which case you need a fourth 100A fuse in series with the DNO one, I would argue that the DNO fuse is sufficient to protect these tails (assuming that the other conditions applicable for the use of the DNO fuse to protect tails are met).

    If you go over a potential flow of more than 100A through a CU then it is your job to do the engineering to confirm that it is acceptable - unless you happen to have insider access to the design arguments that the manufacturer uses to underpin a 100A CU rating, you are unlikely to be able to do that in a way that is considered "competent".

Reply

  • If you are going to have two separate CUs then for this to work, you need at least three fuses, one of which will be the DNO fuse, arranged in a Y.

    If we assume that the DNO supply comes in at the bottom of the Y then you will have a (in this case) 100A fuse there.  You will also need a 100A fuse from the centre of the Y to the main CU (say at the top left) and you will need a fuse, rated at a maximum of (100-max solar current) for the second CU going to the top right of the Y.  The wiring between the fuses will need to be rated at the sum of the DNO fuse and the solar inverter rating.

    Basically, each CU needs to be on its own fuse (just install a standard switch fuse before each CU).

    Note that the wiring between the fuses will include the service head to meter and meter onwards tails, there may be those who would argue that these need to be protected, in which case you need a fourth 100A fuse in series with the DNO one, I would argue that the DNO fuse is sufficient to protect these tails (assuming that the other conditions applicable for the use of the DNO fuse to protect tails are met).

    If you go over a potential flow of more than 100A through a CU then it is your job to do the engineering to confirm that it is acceptable - unless you happen to have insider access to the design arguments that the manufacturer uses to underpin a 100A CU rating, you are unlikely to be able to do that in a way that is considered "competent".

Children
No Data

Community Rules & Guidelines