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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

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  • Does it not depend to some extent upon where the solar is connected?

    On the naive assumption that your 100 A fuse will blow at 101 A, If you have a 4 kW solar array connected upstream of the DB, you could potentially draw 117 A through your main switch and proximal busbar. However, we all know that you could draw 117 A from the grid, at least in the short term (if you can afford it). Presumably manufacturers take this into consideration, so your nominal 100 A board will cope with more.

    What the board cannot do is draw the slight overload from the grid plus the solar output, so the board's rating needs to be observed.

    If you feed the solar in at the distal end of the busbar, only the nominal 100 A goes through the mainswitch and somewhere in between there is no flow in the busbar, but as Alan says, there remains the question of heat dissipation in the board as a whole.

    I suppose that in this instance, diversity is your friend, but I would be very wary of knowingly allowing a DB to be overloaded - it might catch on fire.

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  • Does it not depend to some extent upon where the solar is connected?

    On the naive assumption that your 100 A fuse will blow at 101 A, If you have a 4 kW solar array connected upstream of the DB, you could potentially draw 117 A through your main switch and proximal busbar. However, we all know that you could draw 117 A from the grid, at least in the short term (if you can afford it). Presumably manufacturers take this into consideration, so your nominal 100 A board will cope with more.

    What the board cannot do is draw the slight overload from the grid plus the solar output, so the board's rating needs to be observed.

    If you feed the solar in at the distal end of the busbar, only the nominal 100 A goes through the mainswitch and somewhere in between there is no flow in the busbar, but as Alan says, there remains the question of heat dissipation in the board as a whole.

    I suppose that in this instance, diversity is your friend, but I would be very wary of knowingly allowing a DB to be overloaded - it might catch on fire.

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